Production of d-tagatose, a low caloric sweetener during milk fermentation using l-arabinose isomerase

Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus are used for the biotransformation of milk in yoghurt. During milk fermentation, these lactic acid bacteria (LAB) hydrolyze lactose producing a glucose moiety that is further metabolized and a galactose moiety that they are enable to metabolize. We investigated the ability of L. bulgaricus and S. thermophilus strains expressing a heterologous l-arabinose isomerase to convert residual d-galactose to d-tagatose. The Bacillus stearothermophilus US100 l-arabinose isomerase (US100 l-AI) was expressed in both LAB, using a new shuttle vector where the araA US100 gene is under the control of the strong and constitutive promoter of the L. bulgaricus ATCC 11842 hlbA gene. The production of l-AI by these LAB allowed the bioconversion of d-galactose to d-tagatose during fermentation in laboratory media and milk. We also established that the addition of l-AI to milk also allowed the conversion of d-galactose into d-tagatose during the fermentation process.