*2.2.2 Tannins*

*Innovation in the Food Sector Through the Valorization of Food and Agro-Food By-Products*

The most relevant flavonoids present in strawberries are the anthocyanins due to their high concentration, approximately 20–47 mg/100 g raw strawberry [16]. More than 25 different pigments of anthocyanins have been described in the different varieties of strawberries [5]. Anthocyanins are responsible for the red color in strawberries [16]. The most important anthocyanins of the strawberry belong to the family of pelargonidin aglycones and cyanidin aglycones [23–25]. According to several studies, pelargonidin-3-glucoside is the dominant anthocyanin in strawberries [16, 24, 26–28]. The interest in anthocyanins have recently increased because of its pharmacological and therapeutic properties [5]. Anthocyanins have shown to positive effect toward reduction of coronary diseases, anticancer, antitumor, anti-inflammatory and anti-diabetic effects; as well as improving visual acuity and cognitive behavior [29]. These therapeutic effects of anthocyanins are connected to their high antioxidant activity [29]. In addition, anthocyanins can be used as a

The second most important group of flavonoids in strawberries are flavonols, with approximately 1.5–3.4 mg/100 g raw strawberry [5, 16, 30, 31]. The most important flavonols of the strawberry belong to the family of quercetin and kaempferol, being the quercetin-derivatives the most abundant flavonols in strawberries [5, 17, 25–27]. Quercetin, in particular, is a potent antioxidant, cytoprotective, and

Finally, the third group of flavonoids in strawberries are flavanols. Flavanols are the only class of flavonoids that do not naturally occur as glycosides. They are found in strawberries as monomeric compounds, such as catechins, and in

**274**

**Figure 2.**

pigment in the food industry [29].

*General scheme of phytochemicals contained in strawberries.*

anti-inflammatory [30].

Tannins are classified into two groups: non-hydrolysable or condensed tannins and hydrolysable tannins (**Figure 2**). The condensed tannins are also called proanthocyanins, and are bound to the flavanols [33]. The content of condensed tannins in strawberries is approximately 54–163 mg/100 g raw strawberry [16]. In strawberries, the most relevant condensed tannins are procyanidins from catechin and its polymers. Condensed tannins are commonly found in the pulp of strawberries and achenes [5]. Due to the variety of physiological activities, they have been reported to possess, directly and indirectly, antioxidant, antimicrobial, anti-allergic and antihypertensive properties, as well as to inhibit the activities of some enzymes and physiological receptors [34].

The most common hydrolysable tannins in strawberries are ellagitannins, specifically sanguiin H-6 and ellagic acid [5, 26–28]. The content of ellagitannins in strawberries is approximately 10–23 mg/100 g raw strawberry [16]. Ellagic acid is an ellagitannin present in the secondary metabolism of vegetables, its main characteristic is its antioxidant, antimicrobial, antimutagenic, anticarcinogenic and antiviral capacity [16]. The content of ellagic acid in strawberries is approximately 1–2 mg/100 g raw strawberry [16]. Due to the phenolic nature of ellagic acid, this compound tends to react by forming complexes with other molecules of proteins, alkaloids, and polysaccharides, so that it is usually found as ellagitannins esterified with glucose, because of this it is difficult to find it free [20]. The properties of ellagic acid are also exploited in the food industry, so it is used in the manufacture of nutraceutical drinks and food supplements. Likewise, the application of ellagic acid for food preservation is of great impact for the perishable food industry, using its antioxidant activity for microorganisms inhibition [35].

### *2.2.3 Phenolic acids*

Strawberries contain a variety a of phenolic acids which are presented as derivatives of the hydroxycinnamic acid, such as caffeic acid, and hydroxybenzoic acids such as gallic acid [5]. The content of phenolic acids in strawberries is approximately 0.8–6.7 mg/100 g raw strawberry [16]. The major hydroxycinnamic acid in strawberries is p-coumaroylhexose, but ferulic acid and caffeic acid glycosides have also been identified in strawberries [26, 27]. Hydroxycinnamic acid derivatives are responsible for the bitter taste of the strawberry, and it is used in the manufacture of creams [33, 36]. The primary derivative of hydroxybenzoic acid is p-hydroxybenzoic glycoside [28]. The p-hydroxybenzoic glycoside is widely used in the synthesis of organic compounds and their esters, known as parabens, which are used as preservatives in cosmetics [37].