Denaturation, aggregation and heat stability of milk protein during the manufacture of skim milk powder

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Abstract
The effect of preheat treatment, evaporation and drying in a commercial plant on the denaturation of βlactoglobulin and α-lactalbumin, their incorporation into the casein micelle and the heat stability characteristics of the milks and powders were determined. Preheat treatments between 110 °C for 2 min and 120 °C for 3 min denatured between 80 and 91% of β-lactoglobulin and between 33 and 45% of α-lactalbumin. Evaporation increased the extent of denaturation but spray drying did not increase it further. The incorporation of α-lactalbumin and βlactoglobulin into the micelles was markedly less than the amount that denatured and was not a constant ratio to it. Heat coagulation times at 140 °C of milks, concentrates and powders diluted to the original milk concentration were measured as a function of pH. In general, the greater the collective heat treatment, the shorter the time required to achieve coagulation. Spray drying shifted the peak positions in the pH-heat coagulation time profiles. In contrast, heat coagulation times (measured at 120 °C) of concentrates and powders diluted to 20% total solids content increased with the severity of the preheat treatment. Surprisingly, spray drying markedly increased the heat coagulation times of the diluted concentrates.