Study of Cheese Associated Lactic Acid Bacteria Under Carbohydrate-Limited Conditions Using D-Stat Cultivation

The metabolic pathways alternative to glycolytic energy (ATP) during growth of starter and nonstarter lactic acid bacteria were studied simulating the depletion of carbohydrates during cheese ripening. D-stat cultivation strategy with the gradual decrease of galactose concentration in tryptone-arginine feeding medium was used. With the decrease of galactose feeding, the biomass yield calculated on carbohydrate consumption (Y_X/HEX) and acetate/lactate production ratio of all strains increased. We assume that ATP and biomass yields improved by directing the pyruvate flow from lactate to acetate and that metabolic energy could be obtained either by producing acetate from carbohydrates or from arginine metabolism in ADI-positive strains. Four LAB strains out of eight produced ornithine from arginine indicating active arginine-deiminase (ADI) pathway. These ADI-positive strains achieved 3-10 times higher Y_X/HEX than ADI-negative strains in tryptone-arginine medium. Lactobacillus plantarum also used serine as an energy source. Starters and NSLAB strains using the amino acids arginine and serine or limited amounts of carbohydrates therefore have the potential to influence flavor production in cheese more efficiently.     

Study of Cheese Associated Lactic Acid Bacteria Under Carbohydrate-Limited Conditions Using D-Stat Cultivation

The metabolic pathways alternative to glycolytic energy (ATP) during growth of starter and nonstarter lactic acid bacteria were studied simulating the depletion of carbohydrates during cheese ripening. D-stat cultivation strategy with the gradual decrease of galactose concentration in tryptone-arginine feeding medium was used. With the decrease of galactose feeding, the biomass yield calculated on carbohydrate consumption (Y_X/HEX) and acetate/lactate production ratio of all strains increased. We assume that ATP and biomass yields improved by directing the pyruvate flow from lactate to acetate and that metabolic energy could be obtained either by producing acetate from carbohydrates or from arginine metabolism in ADI-positive strains. Four LAB strains out of eight produced ornithine from arginine indicating active arginine-deiminase (ADI) pathway. These ADI-positive strains achieved 3–10 times higher Y_X/HEX than ADI-negative strains in tryptone-arginine medium. Lactobacillus plantarum also used serine as an energy source. Starters and NSLAB strains using the amino acids arginine and serine or limited amounts of carbohydrates therefore have the potential to influence flavor production in cheese more efficiently.     

Physiological properties of Lactobacillus paracasei, L. danicus and L. curvatus strains isolated from Estonian semi-hard cheese

Growth and survival of Lactobacillus paracasei (six strains), L. danicus sp. nov. (four isolates, two strains) and L. curvatus (two strains) from semi-hard Estonian cheeses were comparatively studied in different environmental conditions of relevance for their growth in cheese and survival in gastric environment. Maximum specific growth rates for L. paracasei strains varied between 0.40 and 0.57 h⁻¹, and all strains were tolerant to low water activities, heating at 60 °C for 30 min and pH 3. The newly discovered genetically distinct species L. danicus was characterized by low maximum growth rates (0.26–0.38 h⁻¹) and low temperature optimum (<30 °C). It was acid and heat sensitive and inhibited at salt concentrations from 4% and water activities below 0.93. L. curvatus was characterized by the highest growth rates (0.65–0.70 h⁻¹), tolerance to high NaCl concentrations, but sensitivity to heating, bile salts and low pH. The study showed that genetically different LAB species isolated from cheese could be distinguished by simple cultivation experiments.     

The effect of temperature and pH on the growth of lactic acid bacteria: a pH-auxostat study

The growth of Streptococcus salivarius subsp. thermophilus, Lactococcus lactis subsp. lactis, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus acidophilus and Lactobacillus paracasei was studied in a pH-auxostat. The computer-controlled smooth change of pH or temperature was used to study the effect of pH and temperature on the culture characteristics (specific growth rate mu growth yield Y(ATP) and specific lactate production rate Q(ATP)). The behaviour of mu and Q(ATP) with an increase of temperature suggested that ATP production capacity was an important factor in determining the maximum growth rate of lactic acid bacteria (LAB). The decrease of Y(ATP) with increase of temperature resulted in the decrease of specific growth rate while Q(ATP) remained constant or even increased. With the decrease of pH, decrease of both Y(ATP) and Q(ATP) was observed. S. thermophilus St20 having the highest maximum specific growth rate of 2.2 h(-1) at 44 degrees C was the most acid sensitive strain. The L. paracasei E1H3 having the lowest mu(max) was the most tolerant strain to pH change. The behaviour of mixed culture of L. bulgaricus and S. thermophilus in milk in a pH-auxostat with pH-decrease was in agreement with the pure culture experiments of the same species. The study showed that pH-auxostat with smooth controlled change of pH and temperature is an effective method for determination of technological characteristics and comparative physiological study of LAB.