**2. Foods as natural resources of phenolics**

Polyphenols have been exhaustively studied in their different natural matrices such as fruits, vegetables, teas, algae and microalgae and more recently agro-food wastes (peels, seeds, pulps, stems and roots) [12–15]. In the three last decades, there has been a prolific publication of scientific studies showing that plant-derived foods and agro-food wastes from industrial transformation have huge quantities of polyphenols. In **Table 1** are summarized some recent studies, and as result from these and other studies, there is a diverse source of polyphenols in plant materials, but both type and amount seem to be highly influenced by their chemical nature, extraction methods, sample particle size, storage time and conditions, as well as by the presence other of interfering substances [25]. Also, their chemical structure and nature vary from simple to highly polymerized substances that include varying proportions of phenolic acids, phenylpropanoids, anthocyanins and tannins, among others [26–28]. Moreover, they might also exist in complex mixtures with carbohydrates, proteins and some quite insoluble high-molecular-weight phenolics [28]. Therefore, the phenolic extraction from plant materials is always a mixture of different steps, and many modifications of a particular method are often needed for the removal of unwanted non-phenolic substances such as waxes, fats, terpenes, pigments (chlorophylls and carotenoids). Solid-phase extraction (SPE) techniques, purification and fractionation based on acidity, are commonly used to remove unwanted non-phenolic substances or even other unwanted phenolics [29].

Although the recent advances in the technology had providing innovative approaches to obtain enriched polyphenol natural extracts, we must ware that their extraction efficiency will always be dependent of several factors in which the nature of samples and solvent, pH, temperature, light, length of extraction period, particle size, solvent/sample ratio and liquid-liquid or solid-liquid extraction process [25], among others, are the most critical.


**Table 1.** Most common types of polyphenols found in foods and plant-derived products [14–24].
