Effect of pH at pressure treatment on the acid gelation of skim milk

Skim milk at pH between 6.4 and 7.3 was pressure treated at 200–600 MPa for 30 min and then slowly acidified with glucono-δ-lactone to form acid gels. Milks at low pH produced acid gels with low elastic moduli (final G′) and yield stresses and those at higher pH produced acid gels with higher final G′ and yield stresses. Pressure treatment disrupted the casein micelles at all pH and transferred high levels of casein to the serum phase. Denaturation of α-lactalbumin occurred at a pressure of 600 MPa only, and the level of denaturation increased with increasing pH. Denaturation of β-lactoglobulin (β-LG) occurred at all pressures, with the level of denaturation increasing with the magnitude of the pressure treatment and with pH. The denaturation of the whey proteins and the disruption of the casein micelles could not entirely account for the changes in the rheological properties of the acid gels, as denaturation of up to 50% of the whey proteins produced acid gels with very low final G′ and yield stresses. It is proposed that the pH and the magnitude of the pressure treatment affect the interactions of the denatured β-LG with the casein proteins in the pressure-treated milks, and that this affects the ability of the denatured β-LG to participate in the acid gel structures.Industrial relevanceThe control and manipulation of the firmness of acid skim milk gels is important in many dairy food applications such as yogurts and some types of cheeses. This study has demonstrated that acid gel firmness can be substantially manipulated when the milk is pH adjusted and pressure treated before acidification, and that these effects are different to those obtained through heating. The commercial uptake of high pressure processing in the dairy industry is dependent on this technology producing unique functional properties in milk when compared with traditional processing. The results of this study indicates that high pressure processing of milk may offer unique functional properties in acid gel applications which could be used for the development of new or improved dairy products.