Variation of water-holding capacity and mechanical properties between the particle size fractions of mineral acid, lactic and rennet casein curds

Abstract

Mineral acid casein, lactic casein and rennet casein curds were prepared either in the laboratory or on pilot-scale equipment. The water-holding capacities of mineral acid casein curd particles prepared in the laboratory were relatively constant for particle sizes >4 mm both after sieving and after centrifugation; however, they increased dramatically at smaller particle sizes to a maximum of 7·2 g water/g dry curd after sieving and 4·3 g water/g dry curd after centrifugation at 4060 g for the size fraction that passed a 0·85 mm screen. The mechanical strength of the centrifugally compressed curd as assessed by a penetrometer probe also decreased dramatically with decrease in particle size. This variation of mineral acid casein curd properties with particle size was attributed partly to variation in particle pH at the instant of curd formation because of imperfect mixing of acid and skim milk, i.e. localized pH gradients. Lactic casein and rennet casein curds and curd formed by acidification of skim milk with glucono-δ-lactone also produced similar but smaller variations in curd properties with particle size. These curd particles are all formed by break up of a quiescent gel and the variation in curd properties with particle size was attributed to variations in mechanical strength in the original milk gel, i.e. weak areas of the gel break up more easily, resulting in small, weak curd particles.