Use of metabarcoding and source tracking to identify desirable or spoilage autochthonous microorganism sources during black olive fermentations.

This study aimed at investigating the influence of the process environment and raw materials as sources of microorganisms during Nyons black table olive fermentations. Fermented olives and/or brine from spoiled fermentation tanks were analyzed and compared to good quality samples from fermentations collected during 3 consecutive harvest years. Fresh olives, salt and different process environment samples were also analyzed. Microbial diversity of all samples was analyzed using 16S and ITS2 amplicon sequencing and SourceTracker tool was used to investigate links between environment, raw materials and fermentation samples. First, comparison of microbial diversity in control and most spoiled fermentations revealed striking differences in bacterial composition with an overall higher diversity in spoiled fermentations especially for lactic acid bacteria with Lentilactobacillus buchneri, Lentilactobacillus parafarraginis dominating in brine and Pediococcus parvulus, Pediococcus ethanolidurans dominating in olive fruits. Fungal communities were similar in composition although higher abundances of Pichia membranifaciens and Penicillium carneum/roqueforti were observed in spoiled samples. Secondly, process environment samples were characterized by high bacterial and fungal diversity, especially compared to fresh olive fruits. Overall, dominant fungal species in control fermentations were also found in most environmental samples revealing a "house mycobiota". SourceTracker analysis further highlighted the contribution of brine and water from the optical sorter as a source of fungi. Most interestingly, spoilage fungi and most bacteria were retrieved in brine and environmental samples while others such as P. ethanolidurans were only found in environmental samples indicating that the studied spoilage originated from a fermentation deviation rather than a punctual contamination. Taken altogether, these results highlighted the positive and negative influence of the process environment and emphasized the relevance of studying it to better understand microbial vectors occurring during food fermentations, especially natural ones.