Anaerobic fermentation ofLactobacillus bulgaricus andstreptococcus thermophilus on 3% nonfat dry milk with pure and mixed culture

Shear stress effects on growth and activity of Lactobacillus delbrueckii subsp. bulgaricus

Cells are frequently submitted to shear stresses during industrial processes. Shear stress can be either beneficial or detrimental to the cells depending on the organism and on the level of intensity. The present work was designed to study the effect of shear stress on cell activity of a widely used lactic acid bacterium, Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus). A constant shear stress bioreactor, based on Couette device, was developed and used to control shear stress from 0 to 72 Pa during a 4-h cultivation of a supplemented whey permeate medium with L. bulgaricus at 42 degrees C. In order to reach high shear stresses and to perform experiments within the laminar flow range where hydrodynamic conditions are accurately defined, the medium was thickened with carboxymethylcellulose (CMC). pH, cell counts, optical density, lactose and lactic acid concentrations were monitored during culture, and cell activity was evaluated after culture and after a freezing treatment at -80 degrees C, using a standardized activity test based on optical density measurement. Cell metabolism was significantly improved by intermediate shear stress levels (36 and 54 Pa) during culture. Furthermore, biomass concentration, evaluated by optical density, was clearly higher at 36 Pa. Cell lengthening was observed, which was mainly related to the presence of CMC and partly to shear stress level, especially at 36 Pa. Hydrodynamic conditions during culture could affect the membrane permeability of the cell and its resistance to freezing. Cells cultivated at 72 Pa were certainly weakened by shearing forces, and these cultures exhibited lag times twice as long after freezing as those grown at 36 Pa. Furthermore, after freezing, cultures grown at 36 Pa had shorter lag times than at 0 Pa (1.1 and 1.3 h, respectively) and higher specific growth rates (1.24 and 0.99 h-1, respectively).

Uncoupling of growth and acids production in Bifidobacterium ssp

Kinetics of batch cultivation of four species of Bifidobacterium in milk were examined in detail. Bifidobacteria could grow well in milk inoculated with cultures prepared in a synthetic medium. Cessation of growth occurred, however, in pH-controlled batch cultures, although incomplete utilization of lactose was observed. Lactate and acetate accumulation caused limitation on growth of bifidobacteria leading to an uncoupling of biomass and product formation. From 70 to 75% of both final lactate and acetate concentrations were produced during the stationary growth phase of Bifidobacterium bifidum, Bifidobacterium breve, and Bifidobacterium longum cultivated in milk, whereas Bifidobacterium infantis produced less acetate or lactate during this phase.

Sugar Utilization and Acid Production by Free and Entrapped Cells of Streptococcus salivarius subsp. thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, and Lactococcus lactis subsp. lactis in a Whey Permeate Medium

Cells of Streptococcus salivarius subsp. thermophilus and Lactococcus lactis subsp. lactis entrapped in k -carrageenan-locust bean gum gel performed similarly to free cells in the conversion of lactose to lactic acid. Bead diameter influenced the fermentation rate. Cells entrapped in smaller beads (0.5 to 1.0 mm) showed higher release rates, higher lactose, glucose, and formic acid utilization, higher galactose accumulation, and higher lactic acid production than did cells entrapped in larger beads (1.0 to 2.0 mm). Values for smaller beads were comparable with those for free cells. Immobilization affected the fermentation rate of lactic acid bacteria, especially Lactobacillus delbrueckii subsp. bulgaricus. Entrapped cells of L. delbrueckii subsp. bulgaricus demonstrated a lower lactic acid production than did free cells in batch fermentation. The kinetics of the production of formic and pyruvic acids by L. lactis subsp. lactis and S. salivarius subsp. thermophilus are presented.