Modeling growth of Lactobacillus helveticus and Kluyveromyces marxianus isolated from fermented camel milk (shubat).

Shubat is a traditional fermented camel milk beverage in which lactic acid bacteria and yeasts jointly determine fermentation behavior, but the kinetic properties of autochthonous strains remain insufficiently characterized. This study aimed to model the growth and metabolite production of Lactobacillus helveticus SH4 and Kluyveromyces marxianus Y45, two most dominant species found in Kazakhstani shubat. Growth was evaluated under different temperatures, pH values, and carbohydrate sources, and described using logistic, cardinal pH-temperature, and Monod models. Lactic acid and ethanol production were measured during cultivation, and co-culture behavior was assessed in modified MRS medium and reconstituted camel milk. L. helveticus SH4 showed optimal growth at pH 7.22 ± 0.11 and 39.66 ± 0.38°C, while K. marxianus Y45 grew optimally at pH 5.39 ± 0.23 and 37.78 ± 0.71°C. SH4 grew efficiently on glucose, galactose, and lactose, whereas Y45 showed the highest growth rate on glucose. Metabolite analysis indicated lactic acid production by SH4 and ethanol production by Y45. In co-culture, both strains followed monoculture-based growth trends in modified MRS, while reconstituted camel milk reduced the final SH4 population. These results provide quantitative parameters for understanding and improving controlled shubat fermentation.