## 2 Bioactive Compounds Commonly Found in Green-Colored Fruits and Vegetables
### General Characteristics, Structure, and Food Sources
The most common ester derivatives of GA are alkyl esters such as methyl gallate, propyl gallate, octyl gallate, dodecyl gallate, tetradecyl gallate, and hexadecyl gallate. Some of the main catechin derivatives of GA are EC, ECG, EGC, EGCG, and gallocatechin gallate [181]. **Protocatechuic acid (PCA)**, 3_A_-dihydroxybenzoic acid, is a phenolic compound found in various green food plants such as calamondin, olives, roselle, and white wine grapes [182]. It is the main secondary metabolite of antioxidant polyphenols and the main metabolite of anthocyanins [183; 182]. The molecular formula of protocatechuic acid is (HO)\({}_{2}\)C\({}_{6}\)H\({}_{3}\)COOH. It exists as white-to-brown crystalline powder that changes color in the air. **Syringic acid (SA)** is a phenolic compound readily found in fruits and vegetables such as olives, dates, pumpkin, and grapes, spices, acai palm, honey, and red wine [184]. It is a white powder with high solubility in ethanol, methanol, and ethyl ether and poor solubility in water. Like other phenolic compounds from plants, such as PCA, gallic acid, and QA, SA is also synthesized by the shikimic acid pathway [185]. SA consists of a single benzene ring linked with two OCH\({}_{3}\) groups, one OH group, and one COOH group, and its molecular formula is C\({}_{9}\)H\({}_{10}\)O\({}_{5}\) (Table 2) [184]. **Vanillic acid (VA)**, 4-hydroxy-3-methoxybenzoic acid, is a benzoic acid derivative, and its molecular formula is C\({}_{8}\)H\({}_{8}\)O\({}_{4}\). The solution process of VA in the solvents is endothermic and spontaneous [186]. It has a pleasant and creamy odor; therefore, its widely used as a flavoring agent, preservative, and food additive in the food industry. It is a phenolic compound that is oxidized from vanillin, which is obtained from several foods, such as avocado, green olive, _Angelica sinensis_, green tea and many other plants [187; 188; 189]. Bioavailability  
Many beneficial biological effects of hydroxybenzoic acids, including GA, PCA, SA, and VA, on the treatment and prevention of cardiovascular and metabolic diseases have been documented [157; 191]. Meanwhile, the health beneficial properties of phenolic acids can be influenced by various factors such as poor stability, restricted bioavailability, and absorption [9; 15; 179]. The bioavailability of different phenolic acids varies and does not reconcile with their ubiquity. Previous animal and human trials have shown that GA can be absorbed in the body, and its effectiveness is hindered because of its rapid metabolism and elimination from the body [180]. After oral administration, approximately 70% of GA is absorbed and then excreted in the urine as 4-O-methylgallic acid [15; 160; 192]. Thus, the usage of GA is restricted because of its low bioavailability and rapid elimination in both animals and humans. Therefore, efforts have been made to enhance its bioavailability. For example, repeated dosing, use of structural analogs or derivative compounds, phospholipid complexation, and microencapsulation have been tested to improve the plasma levels of GA [15]. The water-insolubility of SA could result in a low systemic bioavailability as with other water-insoluble drugs such as other conventional clinical drugs (e.g., simvastatin and fenofibrrate) [193]. Despite its promising biological effects, the poor solubility, fast elimination, and concomitant low bioavailability of SA are the major hurdles for its application. Therefore, efforts have been made to improve its bioavailability by employing emerging technologies, such as incorporation of SA into liposome or micelles and utilization of self-microenulisifying drug delivery systems [194; 195]. Recently, [PERSON] et al. demonstrated that the incorporation of SA in liposomes enhanced its oral bioavailability and antioxidant properties in rats [30]. In addition, another study reports that a nano system for the oral delivery of SA improved its bioavailability and enhanced its hypolipidemic effect in diet-induced hyperlipidemic mice compared with an SA suspension-treated group [194].