These products are similar but can differ markedly from each other e.g. traditional Italian gelato is significantly different than mass produced, commercial ice cream in Ireland, GB and the US. However, high end artisan produced ice cream made using batch processes can be similar.

Traditional Italian gelato generally contains around 8% milk fat, has a low overrun usually (< 18%) and is served from a cabinet held at around -11°-12°C. The physical properties are usually different.  Because of the higher temperature of storage and the concentration of sugars, gelato is served and eaten in a highly viscous, semi-frozen state. The texture is often described as being smooth and dense. Fruit-flavoured gelato is also characterised by intense natural fruit flavours and is quite sweet although natural fruit-like sour notes may be evident.

Traditional gelato is produced using batch production methods and is generally consumed within a short time from manufacture.

While there are differences between these products the scientific principles that underpin their production are similar and in the articles that follow, I will often use the terms ice cream and gelato interchangeably. Please accept that I do not equate the quality and eating experience of traditional Italian gelato with mass-produced commercial ice cream.


Refrigerated storage of raw milk is used to limit the growth of microorganisms in milk prior to processing. It has been known for some time that the quality and yield of cheese produced from bulk cooled milk may be adversely affected by this procedure (Weatherup et al., 1988; Weatherup and Mullan, 1993). The reduced yield and poor quality may be due to physico-chemical changes in the state of several milk components e.g. dissociation of micellar casein, mainly Κ-casein into a soluble phase, occurs during the first 48 h of storage at 4° and 7° C. This results in losses of fat and curd fines, weaker curd, more moist curd and a slightly lower yield. Partial reversal of dissociation occurs after further storage. The reduced yield and quality can also be due to the activity of proteases and lipases produced by psychrotrophic bacteria.

Despite the work that has been done over many years milk is still being stored for extended periods (1-3+ days on some farms) and cheesemakers are again (2019) reporting problems with the yield and quality of cheese produced using this milk.

Following several queries related to milk quality and cheese manufacture I am providing a report written by Wilf Weatherup and me some years ago that may be helpful.

A simple calculator has been provided using the total viable count of milk prior to pasteurisation and a regression equation to predict the grade value of Cheddar cheese.

I get lots of queries from companies and individuals either requesting recommendations for people who might be suitable for dairy science, dairy technologist, food technologist jobs or for advice from food job seekers on vacancies in particular job areas. I regret that I can no longer respond directly to these queries, I simply do not have the time to do so.

I will use  this page to  publicise posts connected with the subject areas covered by this site and will consider publicising the details of people searching for work here also. If you wish to advertise a vacancy on this site please use the contact form to discuss your vacancy. Companies placing adverts normally make a small donation towards the running costs of the Dairy Science and Food Technology website or link to this site.  

You can also obtain information about careers and current vacancies in food science and food technology in the links section of this site. I will try to keep these updated and have added a number of  recruitment company links. Note that there is a world wide shortage of food scientists and technologists and that pay and working conditions in many countries now reflect this new reality. Food scientists and technologists with postgraduate qualifications are particularly well paid in the US and the remuneration situation is now improving significantly in the UK, Ireland and in other parts of Europe.

Typical pots of UHT milk


This article investigates how to calculate the lethal effects of UHT treatment and the usefulness of TTIs for differentiating sterilised, direct and indirectly processed UHT-treated milk. The importance of accessing accurate temperature time-data and knowing holding tube dimensions, flow rate, average and minimum holding time and the flow characteristics (Reynolds number) are discussed. The reliability of a model developed by Claeys et al. (2003) to predict the effects of UHT-processing on hydroxymethylfurfural, lactulose and furosine concentrations in milk is discussed. Free On Line calculators for calculating holding time, average flow rate, holding tube length in UHT and HTST plants are provided. A free On Line calculator programmed using the thermal constants calculated by Claeys et al. (2003) is provided to calculate hydroxymethylfurfural, lactulose and furosine concentrations following heat treatment in skim, semi fat and full fat milks. This calculator also calculates F0, B*, C* and % destruction of thiamine. Two methods of numerical integration are used to measure the cumulative lethal and chemical effects of UHT treatment, namely the Trapezoid and Simpson's rules.


Typical UHT treatments involve heating milk to 137 to 150 in a continuous-flow process and holding at that temperature for one or more seconds before cooling rapidly to room temperature. The milk is then aseptically packaged to give a product that is stable for several months at ambient temperature.

In Europe, UHT treatment is defined as heating milk in a continuous flow of heat at a high temperature for a short time (not less than 135 °C in combination with a suitable holding time, not less than a second) such that there are no viable microorganisms or spores capable of growing in the treated product when kept in an aseptic closed container at ambient temperature (Reg EC 2074/2005).

Why is the freezing curve of an ice cream or gelato mix important?

Freezing curves are useful in developing new ice cream and gelato products and predicting the hardness of the ice cream produced from particular mixes at designated temperatures, including the serving temperature. Curves can also be used to predict sensitivity to heat shock, storage stability and resistance of packaging materials to deformation during distribution and storage.

Many students have problems in understanding the mathematics describing the destruction of microorganisms by heat. Log reductions of pathogens and equivalent time-temperature treatments along with the associated lethalities account for a large part of the harder to understand topics. The quiz below is a simple test of of some of the basic concepts. Note Z value is not dealt with in this quiz. If there is sufficient interest I will provide the answers.

Heat Processing Quiz

Small, and even large companies, frequently find it difficult to contact potential new supplier or service partners. This is a particular problem for some small companies attempting to operate testing laboratories.

However, there are companies who provide exemplary customer service and work hard to meet customer needs. The following is a list of these companies that provide exemplary customer service in the laboratory service area.



This section provides the context to using Excel to calculate the cumulative lethal effects (at all stages during processing) of heat on microorganisms and provides an explanation of how the Excel spreadsheets and On Line calculators available for download from the Dairy Science and Food Technology (DSFT) work.

Here we provide an overview of the background, including a summary of the underlying mathematics, required to produce an Excel spreadsheet for performing basic thermal processing calculations. Note I am not providing a guide to using spreadsheets but basic information that a competent Excel user should be able to use to make their own thermal processing spreadsheet.

Download Excel Thermal Processing Spreadsheets.

Spreadsheets for calculating F, B*, C* values of thermal processes and the concentration of Time Temperature Indicators.


DSFT has an extensive range of free-On Line resources for undertaking a wide range of thermal process calculations including dry heat sterilisation (FH) and depyrogenation (FD or FP). This page provides access to a range of Microsoft Excel spreadsheets that can be downloaded for a small donation and will work on a PC or Mac without access to the Internet. In general these spreadsheets are similar to their corresponding free applications on this website.

These donations are important. They contribute towards the running costs of the website and enable me to provide free access to the website rather than require a subscription.


There will be occasions when a food manufacturer wishes to use a different, but equivalent lethal thermal process. How does the processor calculate the equivalent process?

This article explains how to calculate an equivalent thermal or heat process at a higher or lower temperature and provides access to a free On Line calculator for checking your calculations.

Providing that the F value at Tref and the z value are known then the F value at the required temperature, T, can be calculated using equation 1.

Equation 1.  heq1 articles


Equation 1 has been derived from Stumbo (1973).

Making ice cream for Goldilocks!

Ice cream or gelato should not be too hard or too soft at the serving-temperature.   Hardness is largely dependent on the concentration of frozen water at the serving-temperature which is influenced by the concentration of lactose, sucrose, glucose, other sugars and milk salts in the mix. This article explains the significance of sugar concentration on the freezing point depression (FPD) of ice cream. The effect of individual sweeteners on FPD is discussed. The use of relative freezing point depression (RFPD), which does not include lactose, to control hardness in commercial ice cream production is outlined. A similar method widely used in continental Europe that includes lactose in the calculation, the Potere Anti Congelante (PAC) method, is discussed using worked examples. Examples of how to reformulate ice cream mixes to obtain a controlled hardness whilst maintaining a similar relative sweetness value are also provided.


El helado o helado no debe ser demasiado duro o demasiado blando en la porción temperatura. La dureza es en gran parte dependiente de la concentración de agua congelada en la porción-temperatura que está influenciada por la concentración de lactosa, sacarosa, glucosa, otros azúcares y sales de la leche en la mezcla. En este artículo se explica la importancia de la concentración de azúcar en la depresión del punto de congelación (FPD) de helado. Se discute el efecto de los edulcorantes individuales de FPD. Se describe el uso de la depresión relativa del punto de congelación (RFPD), que no incluye la lactosa, para controlar la dureza en la producción comercial de helados. Un método similar se utiliza ampliamente en Europa continental que incluye lactosa en el cálculo, el anti congelante Potere (PAC) método, se discute el uso de ejemplos prácticos. Ejemplos de cómo reformular el helado se mezcla para obtener una dureza controlada, manteniendo un valor dulzura relativa similar también se proporcionan.


Eis oder Eis sollte nicht zu hart oder zu weich an der Portion-Temperatur. Die Härte ist weitgehend abhängig von der Konzentration des gefrorenen Wassers an der Serving-Temperatur, die durch die Konzentration von Lactose, Saccharose, Glucose, Zucker und Milch anderen Salzen in der Mischung beeinflusst wird. Dieser Artikel erklärt die Bedeutung der Zuckerkonzentration auf der Gefrierpunktserniedrigung (FPD) von Speiseeis. Die Wirkung der einzelnen Süßstoffe auf FPD wird diskutiert. Die Verwendung von relativ Gefrierpunktserniedrigung (RFPD), die keine Lactose, um die Härte in der kommerziellen Herstellung von Eiscreme steuern skizziert. Ein ähnliches Verfahren weit verbreitet in Kontinentaleuropa verwendet die Laktose in der Berechnung berücksichtigt, wird die Anti Potere Congelante (PAC)-Methode, diskutiert mit Beispielen gearbeitet. Beispiele, wie man Eis formulieren mischt, um eine kontrollierte Härte bei gleichzeitiger Wahrung eines ähnlichen relativen Süße Wert sind ebenfalls vorhanden zu erhalten.


Gelato o gelato non dovrebbe essere troppo duro o troppo morbido al servizio-temperatura. Durezza dipende in larga misura la concentrazione di acqua congelata al serving temperatura che è influenzata dalla concentrazione di lattosio, saccarosio, glucosio, altri zuccheri e sali latte nella miscela. Questo articolo spiega il significato della concentrazione zuccherina sulla depressione del punto di congelamento (FPD) di gelato. È discussa l'effetto dei singoli dolcificanti sul FPD. L'uso di relativa depressione del punto di congelamento (RFPD), che non include il lattosio, per controllare la durezza di ghiaccio commerciale produzione crema è delineato. Un metodo simile ampiamente utilizzato in Europa continentale che comprende lattosio nel calcolo, il metodo Potere Anti Congelante (PAC), è discusso con esempi pratici. Esempi di come riformulare il gelato mix per ottenere una durezza controllata mantenendo un simile valore di dolcezza relativa sono anche previste.


La crème glacée ou de glace ne doivent pas être trop dur ou trop mou à la portion température. La dureté est largement dépendante de la concentration de l'eau congelée à la portion de température qui est influencé par la concentration de lactose, de saccharose, de glucose, d'autres sucres et des sels de lait dans le mélange. Cet article explique l'importance de la concentration de sucre sur l'abaissement du point de congélation (FPD) de crème glacée. L'effet des édulcorants individuels sur FPD est discutée. L'utilisation de l'abaissement du point de congélation par rapport (RFPD), qui ne comprend pas le lactose, pour contrôler la dureté de la production de la crème glacée du commerce est décrit. Une méthode similaire largement utilisé en Europe continentale qui inclut lactose dans le calcul, la méthode Potere Anti congelante (PAC), est examinée en utilisant des exemples pratiques. Des exemples de la façon de reformuler mélanges à crème glacée pour obtenir une dureté contrôlée, tout en conservant une valeur de douceur relative similaire sont également fournis.

الآيس كريم أو الجيلاتي لا ينبغي أن يكون من الصعب جدا أو لينة جدا في درجة الحرارة خدمة. صلابة يعتمد إلى حد كبير على تركيز المياه المجمدة في درجة الحرارة التي تخدم يتأثر تركيز اللاكتوز، السكروز، الجلوكوز والسكريات الأخرى وأملاح الحليب في هذا المزيج. وهذا المقال يوضح أهمية تركيز السكر على الاكتئاب نقطة التجمد (حماية الأسرة) من الآيس كريم. وتناقش تأثير المحليات الفردية على حماية الأسرة. ويرد استخدام النسبية تجميد الاكتئاب نقطة (RFPD)، والتي لا تشمل اللاكتوز، للسيطرة على صلابة في إنتاج الآيس كريم التجارية. وهناك طريقة مشابهة تستخدم على نطاق واسع في أوروبا القارية التي تضم اللاكتوز في الحساب، وPotere مكافحة Congelante (PAC) الأسلوب، وتناقش باستخدام أمثلة عملت. أمثلة على كيفية إعادة صياغة الآيس كريم يمزج للحصول على صلابة تسيطر عليها مع الحفاظ على القيمة النسبية حلاوة مماثلة كما تقدم.


Roomys of gelato moet nie te hard of te sag op die bediening-temperatuur. Hardheid is grootliks afhanklik van die konsentrasie van bevrore water op die bediening-temperatuur wat deur die konsentrasie van laktose, sukrose, glukose, ander suikers en melk sout in die mengsel beïnvloed word. Hierdie artikel verduidelik die betekenis van suiker konsentrasie op die vriespuntverlaging (FPD) van roomys. Die effek van individuele versoeters op FPD bespreek word. Die gebruik van relatiewe vriespuntverlaging (RFPD), wat nie laktose sluit, hardheid te beheer in kommersiële roomys produksie word geskets. 'N Soortgelyke metode wyd gebruik word in die vasteland van Europa wat insluit laktose in die berekening, is die Potere Anti Congelante (PAC) metode, bespreek die gebruik van uitgewerkte voorbeelde. Voorbeelde van hoe om roomys te herformuleer meng 'n beheerde hardheid te verkry, terwyl die handhawing van 'n soortgelyke relatiewe soet waarde word ook voorsien.


Ijs of gelato moet niet te hard of te zacht zijn op de portie-temperatuur. Hardheid is grotendeels afhankelijk van de concentratie van bevroren water in de portie-temperatuur die wordt beïnvloed door de concentratie van lactose, sucrose, glucose, andere suikers en melkzouten in de mix. Dit artikel legt de betekenis van de concentratie suiker op de vriespuntdaling (FPD) van ijs. Het effect van afzonderlijke zoetstoffen op FPD wordt besproken. Het gebruik van relatieve vriespuntdaling (RFPD), die geen lactose, hardheid beheersen commerciële productie van ijs wordt geschetst. Een soortgelijke methode op grote schaal gebruikt in continentaal Europa, dat lactose in de berekening opgenomen, de Potere Anti Congelante (PAC)-methode, wordt besproken met behulp van uitgewerkte voorbeelden. Voorbeelden van hoe je ijs herformuleren mixen om een gecontroleerde hardheid te verkrijgen met behoud van een vergelijkbare relatieve zoetheid waarde zijn ook aanwezig.



גלידה או גלידה לא אמורה להיות קשה מדי או רכה מדי בהגשה בטמפרטורה. קשיות היא תלויה במידה רבה את הריכוז של מים קפואים בהגשה בטמפרטורה שמושפעת מהריכוז של לקטוז, סוכרוז, גלוקוז, סוכרים ומלחים אחרים חלב בתערובת. מאמר זה מסביר את חשיבותו של ריכוז סוכר בדיכאון נקודת הקיפאון (FPD) של גלידה.ההשפעה של ממתיקים בודדים על FPD נדונה.השימוש בדיכאון יחסי הקפאת נקודה (RFPD), שאינו כולל לקטוז, כדי לשלוט בקשיות קרם ייצור קרח המסחרי הוא התווה.שיטה דומה בשימוש נרחב ביבשת אירופה הכוללת לקטוז בחישוב, Potere אנטי Congelante (PAC) שיטה, נדונה באמצעות דוגמאות עבדו. דוגמאות כיצד לנסח מחדש את הגלידה מתערבבת להשיג קשיות מבוקרת, תוך שמירה על ערך מתיקות יחסי דומה גם כן.





8th NIZO Dairy Conference, 11-13 September 2013

8th NIZO Dairy Conference
Functional Enzymes for Dairy Applications
11-13 September 2013, Papendal, The Netherlands

Enzymes play a crucial role in achieving and maintaining the desired textural and sensorial properties of almost all dairy products. Prime examples include role of enzymes in the conversion of milk into a cheese and its subsequent flavor formation during ripening and the role of enzyme inactivation in extending the shelf-life of liquid dairy products. In addition, enzymes are widely employed to add further value to dairy ingredients and products, e.g., in the production of oligosaccharides from lactose, lysophospholipids and the various enzymatic tools available to optimize milk protein functionality. All these topics, from fundamental understanding to application in consumer products, will be covered during the conference.

Key Speakers and conference papers

Herwig Bachmann, VU University Amsterdam / NIZO food research, The Netherlands
Enzymes and their role for the selection and improvement of dairy starter cultures

Richard IpsenUniversity of Copenhagen, Denmark
Enzymatic modification to improve milk lipid functionality

Paul JelenUniversity of Alberta, Canada
Abstract: Enzymatic production of lactose derivatives

Alan KellyUniversity College Cork, Ireland
Abstract: Physiological and technological significance of proteases in mammalian milk

Isidra Recio, Universidad Autónoma de Madrid, Spain
Abstract: Dairy protein and technological significance of proteases in mammalain milk

Fred van de Velde, NIZO food research, The Netherlands
Abstract: Enzymatic modification of milk proteins

Luc van LoonMaastricht University Medical Centre+, The Netherlands
Milk protein and muscle maintenance

For more information see the NIZO website.

Mycobacterium avium subsp. paratuberculosis

The article on Mycobacterium avium subsp. paratuberculosis (MAP) was updated in May 2013 to correct  typographical errors in two of the equations used and a formatting error where the first few paragraphs of the article were not shown in some browsers. 

News at April 2013

The calculator for determining the decimal reduction time (D) of a microorganism at a temperature other than the reference temperature has been updated to show the very low values of D at high temperatures. Previously values lower than 0.001 seconds were shown as 0. Values are now shown in decimal notation.

I have had several discussions regarding the scientific assessment of the safety of ESL (processed at around 120 °C) milk stored at 4 °C for extended periods. In one situation an EHO wanted to see the evidence that justified the company claiming a 35 day safe shelf life. I was pleased to have the opportunity to work with the HACCP team to calculate the lethality of the process, decimal reduction values of appropriate pathogens, the concentration of relevant pathogenic spore formers in milk, the effect of heat treatment on residual numbers and the use of free On Line databases to model pathogen growth in the product subjected to temperature abuse. A report on this risk assessment is available from the author on request.

Dough greying and gas production in frozen ready to bake biscuits has been another interesting problem. This illustrated again, at least to me, the importance of companies employing food scientists to deal with complex problems. It also illustrated the need to retain samples of critical raw materials to enable the investigation of product problems at a later time. Interestingly, despite the product being frozen the company incurred significant expense and wasted time on microbiological analysis. The product used encapsulated bicarbonate to prevent the release of carbon dioxide on storage. The gas production was caused by the reaction of the supposedly inert bicarbonate with dough constituents.

Consumer attitudes to in vitro or cultured meat

A student has posted a link to his/her Face Book page on the forum that is turn linked to an On Line survey on attitudes to cultured meat, also known as in vitro or test tube meat. 

I would applaud this student’s use of social media and Survey Monkey as a method of getting consumer feedback on an interesting topic. Obviously there are qualifications about the methodology used and it would be interesting to see how they are dealt with.

I have included a link to an article on in vitro meat. This gives a good overview and there are several useful references. 

It will be interesting to see how vegans and vegetarians view cultured meat. I would also have liked to have seen a question aimed at faith groups to see if there are religious objections.

New ice cream mix spread sheet added

A customised artisanal gelato / ice cream spread sheet offering sweetness and hardness control options for small batch product manufacture using whole milk, cream, skim milk powder, sucrose, dextrose, fructose, a range of glucose powders or syrups, and flavourings is now available and can be downloaded from .

The spread sheet is called: Pack 7. Customised artisanal gelato / ice cream spreads sheet offering sweetness and hardness control options for small batch mix manufacture using whole milk, cream, skim milk powder, sucrose, dextrose, fructose, a range of glucose powders or syrups, and flavourings.

An On Line demonstration version can be tested at .

How do you get bacteriophages to form plaques?

The article discussing factors affecting plaque formation by bacteriophages "How do you get bacteriophages to form plaques?" has been updated and is available at:


Current data indicate that some 1031 bacteriophages exist globally, including about 108 genotypes. Some phages form very tiny or micro plaques. These can sometimes be so small that it is almost impossible to see them. Frequently 'new' phages can be observed using e.g. electron microscopy under conditions where there is strong evidence of a potential host yet it can be very time consuming or in some instances not possible to get the phage to form plaques. Less than 1% of the phages observed using microscopy have ever been grown in culture, this is sometimes called "the great plaque count anomaly".

The conditions required to get a newly isolated phage to form plaques have been reviewed. The importance of testing both logarithmic and stationary phage cells, a range of incubation temperatures, replacing agar with agarose, using low strength agarose top agar overlays in initial experiments, media supplemented with Ca++ and Mg++ that do not contain cation chelators, modifications to the double agar assay method including a) changes to the initial assay step so that phage adsorption takes place at ambient temperature  and b) the use of antibiotics and other activators of the host cell’s SOS system are discussed.

Astratto  IT

I dati attuali indicano che alcuni batteriofagi 1031 esiste a livello mondiale, tra cui circa 108 genotipi. Alcuni fagi formare placche molto piccoli o micro. Questi possono essere a volte così piccolo che è quasi impossibile vederli. Frequenti "nuovi" fagi si può osservare utilizzando ad esempio microscopia elettronica in condizioni in cui vi è una forte evidenza di un potenziale ospite ma può richiedere molto tempo o in alcuni casi non è possibile ottenere il fago a formare placche. Meno dell'1% dei fagi osservati con il microscopio sono mai stati coltivati in coltura, questa è a volte chiamato "il grande numero di targa anomalia".

Le condizioni richieste per ottenere un fago di recente isolato a placche forma sono stati rivisti. L'importanza di testare cellule fagiche sia logaritmiche e fissi, una gamma di temperature di incubazione, in sostituzione di agar con agarosio, con bassa intensità sovrapposizioni agar migliori agarosio in esperimenti iniziali, i media integrati con Ca + + e Mg + + che non contengono chelanti di cationi, modifiche al sistema di duplice agar metodo di dosaggio compreso a) modifiche al passo saggio iniziale in modo che fago adsorbimento avviene a temperatura ambiente e b) l'uso di antibiotici e altri attivatori della cellula ospite SOS sistema sono discussi.


Huidige gegevens tonen aan dat sommige 1031 bacteriofagen wereldwijd bestaan, met inbegrip van ongeveer 108 genotypen. Sommige fagen vormen zeer kleine of micro plaques. Deze kunnen soms zo klein dat het bijna onmogelijk is om ze te zien. Vaak 'nieuwe' fagen kunnen worden waargenomen met behulp van bijvoorbeeld elektronenmicroscopie onder omstandigheden waarbij er duidelijk op een potentiële gastheer maar kan zeer tijdrovend of in sommige gevallen niet mogelijk de faag om plaques te vormen krijgen. Minder dan 1% van de fagen waargenomen met behulp van microscopie ooit zijn geteeld in cultuur, wordt dit ook wel 'de grote plaquette tellen anomalie ".

De voorwaarden om een onlangs geïsoleerd faag naar vorm plaques zijn beoordeeld. Het belang van testen van zowel logaritmische en stationaire faag cellen verschillende incubatietemperaturen, vervangen agar met agarose, met lage sterkte agarose top agar overlays in initiële experimenten media aangevuld met Ca + + en Mg + + die geen kation chelatoren, wijzigingen van de dubbele agar assay werkwijze omvat a) aanpassing van het aanvankelijke stap assay zodat faag adsorptie plaatsvindt bij omgevingstemperatuur en b) het gebruik van antibiotica en andere activatoren van SOS de gastheercel systeem besproken.


Текущие данные показывают, что некоторые бактериофаги 1031 существуют во всем мире, в том числе около 108 генотипов. Некоторые фаги образуют очень маленькие или микро бляшек. Они могут иногда быть настолько мала, что это почти невозможно, чтобы увидеть их. Часто «новых» фагов можно наблюдать с помощью, например электронной микроскопии в условиях, когда имеются убедительные доказательства потенциальной принимающей но он может быть очень много времени, а в некоторых случаях не удается получить фага с образованием бляшек. Менее 1% фагов наблюдать с помощью микроскопа когда-либо выращенных в культуре, это иногда называют "великим аномалии количества налета".

Условия, необходимые для получения новых изолированных фагов в форме бляшек были пересмотрены.Важность тестирования и логарифмической и стационарной фагом клеток, ряд температуру инкубации, заменив агар с агарозы, используя низкую прочность агарозном верхней накладки агар в начальных экспериментах, среде с Ca + + и Mg + +, которые не содержат катион энтеросорбенты, изменения в двойном агар метода анализа в том числе) изменения в начальной стадии анализа, так что адсорбции фага происходит при комнатной температуре и б) использование антибиотиков и других возбудителей SOS клетки-хозяина системе обсуждается.


Aktuelle Daten zeigen, dass einige 1031 Bakteriophagen weltweit existieren, darunter etwa 108 Genotypen. Einige Phagen bilden sehr kleine oder Mikro Plaques. Dies kann manchmal so klein, dass es fast unmöglich ist, sie zu sehen. Häufig "neuen" Phagen beobachtet zB mit werden Elektronenmikroskopie unter Bedingungen, wo es starke Hinweise auf einen möglichen Host dennoch kann es sehr zeitaufwendig sein oder in einigen Fällen nicht möglich, die Phagen-Plaques bilden bekommen. Weniger als 1% der Phagen beobachtet mittels Mikroskopie jemals in Kultur angebaut wird dies manchmal als "die große Plaque Anzahl Anomalie".

Die Voraussetzungen für eine neu isolierten Phagen zu bilden Plaques erhalten wurden überprüft. Die Bedeutung der Prüfung sowohl logarithmische und stationäre Phagen-Zellen, eine Reihe von Inkubationstemperaturen, ersetzen Agar mit Agarose mit niedriger Festigkeit Agarose Topagar Overlays in ersten Versuchen, Medien ergänzt mit Ca + + und Mg + +, die nicht enthalten Kation Chelatoren, Änderungen an der Doppel-Agar Assayverfahren, einschließlich a) Veränderungen der anfänglichen Analysenschritt sodass Phagen Adsorption erfolgt bei Raumtemperatur und b) die Verwendung von Antibiotika und anderen Aktivatoren von der Wirtszelle SOS-System diskutiert.






現在のデータは、いくつかの1031バクテリオファージは約108の遺伝子型を含めて、世界的に存在することを示している。いくつかのファージは、非常に小さなまたはマイクロプラークを形成する。これらは、時にはそれがそれらを参照することはほとんど不可能であるように小さくすることができる。頻繁に"新しい"ファージは例えばを用いて観察することができます潜在的なホストの強い証拠がある条件下では、電子顕微鏡では、まだそれは非常に時間がかかるまたはプラークを形成するために、ファージを取得することはできませんいくつかのインスタンスであることができる。顕微鏡を用いて観察したファージの1%未満では、これまで培養で増殖されているが、これはしばしば "偉大なプラークカウント異常"と呼ばれています。

フォーム斑に新たに単離したファージを得るために必要な条件が見直されました。初期の実験、を補充した培地では、低強度アガローストップ寒天オーバーレイを使用して、アガロースを寒天に置き換える、対数と固定両方ファージ細胞、培養温度の範囲をテストすることの重要性Ca + +およびMg + + doubleに陽イオンキレート剤、修正を含まないファージ吸着は周囲温度で行われ、b)は、抗生物質および宿主細胞のSOSシステムの他の活性化剤の使用が議論されるように、初期の検定ステップへ)の変更を含む寒天アッセイ法。


Les données actuelles indiquent que certains bactériophages 1031 existent dans le monde, y compris environ 108 génotypes. Certains phages former des plaques très petites ou micro. Ceux-ci peuvent parfois être si petit qu'il est presque impossible de les voir. Foire aux «nouveaux» phages peuvent être observés à l'aide par exemple microscopie électronique dans des conditions où il ya des preuves solides de l'hôte potentiel mais il peut être très coûteuse en temps ou, dans certains cas, ne sont pas possibles pour obtenir le phage pour former des plaques. Moins de 1% des phages observés par microscopie ont déjà été mises en culture, ce qui est parfois appelé "l'anomalie grand nombre de plaque».

Les conditions requises pour obtenir un phage nouvellement isolée de plaques de forme ont été examinés. L'importance de tester les cellules de phages à la fois stationnaire et logarithmique, une gamme de températures d'incubation, en remplacement de la gélose d'agarose, en utilisant à faible résistance superpositions d'agarose agar haut dans les expériences initiales, les médias complétés par Ca + + et Mg + + qui ne contiennent pas des chélateurs de cations, des modifications à la double méthode de dosage agar, y compris a) les changements à l'étape de dosage initial de sorte que le phage adsorption a lieu à température ambiante et b) l'utilisation d'antibiotiques et d'autres activateurs de SOS de la cellule hôte système sont discutées.

Training event:"I Laboratori Nazionali di Riferimento per Listeria monocytogenes e Campylobacter. Seminario" - Italy - Teramo, 10 - 12 Dicembre- Montecarlo Room Hotel Sporting

I had the opportunity to present a paper on ‘Bacteriophage and food fermentations. Phage assay and enumeration’ at a training event in Teramo, Italy on the 11th December 2012.One of my former PhD students Dr Giuseppe Aprea is now working at the Istituto G. Caporale in Teramo, Italy. The institute is home to a national, reference laboratory for campylobacter and listeria and undertakes a wide range of food safety work in addition to veterinary research that includes ‘Blue tongue disease’.Giuseppe Aprea is evaluating the effectiveness of listeria bacteriophages in reducing the numbers of listeria in food processing environments, food and their effectiveness in eliminating listeria from biofilms.EM unit in Teramo

The research institute is very well resourced and researchers can access transmission and scanning electron microscopes plus extensive gene sequencing laboratories.Professor Vincenza Prencipe from the Istituto G. Caporale in Teramo has excellent facilities for studying the behaviour of pathogens in foods and I was delighted to have the opportunity to look at their cheese and meat processing facilities.


Bacteriophage and food fermentations. Phage assay

While many food microbiologists are familiar with the disruption of food fermentations by phages some may be less familiar with the more positive applications of phages including their use in preventing bacterial diseases of fruit and vegetables, their use in controlling the concentrations of pathogens in foods e.g. E. coli 0157, their role in nutrient recycling in the oceans and in soil, and in nanotechnology.Despite the high numbers of phages in the environment e.g. concentrations of 107- 108 plaque forming units (PFU) /ml are common in soil whereas concentrations of 106- 107 PFU/g are often found in sea water, it can be difficult to isolate hosts and to get phages to produce plaques. Plaque formation is very important in phage purification and the inability to get plaque formation does limit the application of genetic and other molecular biology studies.It was good to get the opportunity to review the challenges of phage isolation, and subsequent assay using the double agar phage assay method. The work of Abedon and co-workers on the mathematical modelling of plaque production e.g. Abedon and Yin (2009) has revealed the importance of phage diffusivity (the rate of virus particle diffusion in the absence of the host), phage-bacterium attachment, phage latent period, burst size, and host density on plaque production and overall size. Abedon’s work combined with recent work on the tendency of some large Siphophages to aggregate (will not pass through normal filters), not plaque in overlays containing >0.2% agrose combined with the use of antibiotics that activate the SOS response in cells has the potential to get even more phages to plaque and to markedly increase the size of micro plaques.I intend to update the bacteriophage pages to reflect these developments over the next few months.

Nutritional article / tools updated

The article on labelling dealing with the calculation of the energy of food components, the associated energy calculator and the sodium to salt (sodium chloride) calculator has been updated.

Starting up an ice cream business

An article aimed at helping new ice cream 'startups' was added to the Dairyscience and Food Technology website today the 3rd May 2012.

The author, Lee Williams owns and operates Valenti’s Gelato-Artisan, a Company dedicated to promoting Artisan Ice Cream and Gelato making throughout the UK.

Training course at Universita' Degli Studo Di Napoli, Federico 11 in Italy

On the 24th September 2009 I had the opportunity to talk to the “Graduate Quality Course on Health, Hygiene and Technology on milk and milk-product chain production” at the Universita' Degli Studo Di Napoli, Federico 11 in Italy.

Discussing issues in cheese science and technology with postgraduate students from a range of disciplines and companies in Italy was a very pleasant and interesting experience.

My thanks to Professor Nicoletta Murru for the invitation and to Dr. Giuseppe Aprea (Virology and Serology Unit, Animal Health Department, Experimental Zooprophilactic Institute of Southern Italy) who acted as interpreter.

One of the topics discussed was the use of HACCP in milk production at farm level. I look forward to helping to organise a study tour to Northern Ireland to look at HACCP use on farm and the food processing facilities at the Loughry campus of the College of Agriculture, Food and Rural Enterprise.

Students on milk products course


Professor Nicoletta Murru and Dr Giuseppe Aprea with students from the “Graduate Quality Course on Health, Hygiene and Technology on milk and milk-product chain production” at the Universita' Degli Studo Di Napoli, Federico 11 Italy.


Thermal processing report updated

The downloadable report "Thermal processing of acid fruit and vegetable products. Significant microorganisms, recommended processing time / temperatures, and public health significance of spoilage" has been updated.

The report:

1) Identifies the potential spoilage organisms of acidic foods
2) Discusses the decimal reduction times and Z-values of the major spoilage organisms of acid foods
3) Lists F or P values and reference temperatures for ensuring the production of commercially sterile acid foods
4) Explains how to calculate F values and the number of log reductions of spoilage organisms following processing
5) Explains how to calculate equivalent processes e.g. at higher temperatures using published data
5) Explains the importance of measuring pH over the shelf life of acidified products
6) Provides a summary of the major causes of spoilage of acidic foods and their control
7) Lists literature, including a free On Line database containing around 6000 D-values, concerning the manufacture and control of acid foods.

This report contains 17 pages, 5 tables and 19 references (the table of contents is shown in figure 1).

Figure 1. Table of contents for thermal processing report


Worked examples are provided and the author is prepared (within reason) to help users having
problems providing they are posted on the forum. The report will be converted to an Ebook with additional sections including the manufacture of acidic milk products, validation and quality assurance of heat processes. Donors will be advised of updates which will be available at no cost.

More information is available at

Jaap de Jonge visits CAFRE's Loughry Campus

I was delighted to have the opportunity to meet Jaap de Jonge, MD of Jongia (UK), today (15-5-2012) when he visited CAFRE's Loughry Campus. Jongia have supplied soft cheesemaking equipment to Loughry and the visit was an opportunity to explore recent developments in cheese making technology particularly for artisanal producers.

Jaap de Jongia

Jaap pictured with Gary Andrews and Joy Alexander from CAFRE

Congratulations to Dr Giuseppe Aprea

 Congratulations to Dr. Giuseppe Aprea who received his PhD at the Universita' Degli Studo Di Napoli, Federico 11 on Monday the 15th January 2007. I was particularly pleased to be a member of the Commision responsible for examing Guiseppe and the other doctoral candidates.


The author pictured with Professor M. L. Cortesi, Professor N. Murru and Dr G. Aprea at the University of Naples


The author pictured with Professor M. L. Cortesi, Professor N. Murru and Dr G. Aprea at the University of Naples 

Dr Aprea worked with the author and colleagues at Loughry to study fermentation problems with Mozzarella cheese produced using buffalo milk in Southern Italy. The cheeses were manufactured using natural whey or artisanal cultures. 

The research involved the isolation and identification of lactic acid bacteria from the starters and the determination of their sensitivity to bacteriophage.  The work identified that many of the lactic acid bacteria, particularly lactobacilli, present in the whey starters contained inclusion bodies comprised of polyphosphate. This work has been published. Two unusual phages for Lactobacillus brevis, the first reported for this organism were also identified.

 Giuseppe Aprea working with Professor Ger Fitzgerald at University College Cork has characterised the genome of both phages and determined their sensitivity to high pressure and temperature.

Milk protein derived bioactive peptides

An article on 'Milk protein derived bioactive peptides' by Emily Haque and Dr. Rattan Chand from the National Dairy Research Institute in India has been added to the Dairy Science and Food Technology website.

This article reviews recent research that has shown that milk proteins can yield bioactive peptides with opioid, mineral binding, cytomodulatory, antihypertensive, immunostimulating, antimicrobial and antioxidative activity in the human body.

The authors review the properties of bioactive peptides, explain that bioactive peptides are encrypted in milk proteins and are only released by enzymatic hydrolysisin vivoduring gastrointestinal digestion, food processing or by microbial enzymes in fermented products.

Mention is made of the significant research being undertaken on the health effects of bioactive peptides.The naturally formed bioactive peptidesin fermented dairy products, such as yoghurt, sour milk and cheeseare discussed e.g. antihypertensive peptides have been identified in fermented milks, whey and ripened cheese. Some of these peptides have been commercialised.

The authors conclude with their perspectives on the future of bioactive peptides and suggest that "Bioactive peptides have the potential to be used in the formulation of health-enhancing nutraceuticals, and as potent drugs with well defined pharmacological effects".

Calculate the energy density of food

An article on how to calculate the energy density of food has been added.The purpose of this article is to discuss how to calculate the energy density or energy content of foods. A calculator is also included to calculate energy density using the the Atwater factors.

Dairy farming in a new environment

Students studying dairy science, food science, nutrition and food technology should have at least an appreciation of the macro aspects of food production on the farm.

Farming in Europe and in North America is challenging for many farm families at present. Farm incomes are lower than many farmers need to sustain and develop their businesses and the costs of meeting environmental legislation are increasing. All of this combined with the considerable purchasing power of the food retailers who tend to drive down farm prices requires farmers to have excellent business management skills.

What are the business management challenges for farmers? In a challenging but positive article Ian McCluggage discusses 'Dairy farming in a new environment.'

In his article Ian mentions benchmarking. Further information on benchmarking is also available under the Focus Farm Programme. In this innovative DARD-led programme some 80 farmers throughout Northern Ireland share their business expertise through a mentoring programme. Many of these farmers use benchmarking.

Site updated on the 23rd August

The article on the science of modified atmosphere packaging has been updated to include additional information on non-dairy foods. I have also included some advice on how to cite this and other articles. The article and other articles on packaging can be viewed here.

Thanks to James Atherton for sharing a Javascript used on his Teaching and Learning site.

Finally after many months an article on the use of starter cell concentrates has been added. This material had been written quite a few years ago and I was disappointed to learn that facilities for innoculating bulk starter vessels with cell concentrate have not improved much. No wonder we still have phage problems.

Biotherapeutic properties of yeasts

A paper on the 'Biotherapeutic properties of probiotic yeast Saccharomyces species in fermented dairy foods' by Kalpana Dixit and D.N.Gandhi, Dairy Microbiology Division, National Dairy Research Institute, Karnal-132001, INDIA has been added to the Dairy Science and Food Technology site.

Predicting the grade of Cheddar cheese

The production of mature cheese is expensive and involves management of several risks. There is increasing interest in using models to predict grade or to reject those cheeses that are likely to develop faults on storage.

An article on the use of the Lawrence model to predict the grade value of Cheddar cheese has been added.

The article also provides access to a calculator where users can input actual cheese data and use the Lawrence model to predict the cheese grade.

Safe shelf life of smoked vacuum packed cheese

A small UK business wishes to manufacture smoked cheese and to sell the cheese vacuum-packed. The Environmental Health Officer (EHO) dealing with the business demands that the products produced should have a restricted shelf life in accordance with his/her interpretation of guidance from the Food Standards Agency (FSA). However the company is aware of other businesses selling similar products without restriction.

The above query was posted on the forum today the 6th May 2012. Have you a view? Has the EHO made a mistake?

Theoretical yield of Cottage cheese

A calculator to determine the theoretical yield and process efficiency in Cottage cheese manufacture has been added. Using the solids-not-fat, protein or preferably casein concentration in the cheese milk an estimate of theoretical yield can be obtained. This can be compared with the actual yield to obtain values for cheesemaking efficiency.

Posting on Cottage cheese & site news

The latest posting concerns how to predict the theoretical yield of Cottage cheese.

The site forum has been updated from SimpleForum 3.3 to the professional version. The latter enables forum posts to be viewed without having to register.It alsoenables simpler and faster registrationfor posting anda range of other features e.g. personal messaging, posting attachments; these have still to be enabled. One downside of the upgrade is that the original statistics have been lost. On the positive side posters, providing they have enabled the feature in their profile, will receive an Email each time a response to their topic has been posted.

Finally peace and happiness to all users of this website in 2007.

DSFT is a consultancy. Knowledge transfer is our business and there is a charge for providing independent, scientifically valid and ethical advice.

We normally charge £500 per day with discounts for longer projects, £250 for half day projects and £100 per hour for consultations using Skype, Facetime, Email or phone.

We recognise that everyone cannot afford these rates and provide a facility through the forums to respond to queries at no cost. At busy periods it can be a week or longer before we can respond in the forums but the forums are good for non-urgent queries.

There is a limit to how much free consultancy we provide after purchasing spreadsheets or Ebooks and it would be appreciated if you would consider making a donation to cover business costs before asking for advice.

You can use the PayPal donate button below to make a donation.


pixel articles

DSFT provides a range of thermal processing consultancy services to food and pharmaceutical manufacturers. These include:

  • Independent validation of the antimicrobial effectiveness of the heat treatments used in processing.
  • Calculation of the average holding time used in processing HTST and HHST products.
  • Determination of the flow type and calculation of the minimum holding or residence time of the fastest flowing particles in HTST and HHST products.
  • Determination of the F values and the number of logarithmic (log10) reductions of designated microorganisms following heat treatment.
  • Advice on equivalent heat processes to meet legislative and other requirements.
  • Benchmarking of company processes against statutory and international best practice.
  • Advice on alternative methods to microbiological examination for providing additional assurance of adequate heat treatment e.g. the phosphatase test is of no value in providing assurance that a temperature >80°C was used in milk processing. Additional tests that confirm higher temperatures than e.g. normal milk-pasteurization temperatures can be provided. The merits of incorporating these into routine quality assurance testing will be explained.

An article on thermal process modelling has been added. This article calculates the effect of HTST treatment on the number of log reductions of major milk pathogens and discusses the temperature milk should be pasteurized if Mycobacterium avium subsp. paratuberculosis (MAP) was designated as a human pathogen. The log reductions refer to log10 or decimal (10 fold) reductions in the concentration of viable bacteria.

 Effect of HTST treatment on the number of log reductions of major milk pathogens.

The use of high temperature short time heat treatment (HTST) of milk (72°C for 15 seconds) to destroy pathogenic bacteria, reduce the number of spoilage organisms and increase shelf life is well established (Juffs and Deeth, 2007).

The history of pasteurization (pasteurisation is also valid) is fascinating and is notable for its public health success and for the insights of many scientists and engineers. Prior to the introduction of pasteurization, consumption of raw cow milk was a major source of infection by bacteria causing tuberculosis. Pasteurization has eliminated heat-treated-milk as a source of infection. Regrettably raw milk and raw milk products remain a major source of new cases of bovine tuberculosis.

This article calculates the effect of HTST treatment on the number of log reductions of major milk pathogens and discusses the temperature milk should be pasteurized if Mycobacterium avium subsp. paratuberculosis (MAP) was designated as a human

pathogen. The log reductions refer to log10 or decimal (10 fold) reductions in the concentration of viable bacteria. The article does not discuss the effects of heat on the functional properties or the nutritive quality of milk. An updated refereed version of this paper has been published (Mullan, 2019) and is available free in a read only format at .

Help for lethal rate calculator

How does the calculator work? The calculator converts temperature readings to lethal rates, plots the lethal rates against time, and determines the F values for the overall heat-process. The area under the curve is determined using the industry standard method, the Trapezoid rule. The calculator is capable of giving accurate F determinations for most thermal processes. In general the more values, and the smaller the time interval between the values, the more accurate the value for F will be.

Lethal rate is calculated using the formula, lethal rate= 10 (T-Tr/Z) where T is the temperature, in Celsius, at which the lethal rate is required and Tr is the reference temperature. Note lethal rate is a relative term that compares the microbial killing effect at a measured temperature to one minute at the reference temperature.

Tr will vary on whether Fo is being calculated or whether a pasteurisation process or other heat treatment is being assessed. A Tr of 121.1° C is used in the determination of Fo. If F70 or other F value is required then Tr must be set to 70° C or other temperature.

The Z-value, is measured in °C, and is the reciprocal of the slope of the thermal death curve for the target microorganism or spore; 10° C is the value frequently used in Fo calculations performed on low acid foods. Users can vary the Z-value depending on the target organism being considered. The Z-value has a significant effect on the F value of a process. The effect of the Z-value can be seen in the free lethal rate tables that can be downloaded from here.

Tr can be varied by the user. 

Using the calculator

The Reference Temperature and the Z values should be set first. Next the data grid for entering data is produced.

The starting time e.g. 2 minutes from the beginning of heating and the final time e.g. 180 minutes from the start of heating should be set. The time interval between the temperature readings (∆t) must also be set e.g. 0.1 minutes. Once this information has been set, the user should press the “New data grid” button to create a data entry table. Temperature values are entered using the edit control.

The application can be tested using internal test data. This data when used with the set values should give a Fo value of 8.43. You may also change the Tr or Z values once the data has been loaded and use the “Recalculate button” to investigate the effects of changing these variables on the F values generated by your dataset.

Uploading a csv file

Instead of creating a new data grid to enter time and temperature data you can upload a text file containing your data and the application will determine the F value. If you want to use a different Tr or Z value you should set the Tr-value after you have uploaded the data and then use the Recalculate button to determine the F value. The data must be submitted as a comma separated value (CSV) text file. Use a '.' and not a ‘,’ as the decimal separator. The file must have the extension “.csv”.

A csv file looks like:

Time, Temperature

and can be generated using a spreadsheet programme, if you do not want to prepare the data manually. While a csv file will enable users to use actual values, and will work with uneven time intervals, the use of uneven time (t) intervals is not recommended; keep the ∆t constant! Numerical integration (this is what we are actually doing) using the Trapezoid rule gives more accurate results when even time intervals are used.

Note. When a .csv file is imported the data in the table and on the chart will change to reflect the new values entered. Once this data has been entered you can vary the Tr and z-values and use the recalculate button to calculate a new F value.

The number of log cycles that a designated microbial population has been reduced can be calculated by dividing the F value by the D value at Tr. A 12 log cycle reduction is required for spores of Clostridium botulinum in 'commercially sterile' low acid canned foods.

Please post any queries in the forum.

You can also download Excel spreadsheets to calculate F values.

Return to lethal rate calculator

See survivor probability calculator

How to cite this article

Mullan, W.M.A. (2008). [On-line]. Available from: . Accessed: 7 August, 2020. Updated 2010, 2015, 2017. 

Microbial testing is still important but it is critical to understand its limitations in assuring food safety.

Despite the global use of HACCP systems and a legal requirement for the use of HACCP in many jurisdictions' food poisoning remains an endemic problem and large numbers of people continue to be hospitalised, die and as a result companies either face substantial legal costs and / or in many cases are forced to cease trading.

While the use of HACCP systems significantly reduces the need for microbiological end point testing of foods, sampling schemes and microbial analysis have important roles in system validation and quality assurance. This article also provides access to three free On-Line calculators that enable the probability of detecting a pathogen in a food, the number of samples required to test to meet a food standard and how to calculate the prevalence of a pathogen when all the samples taken for testing return negative results.

This raises an issue concerning the adequacy of sampling schemes and microbial analysis in commercial food manufacture.

In September 2015 the US Centers for Disease Control and Prevention reported on a multistate outbreak of listeriosis allegedly caused by Mediterranean-type soft cheeses. Some 30 people were affected, twenty-eight people were hospitalized and three deaths were reported. However, listeria were not isolated from the cheeses produced by the manufacturer concerned.

Mr George DoranGeorge Doran graduated with a 2:1 honours degree in Food Technology from the College of Agriculture and Food Technology in Northern Ireland in 2015. Mr Doran's final year research project was entitled "An Investigation of Biofouling in Two Mozzarella Cheese Manufacturing Plants".  

 George completed his food technology internship at Cottage Catering, Dromore, N. Ireland and gained experience in Quality Assurance, New Product Development and Production.

George has achieved several academic distinctions and has extensive work experience gained through part-time work in the security and retail sectors.

Included amongst George's achievements are:

  • Member of the winning team for the Chesapeake Product Development Challenge in December 2013
  • Represented IFST Ecothrophelia in London at Food Matters Live in November 2014


Gemma Devine graduated with a 2:1 honours degree in Food Technology from the College of Agriculture and Food Technology in Northern Ireland in 2015. Ms Devine's final year research project was entitled "Identifying fault-causing Thermoduric bacteria in cheese". Summary CV to be completed.

The purpose of this article is to show how the calculator for predicting salmonella growth on tomatoes was written in ASP. I would like to encourage lecturers and students to learn how to programme. While ASP is being replaced by ASP.NET the basic premises using in ASP coding can be applied to other languages and even if you decide to learn PHP which is a particularly versatile web language or other language (probably good idea) you will be able to apply the concepts learned.

There are several programming languages used to construct models that will run on web servers. These include Perl, PHP, Python, Classic ASP, JavaScript and ASP.Net. These languages can be used with appropriate databases to develop powerful web-based applications.

PHP has become particularly popular and is fairly easy to learn. Classic ASP is even easier to learn but is increasingly being replaced by ASP.NET. Virtually anyone who can put a spreadsheet together is capable of learning a basic web programming language such as ASP or PHP.

Constructing the data entry form

The HTML and ASP code used to construct the data entry form is shown below. For simplicity web page header information, value information in ASP and some form security information has not been shown. Nevertheless the form and script below will work on a PC or webserver that supports ASP without amendment.

The input form was constructed by writing the code for a table in HTML and adding in HTML text boxes to allow initial number (no), temperature (t) and incubation time (hr) to be entered.

 <%@LANGUAGE="VBSCRIPT" CODEPAGE="1252"%> ' A standard HTML header could be used instead

<form id="form1" name="form1" method="post" action="sam1.asp">

<table width="75%" border="1" cellspacing="0" cellpadding="1">
<th colspan="2" scope="col">Predict the growth of salmonella in cut tomatoes at 10&deg;C to 35&deg;C</th>
<td width="41%">Initial number of salmonella / gram</td>
<td width="59%"><input name="no" type="text" id="no" value="<%=no%>" /></td>
<td>Temperature, &deg;C</td>
<td><input name="t" type="text" id="t" value="<%=t%>" /></td>
<td height="20">Incubation time, hours</td>
<td><input name="hr" type="text" id="hr" value="<%=hr%>" /></td>
<td height="20" colspan="2"><div align="center">
<input type="submit" name="Predict number of samonella" id="Predict number of samonella" value="Predict number of salmonella" />

Writing the data processing script

A simplified, but fully working, version of the processing script is given below. The notes which are preceded by a ' explain how the script works.


<% Option Explicit
'Use of option explicit will ensure that error messages are displayed if there are coding problems. These are helpful in finding solutions. %>


'List variables

Dim no
Dim t
Dim hr

'no, t and hr are values that have been entered on the form
Dim r
Dim g
Dim gen
Dim pop

'r,g,gen, pop are the products of calculations that will be undertaken

'We will now obtain the information from the entry form to perform the calculation


'Taking information from the form we will calculate r,g, gen and pop. This could easily be done in one calculation. The calculation has been broken down into its component parts so that it can be followed more easily.

'For an explanation of the calculation please see
g=1/(r*r)  'note r is the square root of the growth rate, we need to obtain the growth rate by multiplying r by r
pop = no*2^gen

'We next need to provide the results of the calculation for pop which is the total number of salmonella after growth at the temperature chosen for the time inputted.

If t >9.99 AND t <35.01 THEN ' basic validation to ensure that the results are within the temperature parameters modelled

'The  'If Statement' is used to make a decision to execute code if some condition is True.
response.write ("The predicted number of samonella after" & "&nbsp;" & (hr)& "&nbsp;" & "hours has been calculated as" & "&nbsp;" & round(pop,0) & "&nbsp;" & "CFU/g.")

'END IF is used to indicate the end of code execution

IF t < 10 THEN
Response.Write ("&nbsp; Caution. This model as not been validated at temperatures <10&deg;C.")

IF t > 35 THEN
Response.Write ("&nbsp; Caution. This model as not been validated at temperatures >35&deg;C")

Using the scripts

The simplest way of using the scripts is to copy each using a simple text editor e.g. Notepad on a PC or its equivalent on a Mac or Linux machine. Save the form script using whatever "name" you want as name.htm or name.asp. The data processing script must be saved as sam.asp since sam.asp is the name of the script in the input form that has been designated to process the data.

These scripts will work on a Windows web server or a Windows PC running Microsoft's free IIS or PWS components. 


The scripts  have been provided free and are not warranted in any way. They are intended for educational use only. They can easily be adapted to work using PHP or JavaScript. Users use the code provided at their own risk. 


I learned ASP, how to use Access Databases and HTML from a former colleague Dr Raymond Martin. Raymond generously helped me correct concatenation and many other errors and though his help I gradually learned how to write quite sophisticated data-base driven ASP applications. I gratefully acknowledge Raymond's help over many years.

How to cite this article

Mullan, W.M.A. (2015). [On-line]. Available from: . Accessed: 7 August, 2020. Updated December 2019. 

DRAFT Article.

This article is available On Line to enable editing. Its draft status is scheduled to be removed by September, 2015. In the meantime comments are invited from scientists and technologists familiar with the subject area to help improve this article. I wish to acknowledge the generous comments and advice received from workers in this area. As a result of the feedback I will include more information about the principles behind modelling bacterial growth and the limitations of models.

Michael Mullan. 17th April, 2015

The purpose of this article is to explain how to derive a simple Ratkowsky square root model (Ratkowsky et al., 1982) that describes the growth of a fault causing bacterium on cooked meat over the temperature range 10° - 35 °C. 

An example of the growth curve of the fault causing bacterium on tomatoes at 35°C is shown in figure 1.

The curve shows the 4-typical growth phases, lag, logarithmic, stationary and decline.

growth-curve articles


The specific growth rate (k) is calculated for the  logarithmic phase using equation 1.

Equation 1. k=log10(Nt) - log 10 (N0)
                          T x 0.301


Nt = is the number of bacteria at the end of the observation period.
N0 = is the number when the observation period started.
T = is the time that has elapsed over the growth period in hours.

k= 9.146-6.669
        2 x 0.301

k= 4.115 generations h-1 at 37 °C.

Next we need to construct a series of curves at 10°, 20°, and 30 °C to find the specific growth rate at each temperature. We have got the growth curve at 35 °C already (Figure 1 above). The specific growth rates at each temperature are then tabulated using equation 1 against time (Table 1). Note I have not shown the growth curves at the other temperatures. 

  Table 1. The effect of temperature on the specific growth rates of a bacterial isolate on tomatoes
 Temperature, °C  Specific growth rate, h-1
 10  0.023
 20  0.16
 30  0.384
 35 0.518

 We can now use the Ratkowsky square root model (Ratkowsky et al., 1982) to derive the relationship between the growth rate constant, temperature and the initial number of microorganisms.

The equation has been described previously, √r= b (T-To), where r is the growth rate constant.  In particular we need to calculate b, the slope of the square root of specific growth rate versus temperature plot, and, To, the value at which the square root of growth rate intercepts the x axis.

The curve produced using linear regression (black line) and the actual data (blue line) using the data in table 1 is shown in figure 2.

 trend-line articles

Using Excel, the linear regression equation that describes the trend line (black line in figure, for the growth of the fault causing bacterium on cooked meat over the temperature range 10° to 35 °C is:

√r= 0.0228 (T-0.0693) where √r is the square root of the growth rate constant and T is the temperature in °C.

Use of Combase Tools

While the calculations described previously are not difficult it is possible to automate the derivation of growth rate equations using free tools from Combase.  DMFit is an Excel add-in to fit log counts vs. time data and extract parameters such as growth rate. It can be downloaded from Combase.

Model validation

Validation is an important element in model development. The literature cited below provides useful insights into model development and may prove useful during validation. The USDA Pathogen Modelling Program (PMP) and ComBase should be consulted for pathogen-models.


To be added.

Literature cited

Baranyi, J., Pin, C. and  Ross, T. (1999). Validating and comparing predictive models. Int. J. Food Microbiol. 48:159-166.

Baranyi, J.,  Ross, T., Roberts, T.A. and  McMeekin, T. (1996). The effects of overparameterisation on the performance of empirical models used in predictive microbiology. Food Microbiol. 13:83-91.

 Pin, C.,  Sutherland, J. P. and Baranyi, J. (1999). Validating predictive models of food spoilage organisms. J. Appl. Microbiol. 87:491-499.


To be added.

How to cite this article

Mullan, W.M.A. (2015). [On-line]. Available from: . Accessed: 7 August, 2020.  

Considerable effort has been devoted to modelling the growth of pathogenic and spoilage bacteria in food. This is referred to as predictive microbiology. Mathematical equations are used to describe the effect of environment, for example temperature, or in more complex models temperature, pH, available water (Aw) and other factors that affect microbial behaviour.

The advantages of modelling have been described in the article on "Modelling in Food Technology" including reducing the costs and time required in determining the safe shelf life of new products or in undertaking pathogen challenge testing. However, caution and scientific expertise are required e.g. the Food Safety Authority of Ireland (2012) has cautioned the food industry on the use of predictive models.

The purpose of this article and supporting material is to illustrate how published research in predictive microbiology can be used in practice. This is an area in which Food Science and Food Technology undergraduates sometimes find difficulty. Part of the difficulty may exist because the steps in the calculations involved are not usually presented.

This article describes a model for the growth of salmonella on cut tomatoes and a calculator where you can enter the initial numbers of salmonella, the incubation temperature and the incubation time to obtain a prediction of final numbers.

Modelling microbial growth

Because of the critical importance of temperature early work attempted to modify the Arrhenius Law to describe microbial growth but this was either unsuccessful or the relationships derived were generally too complex for routine use. In a classic paper, Australian workers (Ratkowsky et al., 1982) proposed a relatively simple, two–factor empirical equation (equation 1) to describe the influence of temperature on microbial growth up to the maximum growth temperature of an organism, Tmax.  This is often called the square root model.

Cakes are classified as intermediate moisture foods and may be subject to spoilage by moulds. Water activity (aw), the water that is available, unbound or free, for chemical reactions and microbial growth is a major factor that can be utilised to limit or prevent microbial growth. Cakes generally have aw values ranging from 0.65-0.9.

Mould spoilage on cakes tend to occur on the surface and work has been done to model the water vapour pressure above cakes with temperature to derive models for the mould free shelf life (MFSL) of these products. The water vapour pressure above a food is determined by several factors including temperature, the water content of the food, the solutes present and the water activity in the food.

All foods have their own equilibrium relative humidity (ERH). This is the humidity at a given temperature at which the food will neither lose nor absorb moisture to or from the atmosphere.

If the food is held below its ERH it will lose moisture and become drier; above this value, it will absorb moisture from the atmosphere.  The gain or loss of water can have a major effect on a food and can influence shelf life significantly. The EHR is determined by exposing the food to carefully controlled atmospheres containing defined water vapour pressures generated using for example standard solutions of salts.

EHR and water activity (aw) are closely related. Water activity represents the ratio of the water vapour pressure of a food to the water vapour pressure of pure water under the same conditions.  Water activity is expressed as a fraction. If this is multiplied by 100 then ERH is obtained.  Most bacteria cannot grow below an aw of 0.86 (86 % ERH).

Cauvain and Seiler (1992) found that the logarithm of the MFSL had a linear relationship with EHR over the range 74-90% at 21° and 27°C.  The equations derived (equations 1 and 2):

Equation 1. Log10 (MFSL, days at 27°C) =6.42 - (0.065 x ERH%)

Equation 2. Log10 (MFSL, days at 21°C) =7.91 - (0.081 x ERH%)

can be used to determine the shelf life of new cake products rapidly and inexpensively. These equations are available in expensive commercial software for determining the MFSL of cakes.

Determine the mould free shelf life of cakes.

Literature Cited

Cauvain, S.P. and Seiler, D.A.L. (1992). Equilibrium relative humidity and the shelf life of cakes. FMBRA Report No. 150, CCFRA, Chipping Campden, UK.

How to cite this article

Mullan, W.M.A. (2015). [On-line]. Available from: . Accessed: 7 August, 2020.  

Modelling the spoilage of pasteurized milk

Spoilage of pasteurized milk is almost always due to the growth of microorganisms. These are generally introduced after heat treatment and are referred to as post process contaminants (PPCs).

The shelf life of pasteurized milk is largely dependent on the number of PPCs and storage temperature (e.g. Muir, 1996). Muir (1996) has described a simple equation (equation 1) relating the number of number of PPCs and storage temperature to shelf life of pasteurised milk.

Equation 1. Shelf life (h)={0.00621*(T+273-(269.55-0.74))*(CFC15)-0.11 x ( CFC15) x 2} -2.

Where T = storage temperature in °K; CFC15, =log10 count after pre-incubation of pasteurized milk at 15°C for 24 hours enumeration on milk agar containing a selective supplement for pseudomonads called cetrimide-fucidincephaloridine (CFC).

Muir (1996) has explained that the equation can predict shelf life at storage temperatures between 6°C and 14°C to within 1 day for between 60 and 90% of samples. The accuracy of the equation has been reported to increase as the storage temperature of the pasteurized milk increases.

Go to Shelf Life of Pasteurized Milk Calculator .

Literature cited
Muir, D.D. (1996) The shelf-life of dairy products: 2. Raw milk and fresh products. Journal of the Society of Dairy Technology. 49, 44-48.

How to cite this article

Mullan, W.M.A. (2015). [On-line]. Available from: . Accessed: 7 August, 2020.  

The isolation of lactic acid bacteria from raw and pasteurized milk is discussed.

It can be difficult for entrepreneurs to obtain starter cultures for trials. This article provides contact details of some culture suppliers.

Small, and even large companies, frequently find it difficult to contact potential new supplier partners.  This is a particular problem for some small companies attempting to get their new business started or wishing to expand.

However there are companies who provide exemplary customer service and work hard to meet customer needs. The following is a list of these companies that provide exemplary customer service in the ice cream sector.

 Dry heat sterilisation is widely used for glassware and materials that are not suitable for sterilisation using saturated steam. A range of temperatures and times are used.  Currently a temperature of at least 170°C for 30-60 minutes is widely used. The term is not particularly precise since variable concentrations of water may be present in the oven used (Sandle, 2013).

Living microorganisms are widely used for several therapeutic purposes and their beneficial effects as biotherapeutic agents are well known. While certain strains of lactic acid bacteria and bifidobacteria are used as probiotics in pharmaceutical preparations, feed additives and so-called functional foods yeasts also possess some medicinal efficiency.

 DSFT provides a range of consultancy services to ice cream, gelato, sherbet and sorbet manufacturers. These range from help in formulating recipes, reducing costs by replacing expensive imported additives, problem solving, courses in advanced product manufacture, advice on all aspects of HACCP including process validation, providing assistance in discussions with regulatory authorities to independent audits of plant, process and external advice received. Independent nutritional advice on the manufacture and safety of low or sugar-free gelato and ice cream products is also provided.

We can help with the development and commercial production of soya and dairy-free, high protein ice cream-like products with or without sucrose. There are better options than sorbitol and fructose. Science based advice on describing products intended for consumers with health issues can also be provided.

Despite work undertaken over 20-years ago by researchers in Canada many large scale ice cream producers have limited knowledge of the minimum holding time of ice cream mix processed using HTST or HHST heat-processes. Furthermore many lack the evidence that they meet regulatory requirements for holding time. DFST can use your plant data to calculate average holding time, flow type, minimum residence or holding time, the log reductions of major pathogens and advise on any remedial action required.


Contact DSFT

Scoop of ice cream

In this article we will explore how to use mix composition to control the hardness or "scoopability" of ice cream or gelato. The serving temperature which influences the concentration of ice present will also be considered. The volume of air added during freezing (overrun), the manufacturing process and the concentration and type of emulsifier can also affect hardness. 

However, these effects are generally less significant than the concentration of sweeteners used and serving temperature. This article should be read in conjunction with the article on the sweetness of ice cream. A condensed version of this article is available in the Food Science and Technology OnLine Journal (Mullan, 2018).

This article originally had the title "Goldilock's ice cream. Controlling hardness or scoopability." Goldilocks was a character from "Goldilocks and the Three Bears" a British 19th-century fairy tale and I originally thought that everyone would understand if an ice cream was acceptable to Goldilocks it had to be good! I have changed the title to reflect that many readers have not read this fairy tale and I may have been inadvertenly confusing people.

Range of flavoured ice creams

Ice cream and gelato manufacturers produce products with a range of favours. There are often significant variations in sweetness and hardness between flavours. This article provides an explanation of sweetness, how it is measured and how it can be controlled.

Relative sweetness and the Potere Dolcificante method are discussed and calculations are used to explain the differences. The limitations and disadvantages of using numerical values of sweetness are explained. Since sweetness and hardness are closely related the reader is also referred to the article on controlling hardness or resistance to scooping.


The tables of data from the gassy cheese article.


Table 1. Major microbial groups that can produce gas in cheese

Microbial group


Gaseous products

  Clostridium tyrobutyricum


CO2, H2

  E.g. Lactobacillus brevis
  E.g. Lactobacillus casei




  Streptococcus thermophilus1





CO2, H2




 Lactococcus lactis ssp. lactis biovar. diacetylactis



Bacillus species
  Bacillus subtilis


CO2, H2

  E.g. Leuconostoc mesenteroides
  E.g. Leuconostoc dextranicum



  Propionibacterium shermani



Notes:1 Streptococcus thermophilus can also produce gas from other substrates.


Table   2. Microbiological analysis of blown and normal cheese from factory X




Blown cheese-A

Blown cheese-B

Normal cheese

'Total count' on milk agar

1.3 x 10 8

6 x 107

2 x 107

Yeasts and moulds








Catalase-negative citrate   utilisers(1)

8 x 107

4 x 107

<1 x 106


<1 x 10 2

<1 x 102

<1 x 102

Group D streptococci

<1 x 102

1.3 x 103

2 x 102


4.1 x 104

4.1 x 105

1 x 103

Aerobic sporeformers

1 x 102

1 x 102

1 x 102


All cheeses were obtained from the same commercial plant and from the same production run.
Cheeses had been held at 7°C for 8 weeks after manufacture before sampling. Moisture, pH and salt
levels were within acceptable limits and similar in all samples.
(1)Determined using differential agar media13,14.
(2) Determined using Rogosa agar15

TABLE 3.   Relationship between citrate level in cheese, ex-press, and gas production in   Cheddar cheese

Days after manufacture

Starter code

Milk citrate level (% w/w)

Cheese citrate level(2) (% w/w)

Condition of barrier bag(3)









< 0.001







29 (vat 1)





29 (vat 3)









Tight, no gas







(1) Defined multi-strain cultures that did not contain citrate utilising strains.
(2) Cheese analysed immediately after pressing for citrate (12 hours).
(3) Cheese was stored at 7C for 6 weeks before examination.
ND - not determined


TABLE 4.   Characteristics of Rogosa-agar isolates from a commercial mixed-strain   culture







Cocco/ bacillus

Cocco/ bacillus

Cocco/ bacillus

Cocco/ bacillus

Gram reaction










NH3 from arginine





Nitrate reduction





Lactic acid (isomer present)





Acid produced in RSM(1)





Gas from glucose





Gas from gluconate





Acid from arabinose





Acid from xylose





Acid from maltose





Growth at 40ºC





Growth at 10ºC





Growth at 6ºC





Growth in 6% NaCl





Growth in 6.5% NaCl





Citrate utilisation






TABLE 5. Maximum volumes 1,2 (L) of carbon dioxide available from citrate and lactose in cheese (3)

% substrate (w/w)

Volume of CO2(1)




















(1) Volumes at standard temperature and pressure (STP)
(2) No allowance made for adsorption/solution. Values are rounded
(3) Cheese mass 18 kg
(4) Assuming 3 moles of CO2 produced from 1 mole citrate
(5) Assuming 2 moles of CO2 produced from 1 mole lactose
NC. Not calculated. Citrate levels of >0.2% w/w are not normally found in Cheddar cheese.

Return to gassy cheese article.

So you want to start an ice cream and Gelato shop business?

The purpose of this article is to help anyone interested in starting their own ice cream business to get started. I am an artisan ice cream maker with extensive experience of industrial scale ice cream making. While I will discuss both artisanal or traditional and industrial or large scale ice cream manufacture, the main emphasis will be on artisanal production.

lee-williamsLee Williams owns and operates Valenti’s Gelato-Artisan, a Company dedicated to promoting Artisan Ice Cream and Gelato making throughout the UK. He has over 35 years’ experience in the Ice Cream Industry. Lee began his career in a family business, a second generation of Ice Cream makers in SW England. During this time he worked in various locations including Southern Africa, creating a global perspective to his service. More recently he has developed strong working partnerships with European equipment manufacturers and Italian flavour houses. Valenti’s product portfolio includes all types of Artisan Ice Cream making equipment, gelato shop with parlour design and ingredients, together with training, mix formulation and technical support, available throughout the UK and Ireland.

 Valenti’s can provide specialist training at their Academy based in Cornwall the home of Ice Cream making and also on-site training programmes at the clients own premises.


Download Microsoft Excel standardisation calculator for whole milk powder manufacture

The calculator used on this web site for producing milk powder to a target fat concentration was validated using a Microsoft Excel spreadsheet. This spreadsheet is available for download subject to a small donation (£4.99, < €6 or <US $10, conversions based on the exchange rate at March 2012) to contribute towards the running costs of the Dairy Science and Food Technology website.

The download contains one Microsoft Excel spreadsheet and a document in PDF form. The PDF file explains the basis of the calculations. A screenshot of the spreadsheet is shown in figure 1.

 Image of Excel spreadsheet for producing whole milk powder to fat target

Figure 1. Screenshot of the Microsoft Excel spreadsheet for undertaking calculations to determine the fat concentration required to produce a milk powder to a defined fat concentration.

All the cell formulas are unlocked. The spreadsheet is for your own personal educational use. Only make a donation if you accept the Terms and Conditions for the use of Spreadsheets from this site.


Visa MasterCard Discover American Express



Following secure payment you will shortly receive an Email with a download link. The link is valid for 7-days and during this time you can download the spreadsheet 4-times.


Technologists producing acidic foods such as pickles and sauces often find it difficult to get information on the processing conditions required to obtain commercial sterility or how to calculate the processing time at a higher temperature. Following the experience of working with processors experiencing technical issues, including spoilage problems and difficulties in exporting products, I have produced a concise Ebook (Thermal processing of acid fruit and vegetable products. Significant microorganisms, recommended processing time / temperatures, and public health significance of spoilage) that may be helpful. Currently the Ebook (figure 1):

The purpose of this article is to help anyone wishing to download an ice cream mix spreadsheet to select the correct spread sheet and to provide some additional information on how to use the spreadsheets.

Users must first log on to the calculator using the user name selected and the password that has been sent to your  Email-address. If you can see the "log on" button as shown in the screen shot below you have not logged in!


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