psychrotrophic Bacillus species are becoming increasingly associated
with the spoilage of refrigerated milk.
To extend the shelf life of milk beyond 5–15 days requires measures to
reduce the number of spores present. UHT processing achieves this but
introduces a cooked flavour which is not always appreciated. To avoid
this problem, alternative techniques to produce so-called extended shelf
life (ESL) milks which retain the taste of pasteurised milk have been
explored. These include direct heating by steam injection to temperatures
of 85–127 1 C for 2 seconds, the removal of spores by centrifugation
combined with a pasteurisation treatment, or microfiltration of skim
milk and its subsequent recombination with UHT treated cream. Such
techniques combined with hygienic handling and an effective cold chain
can achieve shelf lives of 21 days.
5.2.4 Milk Products
Fresh milk is the starting point for a number of other food products,
some of which are shown in Figure 5.3. A number of these are described
in detail elsewhere in this book. Yoghurt, cheese and other fermented
products are dealt with in Chapter 9, dried milk in Section 4.7, and the
principles behind the production of evaporated milk and UHT milk are
described in Section 4.1.
Fat can be concentrated from milk by centrifugal separation to
produce a number of different types of cream. These are distinguished
Figure 5.3 Milk and milk products.Tindicates elevated temperature; pH, reduced pH;aw,
reducedaw; sep., separation, comp., compartmentalization; and * stored at
chill temperatures
130 Microbiology of Primary Food Commodities