Abstract
Fruits and vegetables are an important supplier of biological active substances, such as vitamins and secondary metabolites (SM) for human nutrition, but also for further industrial applications. Pepper (Capsicum annuum) is grown and consumed all over the world as a fresh vegetable or dried as a spice. It is also used as a coloring agent, as well for medical purposes. Pepper fruits are considered as an attractive source of health-related compounds, but other organs like the leaves and stem also contain considerable amounts of antioxidants, e.g., phenolic compounds. This indicates potential for valorization of residual biomass from horticultural production by using innovative bioeconomic concepts. Herein, we present an overview about the biosynthesis of phenolic compounds, with a special focus on flavonoids and their regulation in pepper, the current knowledge of amounts and distribution of these valuable substances, as well as possible strategies for: (1) increasing flavonoid contents in pepper, (2) improving the nutritional value of fruits, and (3) new concepts for utilization of residual biomass from horticultural production.
Highlights
This study showed that flavonoid QTL hotspots exist in the Capsicum genome and that the biosynthesis of these compounds is complex and involves a wide number of genes [84]
Some authors like Kim et al [107], Marin et al [108] and Howard et al [109] used liquid chromatography to determine the total flavonoid content, based on the addition of the different flavonoid groups identified. This can be a very imprecise estimation of the true flavonoid amount since the identification and quantification is made by comparison with two or three commercially available flavonoid standards which do not fully represent the total amount of flavonoids that might exist in pepper
The authors reported that these values varied according to maturity stage, being higher when the fruit was in the immature state and decreased as the fruit went from green to red ripe state, suggesting that a change in the fruit’s flavonoid composition occurs during fruit ripening [108]
Summary
They are derive from the shikimate pathway with phenylalanine or tyrosine as intermediates [41] They are based on the structure of the family’s central element, respectively, the aromatic or phenolic ring, often in combination with a propyl group (as it is the fact for phenylpropanoids) [42]. Polyphenols appear rarely as free compounds, but can be found in the form of esters or glycosides in plants [42,43] They possess antioxidant properties, and their activity depends on the number and position of the hydroxyl groups and on the surrounding pH [44]. Aurones result from the cyclization of chalcones when a meta-hydroxyl group reacts with the α-carbon of the double bond to form a heterocycle with five atoms between ring A and B [42]. Conversion of flavanones into flavonols, and may be known as flavanonols [42]
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