Milk Salts: Technological Significance - Advanced Dairy Chemistry - page 369

9.7.4. Cold-Set Whey Protein Gels

Whey protein gels can also be produced using a two-step process that
involves heat treatment at low ionic strength and/or far from the isoelectric
point, followed by an increase in ionic strength and/or adjustment of pH
(Barbut and Foegeding, 1993; Britten and Giroux, 2001). These gels are called
cold-set gels, as the initial heat treatment produces a polymerized solution
and gelation can occur at low temperatures ( ambient) if the repulsive forces
are screened by the addition of mono- or polyvalent cations (e.g. Ca 2+ )ora
decrease in pH (e.g. through the addition of GDL or by bacterial fermenta-
tion). To obtain gels via the cold-set gelation method, it is necessary to first
prepare a solution of heat-denatured proteins, with a protein concentration
below the critical gelation concentration. Heating (e.g. 808C for 30 min)
results in the formation of soluble, denatured whey protein aggregates.
Whey protein fibril-type gels are formed at very low pH values (e.g. 2) and
cold-set fibril gels can also be made by the addition of Ca 2+

(Bolder et al.,

2006).

9.7.5. Emulsions

Caseins, especially caseinates, are widely used as emulsifiers (Dickinson,
1997). The aggregation state of casein greatly influences surface activity with
sodium caseinate (non-micellar), having greater surface activity than micellar
or Ca caseinate (Mulvihill and Murphy, 1991). Dalgleish (1987) reported that
emulsions prepared with s -or -casein were sensitive to precipitation by Ca
but emulsions prepared with -casein did not aggregate on Ca addition. The
phosphoserine residues in -casein helped that molecule maintain a thick steric
stabilizing monolayer on emulsion interfaces (Dickinson, 1997). Increasing
ionic strength by the addition of electrolytes screens out the double-layer
repulsion and therefore reduces the electrostatic stabilization of proteins.
Therefore, emulsions prepared with commercial milk protein ingredients of
high salt content may be more flocculated than model systems prepared with
pure proteins dissolved in low ionic strength buffer solutions (Dickinson,
1997). Calcium ions influence the stability of sodium caseinate-stabilized emul-
sions (Ye and Singh, 2001). Addition of CaCl 2 before or after homogenization
caused a decrease in the creaming stability of emulsions made with 0.5% case-
inate. In contrast, addition of CaCl 2 up to 10 mM increased the creaming
stability of emulsions made with 3% caseinate, although the stability decreased
again > 20 mM CaCl 2 . There was an increase in the surface protein concentra-
tion with an increase in the level of CaCl 2 , which was due to enhanced
adsorption of the s -caseins (Ye and Singh, 2001).