چكيده به لاتين
Yogurt is a simple ecosystem whose successful manufacture relies on interactions between two thermophilic lactic acid bacteria, Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. Due to the nutritional and economic significance of yogurt, this study aimed to investigate the factors affecting the characteristics of the final product, by the analyzingf the metabolic fluxes of the microbial community, the key player in the yogurt fermentation. To this end, metabolic model of Streptococcus thermophilus LMG311 (with 955 reactions) and Lactobacillus delbrueckii subsp.bulgaricus ATCC1182 (with 1118 reactions) were employed. Milk contains casein, amounts of all free amino acids, and lactose as the primary sugar. Therefore to simulate the growth of each microorganism in milk, the distribution of metabolic fluxes in each of the microorganisms was investigated in two different conditions: 1) with the entry of essential amino acids for growth and 2) with the entry of all amino acids, using lactose as the sole carbone source. The results demonstrated that during growth on 5 mmol/gdw/h lactose with the introduction of essential amino acids for growth, the growth rate of S. th (0.3315) was significantly higher than that of L. b (0.07) ) in both models. Based on experimental studies conducted on the microbial community of yogurt and the three phase growth curve of these two microorganisms in milk, a comprehensive simulation of the yogurt microbial community was carried out in two parts. The first part focused on phase Ⅰ growth of the yogurt microbial community. During phase I, S. thermophilus can utilize free amino acids in milk more rapidly and efficiently, resulting in a much higher growth rate compared to L. bulgaricus, whose growth rate is close to zero. Therefore, to simulate phase I growth, the growth of L. bulgaricus was assumed to be zero, and the single-organism model of S. thermophilus was used. From phase Ⅱ, the simultaneous growth of two microorganisms begins. At this time, with the depletion of free amino acids in milk, casein is broken by L. bulgaricus, and both microorganisms benefit from the released amino acids. Therefore, in the second part, the two-membered microbial community of yogurt was simulated using the SteadyCom algorithm, then the effect of casein introduction, microbial exchanges, and also the effect of changes in lactate as the main product in the milk fermentation process were investigated. The results of the simulation showed that the growth rate and lactate production rate in the microbial community increased compared to the pure culture of each microorganism. In single-species growth, the highest lactate production rate was attributed to L. bulgaricus at 15 mmol/gdw/h, but this value increased to 19.5 mmol/gdw/h in the simultaneous growth of the two bacterial species. Additionally, the results showed that these two members can exchange amino acids, folate, formate, and carbon dioxide with each other, and despite competition for the lactose as carbon source, they can establish a symbiotic relationship with each other by exchanging these metabolites.
