The formation of volatile compounds in extra virgin olive oil due to the maturity of the fruit

Virgin olive oil, a food with high nutritional value and unique sensorial properties, is associated with the Mediterranean diet, whose benefits for human health have been described in several scientific works. Among the positive sensory properties of the oil, in addition to spiciness and bitterness, there is fruitiness, described as the set of aromatic properties that characterize the oil obtained from healthy and fresh fruits, green or ripe, depending on the cultivar , and observation can be direct or indirect, i.e. retronasal. The unique smell of extra virgin olive oil is the result of a complex mixture of volatile compounds that form during and after the extraction of the oil from the olive fruit.

Most of these volatiles belong to the aldehydes, alcohols, esters, hydrocarbons, ketones, and furans, with the volatile compounds C6 and C5 being the most common. Analysis of volatiles in the VOOs of several Italian and Spanish olive cultivars revealed that C6 aldehydes (hexanal, Z-3-hexenal, E-2-hexenal), C6 alcohols (hexanol, Z-3-hexenol, E -2-hexenol) and their acetate esters (hexyl acetate and Z-3-hexenyl acetate) represent 60% to 80% of the total volatiles.

Volatiles with desirable odorous properties belong to specialized plant metabolites and are formed in a series of biochemical reactions known as the lipoxygenase (LOX) pathway. The enzymes of the LOX pathway are released during processing, after the breakdown of the olive fruit tissue. The precursors of the volatile C6 compounds are linoleic acid (18:2) and linolenic acid (18:3). The corresponding hydroperoxides of these acids are formed by the action of the LOX enzyme. To date, four isoforms of the LOX enzyme have been isolated from olive fruits, two each representing the LOX-1 and LOX-2 superfamilies. Isoforms of the LOX-1 superfamily show a higher affinity for linoleic acid and synthesize 9- and 13-Z,E-HPOD in a ratio of 2:1 and 4:1, respectively, while representatives of the LOX-2 superfamily they show a greater affinity for linolenic acid and exclusively synthesize 13-hydroperoxides of fatty acids. Fatty acid hydroperoxides are cleaved by hydroperoxide lyase (HPL), and the resulting aldehydes are reduced to alcohols by alcohol dehydrogenase (ADH) and converted to the corresponding esters by acyltransferase (ATT). In addition to this main action, the LOX enzyme can also cleave the resulting fatty acid hydroperoxides, and the released alkoxy radicals participate in chemical reactions whose products are volatile C5 compounds, of which only 1-penten-3-one contributes significantly independent to the desirable odorous characteristics of extra virgin olive oil, while the other C5 compounds contribute to the aroma through a synergistic effect with other volatile compounds. Ultimately, the content of volatile compounds in the oil strongly depends on the cultivar, the degree of ripeness of the fruit, the geographical area of ​​cultivation, the agronomic factors and the processing method.

The formation of volatile compounds in extra virgin olive oil due to the maturity of the fruit

Lipoxygenase activity varies among cultivars.

Despite the differences, Croatian researchers found that ripening had a very similar effect on the LOX enzyme activity of the various cultivars examined.

With ripening, the specific activity of the LOX enzyme in fruits increases and reaches maximum activity in the period of fruit discoloration (MI~4), followed by a decrease in the polyphenol content of more than 75% in Oblica, by 60 % in Levantinka and 70% in Lastovka.

Analysis of the primary products of linoleic acid oxidation revealed that the LOX enzyme synthesizes a mixture of hydroperoxides, namely 9- and 13-Z,E-HPOD in a ratio of approximately 1:2 in cultivars, which classifies it as a nontraditional lipoxygenase of the LOX-1 superfamily.

The dual-specificity isoform of the LOX enzyme has a high affinity for linoleic acid and synthesizes 13-Z,E-HPOD, a precursor of the volatile C6 targets hexanal, hexanol, and hexyl acetate. In all three cultivars studied, a positive correlation was observed between MI, LOX enzyme activity and hexyl acetate concentration. The highest concentration of the target components isolated in this study, as well as most other desirable C6 volatile compounds was found in oils obtained from processing fruits with 2/3 purple skin (MI~3). Quantitative differences in the concentrations of individual aldehydes, alcohols and esters were found.