Rennet coagulation of skim milk and curd drainage: effect of pH, casein concentration, ionic strength and heat treatment

Abstract

The effect of the pH of skim milk (6.4 to 6.0), the casein concentration (27 to 36 g . kg -1 ), a reduction in the ionic strength to 0.6 compared to that of unmodified milk and the effect of heat treatment on rennet coagulation, gel syneresis and drainage were tested. Coagulation was measured using a formagraph and by viscoelasticimetry, while syneresis was studied using a centrifugation test. Drainage was followed under conditions similar to those of cheesemaking, using either a mesophilic or thermophilic process. The effects of pH and casein concentration on rennet coagulation, gel syneresis and drainage were in accordance with those reported in the literature. Reduction in ionic strength led to shorter coagulation time and firming time. It also led to an increase in gel firmness, gel syneresis, drainage rate, final amount of expelled whey, solids content and hardness of drained curd. Ionic strength appeared to have an interactive effect with pH on coagulation and with the concentration of casein on syneresis and drainage. This effect of the reduction in ionic strength is discussed. rennet coagulation / pH / casein concentration / ionic strength / drainage Rsum -La coagulation prsure du lait crm et l'gouttage : effet du pH et de la concentration en casine, de la force ionique et du traitement thermique. L'effet du pH du lait crm (6,4-6,0), de la concentration en casines (27-36 g . kg -1 ), de la rduction de la force ionique du lait 0,6 fois celle du lait natif et du traitement thermique a t test sur la coagulation prsure, l'aptitude la synrse et l'aptitude l'gouttage des gels. La coagulation tait suivie l'aide du formagraph et d'un viscolasticimtre, tandis que la synrse tait suivie par un test de centrifugation et l'gouttage tait tudi par une mthode proche d'une fabrication fromagre. Les rsultats confirment ceux de la littrature pour l'effet des facteurs pH et concentration en casine sur la coagulation, la synrse et l'gouttage. La rduction de la force ionique a entran une rduction du temps de prise et de