Effect of inulin as prebiotic and synbiotic interactions between probiotics to improve fermented milk firmness

► Skim milk is fermented by binary cultures and microbial cocktail of lactic bacteria. ► We use Streptococcus thermophilus, Bifidobacterium lactis, Lactobacillus acidophilus, Lactobacillus rhamnosus and Lactobacillus bulgaricus. ► Inulin improves firmness and viable counts of fermented product after cold storage. ► We note metabolic interactions and synergistic effects among bacteria. ► Metabolization of inulin is one of the possible causes of its prebiotic effect. Inulin behaved as a prebiotic to improve firmness of skim milk fermented by (a) pure cultures of Lactobacillus acidophilus (La), Lactobacillus rhamnosus (Lr), Lactobacillus bulgaricus (Lb) and Bifidobacterium lactis (Bl), (b) binary co-cultures of them with Streptococcus thermophilus (St), or (c) a cocktail containing all them. Inulin addition to co-cultures and cocktail enhanced products firmness, either after 1day (D1) or 7days (D7) of cold storage, likely due to the increase in microbial growth induced by metabolic interactions among lactic acid bacteria and partial inulin metabolization. Co-culture firmness did in fact range from 0.33N without inulin (St–Lb) after D1 and 0.55N with inulin (St–Lr) after D7. Also cocktail cultures exhibited high values of firmness, ranging, as an average, from 0.43N (D1) to 0.46N (D7), which suggests that they could have been potentiated by the reciprocal synergistic effects of microorganisms in complex mixture.