Microbial Fermentation is a traditional method for food preservation, extending shelf life, and ensuring microbial stability. This study aimed to assess the microbial safety of a fermented pea protein-based product using a mixed starter culture of Lactococcus lactis LTH 7123 and Yarrowia lipolytica LTH 6056. Safety evaluation was conducted through challenge tests with surrogates of common foodborne pathogens to determine a potential inhibition by the starter cultures.
Experiments were performed in fermentation mixtures consisting of commercial pea protein isolate and glucose. The fermentation samples were first inoculated with L. lactis or the mixed starter culture and additionally inoculated with either Escherichia coli ATCC 11229, Listeria innocua ATCC 33090, Salmonella enterica subsp. enterica serovar Senftenberg LTH 5703 and Bacillus cereus DSM 31T. As a growth control, pea protein suspensions were inoculated only with the tested surrogates. Viable counts and pH-values were determined after 16 hours and subsequently every 24 h throughout a 96-hour fermentation period.
All surrogates exhibited an initial increase in their viable counts by about 3 log units within 16 h of fermentation. While the bacterial counts of the surrogates in the mixed culture fermentation were comparable to their growth control, their bacterial count after 24 h decreased in presence of L. lactis alone after 24 h. However, no complete elimination was achieved after 96 h, except for S. Senftenberg. B. cereus counts in both fermentations remained constant throughout the fermentation time with about 10^6 cfu/ml after 24 h. The weaker growth of L. lactis and the slightly higher final pH-value in the mixed culture may explain variations in pathogen inhibition.
The results indicated insufficient inhibition of surrogates using L. lactis and Y. lipolytica as a mixed culture. However, growth inhibition of E. coli, L. innocua and S. Senftenberg could be achieved with L. lactis as a single strain. Y. lipolytica probably limited the inhibition of the surrogates during the fermentation of pea protein. To ensure process safety, further hurdles should be implemented.