Effect of oil-born yeasts on the quality of extra-virgin olive oils of Basilicata region

The olive oil microbiota mainly consists of yeasts, which may positively or negatively affect the physicochemical and sensory features of product. In this study, 17 yeast strains belonging to Candida boidinii, Lachancea fermentati, Nakazawaea molendinolei, N. wickerhamii and Schwanniomyces polymorphus species were collected during olive oil production, identified and tested for the ability to ferment sugars, to grow at low temperatures, for the occurrence of different enzymatic activities, for the tolerance and degradation of phenolic compounds, radical scavenging activities, biofilm formation, survival to simulated gastro-intestinal (GIT) tract. Yeasts were also inoculated in extra virgin olive oils (EVOO; from Leccino and Coratina cultivar) to evaluate their survival and their effect on EVOO quality (changes in analytical indices) during 6-months of storage. Most of strains were able to grow at 15◦C, while the ability to ferment different sugars was strain-specific. All strains had β-glucosidase activity, while none exhibited lipolytic activity; peroxidase was widespread among the strains, while protease activity was strain-dependent. Esterase and the ability to hydrolyse oleuropein and form hydroxytyrosol was present only in N. wickerhamii strains. All strains were able to survive in olive mill wastewater, used as a model of phenolic compounds-rich matrix. A potential biofilm formation was observed only in N. wickerhamii, while the ability to scavenge radical and to cope with GIT-associated stresses were straindependent. High levels of survival were observed for almost strains (except S. polymorphus), in both Leccino and Coratina samples. Yeasts limited the acidity rise in olive oils, but overtime they contributed to increase the parameters related to oxidative phenomena (i.e. peroxides, K232, K270), resulting in a declassification of EVOOs. The total phenolic content (TPC) was correlated to the presence of yeasts and, at the end of storage period (6 months) inoculated samples had significantly lower concentrations compared to the control oils. This study confirms that yeasts are able to survive in olive oils and, therefore, the control of their occurrence during extraction process and storage conditions is needed to obtain high-quality products and to maintain the standards of EVOO classification.