Food Biochemistry and Food Processing (2 edition)

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25 Biochemistry of Milk Processing 469

age gelation, which refers to a progressive increase in viscosity

during storage, followed by the formation of a gel that cannot be

redispersed. The mechanism of age gelation is not completely

understood, but is probably due to one or both of the following:

1. Physicochemical factors:For example dissociation of ca-
   sein/whey protein complexes; changes in casein micelle
   structure and/or properties; cross-linking due to the Mail-
   lard reaction; removal or binding of Ca^2 +.


2. Biochemical factors:For example action of proteolytic en-
   zymes, such as plasmin or heat-stable bacterial proteinases
   (i.e., fromPseudomonasspecies), which may hydrolyse
   κ-casein, inducing micelle coagulation.

Recent reviews on this subject include Nieuwenhuijse (1995),

Datta and Deeth (2001) and Nieuwenhuijse and van Boekel

(2003).

Heat-Induced Changes in Lactose in Milk and

the Maillard Reaction

Heating lactose causes several chemical modifications, the na-

ture and extent of which depend on environmental conditions

and the severity of heating; changes include degradation to acids

(with a concomitant decrease in pH), isomerisation (e.g., to lac-

tulose), production of compounds such as furfural and interac-

tions with amino groups of proteins (Maillard reaction).

In the Maillard reaction, lactose or lactulose reacts with an

amino group, such as theε-amino group of lysine residues, in a

complex (and not yet fully understood) series of reactions with a

variety of end products (O’Brien 1995, van Boekel and Walstra

1998; O’Brien 2009). The early stages of the Maillard reaction

result in the formation of a protein-bound Amadori product,

lactulosyllysine, which then degrades to a range of advanced

Maillard products, including hydroxymethylfurfural, furfurals

and formic acid. The breakdown of lactose to organic acids

reduces the pH of milk. The most obvious result of the Maillard

reaction is a change in the colour of milk (browning), due to the

formation of pigments called melanoidins, or advanced-stage

Maillard products; extensive Maillard reactions also result in the

polymerisation of proteins. The Maillard reaction also changes

the flavour and nutritive quality of dairy products, in the latter

case through reduced digestibility of the caseins and loss of

available lysine.

Moderately intense heating also causes the isomerisation of

lactose to lactulose. Lactulose, a disaccharide of galactose and

fructose that does not occur naturally, is of interest as a bifi-

dogenic factor and as a laxative. More severe treatments (e.g.,

sterilisation) will result 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).

Heat-Induced Changes in Milk Lipids

There are little effects of heating on milk triglycerides at the

temperatures commonly encountered in dairy processing. Heat-

ing at modest temperatures can promote oxidation of lipids, but

more severe heating inhibits oxidation (van Boekel and Walstra

1998). However, heat-induced denaturation of whey proteins can

result in their interaction with the milk fat globule membrane

(MFGM) (Huppertz et al. 2009). Because of denaturation of im-

munoglobulins, creaming due to cold agglutination of milk fat

globules is prevented in heated milk.

Inactivation of Enzymes on Heating of Milk

Heat treatment of milk inactivates many enzymes, both indige-

nous 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 indices of heat treatment. Ther-

mal inactivation characteristics of a number of indigenous milk

enzymes are summarised in Table 25.3.

Because of its importance, the thermal inactivation kinetics

of several milk enzymes have been studied in detail (Andrews

et al. 1987). A case of particular interest is that of alkaline

phosphatase, which has been used since 1925 as an indicator

of the adequacy of pasteurisation of milk, because its ther-

mal inactivation kinetics in milk closely approximate those of

Table 25.3.Thermal Inactivation Characteristics of Some Indigenous Milk Enzymes

Enzyme Characteristics

Alkaline phosphatase Inactivated by pasteurisation; index of pasteurisation; may reactivate on storage

Lipoprotein lipase Almost completely inactivated by pasteurisation

Xanthine oxidase May be largely inactivated by pasteurisation, depending on whether milk is homogenised or not

Lactoperoxidase Affected little by pasteurisation; inactivated rapidly around 80◦C; used as an index of flash

pasteurisation or pasteurisation of cream

Sulphydryl oxidase About∼40% of activity survives pasteurisation, completely inactivated by UHT

Superoxide dismutase Largely unaffected by pasteurisation

Catalase Largely inactivated by pasteurisation but may reactivate during subsequent cold storage

Acid phosphatase Very thermostable; survives pasteurisation

Cathepsin D Largely inactivated by pasteurisation

Amylases (αandβ) Largely inactivated;βmore stable thanα

Lysozyme Largely survives pasteurisation

Source: Adapted From Farkye and Imifadon 1995 UHT, ultra-high temperature.