No abstract

The effect of control of culture acidity on growth kinetics of one homofermentative strains of lactic acid bacteria, namely., Lactococcus lactis subsp lactis was evaluated. The bacterial growth was estimated on the base of colony forming units, developed titratable acidity and incubation time .Sterilized skim milk was used as a medium with incubation at 34°C.Sampling had been carried out at zero time and each 2 hours intervals. After each sampling, a calculated volume of a liquid of 0.1 N sodium hydroxide was added to the culture to adjust the acidity of the remaining volume of the cultures to its initial value at zero time. The obtained results indicated that:1-When Lactococcus lactis subsp lactis grown under control of culture acidity lag phase was not observed.2-Significant effect of acidity control was noticed on the growth rate. Exponential phase of growth was between 2 ed and 12 th hour of incubation. However, the same culture being grown with control of culture acidity the exponential phase of growth was at interval of time from zero up to 24 th hour of incubation. 3-The maximum cell population in case of the control sample was 124 x 10 5 CFU /ml after 12 hours of incubation. The corresponding values when growth was under control of culture acidity was by a maximum cell population of 35x10 6 CFU/ml after 32 hour of incubation.

The purpose of this study was to improve the functional properties of bioyogurt, which is one of the most popular fermented dairy products widely, consumed worldwide, and to evaluate the effect of whey protein concentrate (WPC) and Arabic Gum (AG) on promoting growth and survival of Bifidobacterium longum. Bio-yogurt was manufactured by using 2% Bifi. longum and enriched with 1% WPC or 2% AG, after milk inoculation by bacterial culture it was incubated at 42±1°C until complete coagulation and then stored at 5±1°C. Chemical, bacteriological analysis and sensory evaluation had been carried out in fresh and after 7 & 14 days of storage. The obtained results indicated that, acid development in control samples without probiotic bacteria or WPC & AG was less compared with Bio-yogurt containing Bifi. longum, WPC and AG, which scored highest acidity and lowest pH values during storage. Moreover, during storage there were an increase in total solids, soluble nitrogen, total nitrogen and acidity and slight decrease in pH values of cheese made with 1% WPC or 2% AG in all treatments. Concerning the bacteriological determinations, there were a slight increase in total bacterial, Streptococci, Lactobacilli and Bifi. longum counts during first 7 days of storage, and then decrease gradually during the second week of storage. In addition, WPC and AG increased the total bacteria, Lactobacilli, Streptococci and Bifi. longum counts comparing with control samples. Generally, the addition of WPC increased organoleptic properties of the product, while the addition of AG did not.

Effect of some growth promoting substances, i.e., yeast extract, fructose, and peptone on the growth rate and acid production by Bifidobacterium bifidum in sterilized skim milk at 40°C was studied. Growth promoting substances were added to milk in three different concentrations, i.e., 0.1, 0.3, and 0.5%. The obtained results indicated that the three substances resulted in an increased of growth and lactic acid production by Bifidobacterium bifidum. The maximum cell population in the presence of yeast extract reached were 25x10 4 , 12x10 5 and 21x10 5 CFU/ml after 12 hours. In the presence of 0.1, 0.3 and 0.5%, respectively. The same effect was found in the presence of fructose and peptone, where the maximum cell population after 12 hours of incubation reached 35x10 4 , 46x10 4 and 53x10 4 and 25x10 4 , 51x10 4 and 40x10 5 CFU/ml, respectively. The obtained results showed a direct relationship between the rate of increase in C.F.U and DTA (developed titratable acidity). Direct relationship could be established between the growth rate of Bifidobacterium bifidum in skim milk and the availability of utilizable nitrogenous compound, which might enhance the rate of growth. As it was expected, the highest population would reach in milk supplied with a good source of nitrogenous compounds such as peptone.

The effect of reduction in water activity on growth and acid production by Lactococcus lactis subsp lactis, Lactococcus lactis subsp cremoris and Lactobacillus casei was studied at water activity values of 0.998, 0.975, 0.967 and 0.959. Reduced water activity resulted in extended lag time of varied magnitude, varied growth rates, different cell population and acid production. The highest growth rates were observed at an a w of 0.998 (control). Lactococcus lactis subsp lactis, Lactococcus lactis subsp cremoris and Lactobacillus casei reached a maximum cell population of 80x10 6 CFU/ml, 60x10 6 CFU/ml and 60x10 6 CFU/ml after 24 th hour of incubation respectively.At water activity value of 0.975, Lactococcus lactis subsp lactis, Lactococcus lactis subsp cremoris and Lactobacillus casei reached a maximum cell population of 40x10 5 CFU/ml, 51x10 5 CFU/ml and 51x10 5 CFU/ml respectively after 24 th hour of incubation , which indicate that Lactococcus lactis subsp lactis reached a maximum cell population which is lower than that observed for the other two strains. Which indicate that for each bacterial strain there is an optimum water activity at which the growth rate is faster comparing with the growth rate at lower or higher water activity. In addition the sensitivity to culture water activity varied between different bacterial strains. Also the obtained results indicated that the value of culture water activity had approximately simlar effect on rate of increase in developed titratable acidity as that found for the rate of increase in colony forming units.