THE previous section of this study in which the titration curves of various milk constituents were discussed approximately denned their influence on the titration. Some of the conclusions arrived at are amplified and more clearly defined when the buffer index of the curves is considered.In this discussion Van Slyke's(i) buffer index will be used, i.e. dBjdpK, the ratio of the change in normality of acid or base to the corresponding change in pE. when the change of pH. is infinitely small. The values recorded in this paper were obtained by measuring the slopes of the corresponding titration curves.Buchanan and Peterson (2) used this method of expressing the buffer capacity of milk. They found a maximum value of 0-0326 in the region 5-0-5-5 falling to a minimum of 0-00635 between pTL 8-0 and 8-5. While the general shape of their buffer curve has been confirmed by the following work they draw some rather surprising conclusions from it. They state that there is a great similarity between the titration curves of milk and sodium phosphate and that therefore "phosphates are at least among the most important buffers of milk." While the conclusion is undoubtedly correct, sufficient work has been described in the previous section of this study to show that there is little similarity between the two curves. They also claim that, as the value of the buffer index is not affected greatly near the isoelectric point of the casein, the casein can have little effect on the buffering in this region, pK 4-5-5-0. On the other hand, Whittier (3) comes to the opposite conclusion, i.e. that the buffer action of the casein is exerted principally between pH 4-50 and 5-70 with a maximum at approximately pB. 5-20. This follows from a consideration of the buffer curves of milk and of the acid whey prepared from it. Whittier considers that the difference between these two curves shows the effect of casein.Whittier was concerned particularly with the region between pH. 4 and 7 and did not consider the more alkaline portion of the curves. Moreover, he reached a condition of approximate equilibrium in making the titrations, so that his results are not applicable to the rapid titration of milk to the phenolphthalein end-point. Unless otherwise noted the curves in the following work were obtained from titrations made in the manner described in the previous
The titratable acidity of fresh milk has been studied by several investigators, but the effects of the various buffers present have not been precisely defined. Early statements as to the cause of the titratable acidity were expressions of opinion unsupported by experimental evidence; the theories of indicators and buffer action were not considered. The acids and bases present were calculated by arbitrary rules to various combinations of salts, and the acidity was stated to be due to the presence of acid salts in these hypothetical mixtures.
IT is often stated that the addition of a citrate to milk prevents clotting by rennin because an un-ionised calcium citrate is formed, thus removing the calcium ions from solution, these ions being considered essential for the clotting process. The prevention of the clotting of blood by citrates is explained in the same way.It is not the purpose of this paper to discuss the clotting phenomenon, but to record some measurements of the dissociation of calcium citrate. However, it must be pointed out that there is some confusion on the subject. A typical analysis of milk-serum [Van Slyke and Bosworth, 1915] shows a concentration of 0-465 g. of calcium per litre and 2-37 g. of citric acid. The citrate is present in a larger proportion than is necessary for equivalence with the calcium, so if un-ionised tricalcium citrate were formed there would be no calcium ions present. Analyses in this laboratory have shown the above figures to be typical of milks which clot normally with rennin. If the theory mentioned in the first paragraph were quite correct, most milks would not clot, for there is normally enough citrate present to prevent this. Shear and Kramer [1928] and Shear, Kramer and Resnikoff [1929] give results of conductivity titrations of calcium salts with sodium citrate and claim that the results give "direct evidence for the binding of calcium ions by sodium citrate in some kind of soluble complex." They obtained a minimum conductivity when the added sodium citrate was approximately equivalent to the calcium present. As they adjusted the PH of their solutions with uncertain proportions of acid and alkali, it is impossible to make an estimate of the conductivity of the solution due to any calcium ions present. The only safe deduction from their experiments seems to be that calcium citrate is less ionised than calcium chloride or sodium citrate at the same dilution. Stewart and Percival [1928] state that they have found by conductivity measurements that calcium citrate is almost undissociated in solution, but they give no particulars. It was, therefore, decided to publish the following measurements of the conductivity of simple calcium citrate solutions. Method and apparatus.Conductivities were measured by the usual method, using a metre bridge and post-office box with a small induction coil as source of current. The con-
The oxidation of the fat of butter during cold storage has been studied by measuring the fat-aldehyde value of the fat. A direct relationship was found between this value and the grade after storage.It was found that acidity, starter organisms, salt and low-pasteurization temperatures each favour the oxidation. Neither diacetyl nor acetoin influence the oxidation.The results indicate the presence in ripened pasteurized cream and in unripened raw cream of a fat-oxidizing enzyme which is most active at low pH values (about 5) and high salt concentrations.
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