Influence of pH,lactose and lactic acid on the growth of Streptococcus cremoris: a kinetic study

Summary The effect of various culture conditions on growth kinetics of an homofermentative strain of the lactic acid bacterium Streptococcus cremoris were investigated in batch cultures, in order to facilitate the production of this organism as a starter culture for the dairy industry. An optimal pH range of 6.3–6.9 was found and a lactose concentration of 37 g·l^-1 was shown to be sufficient to cover the energetic demand for biomass formation, using the recommended medium. The study of the effect of lactic acid concentration on growth kinetics revealed that the end-product was not the sole factor affecting growth. The strain was characterized for its tolerance towards lactic acid and a critical concentration of 70 g·l^-1 demonstrated. With the product yield of 0.9 g·g^-1 at non-lactose limiting conditions the lactic acid concentration of 33 g·l^-1 could not explain the low growth rates obtained, implicating a nutritional limitation.Symbols t _f fermentation duration (h) - X Biomass concentration (g·l^-1) - X _m maximum biomass concentration (g·l^-1) - S lactose concentration (g·l^-1) - S _r residual lactose concentration (g·l^-1) - P produced lactic acid concentration (g·l^-1) - P _a added lactic acid concentration (g·l^-1) - P _c critical lactic acid concentration (g·l^-1) - specific growth rate (h^-1) - _max maximum specific growth rate (h^-1) - R _x/S biomass yield (g·g^-1) calculated when =0 - R _P/S product yield (g·g^-1)