FREE LACTIC ACID IN SOUR MILK

Abstract

It has been the universal custom to speak of the acid constituents present 'n sour milk as lactic acid, just as if the acidity were due directly and entirely to the presence of free lactic acid, as such.The usual method of determination has been to titrate the milk to phenolphthalein with 0.1 N alkali and to express the results either as percentage of lactic acid or as the number of cc. of 0.1 N alkali required to neutralize 100 cc. of milk.When milk sours under ordinary conditions, lactic acid is first formed but this reacts at once, practically as fast as formed, with the basic constituents present in the milk.'In fresh cow's milk, the compounds reacting with acids are, first, basic phosphates and citrates of calcium, magnesium, sodium, and potassium, and second, calcium caseinate.2When the lactic acid that is formed in the souring of milk reacts with these compounds, there are formed mono-basic or acid phosphate and citrate salts, free casein, and lactate salts, especially of calcium.It is sufficient for our purpose to represent more simply the essential results of the reactions as consisting of the formation of mono-calcium phosphate, free casein, and calcium lactate.In view of this condition, it cannot be expected that there will be considerable amounts of free lactic acid in souring milk until these reactions are practically completed.In previous work carried on in t'his laboratory, efforts were made to determine separately t'he amounts of free lactic acid and combined lactic acid.It was found, however, in a solution containing lactic acid, that lactates and mono-basic phosphates