Importance of some minor compounds in olive oil authenticity and quality

Highlights

• Minor compounds of importance for olive oil authenticity and quality are reviewed.

• Diacylglycerols and fatty acid ethyl esters are useful for freshness determination.

• Pigments could be used in differentiation based on geographical location.

• Proposed parameters are effective in determination of some frauds in olive oils.

Abstract

Background

Consumption and production of olive oils have been increasing steadily worldwide mainly due to proven health benefits and sensorial characteristics of olive oil. At the same time, rising demand makes it harder to protect olive oil genuineness; therefore, inauthentic products have been always a serious problem in olive oil industry.

Scope and approach

Some minor compounds such as pigments (chlorophylls and carotenoids) including their derivatives pyropheophytins (PPPs), diacylglycerols (DAGs) and fatty acid ethyl esters (FAEEs) are all prominent compounds with their discriminatory and descriptive properties. Among several different approaches, use of these components to differentiate genuine and adulterated olive oils could be a promising choice since it is harder to mimic these compounds in fake mixtures. Recent studies focus on these compounds as authentication and quality tools for olive oil and potential of these compounds are aimed to be reviewed.

Key findings and conclusions

Results from literature indicated that these parameters could be used in both authenticity and quality determination of olive oils with some limitations. Pigments were found to be more promising in geographical and/or varietal classification. All of the discussed components have successful applications in determination of olive oil quality with respect to storage history and oil grades. However, in detection of certain types of adulteration techniques such as soft deodorization, reviewed parameters did not work effectively alone. Regulations could be updated with these findings and use of combined parameters including discussed compounds could be further investigated for unsolved authentication problems.

Introduction

Well-established health effects and desirable sensory properties of olive oil are the major driving forces for the high economical value of this product. Major components of olive oil are triacylglycerols and this oil also contains various minor components such as chlorophylls, carotenoids, phenolic compounds and squalene (Yan, Oey, van Leeuwen, & van Ruth, 2018).

Minor components of virgin olive oil which does not need to go through refining steps are highly preserved during mechanical extraction (Olmo-García et al., 2019). Minor compounds are not only significant for physicochemical characteristics of the product, but they are also correlated with taste and nutritional value (Olmo-García et al., 2019). In addition, they are important markers for olive oil quality, purity and authenticity (Olmo-García et al., 2018; Tena, Wang, Aparicio-Ruiz, García-González, & Aparicio, 2015). Therefore, the concentrations and types of minor compounds are of great importance for both the consumers and the manufacturers (Olmo-García et al., 2018). The quality and quantity of these metabolites are affected by olive variety, growth conditions of olives, extraction and refining procedures of oil as well as storage conditions (Dais & Hatzakis, 2013).

Besides their health-promoting effects, minor components (volatiles, phenolic compounds, terpenoids, sterols, etc.) are also found to be more successful descriptors of olive oil compared to major metabolites due to the fact that it is hard to mimic minor compounds during preparation of illegal formulations (Dais & Hatzakis, 2013). Importance of minor compound composition has become even more significant since olive fruits have been started to be cultivated outside the Mediterranean zones. Even for the same olive type, differences in olive growth locations are also leading to compositional differences between oils obtained from relatively new areas and the products from traditional olive producer countries (Aparicio, Morales, Aparicio-Ruiz, Tena, & García-González, 2013). As a result, olive oils from new cultivation areas could be out of the limits set by official regulatory agencies mainly based on Mediterranean countries (Uncu, Ozen, & Tokatli, 2019). In addition, some traditional but minor cultivars, even grown in the Mediterranean region could still have chemical compounds out of the described limits (García González, Aparicio, & Aparicio-Ruiz, 2018). Thus, the data of the minor compounds of olive oils have become more valuable for statistical evaluation as a significant part of authentication studies (Dais & Hatzakis, 2013).

As a solution to these emerging problems, new chemical parameters mainly exploiting minor compounds of olive oil have been put into action as quality and/or authenticity indicators (Dais & Hatzakis, 2013). If the official and recently proposed methods are examined, it could be seen that methods that determine quality and adulteration in general are intertwined with each other. Therefore, effect of the various constituents of olive oil on the quality and authenticity are examined together in this review. Fatty acid alkyl esters (FAAEs), diacylglycerols (DAGs), natural color pigments, particularly pyropheophytins (PPPs) as the degradation product of chlorophylls and phenolic compounds are regarded as some of the potential quality and authenticity indicators of olive oil (European Commission, 2013).

Several recent reviews and studies in the literature provide information regarding the current regulations about olive oil as well as their methods of analysis (Bajoub, Bendini, Fernández-Gutiérrez, & Carrasco-Pancorbo, 2018; Conte et al., 2019; Tena et al., 2015). Some novel techniques (Surface-enhanced Raman spectroscopy (SERS), biosensors, microfluidic devices) have also been applied in olive oil authentication (Bremer, Smits, & Haasnoot, 2009; Deng et al., 2018; Du et al., 2019; McIntosh et al., 2016) and in determination of some bioactive compounds of olive oils especially polyphenolic content (Al Mughairy, Al-Lawati, & Suliman, 2019; Camerlingo, Portaccio, Delfino, & Lepore, 2019; Hammami, Kuliček, & Raouafi, 2016; Ramos, Contreras, & Macías, 2020). Some well-known minor compounds such as sterols, stigmastadienes, aliphatic hydrocarbons and phenolic compounds along with major compounds (triacylglycerols, fatty acid contents) which have official limits in regulations were evaluated in detail in the previous reviews (Aparicio, Conte, & Fiebig, 2013; Arvanitoyannis & Vlachos, 2007; Ben-Ayed, Kamoun-Grati, & Rebai, 2013; Boskou, 2008; García González et al., 2018; Montealegre, Alegre, & García-Ruiz, 2010) Olive oil is very rich in terms of phenolic compounds such as hydroxytyrosol, tyrosol and oleuropein. Because of the well-established health effects of these compounds they have been extensively studied and there are several reviews about these compounds (Alu'datt et al., 2017; Boskou, 2015) and literature reviews are even available on certain individual phenolic compounds (Parkinson & Keast, 2014; Wani et al., 2018). However, there is not any comprehensive and critical review in the literature focusing on emerging minor compounds, FAAES, DAGs and pigments, and emphasizing their importance in olive oil studies although many studies in the literature indicated their potential on different quality issues such as detection of different types of adulteration and determining olive grade or storage history. Therefore, it was aimed to review the several minor compounds (FAAEs, color compounds with their derivatives (e.g. PPPs), DAGs with derivatives (e.g. monochloropropanediol esters (MCPDEs) and glycidyl esters (GEs)) that have been studied in recent years in terms of the authenticity and quality of olive oil.