Biacetyl Production by Cultures of Lactic Acid-Producing Streptococci

Abstract

Summary Certain cultures of S. lactis and S. cremoris grown in milk produced a flavor resembling a mixed culture of lactic acid-producing and citric acid-fermenting streptococci. The flavor and aroma of these cultures suggested formation of biacetyl, volatile acids and CO 2 . The incubation temperature, incubation time and pH required for maximum biacetyl production were variable. The greatest amount of biacetyl produced by a lactic acid-producing streptococcus was 4.0ppm. Many cultures produced little or no biacetyl in milk. No direct relationship existed between the amount of acid produced by a culture and its ability to produce biacetyl. However, there was a correlation between the pH of milk and biacetyl production by cultures. Cultures of lactic acid-producing streptococci formed the largest amounts of biacetyl in milk after they had decreased the pH to 5 or less. With most cultures, maximum biacetyl production was evident in the range of pH 4.2 to 4.75. A culture of S. lactis obtained from a commercial mixed culture by plating and further purified by plating and picking isolated colonies contained both high and low biacetyl-producing strains. The addition of citric acid (0.15 per cent) to milk cultures of lactic acid-producing streptococci generally resulted in slightly higher biacetyl production, although the increases were not as conspicuous as those obtained with mixed cultures of lactic acid-producing and citric acid-fermenting streptococci. Acetylmethylcarbinol production was not increased appreciably by addition of citric acid to milk cultures of lactic acid-producing streptococci but volatile acid production was increased considerably. When citric acid was added to milk, the biacetyl content did not decrease as rapidly as it did in plain milk after maximum production was reached. It is suggested that cultures of S. lactis which produce appreciable amounts of biacetyl be recognized as a variety of S. lactis and be designated S. lactis var. aromaticus in accordance with the method of classification of this group proposed by Hammer and Baker (6).