The aromatic profile of Extra Virgin Olive Oil (EVOO) is what makes it unique, it is its fingerprint. It is what differentiates a truly exceptional EVOO from a mediocre one.
The first and most common question in all the olive oil seminars is “Where do all the aromas of olive oil come from?”. Participants are always amazed by the aromatic complexity of a good EVOO. Aromas like freshly-cut grass, tomato, banana, apple, artichoke, thyme, vanilla, or almond are a pleasant surprise to all the participants. Many believe that these aromas are additives, or endogenous to the fruit or are related to various plants growing around the olive tree. Actually, these delicate aromas and flavors are attributed to several volatile compounds like aldehydes, alcohols, esters, hydrocarbons, ketones, furans, and many more.
The chemistry of olive oil flavor is an amazing, not completely explored, field that scientists still study and try to understand.
Not all the compounds involved in the organoleptic characteristics are known, as they are the result of a complex and heterogeneous mixture of molecules. In addition, the composition of the olives and, therefore, the organic and sensory parameters of the resulting oil can vary greatly depending on intrinsic factors (olive variety, cultivation conditions) and extrinsic factors (sun exposure, irrigation, production practices, storage, packaging, etc.). Thus, considering all these factors and many others, the study of organoleptic properties of EVOO and the related compounds involves a great complexity.
Volatile compounds formed during the growth and processing of olive fruit contribute to a combined sensation of smell and taste of the resulting oil, commonly called flavor. Volatile compounds are molecules with less than 300 Da, which vaporize easily at room temperature. These compounds that are released into the headspace, stimulate the olfactory receptors in the nasal cavity by dissolving into the mucus and bonding to olfactory receptors, after passing through the external nostrils, giving an odor sensation. Furthermore, while tasting the EVOO, the aromas are also perceived when the compounds interact with the receptors in the nasal cavity after migrating from the mouth through the nasopharynx.
The formation of volatile compounds in olive fruit is related to cell destruction. An enzymatic process occurs that involves hydrolysis and oxidation. The reactions proceed at a high rate, depending on pH and temperature.
The major volatile compounds found in EVOO belong to C6 and C5 compounds. C6 aldehydes and alcohols and their corresponding esters are considered, both qualitatively and quantitatively, the most crucial and influential aroma compounds of EVOO. They are related to sweetness and green notes and contribute favorably to the aroma. C6 aldehydes (hexanal, 3(Z)-hexenal and 2(E)-hexenal), alcohols (hexanol, 3(Z)-hexenol and 2(E)-hexenol), and their acetyl esters (hexylacetate and 3(Z)-hexenyl acetate) constitute 60 –80% of total volatile fraction, while 2(E)-hexenal being the most remarkable component.
All of them contribute with green notes to the oil and are formed from polyunsaturated fatty acids (PUFAs) through a cascade of biochemical reactions (lipoxygenase pathway) in which enzymes transform PUFAs to aldehydes, which are subsequently reduced to alcohols and esterified to produce esters, another large group of compounds with relevance to the sensory quality of EVOO. Although alcohols are usually contained in higher percentages than aldehydes, the latter is more relevant in EVOO’s flavor because they present lower detection thresholds. Regarding C5 compounds, they are also contained in EVOO in reasonable amounts, contributing to their flavor. C5 aldehydes and alcohols provide pungent sensations in correlation with bitterness. Taking into account the information reported by numerous studies, it can be concluded that the C6 and C5 volatile compounds are powerful odorants, but they can be found in EVOO in a wide range of concentrations, according to differences in olive varieties and extraction methods applied.
However, minor and major volatile compounds are crucial to olive oil quality. Even the volatiles that are below the olfactory level of detection are important because they can explain the formation of future degradation products, which will later have significant importance in the organoleptic characteristics of the oil and they also provide useful quality markers.
Fig. 1 The main pathways involved in the formation of volatile compounds in virgin olive oils
Another group of molecules of great influence in EVOO’s flavor is certain phenolic compounds, as there is a positive connection between EVOO’s aroma and flavor and its phenolic content. The phenolic composition of EVOO was found to be one of the most diverse because it depends on all of the factors mentioned previously, such as olive variety, climatic conditions, maturity, production processes, etc. The variations in the phenolic composition are responsible for some of the different organoleptic characteristics found in EVOO.
Numerous studies have been carried out to clarify the relationship between taste attributes in Virgin Olive Oils (VOOs) and their phenolic compound contents: bitterness intensity was initially related to the presence of oleuropein derivatives, but other researchers, based on sensory evaluations and the amount of some phenolic compounds, have attributed the bitter sensory note to both oleuropein and ligstroside aglycons, or only to ligstroside derivatives such as p-HPEA-EDA. One of the studies reported that the most representative complex of phenols in EVOO were three oleuropein aglycones (OleA), four ligstroside aglycones, and their derivatives, belonging to the secoiridoid group. Other secoiridoids described were decarboxymethylated, hydroxylated, and methylated forms of ligstroside aglycone. Regarding lignans and flavones, pinoresinol, syringaresinol, acetoxypinorexinol, luteolin, and apigenin were identified in EVOO samples. Quinic acid was also found in all the analyzed samples. Sensory descriptors such as fruitiness, bitterness, pungency, and astringency are flavors linked to the concentration of phenolic compounds. For a comprehensive specification, single flavor descriptors such as freshly cut grass, tomato, artichoke, leaves, nuts, apple, banana, tropical fruits, and herbs that are part of the main attribute of fruitiness, are considered.
Fig. 2 Volatile compounds and odor descriptors attributed to Virgin Olive Oils
Factors affecting flavor compounds
Enzyme levels are genetic characteristics and therefore typical of each cultivar, their activity is affected by several but not less important factors, related to ripeness and growing area of fruits, time and conditions of their storage, and technological aspects of oil extraction. Therefore these parameters quantitatively modify the composition of flavor compounds. Some important factors affecting the compounds responsible for virgin olive oil (VOO) flavor are the cultivar, the ripening stage of the fruit, pedoclimate, altitude, water stress and irrigation, time and conditions of preservation of fruits, technological – processing factors like crushing (destoning practices also affect the flavor profile), malaxation and separation.
These factors influence the proportions in which the compounds responsible for the flavor of VOO are found. The determination of all these compounds related to the organoleptic properties of VOO is a complicated matter, since there are numerous variations produced by the mentioned factors, and since possible synergistic or antagonistic effects also come into play.
Strategies to preserve the flavor of EVOO
Oxidative stability varies between oils as a result of cultivar, crop management during fruit development, harvest timing and the seasonal climate and can have a marked influence on fruit quality. Generally, this is related to the composition of the oil and particularly to the level of antioxidants and the degree of polyunsaturation. Higher levels of polyunsaturation result in reduced oxidative stability. Olive oil contains antioxidants such as phenolic compounds and tocopherols, such as vitamin E (α-tocopherol), and these are implicated with nutritional benefits for consumers. In addition, phenolic compounds provide pungent sensory characteristics in olive oil. Extending the shelf life of olive oil is important for the benefit of the consumer but also for the long-term viability of the producer. The sensory characteristics of olive oil are expected to be of high quality during storage and up to the time of consumption. The assessment of stability is important for the prediction of shelf life and used-by-dates. Measurement of all components such as fatty acid profiles, phenolic content, chlorophyll, and tocopherols, can assist in predicting the stability of the oil. New tests, pyropheophytin a and 1,2-diacylglycerols show good potential for predicting and determining olive oil quality and aging.
The most common preservation practices are filtration, package design, the application of a modified atmosphere, and certain storage conditions.
The filtration step reduces the phospholipid and water content of the oil. This lower content also reduces the fermentation rate and the oil’s cloudiness, which ultimately improves its stability. Other strategies, that have been demonstrated to be useful for preventing the lipid oxidation of EVOO while preventing the modification of its organoleptic properties are based on new packaging designs. Packaging designs aimed to reduce photo-oxidation and contact with oxygen (ex. bag in a box). Packaging practices that apply modified atmospheres that minimize the presence of oxygen in the final product can significantly improve EVOO shelf-life. Therefore, different approaches have been used to reduce oxygen presence in the headspace, nitrogen being one of the main inert gases of choice. The inclusion of nitrogen in the headspace of EVOO slows down the rising of all lipid oxidation parameters over 18 months when compared with no treated oils. Alternative to the use of nitrogen, other inert gases such as argon or carbon dioxide has been evaluated in various studies. Among them, argon showed better protective properties in terms of phenolic content and antioxidant capacity but also slowed down lipid oxidation. Even though dioxide carbon retarded with higher efficacy with the increment of the peroxide value, it induced a significant negative aftertaste in EVOO, so its use is not profitable. Storage temperature is crucial and should be between 10-24oC, with an ideal temperature of 15 oC.
Furthermore, new and innovative strategies are being studied, such as the addition of antioxidants into the oil, microencapsulation, or active packaging application, which favor the chemical stability of the flavor compounds, willing to preserve and ensure the sensory quality of EVOO throughout its useful life.
The flavor compounds of olive oil are its identity, it is what makes a good EVOO a unique and delicious product. All of us who work in the sector we should educate everyone involved in olive oil production about the preservation of those unique characteristics.
It is not only important to produce a product with great complexity of flavors but also to preserve it over time.
Preservation is the word that will concern conscious producers, oleologists and bottlers a lot in the future and it’s the key to consumer satisfaction and EVOO’s worldwide acknowledgment.
Agricultural Engineer - Expert Olive Oil Taster
The article was published in Zeytin Dair Magazine issued by the Turkish Olive Oil Academy in November 2021
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