The phenolic compounds or polyphenols come from the secondary metabolism of plants. Chemically they are compounds having at least one aromatic ring to which one or more hydroxyl groups are attached. There is a great variety of phenolic compounds, and they are classified as flavonoids, formed by two aromatic rings joined by an oxygenated heterocycle and depending on the degree of hydrogenation and the substitution of the heterocycle, flavonols, flavones, isoflavones, anthocyanins, proanthocyanidins, flavanones. , etc. and they are generally in the form of glycosides, and non-flavonoids, benzoic and cinnamic compounds, commonly called phenolic acids, which contain an aromatic ring with different functional groups, and which may be forming esters with organic acids. Other polyphenolic compounds are stilbenes, tannins, lignins and lignans. Some of the properties of products of vegetable origin, such as color, astringency and aroma are due to the presence of compounds of this type. In recent years polyphenols have gained great interest because of their beneficial properties for health, especially as antioxidants.
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Two concepts of antioxidants have to be considered, on the one hand the substances that are added to foods are able to preserve these by retarding their deterioration, rancidity or discoloration, due to oxidation, and on the other hand, the compounds originally present in food and as Due to their antioxidant properties, they have beneficial effects on health.
The food industry uses antioxidants to prevent the deterioration of the quality of some products, especially those high in fat and lipids, and thus maintain its nutritional value. These antioxidants, mainly phenolic in nature, are mostly synthetic, such as tertiary butyl hydroxytoluene (BHT), tertiary butyl hydroxyanisole (BHA), propyl gallate (PG), dodecyl gallate (DG) and tertiary butyl hydroquinone (TBHQ).
Greater interest has natural antioxidants, which are natural components of foods of vegetable origin, mainly polyphenols or phenolic compounds, which are naturally in the initial products, or that are formed as a result of their processing. The flavonoids and the phenolic acids are those that receive greater attention as potential antioxidant agents, mainly due to their wide presence in a high number of foods of great consumption.
The antioxidant activity of polyphenols is due to its ability to reduce the production of free radicals, either by inhibition of the enzymes involved, or by chelation with the transition metals responsible for the generation of free radicals. In addition, flavonoids, due to their low re-dox potential, are capable of reducing highly oxidized reactive oxygen species (ROS). In general, polyphenolic compounds as antioxidants are multifunctional and act according to most of the mentioned mechanisms. Flavonoid polyphenols, such as flavonols, flavones, isoflavones, anthocyanins, flavanones, catechins and proanthocyanidins, are the most potent antioxidants present in plant foods.
One of the most important factors that determines the antioxidant activity of polyphenols is the degree of hydroxylation and the position of the hydroxyl in the molecule. Flavonoids due to their oxygenated heterocycle show higher activity than non-flavonoids. In turn, the solubility and steric effects of each molecule can be affected by the type of structure of said molecule, as is the case of glycosylated derivatives and other adducts, which can increase or decrease the antioxidant activity. Flavonoid compounds are usually found in plants in the form of glycosides, but the action of enzymes or some processes can release the corresponding aglycone. The activity of the phenolic acids is also a function of the hydroxyl groups of the aromatic ring and of the binding of these compounds to organic acids and / or to sugars to form esters. The mechanisms by which all these compounds act vary depending on their concentration and types of compounds present in food.
Oxidative stress and lipid peroxidation are the cause of a large number of chronic diseases that include cancer, cardiovascular diseases, cataracts and dementia. Some studies have shown that the consumption of fruits and vegetables can reduce the incidence and mortality of these diseases and, as far as is known, this protective effect is determined by the presence of antioxidants in these foods, mainly polyphenols. An antioxidant prevents oxidative damage by inhibiting the generation of reactive species, capturing free radicals or increasing the level of protective endogenous antioxidants.
The fact known as "French Paradox" recognizes the low incidence of cardiovascular events among the French population, despite a diet rich in saturated fats, which is attributed to a regular and moderate consumption of red wine, which contains a considerable concentration of antioxidant phenolic compounds.
In recent years, several studies on the in vivo effects of these compounds have shown that a small fraction of the polyphenols ingested in the diet are absorbed in their initial form, aglycone or glycoside, while most of them are degraded to different metabolites. Both the absorbed compounds and the metabolites that they produce show antioxidant capacity in vivo, which indicates the existence of a cascade of reactions in which antioxidants intervene differently.
Phenolic compounds are found in a wide variety of edible plants, fruits, vegetables, beverages such as tea, coffee, beer and red wine, in olive oil, in cereals and in some seeds such as legumes. The fruits contain relatively high concentrations of quercetin, kanferol, hesperetin, etc. and phenolic acids, among them cinnamic derivatives. Tea and, to a lesser extent, red wine, contain appreciable amounts of procyanidins and catechins. Vegetables such as tomatoes, onions, garlic, peppers, etc. they contain mostly high concentrations of quercetin and myricetin derivatives. The growing importance of soy consumption is associated with the presence of isoflavones, not only because of its effect as phytohormones, but also as an antioxidant. Compounds of this type are also found in other legumes such as lentils, beans and peas.
The contribution of polyphenols in the diet can be between 50 and 800 mg / day, depending on the consumption of products that contain it. An important level of antioxidants, is reached when the consumption is about 800 mg / day, which can be achieved with a diet rich in fruits and vegetables.