## 2 Bioactive Compounds Commonly Found in Green-Colored Fruits and Vegetables
### General Characteristics, Structure, and Food Sources  
**Caffeic acid (CA)**, a natural polyphenic compound, is widely found in cabbage, buckwheat, dandelion, artichoke, olive, mint, and other green plants. It is a typical representative of HA and has phenolic hydroxyl and acrylic acid functional group structures. In addition, it is a significant micronutrient in the human body and the main source of HA in human diet. It has good biological activity [145]. It is also known as 3,4-dihydroxy cinnamic acid and exists as yellow crystals at room temperature. It is slightly soluble in cold water and easily soluble in hot water and organic solvents. The chemical formula of CA is C\({}_{9}\)H\({}_{8}\)O\({}_{4}\), and its molecular weight is 180.15 Da. Its melting point is 223-225 \({}^{\circ}\)C.  
**Caffeoylquinic acids (CQA)**, a natural phenolic compound widespread in the plant kingdom, is a combination of quinic acid (QA) and CA via an esteratic linkage. In recent years, CQA has received great attention because of its high content in coffee, tea, certain fruits, and vegetables. In nature, CQA includes monocaffeic, dicaffeic, tricaffeic, and polycaffeic quinic acids, of which monocaffeic and dicaffeic quinic acids are widely present in plants. Another vital derivate of CQA is 3(5)-O-caffeoylquinic acid (5-CQA) or 3-CQA (pre-IUPAC nomenclature), generally termed chlorogenic acid (CGA), which is one of the most abundant HAs and available phenolic acid compounds present in foods, such as green bean coffee, avocado, broccoli, artichoke, green tea, and _Angelica officinalis_ L. fruits [146, 147, 148, 149, 150]. **Ferulic acid (FA) and feruloylquinic acid (FQA)**. Ferulic acid (FA, also known as 4-hydroxy-3-methoxycinnamic acid), is a ubiquitous phenolic compound found in numerous fruits and vegetables, including bananas, citrus fruits, eggplant, and cabbage, as well as in seeds, leaves, and wheat brans [151, 152]. It is also considered one of the most effective agents in the Chinese medicinal plant _Angelica sinensis_ and has been approved as a food additive in many European countries. Feruloylquinic acid (FQA) has an additional quinyl ester of HA, which is synthesized by the condensation of FA and QA [153]. It has a wide range of sources, including kiwi fruit, tomato, avocado, _Angelica sinensis_ and green coffee beans, as well as in the roots of nutrient-deprived _Brachiaria_ species [154]. Although FQA has been reported to have anti-tumor properties, its biological functions have not been studied thus far. _p_**-Coumaric acid (_p_-CA), also known as 4-hydroxycinnamic acid, is a phenolic acid that functions as a precursor for other phenolic compounds [155]. It belongs to the HA family, which is synthesized from phenylalanine and tyrosine via the shikimate pathway. It plays a critical role in secondary metabolism because it can be subsequently transformed to other phenolic acids, such as CA, FA, CGA, and sinapic acid, and other secondary metabolites [156]. _p_-CA is found ubiquitously in fruits (e.g., apples, pears, grapes, oranges, tomatoes, and berries), vegetables (e.g., beans, potatoes, and onions), and cereals (e.g., maize, oats, and wheat), as listed in Table 1[155]. In addition, it can be also found in green-colored foods including avocado, kiwi, green grape, basil, and spinach. Bioavailability  
Hydroxcinnamic acids (HAs) are one of the major classes of phenolic compounds found in green-colored foods and cereal grains. However, the majority of phenolic compounds in foods are present in bound forms with low bioaccessibility which affect their poor bioavailability [157]. The structure of the fiber matrix and the functions of bioactive phenolic compounds found in green-colored vegetables or cereals strongly affect their physiological functions [158]. A low proportion of ingested conjugated HAs was shown to be absorbed in the small intestine, and the remainder reached the larger intestine and were modified or degraded by the gut microbiota [159].