VARIATION OF WATER-HOLDING CAPACITY AND MECHANICAL-PROPERTIES BETWEENTHE PARTICLE-SIZE FRACTIONS OF MINERAL ACID, LACTIC AND RENNET CASEINCURDS

Mineral acid casein, lactic casein and rennet casein curds were prepar ed either in the laboratory or on pilot-scale equipment. The water-hol ding capacities of mineral acid casein curd particles prepared in the laboratory were relatively constant for particle sizes > 4 mm both aft er sieving and after centrifugation; however, they increased dramatica lly at smaller particle sizes to a maximum of 7.2 g water/g dry curd a fter 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 st rength of the centrifugally compressed curd as assessed by a penetrome ter probe also decreased dramatically with decrease in particle size. This variation of mineral acid casein curd properties with particle si ze 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 cu rd formed by acidification of skim milk with glucono-delta-lactone als o produced similar but smaller variations in curd properties with part icle size. These curd particles are all formed by break up of a quiesc ent gel and the variation in curd properties with particle size was at tributed 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.