20 Biochemistry of Milk Processing 459indices are more usually used (e.g., the concentra-
tion of lactulose or the extent of denaturation of -
lg). The use of a range of indicators for different
heat treatments applied to milk was reviewed by
Claeys et al. (2002).
The thermal inactivation of bacterial enzymes in
milk is also of considerable significance. For exam-
ple, psychotrophic bacteria of the genus Pseud-
omonasproduce heat-stable lipases and proteases
during growth in refrigerated milk; while pasteur-
ization readily kills the bacterium, the enzymes
survive such treatment and may contribute to the
deterioration of dairy products made from such milk
(Stepaniak and Sørhaug 1995). Pseudomonasen-
zymes partially survive in UHT milk and may be
involved in its age gelation (Datta and Deeth 2001).
CHANGES ON EVAPORATION
AND DRYING OF MILK
Dehydration, either partial (e.g., concentration to
40–50% total solids, TS) or (almost) total (e.g.,
spray-drying to 96–97% TS), is a common
method for the preservation of milk. Reviews of the
technology of the drying of milk and its effects on
milk quality include Caríc and Kalab (1987),
Písecky (1997), and Kelly et al. (2003).
CONCENTRATION OFMILK
The most common technology for concentrating
milk is thermal evaporation in a multieffect (multi-
stage) falling-film evaporator, which is used either
as a prelude to spray-drying, to increase the efficien-
cy of the latter, or to produce a range of concentrat-
ed dairy products (e.g., sterilized concentrated milk,
sweetened condensed milk).
In many processes, a key stage in milk evapora-
tion is preheating the feed. The minimum require-
ments of this step are to pasteurize the milk (e.g.,
skim milk, whole milk, filled milk) to ensure food
safety and to bring its temperature to the boiling
point in the first effect of the evaporator, for thermal
efficiency. However, the functionality of the final
product can be determined and modified by this pro-
cessing step; preheating conditions range from those
for conventional pasteurization to 90°C for 10 min-
utes or 120°C for 2 minutes. The most significant
result of such heating is denaturation of -lg, to a
degree that is dependent on the severity of heating.
During subsequent evaporation and, if applied, spray-
drying, relatively little denaturation of the whey pro-
teins occurs, as the temperature of the milk gener-
ally does not exceed 70°C (Singh and Creamer
1991). However, further association of whey pro-
teins with the casein micelles can occur during evap-
oration, probably because the decrease in pH reduces
protein charge, facilitating association reactions
(Oldfield 1998)
Evaporation also increases the concentrations of
lactose and salts in milk and induces a partially
reversible transfer of soluble calcium phosphate to
the colloidal form, with a concomitant decrease in
pH (Le Graet and Brule 1982, Nieuwenhuijse et al.Table 20.3.Thermal Inactivation Characteristics of Some Indigenous Milk EnzymesEnzyme Characteristics
Alkaline phosphatase Inactivated by pasteurization; index of pasteurization; may reactivate on storage
Lipoprotein lipase Almost completely inactivated by pasteurization
Xanthine oxidase May be largely inactivated by pasteurization, depending on whether milk is
homogenized or not
Lactoperoxidase Affected little by pasteurization; inactivated rapidly around 80°C; used as index
of flash pasteurization or pasteurization of cream
Sulphydryl oxidase About 40% of activity survives pasteurization, completely inactivated by UHT
Superoxide dismutase Largely unaffected by pasteurization
Catalase Largely inactivated by pasteurization but may reactivate during subsequent cold
storage
Acid phosphatase Very thermostable; survives pasteurization
Cathepsin D Largely inactivated by pasteurization
Amylases (and ) Thermal stability unclear; more stable than
Lysozyme Largely survives pasteurization
Source:Adapted from Farkye and Imafidon 1995.