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

In this paper, Lactobacillus casei growth, sugar utilisation, exopolysaccharides (EPSs) formation and EPSs characterisation during fermentation of milk with or without litchi juice were evaluated. During L. casei fermentation of litchi-milk samples, the EPSs 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 EPSs, and which positively correlated with the EPSs content (R-2 = 0.9782). The EPSs from the fermented litchi-milk samples was isolated for further structural characterisation. Monosaccharide and molecular weight analysis of purified EPSs sample proved that it contained no carbohydrate other than glucose, and its molecular weight was calibrated as 3.03 x 10(6) Da. FT-IR and NMR (H-1 and C-13) spectroscopy analysis confirmed the synthesised EPSs sample was alpha-glucans (dextrans) with mainly alpha-(1 -> 6) linkages. To the best of our knowledge, this is the first study to report that litchi juice can enhance the EPSs (dextrans, mainly alpha-(1 -> 6) linkages) production in fermented milk.