The effect of litchi juice on exopolysaccharide production in milk fermented by Lactobacillus casei

Summary In this paper, Lactobacillus casei growth, sugar utilisation, exopolysaccharides ( EPS s) formation and EPS s characterisation during fermentation of milk with or without litchi juice were evaluated. During L. casei fermentation of litchi‐milk samples, the EPS s content increased with the increase of litchi juice levels (0–25%, v/v), and the both relationship appeared in S ‐type curve. The viscosity of fermented milk also significantly increased with the formation of EPS s, and which positively correlated with the EPS s content ( R 2  = 0.9782). The EPS s from the fermented litchi‐milk samples was isolated for further structural characterisation. Monosaccharide and molecular weight analysis of purified EPS s sample proved that it contained no carbohydrate other than glucose, and its molecular weight was calibrated as 3.03 × 10 6  Da. FT ‐ IR and NMR ( 1 H and 13 C) spectroscopy analysis confirmed the synthesised EPS s sample was α‐glucans (dextrans) with mainly α‐(1→6) linkages. To the best of our knowledge, this is the first study to report that litchi juice can enhance the EPS s (dextrans, mainly α‐(1→6) linkages) production in fermented milk.