Résumé
Recently, especial therapeutic effects and nutritional values of probiotic dairy products have made them as one of the most interesting research topics in the field of production and maitenance methods and the nutritional and therapeutic aspects of these products. The technological problems encountered in the production of these products can be overcome by using methods to improve the growth of probiotic bacteria in fermented milk. In this paper the effect of milk composition [skim milk and whey powder], percent of inoculum [Lactobacillus acidophilus La-5 and starter culture] and incubation temperature on the growth of the probiotic bacteria in yogurt was investigated, and suitable conditions to reach maximum growth were determined using the Taguchi design. The effect of five independent variables [at three levels] on the growth of Lactobacillus acidophilus La-5 [6 hr incubation] in probiotic yogurt production was investigated by Taguchi approach. The variables included: skim milk powder [10, 12, 14% w/w], whey powder [0, 0.75, 1.5% w/w], size of yogurt culture inoculum [0.2, 0.4, 0.8% v/v], L. acidophilus inoculum [0.1, 0.3, 0.5% w/v], and incubation temperature [37, 40, and 43°C]. Among the variables evaluated, yogurt culture inoculum had the maximum effect on the growth of probiotic bacterium, while the incorporation of L. acidophilus inoculum and the incubation temperature had no significant effect on the bacterial growth stimulation. Anyway the most suitable conditions for enhancing the probiotic bacterial growth include: skim milk powder, 10% w/w; without adding whey powder and size of yogurt culture and L. acidophilus inoculum, 0.1%w/v and 0.4% v/v, respectively incubation temperature at 37°C. By exact determination of milk composition, inoculum size and incubation temperature, the maximum number of the probiotic bacterium can be achived in the product. Data of this research work, that are very useful for the industrial production lines, show that by using the minimum level of milk total solid, without the enrichment and by adding the standard yogurt inoculum size and minimum probiotic inoculum size and by low incubation temperature, the growth rate increased and the production of probiotic yogurt with high number of Lactobacillus acidophilus become possible