Rheological properties at small (dynamic) and large (yield) deformations of acid gels made from heated milk

The effect of a range of milk heat treatments on the rheological proper ties, at small and large deformations, of acid skim milli gels was investigated. Gels were made from reconstituted skim mild heated at 75, 80, 85 and 90 degrees C for 15 or 30 min by acidification with glucono-delta-lactone at 30 degrees C. Gels were also made from skim milk powder (SMP) samples that had been give a range of preheat treatments during powder manufacture. Heating milks at temperatures greater than or equal to 80 degrees C for 15 min increased the storage moduli (G') compared with unheated milk and produced gels with G' in the range 300-450 Pa. Acid gels made from high-heat or medium-heat SMP had higher G' than gels made from low-heat or ultra-low-heat SMP. Cooling gels to low temperatures resulted in an increase in G'. The yield stress of gels slightly decreased with mild heat treatments of milk, and then increased again to a maximum, finally decreasing slightly with very high heat treatments of milk. The strain at yielding decreased markedly with increasing heat treatment of milk, making these gels brittle and easier to fracture. We propose that denatured whey proteins aggregated with casein particles during the acidification of heated milk and were responsible for most of the effects observed in this study.