Microbiological Safety and Functional Properties of a Fermented Nut-Based Product

Abstract

Fermented nut-based products, obtained after soaking and fermentation, are gaining increasing interest as animal food substitutes because of ethical, environmental and health reasons. In these products, Lactic Acid Bacteria (LAB) perform the fermentation, leading to matrix acidification and contributing to controlling spoilage and pathogenic microbiota. In this work, LAB strains isolated from an artisanal product and combined with a commercial strain were added as starter cultures during nut soaking to produce a cheese-like fermented plant-based product. Three different LAB consortia were used in challenge tests at laboratory scale against Listeria monocytogenes, Escherichia coli or Salmonella Enteritidis, inoculated in nuts at 5 log CFU/g, and monitored for pathogen survival and matrix acidification. The combination of Lactiplantibacillus plantarum 82 and Leuc. carnosum 4010 resulted in faster acidification (pH value < 4.4 after 18 h instead of 48 h) and the reduction of target pathogens; L. monocytogenes was already absent after seven days from production, and the counts of E. coli or S. Enteritidis were lower with respect to other samples. Thus, this microbial consortium was used for a pilot-scale production in which, beyond safety, the fermented plant-based product was also characterized for aroma profile and phenolic compounds, parameters that are known to be affected by LAB fermentation. The results showed an enhancement of the aroma profile, with an accumulation of molecules able to confer cheese-like notes (i.e., acetoin and diacetyl) and higher phenolic content, as well as the presence of compounds (i.e., phenyllactic acid and hydroxyphenyllactic acid) that could exert antimicrobial activity. This study allowed us to set up a guided fermentation for a cheese-like vegan product, guaranteeing safety and improving aromatic and functional features.

Keywords: Escherichia coli; Listeria monocytogenes; Salmonella Enteritidis; bioprotective cultures; cheese analogues; lactic acid bacteria; phenolic content; vegan product.

Figure 1

Flow sheet of fermented cashew and almond nut production during challenge test. The sampling points are also reported.

Figure 2

Microbial counts of inoculated pathogens during pilot-scale production optimization trial. The graph is divided into three phases: phase 1 includes nuts after treatment in a hot water bath and after the soaking phase, phase 2 comprises spreadable samples during fermentation and phase 3 includes spreadable products during storage. The presence of an asterisk indicates that S. Enteritidis was present in 25 g of sample.