Effect of urea on heat coagulation of milk

Addition of 5-10 mmol/l urea to milk is known to have a strong stabilizing effect on heat coagulation, except at pH 6.7-6.9. The mechanism is, however, unknown. The heat-induced effects of urea were therefore investigated in more detail. The following effects were found: urea slowed down the heat-induced pH decrease, due to pH buffering by ammonia formed from urea; urea stimulated dissociation of casein, especially kappa-casein, probably due to pH buffering urea decreased the Ca2+-activity urea reacted with lysine via cyanate, probably to form homocitrullin. Heat coagulation is assumed to be due to two reactions: (1) salt-induced coagulation, the rate of which is governed by initial pH, Ca2+-activity, extent of kappa-casein dissociation, and (2) chemical crosslink formation, the rate of which is governed by the heat-induced pH decrease. It appeared from kinetic studies that urea slowed down the rate of both reactions (1) and (2). The reason urea does not stabilize at pH 6.7-6.9 is that in this range dissociation of kappa-casein is strong, so time is too short for urea to be able to react with casein.