Ultrasonication of reconstituted whole milk and its effect on acid gelation.

Ultrasonication (US) of whole milk at 22.5kHz and 50W homogenized fat globules. Extended US without temperature control (attaining >90°C at longest times), or with control at temperatures ⩾60°C caused denaturation of the whey proteins and aggregation of the fat globules and proteins. Acidification of US milk produced gels with increased firmness and reduced gelation times compared to untreated milk. Below 60°C, US of milk produced acid gels with very high firmness without whey protein denaturation; the firmness was similar to gels from heated whole milk. Extensive US without temperature control or with control at ⩾60°C decreased acid gel firmness compared to shorter times or lower temperatures. Higher acid gel firmness could be achieved by subjecting the milk to separate heat (80°C/30min) and US treatment (at 20°C) before acidification when compared with either heating or US alone. This was independent of the order of heating and US treatment.

The protein interactions and rheological properties of skim milk heated in the presence of low levels of reducing agent.

Skim milk with low levels of added ß-mercaptoethanol (SM-ME) and untreated skim milk (SM) were heated and then made into acid gels. Acid gels prepared from heated SM-ME had markedly higher firmness and contained more protein connections than acid gels prepared from heated SM. Electrophoretic analyses of the milks showed that the levels of ß-lactoglobulin and α-lactalbumin associated with the casein micelles increased with increasing ß-ME concentration. The levels of disulphide-linked whey proteins were higher in SM-ME than in SM. This suggested that there may be higher levels of initiators for thiol-disulphide exchange reactions, resulting in an increase in the rate of the reactions and the formation of greater numbers of small aggregates, in SM-ME than in SM. Consequently, acid gels made from SM-ME may have more bonds and more particles participating in the network, resulting in firmer gels, than acid gels made from SM.

Effects of adding low levels of a disulfide reducing agent on the disulfide interactions of ß-lactoglobulin and κ-casein in skim milk.

Low concentrations of a disulfide reducing agent were added to unheated and heated (80 °C for 30 min) skim milk, with and without added whey protein. The reduction of the ß-lactoglobulin and κ-casein disulfide bonds was monitored over time using electrophoresis. The distribution of the proteins between the colloidal and serum phases was also investigated. κ-Casein disulfide bonds were reduced in preference to those of ß-lactoglobulin in both unheated and heated skim milk (with or without added whey protein). In addition, in heated skim milk, while the serum κ-casein was reduced more readily than the colloidal κ-casein, the distribution of κ-casein between the two phases was not affected.