From cow's milk to soy juice, when yoghurt cultures have to adapt

Plant-based yoghurts are not very popular with Western consumers, for two main reasons: their organoleptic qualities are considered mediocre compared to those of their dairy equivalents, and they may contain compounds that cause intestinal discomfort. Fermentation, through the careful selection of starter cultures, can help to limit these problems. In this study, we investigated two lactic acid bacteria: Lactobacillus delbrueckii suspb delbrueckii CIRM-BIA865 and Lactiplantibacillus plantarum CIRM-BIA777 to decipher their metabolism during soy juice fermentation. We used genomic, transcriptomic and metabolomic analyses to identify the genetic basis for growth and metabolism in soy juice. We have thus shown that:

• CIRM-BIA865 possesses the genes for transporting and degrading sucrose, the main sugar in soy juice, but not those enabling the degradation of stachyose and raffinose, oligosaccharides often associated with digestive discomfort.
• During fermentation, CIRM-BIA865 exhibits atypical behaviour. While it acidifies the medium as expected, reflecting an active metabolism, its cells elongate, reaching a size 20 times greater than that observed in a laboratory culture medium.
• At the same time, the expression of numerous stress genes is induced, suggesting that soy juice constitutes an unfavourable environment for the growth of this bacterium.
• CIRM-BIA777 possesses all the genes encoding the enzymes necessary for the degradation of sucrose, stachyose and raffinose (alpha-galactosidase).
• However, during fermentation, only sucrose is consumed, without the factors preventing the degradation of the other two sugars having been identified.
• Fermentation by CIRM-BIA777 increases the levels of certain aromatic compounds, such as phenylethanol and 2,3-butanedione, associated with floral and buttery notes, respectively, while decreasing the levels of certain volatile compounds associated with undesirable green and bean aromas, such as hexanal.

Outlook: These results highlight the need for a better understanding of the metabolism of lactic acid bacteria in plant juice fermentation in order to understand the stress factors that can affect bacteria during fermentation and to identify fermentation conditions that would be favourable for the degradation of compounds such as stachyose and raffinose.