REVERSIBILITY OF SHRINKAGE OF MINERAL ACID CASEIN CURD AS A FUNCTION OF IONIC-STRENGTH, PH AND TEMPERATURE

Mineral acid casein (MAC) curd was subjected to a range of ionic stren gth, pH and temperature conditions to explore their effects on curd wa ter-holding capacity, particularly the reversibility of shrinkage. The shrinkage of MAC curd on whey removal during the washing stage of cas ein manufacture could be attributed almost completely to the reduction in ionic strength and not specifically to either calcium or lactose r emoval. This shrinkage on whey removal during washing could not be rev ersed by adding whey back to the curd to attain the original ionic env ironment. In this sense it was irreversible. The pH during formation o f MAC curd was more important than later pH adjustments in controlling curd water-holding capacity. The effect of a high precipitation pH in creating a tough, compact curd could not be reversed by subsequently decreasing the pH. However, the effect of a low precipitation PH in cr eating a soft, open curd could be partly reversed by subsequently incr easing the pH. MAC curd shrank by 45 % when the temperature was increa sed from 20 to 80 degrees C. This temperature-dependent shrinkage was virtually fully reversible on cooling. It is suggested that the water holding capacity of casein curd is governed largely by hydrophobic int eractions and that once such interactions have occurred they cannot ea sily be disrupted by small changes in either pH or ionic strength.