Yeasts characterization of some commercial extra virgin olive oils

This research was carried out on some yeasts which negatively influence the quality of oil, isolated from extra virgin olive oil sold in supermarkets by some international companies. The total yeasts, taken from each sample of extra virgin olive oil, were first divided into groups on the basis of the colouring assumed by the cultivated colonies on the CHROMagar Candida medium and then studied according to their lipolytic activity (Ciafardini et al., 2006a,2006b). For each chromogenic group, the amount of the lipaseproducing yeasts was evaluated and the more active strains were classed using biochemical tests and genetic analyses (Ciafardini et al., 2002, 2004). The chromogenic yeast groups varied according to the origin of the sample: in the poly-varieties extra virgin olive oil mixture, more than three chromogenic yeast groups were observed in about equal amounts, whereas in the mono-varieties one, among the five chromogenic groups considered, one group prevailed according to the type of olive variety. The ratio of the lipase-producing yeast was very high in the wrinkly bluish chromogenic group (about 80%) and low in the smooth brown one (less than 35%). From the more active lipase-producing yeasts belonging to the wrinkly red and wrinkly bluish chromogenic groups, we classed two strains of Candida parapsilosis, whereas from the smooth white chromogenic group we classed a strain as Candida wickerhamii. The strains isolated respectively from the wrinkly white and the smooth brown chromogenic groups were not classed because the ribosomal (26S) D1/D2 region sequencing results were not identical to that of any known yeast species. The results obtained from this study demonstrated for the first time the presence of the human pathogen C. parapsilosis species in two commercial extra virgin olive oils. On the basis of these obtained results, it seems that the microbiological analyses of the yeasts present in the oils contributes towards establishing the technological and health quality of the commercial extra virgin olive oil.