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The Dairy Science and Food Technology website provides scientific and technological information for students studying food science and technology and may also be of interest to those working in the industry. A discussion forum and interactive content through "On Line" calculators are also provided.
There are sections on starter cultures, probiotics, cheese science and technology, bioactive peptides, ice cream, wine making, modeling in food technology, thermal processing and modified atmosphere packaging and labelling. Some general health information including reference to allergy and food intolerance is also presented.
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The double agar method as described by Adams(1959) is widely used to enumerate lactococcal and other phages. In this method a small volume of a dilution of phage suspension and a small quantity of host cells grown to high cell density, sufficient to give 107-108 CFU/ml, are mixed in about 2.5 ml of molten, 'soft' agar at 46°C. The resulting suspension is then poured on to an appropriate 'nutrient' basal agar medium e.g. M17 (Terazaghi and Sandine, 1975) to form a thin 'top layer' which hardens and immobilises the bacteria. Refer to figure 1 below.
T
he basal agar should be free of surface moisture to avoid problems, occasionally encountered, of the soft agar not adhering to the basal agar. Solid, thermostatically-controlled heating blocks are useful for maintaining the soft agar, see figure 2, at the correct temperature and are more useful than water baths. The assay can also be undertaken by adding phage, host and calcium ions to a sterile test tube at ambient temperature. After several minutes molten soft agar at 46°C is added and the mixture poured on to the basal agar as discussed previously. During incubation usually at 30°C, uninfected bacteria multiply to form a confluent film of growth over the surface of the plate.
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In Northern Europe and North America there is relatively little use made of whole milk powder (WMP) in commercial ice cream manufacture. However small scale ice cream or gelato manufacturers sometimes use these ingredients. In Vietnam and other Asian countries ice cream is frequently made from whole milk powder and cream. To the best of my knowledge there has been no information published on how to calculate the weights of cream and whole milk powder required to make ice cream mixes to a product specification. However the basic principles of mix calculation have been well described.
The aim of this tutorial is to explain the calculations required to determine the weight of cream and WMP in ice cream mixes. The significance of being able to undertake these calculations and an explanation of the concept of a balanced ice cream mix has been explained previously. We will start by first defining the mix specification that is required.
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All tailed bacteriophages have a nucleic acid core surrounded by a protein coat. Phages active against lactic acid bacteria are approximately tadpole or sperm shaped and have a distinct head terminating in a tail with a hollow core.
Bradley (1967), in a classic review paper, summarised the principles of phage morphology and outlined the basic morphological types which are still valid (fig. 1). Note phages with a contractile tail are found only in Group A. Phages attacking lactic acid bacteria belong to Groups A, B and C. Phages in Groups A, B and C contain double stranded DNA.
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There are many ways in which traditional cheeses can be classified. Criteria such as country of origin, type of milk used, species of animal used to produce the milk, fat content, moisture content, texture, whether mould ripened or not, cheese making process used, moisture in the non-fat solids have been and continue to be used. These criteria have been used either singly or in combination.
These descriptive approaches are limited in that they provide no theoretical insight into why one cheese is different from another. In other words, why is Gouda cheese different than Cheshire or what makes Emmental different than Cheddar cheese?
For many years researchers in New Zealand, the UK, Ireland, the Netherlands and elsewhere were aware that there were significant differences in the pH and mineral concentrations of the major cheese varieties.
Lawrence et. al. (1984) first suggested that cheeses could be classified on the basis of two criteria, pH and calcium content. This approach is illustrated in fig. 1 for Swiss, Gouda, Cheddar and Cheshire cheeses.
