Thermal Food Processing

304 Thermal Food Processing: New Technologies and Quality Issues
kappa-casein can be hydrolyzed by rennin to give para-kappa-caseinate that is
insoluble in the presence of calcium ions and precipitates as a curd. Usually
casein exists in colloidal particles, so-called casein micelles, containing calcium
and phosphorus, some enzymes, citrate, and milk serum.
Whey proteins do not precipitate at pH 4.6; they are water soluble and sensible
to temperature. The main components of whey protein are beta-lactoglobulin,
alpha-lactalbumin, bovine serum albumin, and immunoglobulins. Beta-lactoglobulin
represents about 50% of the total whey protein and is the main whey protein
in bovine, ovine, caprine, and buffalo milks, while alpha-lactalbumin is the
principal protein in human milk.
10.2.2.1 Denaturation of Whey Protein
Several physical and chemical changes occur in whey protein during thermal
processing of milk, and these changes (denaturation) affect the functional and
sensory properties of milk. During the heating process, whey proteins containing
sulfhydryl residues undergo various changes resulting in the formation of (1) a
protein complex between beta-lactoglobulin and kappa-casein, with consequent
modification of rennet coagulation behavior and heat stability, (2) typical offflavors,
and (3) unusual amino acids (lysinoalanine). 19 In particular, under certain
conditions of concentration and pH, beta-lactoglobulin is able to form gels. 20,21
Whey proteins show different thermal stabilities: alpha-lactalbumin > betalactoglobulin
> bovine serum albumin > immunoglobulins. While denaturation of
alpha-lactalbumin can be a good parameter to describe high-temperature treatments,
such as sterilization, denaturation of beta-lactoglobulin is useful to describe thermal
treatments from pasteurization to UHT processing. 4,22
Alpha-lactalbumin is frequently chosen as an indicator of heat treatment. To
characterize its native and heat-denatured forms, monoclonal antibodies specific in
inhibition enzyme-linked immunosorbent assay (ELISA) have been proposed. 23
Using this method, it is possible to differentiate among raw, pasteurized, UHT, and
sterilized milk even if the alpha-lactalbumin concentration of the original raw milk
is unknown. However, as mentioned above, this technique is mainly suited to detect
UHT treatment and sterilization because of the heat stability of alpha-lactalbumin. 23
Fukal and coworkers 24 investigated the use of immunochemical probes to
discriminate among different heat treatments of milk. They raised polyclonal
antibodies against 10 immunogens: 5 native milk proteins, alpha- + beta-casein,
kappa-casein, whole casein, alpha-lactalbumin, and beta-lactoglobulin, and the
corresponding 5 pasteurized milk proteins. Their results showed no significant
differences in the immunoreactivity of anticasein antibodies in heat-treated milk
compared to raw milk. In contrast, using a combination of antibodies against
native alpha-lactalbumin and beta-lactoglobulin, immunoreactivity decreased in
heated samples and allowed (P < 0.001) a categorization of milk as raw, pasteurized,
UHT treated, or bath sterilized. 24
Irreversible denaturation of whey proteins during heating of milk under commercial
processing conditions has been widely studied. Oldfield and coworkers 25