Fig. 1.

Classification and main sources of phytochemicals [12–17]. Polyphenols, carotenoids, organosulfur compounds, and alkaloids represent examples of large classes of phytochemicals. Polyphenols (plant phenolics) are further divided into flavonoids, phenolic acids, coumarins, stilbenes, hydrolyzable, condensed tannins, lignans, and lignins [13]. Indeed, current evidence state over 8000 identified polyphenols, but the number is largely underestimated [18]. Carotenoids are naturally occurring liposoluble pigments (red, orange, yellow) of fruits and vegetables and can be found also in fungi, algae, and bacteria. Nearly 700 carotenoids have been identified and can be further divided into carotenes, xantophylls, and apocarotenoids (derivatives) [14]. The presence of nitrogen atoms/s is a basic characteristic of alkaloids [19]. Alkaloids can be subdivided into numerous sub-classes. As stated by Heinrich et al. in October 2020, the Dictionary of Natural Products included 27,683 alkaloids [20]. Organosulfur compounds include isothiocyanates, indoles, allylic sulfur compounds, and sulforaphane [15]