The Heat Coagulation of Milk: II. The Influence of Various Added Salts Upon the Heat Stabilities of Milks of Different Concentrations

Summary The variations in the time of coagulation with variations in the temperatures used have been shown to be of a logarithmic nature. However, these variations in the time of coagulation are less with milk of 9 per cent solids-not-fat than they are when milk of 18 per cent solids-not-fat is used. The results indicate also that the use of temperatures of from 110° to 120° C. yield results of greatest practical value. Higher temperatures are apt to give results wherein the errors are large. 2. The data presented upon forewarming also indicate that the concentration of solids-not-fat is an important factor in varying the relative effects of this procedure. The results show that forewarming at high temperatures lowers the stability of those of higher solids-not-fat concentration. Hence, in any comparison of results obtained with milks of different solids-not-fat concentration similar treatment with respect to forewarming must be assured. These results indicate also that the relationships established by forewarming are not altered by the addition of electrolytes except in degree of stability. It appears, therefore, that forewarming does not affect stability merely through changes in the equilibria of the soluble salts, as the stability cannot be restored by the addition of electrolytes in those cases wherein it has been reduced. 3. The results show conclusively that the heat stability of a milk of 18 per cent solids-not-fat concentration cannot be predicted from the heat stability of the original milk. They show further why the use of phosphates as a "neutralizer" has not led to serious trouble. In those cases wherein its use has not materially improved stability its use has not greatly reduced it. 4. The existence of two types of milk, one stabilized by the addition of positively charged electrolytes and the other stabilized by the addition of negatively charged electrolytes, has been confirmed. The effect of the addition of various electrolytes to these types of milk has been studied over a wide range of concentration and it has been shown that in the lower concentrations they are most sensitive to the effect of di- tri- and quadrivalent ions and the monovalent H-ion while in the higher concentrations the monovalent K-ion is also very effective. The relationships of these effects are similar to those found for As 2 S 3 sols. Hence it again seems to have been shown that the calcium caseinate suspension in milk is analogous to hydrophobic sols in its reactions with electrolytes at higher temperatures. Increased stability has been shown to result from the addition of citrates and phosphates to a milk stabilized by negatively charged electrolytes. Finally, complete data have been presented upon the effect of various electrolytes upon the two types of milk at 9 and 18 per cent solids-not-fat concentrations. In general, a study of the data presented shows clearly that in studies on coagulation careful consideration must be given to phase concentrations and that conclusions drawn from work on milk of one concentration are not applicable to other milks of widely different concentrations. Furthermore, at the lower concentrations of solids-not-fat various electrolytes exhibit critical.concentrations or ranges wherein their effect is great. Hence, any work which does not include results over a range of concentration of electrolyte as well as range of concentration of the sol used is apt to lead to erroneous conclusions.