# Flavor Chemistry of Virgin Olive Oil: An Overview
## 2 VOO Flavor Perception
### Off-Flavors  
Parallel to the LOX pathway, processes such as possible sugar fermentation, metabolism of some amino acids (leucine, isoleucine and valine) or oxidative processes lead to the formation of other volatile compounds which, if in excess, can generate off-flavor. The onset of organoleptic defects depends on several factors: health status and ripeness degree of the olives, methods of harvesting and post-harvesting, storage conditions, technology used for the extraction, storage, decanting and filtration of the product. Indeed, each phase of the production process can influence the composition of volatile substances and the appearance of off-flavor. The sensory detection of even a single negative attribute (defect) by an official panel of tasters downgrades the product from EVOO to any lower category, depending on its intensity [21]. The \"mold-humidity\" defect is the characteristic aroma of oils from olive fruits in which fungi and molds have developed, caused by a long period of post-harvest storage in humid and poorly ventilated environments. The most present species are Penicillin and Aspergillus which oxidize free fatty acids, with the formation of methylketones such as 2-heptanone and 2-nonanone (Table 2). Yeasts, such as _Candida_ and _Saccharomyces_, in parallel, are able to reduce carbonyl groups thanks to esterification, contributing to the production of an oil with a damp-mold defect. The volatile molecules that most contribute to this defect are 1-octen-3-one, 1-octen-3-ol and 2-heptanol (Table 2). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sensory Defect** & **Chemical Compound** & **Chemical Structure** & **Odor Property** \\ \hline \multirow{4}{*}{Mold-humidity} & 1–octen–3–one & & mushroom, mold, pungent \\  & 1–octen–3–ol & & mold, earthy \\  & 2–heptanol & & earthy, sweety \\  & 2–heptanone & & sweet, fruity, cinnamon \\ \hline \multirow{4}{*}{Winery-vinegar} & ethanol & & alcohol \\  & acetic acid & & sour, vineary \\ \cline{1-1}  & ethyl acetate & & sticky, sweet \\ \cline{1-1}  & 3–methylbutanol & & woody, whiskey, sweet \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Some of the chemical compounds responsible for the main sensory defects detected in VOO and their odor descriptors. Winery-vinegar is the typical defect of oils obtained from non-fresh olives that have undergone alcoholic and acetic fermentation. The main bacteria involved in this process are the lactic (_Lactobacillus_) and acetic ones, which give rise, in the absence of oxygen, first to ethanol and then to acetic acid and ethyl acetate, volatile compounds which are the main responsible for this defect together with 3-methylbutanol [59; 60] (Table 2). The \"fusty\" defect is the aroma originated in olives stored for a long time in thick layers or in bags, which have undergone various fermentations, the main one being lactic one. A similar defect, which for practicality is grouped together but has a very different biochemical origin, is the \"muddy-sediment\". This defect is the characteristic aroma of oils that have remained in contact with their own muddy sediment for a long time, which have undergone an anaerobic fermentation, mainly butyric. The main microorganisms identified in oils affected by heating are part of the Enterobacteriaceae family, whose proliferation starts at the beginning of the storage phase, and Pseudomonas, _Cistoridium_ and _Serratia_, whose growth takes place later. Ethyl butanoate, ethyl propanoate and butyl acetate are generally found at higher levels in the fusty VOO. The high concentration of 6-methyl-5-hepten-2-one can be explained by the presence of _Pseudomonas_ that degrades terpene alcohols. Instead, the abundance of butanoic and propanoic acids is due to the metabolism of _Cistoridium_ and propionic bacteria (Table 2).