Thermal Food Processing

268 Thermal Food Processing: New Technologies and Quality Issues
As stated above, the primary function of the thermal processing of milk is to
kill undesirable microorganisms. Modern pasteurization is a very effective means
of ensuring that liquid milk is free of the most heat resistant pathogenic bacteria
likely to be present in raw milk; today, most health risks linked to the consumption
of pasteurized milk are probably due to postpasteurization contamination of the
product. UHT or in-container sterilized milks, on the other hand, are virtually
free of even spore-forming thermophilic bacterial species.
9.3 OVERVIEW OF HEAT TREATMENTS
APPLIED TO MILK
As with any food product, the choice of heat treatment applied to milk depends
on a trade-off between, first, the degree of microbial inactivation required to
ensure safety and extend the shelf life by an acceptable factor and, second,
changes in quality of the product; these two consequences of heating are usually,
in effect, inversely correlated.
Overall factors that should be considered in determining the heat treatment
required for a particular product include:
• Postheating growth potential of spore-forming bacteria. For example,
low pH conditions or storage at low temperatures suppress outgrowth
of spore-forming bacteria; thus, products that include these additional
hurdles, e.g., pasteurized or fermented milks, may not require as extensive
heat treatment as higher pH products or products to be stored at
room temperature.
• Consumer preference. For example, in some countries, such as Ireland
and Australia, 11 consumer preference is clearly aligned toward pasteurized
milk, and the more strongly cooked flavors associated with UHT
milk are undesirable.
• Target market. For example, the requirements for processing of infant
formulae are among the most stringent in the dairy industry, due to the
vulnerability of that group of consumers to food-borne illness.
Heat treatments applied to milk are generally defined by the maximum temperature
of heating and the holding time at that temperature. The processes most
commonly applied to milk and dairy products are summarized in Table 9.1. 12,13,16
9.4 EFFECTS OF HEATING ON MILK
CONSTITUENTS
The primary constituents of milk include fat (as an emulsion), protein (a heterogeneous
population, some of which are in colloidal form, while others are dissolved
in the aqueous phase, including enzymes), lactose, salts, and vitamins. Milk is an
exceedingly complex raw material for processing; it has a range of constituents
whose nature, stability, and properties are changed by the types of heat treatments