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Typical pots of UHT milk

Summary

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.

Introduction

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).

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.

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

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.

Can you destroy Mycobacterium avium subsp. paratuberculosis (MAP) by pasteurization? How important is holding time compared with holding temperature?  Use the powerful free tools in this section to answer these questions.

Technologists must be able to calculate the cumulative lethality of a heat process normally referred to as F. This is done by defining a reference temperature, e.g. 121.1 °C for a F0 calculation using a low acid food or e.g. 93.3 °C for an acid product, at which the equivalent lethal effects experienced during heating and cooling at lower temperatures are calculated.

The area under the lethality curve is normally calculated using numerical integration. The most commonly used method is the trapezium or trapezoid method. An alternative, more accurate, but slightly more complicated method, is to use Simpson's rule or to be more correct Simpson's rules.

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 calculator converts temperature readings to lethal rates, plots the lethal rates against time, and determines F or P values for a heat process whether using hot water, saturated steam or dry heat. The area under the curve is determined using the trapezoid rule. Accurate F or P determinations for most thermal processes can be obtained. In general the more values, the more accurate the value for F or P will be. 

 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.

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.

 

 

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