Rennet coagulation properties of skim milk concentrated by ultrafiltration: effects of heat treatment and pH adjustment

Abstract

Skim milks were ultrafiltered to produce concentrates with protein levels of ∼6 and ∼9%, and were then heated at 140°C for 4 s in a UHT plate heat exchanger. In some cases, skim milks were heated at 140°C for 4 s prior to ultrafiltration. Rennet coagulation properties were assessed using low amplitude oscillatory rheometry by measurements of storage modulus ( G′) as a function of time after rennet addition. Concentration of skim milk by UF had no effect on gelation time ( GT) when measurements were made at the natural pH. However, when measurements were made at pH 6.5, concentration of milk resulted in a decrease in GT. Curd firming rates and the final G′ values increased with increase in milk concentration by UF. UHT-treatment of UF concentrated milk resulted in increased GT, decreased curd firming rates and decreased G′ values; the effects of heat treatment on these properties were greater when UHT treatment was given to skim milk prior to UF. Acidification of UHT-treated skim milk and UF concentrates to pH 5.5, followed by neutralization to pH 6.5 (‘pH cycling’), resulted in an improvement of rennet coagulation properties of normal milk, but had no effect on 2X concentrated milk. In contrast, the rennet coagulation properties of 3X concentrated milks were adversely affected by pH cycling. Similar trends were noted when pH cycling was carried out on heated skim milk prior to UF. ©