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Background and Objectives: Enhancing the growth and function of lactic acid bacteria through fortifying the basic ingredient of yogurt, milk ,have always been a key factor in the application as an industrial starter culture. The purpose of the present study is to introduce the application of certain materials in yoghurt milk with the aim of enhancing the growth and acid production, and reduction of fermentation time, optimizing the structure and texture in fermented products as well as reducing starter culture consumption which results in balancing the corresponding expenditures.
Materials and Methods: By using local starter culture consisting Lactobacillus delbruecki ssp bulgaricus and Streptococcus thermophilus in 50:50 ratio, four samples of set yoghurt were prepared in triplicated tests.  One sample as a control and three samples were supplemented by L-lysine, L-cysteine and the mixture of the two by 100mg/L for each one respectively. The factors were studied based on fermentation time, bacterial viability, acid forming, sensory evaluation and the amount of syneresis  and water holding capacity (WHC).
Results: The results showed a significant effect on reducing fermentation time, this factor showed reduction from 260 minute in control sample to 210 minute in samples that were enriched by Lysine and Lysine plus cysteine and 180 minute in cysteine p≤0.05, significantly increasing titratable acidity and optimizing viability in enriched samples besides, the end product had firmer structure and a lower syneresis amounts as it was decreased from 4.71% in control sample to 2.83, 2.76 and 3.06% in samples that were enriched by lysine, cysteine and both of them respectively (p≤0.05) . the same results were found for the best WHC in cysteine fortification(p≤0.05).
Conclusions: Lysine has been responsible for optimization of Streptococcus thermophilus growth. Accordingly, there was an increase in the number of Streptococcus thermophilus to the advantage of Streptococcus thermophilus ratio which later led to a stronger start for fermentation process. On the other hand, as a result of cysteine reduction property enhanced condition for Lactobacillus bulgaricus led to a higher acid production, hence a stronger texture and lower wheying off and better firmness and thickness and finally better sensory evaluation score.