Colloidal gels from proteins and peptides - Physical Gels from Biological and Synthetic Polymers - page 265

this finding: one might expect divalent ions to alter gelation characteristics, just as they

do for polysaccharide gels ( Chapter 5 ). At the same time, changing the ionic strength
will also encourage gelation; this is an interesting area which may develop further in
the future.

9.3 Gels from milk proteins

Of course -Lg is a milk protein, but this section is limited to work on casein and whey

protein isolate systems. Casein gels have long been associated with products such as
cheese and yoghurt. Some are purely physical gels, but most involve the enzymatic
action of rennet, the common name for the enzyme chymosin, usually extracted from the
fourth stomach of young calves. Caseins remain the most important proteins in milk, and
one of the four main caseins,

β

- casein, located on the surface of spherical casein micelles,
stabilizes casein micelles by steric repulsion. Chymosin cleaves the

κ

on the surface
of the casein spherical particles, disrupting the steric stabilization, and then aggregation
occurs.

' hairs '

9.3.1 Casein micelles

The phosphorylated caseins, including - derivatives, make up a

large part of the protein content of milk. The major component of the proteins (90%)
occurs as casein micelles, and both

α S1-, α S2-, β -and κ

κ -caseins and phosphate groups occur widely at the

micelle surface, while -caseins dominate the inner part ( Figure 9.5 ). Since the

stability of casein micelles depends on both electrostatic and steric effects, at pH 6.7
electrostatic conditions alone are not suf

α -and β

cient to maintain the micelles in milk
(Payens, 1979 ). Instead it is understood that the hydrophilic C-terminal part of

Submicelle

Protruding
peptide chain

Calcium phosphate

50 nm

Schematic cross-section of a casein micelle. Reprinted with permission from Walstra ( 1999 )
© 1999 Elsevier.

Figure 9.5