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The Relation of Natural Acidity in Milk to Composition and Physical Properties

Department of Chemistry, Cornell University, Ithaca, New York

ABSTRACT

Determinations of acidity in milk from a large number of cows have shown variations from 0.086 to 0.229 per cent calculated as lactic acid, and in terms of pH from 6.898 to 6.432.

In general, high acid milk contains more of all the nutrients and is particularly high in phosphates; it has greater food value, therefore, than milk of low acidity.

On account of the fact that it contains a larger proportion of buffer substances, phosphates and casein especially, it would be expected to increase less in hydrogen ion concentration with a given addition of acid or alkali than low acid milk. This was found to be the case. From the point of view of commercial handling of milk this fact is of some importance. High acid milks under the same condition of storage reach the coagulating point later than milk of lower acidity. The following observation illustrates that point: Two samples of fresh milk, one with natural acidity of 0.144 per cent and the other 0.175 per cent were allowed to stand side by side at room temperature. At the end of
hours the first sample coagulated on heating, while the latter did not respond to this test until thirteen hours had elapsed.

Under ordinary conditions of handling, the carbon dioxide varied but little in the different samples. The low acid milk contained, perhaps, a little more. It was calculated that enough free carbon dioxide is usually present to account for the acidity equivalent to 0.01 to 0.02 per cent lactic acid.

Between citric acid and acidity there was found no regular variation. But from experiment it was calculated that acid citrates might account for 0.01 per cent acidity in all samples.

High acid milk contained somewhat more casein yet there was not enough difference to account for more than a small part of the difference in acidity between the high and low acid samples. It was concluded that the acidity due to casein is 0.05 to 0.08 per cent.

The albumin accounts regularly for a little less than 0.01 per cent.

The difference between the total acidity and that due to the sum of the factors just enumerated is attributed to phosphates. Practically the entire difference in acidity between the various samples was shown to be due to this constituent.

The hypothesis is advanced that a condition of equilibrium exists between certain of the milk constituents, particularly citric acid, phosphoric acid, casein, and the bases, and that the acidity due to any one of these depends upon its relation to the others. It is this equilibrium which fixes the hydrogen ion concentration; and a change in the proportion of a constituent or ion will result in a shift in the equilibrium and a change in the influence which each has upon the acidity. The calculations just made, therefore, should not be considered arbitrarily. They represent merely the result of the necessity for using alkali to bring the hydrogen ion concentration of each constituent from that of milk to that point at which the phenol phthalein end-point happens to be.

The decrease in acidity of milk on dilution is due to the hydrolysis of both phosphates and caseinates.

The osmotic pressure of milks of varying degrees of acidity was found to be the same. In low acid milks where lactose is low, the chlorides particularly were high. Thus there is a reciprocity which accounts for the constancy of osmotic pressure.

A fairly close relation between chlorine and conductivity was found as might be expected, though, of course, other salt constituents may conduct the current as well. There is, consequently, an approximate relation between acidity and conductivity, milk with low acidity running high in conductivity.

There was no relation between natural acidity (or pH) and coagulability with rennet and with alcohol. These properties were found, however, to run hand in hand, both probably depending upon certain relations between the constituents in milk which are independent of acidity.

The aldehyde reductase reaction is somewhat more rapid in low acid milks but the relation is only general and is believed to be secondary.