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Effect of pH on the Production of Acetylmethyl-Carbinol Plus Diacetyl in Milk by the Citric Acid Fermenting Streptococci

Iowa State College, Ames, Iowa

ABSTRACT

The graphs presented, which represent data obtained on seven cultures of the citric acid fermenting streptococci, emphasize the production of acetylmethylcarbinol plus diacetyl from citric acid. When this acid was used to acidify milk cultures of the organisms, the yield of the carbinol plus diacetyl was always much higher than when other acids were used and a significant production occurred over a wide pH range; the maximum yields were obtained at pH values from 3.9 to 3.7.

With lactic acid as the acidulant, comparatively low yields of acetylmethylcarbinol plus diacetyl were obtained and a significant production occurred over a rather narrow pH range. The maximum yields were obtained at pH values from 4.4 to 4.1 which approximate the pH values commonly found in satisfactory butter cultures. Since lactic acid is the principal acid involved in establishing the pH in butter cultures, the frequent failure to obtain a high yield of acetylmethylcarbinol plus diacetyl in such cultures may be due to the relatively unfavorable character of lactic acid. Sulphuric acid gave higher yields of acetylmethylcarbinol plus diacetyl than lactic acid and also gave a significant production over a wider pH range. The maximum yields occurred at comparatively low pH values which ranged from 3.6 to 3.2. The favorable character of sulphuric acid compared to lactic acid for the production of acetylmethylcarbinol plus diacetyl may explain why the special butter culture made by growing one of the citric acid fermenting streptococci in milk and then acidifying with citric and sulphuric acids (1) are so generally satisfactory. A mixture of 0.15 per cent lactic and variable amounts of sulphuric acid, depending on the pH desired, gave yields of acetylmethylcarbinol plus diacetyl comparable to those obtained with sulphuric acid alone. The maximum yields occurred at pH values from 3.9 to 3.5. In general, these values were between the corresponding values for lactic and sulphuric acids.