Growth rate, proteolysis and acid production of Streptococcus faecalis subsp. liquefaciens in skim milk with some additives

Abstract

Growth rate, proteolysis and acid production of Streptococcus faecalis subsp. liquefaciens were examined in skim milk with and without 0.15% calcium lactate, 0.03 M Na, K phosphate, 5% CaCO3 or 2-8% NaCl, at 30 degrees C. Calcium lactate affected neither the growth rate nor the breakdown of casein. The highest acid production and degradation of casein occurred in phosphate-buffered skim milk (pH 7.1). Inorganic phosphate did not have any influence on the generation time and hydrolysis of casein. CaCO3, on the contrary, slightly reduced the growth rate and the acid production; proteolysis was markedly decreased. Although 2% NaCl brought about ca. 17% reduction in the number of viable cells, it increased not only the breakdown of casein but the acid production during the exponential phase as well. Higher salt concentrations inhibited growth, acid production and proteolysis. The effect increased with increasing the salt level. An adaptation period of 24 h followed by little growth was observed in milk containing 8% NaCl.