*6.7. Phenols*

Phenols are the main phytochemical group and comprise the broad term "polyphenols", which are molecules with one or more phenolic groups and one or more hydroxyl groups and comprise a large and heterogeneous group of secondary plant metabolites. They are synthesized from carbohydrates and are generally produced as defense mechanisms against pathogens and the excess of ultraviolet radiation and to attract pollinators [104]. A general description of the biosynthetic pathways and regulation of phenolic compounds in stone fruits appears in the review by Lara et al. [105]. They are responsible for the sensorial and nutritional quality and antimicrobial, antiviral and anti-inflammatory properties are also attributed to them. Beyond antioxidant properties, they also have a variety of biological activities, including antioxidant, anti-inflammatory, vasodilatory and anticarcinogenic actions and also reduce cholesterol [82,97]. In recent years, there has been an increasing interest in biological properties of natural phenolic compounds as actors in the prevention of diseases in which oxidative stress reactions are involved [106].

In an extensive review, Bolling [107] reported a total phenolic content in whole almond that ranged from 0.47 to 13.40 mg/g gallic acid equivalents (GAE); meanwhile, skinless kernels varied between 0.64 and 0.71 mg/g (GAE). Approximately 130 different polyphenols have been identified in almond, although not all of these have been quantitated. Table 5 shows the range of variability found in the scientific literature about total phenolic content, total flavonoids content and total proanthocyanidins content, and Table 6 reflects the main quantitative results about phenolic compounds in almonds. Unpeeled almond kernels have a content of total phenols higher than peeled almond kernels [97,108]. The skin represents approximately 4% of the total almond weight and contains 70–100% of total

phenols present in the nut [109]. The residual cakes could be expected to possess an added value for applications in food formulations since they are a good source of phenolic compounds that concentrate in the by-product due to their polar properties [35].

The phenolic content of almond skins depends on the industrial processing used. High temperatures (i.e., blanching, drying, roasting) could promote degradation of polymeric compounds such as proanthocyanidins, hydrolysis of glycosylated flavonoids and the decomposition of aglycones, which could explain the increase observed in the content of monomeric and oligomeric flavan-3-ols after drying or roasting, and the decline in flavonol and flavanone aglycones found after these treatments [110]. The total contents of phenolic compounds identified were significantly ( *P* < 0.05) higher (about 2-fold) in the roasted samples than in the blanched almonds (freeze-dried). Roasting is the most suitable type of industrial processing of almonds to obtain almond skin extracts with the greatest antioxidant capacity [111].

Cultivars, climate and geography can a ffect total phenols concentration in almond kernels [57,58,112,113]. For Rabadán et al. [12,70], the variability of total polyphenol content depended mainly on the crop year. The use of pesticides reduces the phenols content, so it is advisable to implement organic production [114]. Among cold-pressed oils, the press system (screw or hydraulic) and the di fferent extraction conditions considered did not generate significative di fferences [11,31].

Inside the phenol groups, mainly tannins, flavonoids, phenolic acids and stilbenes can be found.


**Table 5.** Total phenolic content (mg/g) gallic acid equivalents (GAE), total proanthocyanidins (mg/100 g) and total flavonoids (mg/100 g) in almonds, almond oil and defatted almond cake.


**Table 5.** *Cont.*

a: blanched kernels without skin; b: caffeic acid equivalents; c: total tannins; d: hull extract; n.d.: not detected.


**Table 6.** Phenolic compounds quantified in almonds and almond skins (mg/100 g).


**Table 6.** *Cont.*
