Effects of some oil-born yeasts on the sensory characteristics of Italian virgin olive oil during its storage.

The olive oil microbiota, mainly composed of yeasts, is associated with the suspended fraction of freshly produced olive oils. Some olive oil yeasts are considered useful as they are able to hydrolyse the bitter tasting secoiridoid compound of the oil, whereas others are considered harmful as they can damage the quality of the oil. Present research demonstrated the influence of some yeast strains belonging to Candida adriatica, Candida diddensiae and Candida wickerhamii species on the olive oil sensory characteristics during its storage. All the tested yeasts survived in the inoculated extra virgin olive oil and, after four months of storage, the suspended yeast cells recovered from the olive oil varied between 50% and 80% of the initial total yeasts, according to their sedimentation capacity. The mean of five analytical indices (free fatty acids, peroxide value, K232, K270 and ΔK) were quite similar and about 60% of the treated samples analysed after four months of storage, on the basis of these indices, were still classed as extra virgin. Completely different results were obtained from the analyses of volatile and non volatile carbonyl compounds according to the yeast used. In the samples of oil treated with C. adriatica and C. wickerhamii, instead of some strains of C. diddensiae, a lower concentration of C6 volatile carbonyl compounds and polyphenols, responsible for positive oil attributes, were found. The sensory attributes of the treated olive oils varied according to the composition of the volatile and non volatile carbonyl compounds produced with the treatments. "Muddy-sediment", "rancid" or both defects were found in olive oil samples treated with C. adriatica DAPES 1933, C. wickerhamii DAPES 1885 and C. diddensiae DAPES 1912 and 1913 strains, whereas olive oil samples treated with C. diddensiae DAPES 1918 and 1922 after four months of storage were defect-free, and still categorized as extra virgin, according to the requirements of both chemical and sensory quality indices of European Community Regulations.