The effect of 15 amides and related compounds on the heat stability of milk was investigated; of these urea, biuret, triuret, methyl urea and ethyl urea had a similar stabilizing effect. These 5 compounds reacted with lysine to form a ninhydrin-positive compound, possibly homocitrulline, and with lactose produced Maillard-type browning, but some of the other compounds studied were also capable of participating in one or both of these reactions. The only effect which the 5 stabilizing amides had in common and which the other compounds did not share was a significant pH-buffering capacity in synthetic systems and in milk. It is suggested that urea exercises its stabilizing influence in milk principally through its ability to buffer the pH of the system during heating.Addition of 002 % (3-6 mM) or 0-05 % (8-4 mM) urea to milk before heating increased its coagulation time from 35 to 62 and 104 min respectively (Pyne, 1958). A significant positive correlation between the heat stability at 120 °C of bulk milk at its natural pH and the level of non-protein nitrogen (NPN)
The heat stability and rennet coagulation time (second stage) of milk were reduced by brief dialysis against water. Destabilization appears to arise from a developed imbalance between Ca and phosphate plus citrate due to the very slow diffusion of Ca on dialysis. Average micelle size as indicated by permeation chromatography in porous glass CPG 10 was slightly reduced by dialysis for 24 h. Direct addition of low levels (10-100 mM) of NaCl to milk markedly reduced heat stability at pH > 7-0 (normal minimum) possibly due to dissociation of K-casein, but increased rennet coagulation times; higher levels of NaCl decreased heat stability throughout the pH range 6-4-7-4.
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