458 Part IV: Milk
Recent reviews on this subject include Nieuwen-
huijse (1995), Datta and Deeth (2001) and Nieuwen-
huijse and van Boekel (2003).
HEAT-INDUCEDCHANGES INLACTOSE IN
MILK AND THEMAILLARDREACTION
Heating lactose causes several chemical modifica-
tions, the nature and extent of which depend on
environmental conditions and the severity of heat-
ing; changes include degradation to acids (with a
concomitant decrease in pH), isomerization (e.g., to
lactulose), production of compounds such as furfur-
al, and interactions with amino groups of proteins
(Maillard reaction).
In the Maillard reaction, lactose or lactulose reacts
with an amino group, such as the -amino group of
lysine residues, in a complex (and not yet fully un-
derstood) series of reactions with a variety of end
products (O’Brien 1995, van Boekel 1998). The early
stages of the Maillard reaction result in the formation
of the protein-bound Amadori product, lactulosylly-
sine, which then degrades to a range of advanced
Maillard products, including hydroxymethylfurfural,
furfurals, and formic acid. The degradation of lac-
tose to organic acids reduces the pH of milk. The
most obvious result of the Maillard reaction is a
change in the color of milk (browning), due to the for-
mation of pigments called melanoidins, or advanced-
stage Maillard products; extensive Maillard reac-
tions also result in polymerization of proteins (van
Boekel 1998). The Maillard reaction also changes
the flavor and nutritive quality of dairy products, in
the latter case through reduced digestibility of the
caseins and loss of available lysine.
Moderately intense heating processes cause pri-
marily the isomerization of lactose to lactulose. Lac-
tulose, a disaccharide of galactose and fructose, is of
interest as a bifidogenic factor and also as a laxative.
More severe treatments (e.g., sterilization) will re-
sult preferentially in Maillard reactions.
There is particular interest in the use of products
of heat-induced changes in lactose, such as lactulose
and forosine, as indices of heat treatment of milk
(Birlouez-Aragon et al. 2002).
INACTIVATION OFENZYMES ONHEATING OF
MILK
Heat treatment of milk inactivates many enzymes,
both indigenous and endogenous (i.e., of bovine or
bacterial origin). The inactivation of enzymes is of
interest both for the stability of heated milk products
and as an index of heat treatment. Thermal inactiva-
tion characteristics of a number of indigenous milk
enzymes are summarized in Table 20.3.
Because of their importance, the thermal inactiva-
tion kinetics of several milk enzymes have been
studied in detail. A particular case of interest is that
of alkaline phosphatase, which has for decades been
used as an indicator of the adequacy of pasteuriza-
tion of milk, because its thermal inactivation kinet-
ics in milk closely approximates those of Myco-
bacterium tuberculosis. A negative result (residual
activity below a set maximum value) in a phos-
phatase test (assay) is regarded as an indication that
milk has been pasteurized correctly (Wilbey 1996).
Detailed kinetic studies on the thermal inactivation
of alkaline phosphatase under conditions similar to
pasteurization have been published (McKellar et al.
1994, Lu et al. 2001).
To test for overpasteurization (excessive heating)
of milk, a more heat stable enzyme, for example,
lactoperoxidase, has been used as the indicator en-
zyme (Storch test); lactoperoxidase may also be
used as an index of the efficacy of pasteurization of
cream, which must be heated more severely than
milk to ensure the killing of target bacteria, due to
the protective effect of the fat therein.
Recently, other enzymes have been studied as
indicators (sometimes called time temperature inte-
grators, TTIs) of heat treatments (particularly at
temperatures above 72°C) that may be applied to
milk; these include catalase, lipoprotein lipase, acid
phosphatase, N-acetyl--glucosaminidase and -
glutamyl transferase (McKellar et al. 1996, Wilbey
1996).
The alkaline milk proteinase, plasmin, is resistant
to pasteurization; indeed, its activity may increase
during storage of pasteurized milk due to inacti-
vation of inhibitors of plasminogen activators (Rich-
ardson 1983). Treatment under UHT conditions great-
ly reduces its activity (Enright et al. 1999); there is
some evidence that the low level of plasmin activity
in UHT milk contributes to the destabilization of the
proteins, leading to defects such as age gelation,
although this is not universally accepted (for review
see Datta and Deeth 2001).
Since most indigenous milk enzymes are inacti-
vated in UHT-sterilized milk products and in all in-
container sterilized products, enzymes are not suit-
able indices of adequate processing, and chemical