Alchil esteri e composti correlate in oli d'oliva vergini: loro evoluzione nel tempo

The quantitative determination of alkyl esters in extra virgin olive oil has been adopted in Europe by Reg. (EU) No 61/2011 as an official quality parameter, after being previously adopted by the International Olive Council (IOC), after a collaborative experimentation. The adoption of the limit aroused a lively debate, while for some it was considered too high, others reported the possibility of an increase of alkyl esters over the storage time of the oil. To investigate the matter, the "Società Italiana per lo Studio delle Sostanze Grasse" (SISSG - Italian Society for Fat Researchers) organized an experimental study in cooperation with several Italian olive oil companies: each one giving samples, filtered or not filtered, in bulk or bottled, of different origins. 18 samples of oil (13 in bulk and 5 bottled) were stored for 1 year. Every three months a number of analytical parameters were carried out to determine their quality: moisture, insoluble impurities, methyl esters, ethyl esters, methanol, ethanol, peroxide value, free acidity, UV absorption and sensory evaluation (panel test). All the analytical evaluations, with the exception of methanol and ethanol content (cited procedure), were carried out according to IOC, ISO or EU Official methods. The results highlighted that, within the samples used for this research, the concentration of ethyl alcohol was usually higher in Spanish oils (30-90 mg/kg) rather than Italian ones (1, 2-10, 5 mg/ kg), also free methanol was generally lower (never higher than 16, 5 mg/kg). The same trend has been observed for the fatty acid ethyl and methyl esters. It was also observed, only in oils containing amounts of ethanol and methanol respectively higher than 20 and 10 mg/kg and particularly for those of a low quality (virgin from the beginning of the study), had a relevant rise of alkyl esters during storage. On the contrary, it should be highlighted that samples of a high quality (characterized at T0 by a low content of ethyl esters) present, at the beginning and over the time of the study, amounts of free ethanol lower than 10 mg/kg (0, 3 mill mol) did not show any increment of ethyl esters during the conservation. Free acidity, even if different in the oil samples, seemed not to be a limiting or promoting factor for the esters increasing, suggesting that they could be both generated by ethanol or methanol esterification (with fatty acids) or transesterification (with triglycerides or partial glycerides). The formation during the conservations of ethyl and methyl esters appeared to be strongly related with the concentration of free alcohols in the oils, and, as long as ethanol can be solely produced by fermentation, ethyl esters are confirmed, between the two, the most important markers to determine the oil quality. This study confirms that the recent decision of IOC to delete the parameters "methyl esters" and related limit, as well as the "ratio ethyl esters/methyl esters" and maintain ethyl esters only, in the meantime lowering the limit . The occurence of ethyl esters infact, as well as free ethanol are markers of fermentation (low quality of olive fruits and of the extracted oil). Further studies are needed: - to verify the exact mechanism/s of esters formation in oil, - what are the limits of the related volatile compounds (in particular, ethanol and ethyl acetate) that can be suggested for extra virgin olive oils, - the most robust and simple method that should be used to determine these diagnostic volatiles.