Food Biochemistry and Food Processing

20 Biochemistry of Milk Processing 457

creasing stability on the alkaline side of the mini-
mum. The rare type B HCT-pH profile has no mini-
mum, and heat stability increases progressively
throughout the pH range 6.4–7.4.
In addition to pH, a number of factors affect the
heat stability of milk (O’Connell and Fox 2002):

• Reduction in the level of Ca^2
  or Mg^2
  increases
  stability in pH range 6.5–7.5.


• Lactose hydrolysis increases heat stability
  throughout the pH range.


• Addition of -casein eliminates the minimum in
  the type A profile.


• Addition of
  -lactoglobulin to a type B milk
  converts it to a type A profile.


• Addition of phosphates increases heat stability.


• Reducing agents reduce heat stability and convert
  a type A to a type B profile.


• Alcohols and sulphydryl-blocking agents reduce
  the heat stability of milk.

Concentrated (e.g., evaporated) milk is much less
thermally stable than unconcentrated milk, and its
HCT-pH profile, normally assayed at 120°C, is quite
different. It shows a maximum at approximately pH
6.4, with decreasing stability at higher and lower pH
values (Fig. 20.1).
Severe heating has a number of effects on the
casein molecules themselves, including dephospho-

rylation, deamindation of glutamine and asparagine

residues, cleavage and formation of covalent cross-

links; these changes may result in protein-protein

interactions, which contribute to thermal instability.

STABILITY OFUHT MILK ONSTORAGE

UHT milk is stable in long-term storage at ambient

temperatures if microbiological sterility has been

achieved by the thermal process and maintained by

aseptic packaging in hermetically sealed containers.

The shelf life of UHT milk is often limited by age

gelation, which refers to a progressive increase in

viscosity during storage, followed by the formation

of a gel that cannot be redispersed (Lewis and Hep-

pell 2000). The mechanism of age gelation is poorly

understood, but it is probably due to physicochemi-

cal or biochemical factors, or both:

• Physicochemical factors.For example,
  dissociation of casein/whey protein complexes;
  changes in casein micelle structure and/or
  properties; cross-linking due to the Maillard
  reaction; removal or binding of calcium ions.


• Biochemical factors.For example, action of
  proteolytic enzymes, such as plasmin or heat-
  stable bacterial proteinases (i.e., from
  Pseudomonasspecies), which may hydrolyze -
  casein, inducing micelle coagulation.

Figure 20.1.Heat coagulation time(HCT)-pH profiles of typical type A bovine milk (solid line), type B or serum
protein–free milk (dashed line), as determined at 140°C, or concentrated milk (dotted line), as determined at 120°C.