Xanthan Gum in Skim Milk: Designing Structure into Acid Milk Gels - Food Colloids: Self-Assembly and Material Science - page 294

(a) (b)

2R 2R

2R g

Figure 2 Schematic picture of two spherical colloidal particles of radius R in a solution
containing non-adsorbing polymers of radius of gyration R g , with polymer coils
excluded from a depletion zone (dashed line) near the particle surface: (a) no
overlap of depletion layers (osmotic pressure on the spheres is isotropic); (b)
overlapping depletion layers (net attractive force between particles from unbal-
anced osmotic pressure). The excess osmotic pressure is indicated by the arrows
(After Ref. 10.)

entrapping the serum phase. Thus, a prerequisite for gel formation to occur is
acid-induced destabilization of the casein particles.
In unheated milk at physiological pH (

6.7), the stability of casein micelles
against aggregation is due to the presence of a 'hairy' layer of k-casein
molecules protruding from the casein micelle surface, providing primarily steric
repulsion upon collision of two casein micelles. 14, 15 Considering this 'hairy' k-
casein layer as a salted brush of weak polyelectrolyte molecules, de Kruif and
Zhulina applied 16 the scaling relation derived for the sharp conformational
transition from a highly extended to a strongly collapsed state of the brush.
Within the emerging casein brush model, 17, 18 acidification is seen to cause
brush collapse below pH

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5, resulting in the loss of polymeric stabilization of
the casein micelles, which finally aggregate.
The development of elasticity during gelation is illustrated in Figure 3 for
acid-induced gel formation in unheated skim milk. Figure 3(a) shows the
acidification kinetics common to all the gelation experiments performed. When
pH had dropped to a critical value (4.95), the storage modulus G 0 increased from
its baseline value and continued to develop in the manner shown in Figure 3(b).

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19.3.3 Effects of Pre-Heating of Skim Milk
Heat treatment of milk at temperatures above ca. 701C induces denaturation of
the whey proteins, which subsequently aggregate and also interact with the
casein micelles and more specifically with k-casein. 19 This results in the