## 3 Bioactive Compounds in Fresh Fruits and Vegetables
### Bioavailability of Bioactive Compounds  
Bioactive compounds are known to modulate the metabolic processes, through their antioxidant, antimicrobial, anticancer, or anti-inflammatory activities. These functions vary with the metabolic pathway and bioavailability in the body, as well as chemical structure and amount of dosage [3; 9]. This means that bioactive compounds need to be bioavailable to exert beneficial health effects. Bioavailability can be defined as the fraction of a given food that the body can utilize, providing nutritional efficacy [56]. This is a crucial point to ensure the efficacy of bioactive compounds as limited bioavailability hinders the use of bioactive compounds as functional ingredients [9; 57]. In this context, bioaccessibility, the fraction of a compound available for intestinal absorption [58], depends on several factors, including solubility, transporters, molecular structures, and metabolizing enzymes [9; 59]. Bioaccessibility is also influenced by the composition of the food matrix and its physico-chemical properties, such as pH, temperature, and texture of the matrix [56]. The caloric content and the volume of the food matrix, for instance, can cause physiological changes in the gastrointestinal tract, affecting the bioaccessibility of digested compounds [56]. The different mechanisms of absorption are another factor that influences the bioavailability offood components. The absorption of bioactive compounds can be influenced by the interaction with other dietary ingredients, molecular transformations, transporters, metabolism, gut microbiota, and the solubility of lipophilic and hydrophilic compounds [35]. The molecular structure of a bioactive compound and its isomeric configuration can also affect its absorption considerably. Furthermore, the rate and extent of absorption can vary widely between individuals, depending on key factors, such as diet, genetic background, the composition of gut microbiota, and physical activity. All these factors result in changes to bioavailability [9]. The complex nature of bioactive compounds also has an important role in the bioavailability of food components [35]. For instance, the physiological activity of polyphenols does not depend directly on their prevalence in the human diet [35]. These compounds are, in fact, relatively poorly absorbed (0.3% to 43%) [60], thus limiting their biological effect on the body. The low intestinal absorption of polyphenols is determined by their chemical structure, while molecular weight, glycosylation, and esterification determine their bioavailability [61]. The source of food determines the form in which polyphenolic compounds occur [35]. Many phenolic aglycones are hydrophilic and can be absorbed through biological membranes by diffusion. However, most polyphenols occur in the glycosidic form, affecting their absorption [35]. Phenolic acids from the hydroxybenzoic family occur in low concentrations in food and it has been demonstrated that gallic acid is very well absorbed in the stomach, small intestine, or both [62]. The hydroxycinnamic acids (e.g., caffeic, ferulic, synaptic, or p-coumaric acids) often occur in their esterified form with quinic and tartaric acids or carbohydrate derivatives, being absorbed in the colon [61,63]. When ingested in their free form, they are rapidly absorbed from the stomach or small intestine [35]. Regarding flavonoids, the type of the substituted sugar moiety is an important factor and most flavonoids, except catechins, occur naturally in plants in conjunction with \(\beta\)-glycoside sugars [64]. The first step in the metabolism of flavonoids is the hydrolysis of the glycosidic bond, which may take place in the lumen of the intestine or enterocytes directly before absorption. Moreover, the efficiency of absorption is reduced when bacterial microflora is involved in the flavonoid metabolism and some studies have proved that the activity of microflora can make certain metabolites more active than their precursors [65].