Effect of Streptococcus thermophilus extract on Lactiplantibacillus plantarum acidification and propagation in milk fermentation

The probiotic Lactiplantibacillus plantarum could not independently ferment milk to form yoghurt. In this study, the effect of Streptococcus thermophilus extract on the acidification and propagation capabilities of L. plantarum during milk fermentation was investigated. The results showed that the addition of S. thermophilus crude cell extract significantly shortened the fermentation time and promoted the propagation of L. plantarum. Moreover, the addition of 0.2% S. thermophilus cell extract resulted in a higher acidification rate (final pH: 4.53) at 72 h of fermentation and a higher L. plantarum cell count (log 8.31 cfu/g) at 24 h of fermentation at 37°C. It was speculated that the presence of amino acids and free nucleotides in S. thermophilus lysate might have played an indispensable role during milk fermentation by L. plantarum. This study provided a novel strategy for utilising L. plantarum to ferment milk without using exogenous food ingredients.