Mediterranean aromatic plants are generally rich in phenolic compounds (also known as phenols or phenolics), which are a heterogeneous group of plant secondary metabolites, having in common the presence of at least one aromatic ring and hydroxyl group (OH), which is the root for their name, as phenol is the simplest compound with such features. The designation “phenolic compounds” is broad enough to include from simple molecules to polymers, diverging not only in size but also in chemical properties (as polarity) due to differences in functional groups (such as ester and acid). Given the heterogeneity of this group of compounds, they have been named and categorized differently by different authors according to several viewpoints [55].They can be found in variable amounts in plant foods and are involved in colour (e.g., pigments), flavour (e.g., responsible for astringency, bitterness), and food safety (due to antimicrobial activity). Phenolic compounds from foods are most often valued for their general antioxidant character and they have been recently categorized as phytonutrients because of the mounting evidence and growing awareness of their health-promoting features [47,48,49]. Phenolic compounds exist in fresh vegetables (e.g., leafy vegetables, aromatic herbs, nuts) and processed (fruit juice, tea, coffee, wine) and, although their release kinetics during food digestion and bioavailability afterwards are still unclear, they seem to depend on the food matrix, on the interactions with other nutrients and more [50,51,52].

Figure 2 summarises the main groups of plant phenolic compounds also found in Mediterranean aromatic herbs. Some examples are included and more information (e.g., culinary uses, bioactivities) can be found in Table 1.

When categorizing phenolics regarding plant metabolic pathways, three large groups can be first considered: phenolic acids, chalcones, and coumarins (Figure 2) because they can be precursors of others [55]. Phenolic acids can be precursors in the synthesis of lignins. In another pathway, chalcones may originate flavonoids, which, in turn, may condensate. Polymerization proceeds; proanthocyanidins are first oligomers and then polymers composed of units of flavanols. These polymers may ultimately originate tannins, of high molecular mass. Based on different properties, two subclasses can be considered, hydrolysable and condensed tannins [56]. Coumarins, the third large group above-referred, do not undergo any further transformation [57].

When considering overall function, structure, and occurrence, some types of compounds stand out, as is the case of flavonoids.

As roughly overviewed in Figure 2, flavonoids can be subdivided into flavonols, dihydroflavonols, isoflavones, and flavanols, according to the degree of hydrogenation and hydroxylation of their three-ring structure [56,58]. Flavonoids bind easily to sugars resulting in a pigment whose colour grade depends on the nature of the chemical bond established between the phenol moiety and the sugar residue [57].

Flavanols, also referred to as flavan-3-ols or catechins (Figure 2), are derivatives of flavans and include catechin, epicatechin gallate, epigallocatechin, epigallocatechin gallate, proanthocyanidins, and thearubigins [59]. According to the same authors, catechin is water soluble and can chelate heavy metals and bind to proteins, including bacterial toxins of proteinaceous nature, which may explain its antimicrobial and detoxifying properties. Catechin is mostly found in green tea but is also present in rosemary. Catechin and epigallocatechin may polymerize together in different proportions, and the levels of antioxidant activity seem to depend on the degree of polymerization (Figure 2). Condensed tannins are obtained after a series of condensation and polymerization reactions [57,63].

Condensed tannins are thus flavanol polymers not readily hydrolysed that are responsible for the astringency taste of many plant foods [57]. As polyphenols in general, they were regarded by nutritionists as antinutrients, to be avoided. However, state-of-the-art knowledge, especially about the human microbiome, has been disclosing their relevance as phytonutrients [64]. According to Selma et al. [65], dietary polyphenols act as phytonutrients by first interacting with the gut microbiota and often being transformed by bacteria before entering the bloodstream. Still, according to the same researchers, some health benefits from phenolic compounds may result, to a large extent, from such microbial bioactive metabolites.

Role of Aromatic Plants in the MD Cuisine

Culinary traditions between Mediterranean countries changed significantly over the past decades, but the habit of enriching food with flavours and aromas remains across the Mediterranean countries [41,43]. Aromatic herbs are essential ingredients of MD, used as food additives and condiments, and as herbal teas [44]. Aromatic herbs can be a pleasant and healthier substitute for salt (NaCl) in cooking. Oregano (Origanum vulgare L.), thyme (Thymus vulgarisL.), sage (Salvia officinalis L.), and rosemary (Rosmarinus officinalis L.) are well-known aromatic herbs, which belong to the Lamiaceae family. On the other hand, the mints (a large number of species and subspecies) are widely used as medicinal aids (in folk medicine) because of their reported health-promoting actions on top of their aromas. Lemon balm (Melissa officinalis L.), Peppermint (Mentha piperita L.) and kitchen mint (Mentha spicata L.) are examples of species with subspecies found only in specific habitats. Aromatic herbs used in Mediterranean cuisine are rich in bioactive compounds and, thus, contribute to the MD’s health benefits. Some documented health outcomes of spices and herbs used in Mediterranean cuisine are highlighted in Table 1, which is focused on the native plants of the region. 

Table 1. Prominent phenolic compounds, health-promoting actions and other properties of aromatic herbs commonly used in Mediterranean cuisine.

[please insert table 1 here]

As can be observed in Table 1, the ability to neutralize free radicals and the antimicrobial character are common features of phenolic compounds, which can easily be proven in vitro. Studies on the mechanism of action of certain phenolics are also referenced but the number of published clinical studies is reduced. Beneficial health outcomes are rarely attributed to a single compound but rather to whole foods and notably to the Mediterranean dietary pattern (MD), in which aromatic herbs play a key role. 

Wild Mediterranean Aromatic Plants and Conservation Concerns

The causes contributing to decline or extinction of plant species are mainly uncontrolled overcollection, urban and tourism pressure, and climate change.