Compounds of flavonoids

Flavonoids are plant compounds that are found in almost all fruits and vegetables. Flavonoids are classified into subgroups based on their chemical structure: flavanones, flavones, flavonols, flavan-3-ols, anthocyanins and isoflavones. In plants, flavonoids have long been known to be synthesized in particular sites and are responsible for the color and aroma of flowers, and in fruits to attract pollinators and consequently fruit dispersion to help in seed and spore germination, and the growth and development of seedlings.

Flavonoids are the largest group of phytonutrients, with more than 6,000 types. Flavonoids have roles against frost hardiness, drought resistance and may play a functional role in plant heat acclimatisation and freezing tolerance.

In addition, they have antioxidant, antiviral and antibacterial properties. They also regulate gene expression and modulate enzymatic action.

Flavonols are flavonoids with a ketone group. They are building blocks of proanthocyanins. Flavonols occur abundantly in a variety of fruits and vegetables. The most studied flavonols are kaempferol, quercetin, myricetin and fisetin.

Flavonoids are now considered as an indispensable component in a variety of nutraceutical, pharmaceutical, medicinal and cosmetic applications. Research on flavonoids received an added impulse with the discovery of the low cardiovascular mortality rate and also prevention of coronary heart disease.
Compounds of flavonoids

Antioxidants in fruit

Fruits, nuts, and vegetables in the daily diet have been strongly associated with reduced risk for some forms of cancer, heart disease, stroke and other chronic disease. This is attributed, in part, to their content of antioxidant phytochemicals.

It has been estimated that one-third of all cancer deaths in the United States could be prevented through appropriate dietary modification, especially by enriching diets with fruits and vegetables, because various dietary antioxidants have shown considerable promise as cancer-preventing agents.

Antioxidant vitamins including vitamin A, carotenoids, vitamin C and vitamin E, potentiate the antioxidant status and reduce lipid oxidation.

Besides the vitamin antioxidants, fruits and vegetables also contain non-nutrient antioxidant such as the flavonoids, polyphenols, and terpenes.

People who eat the highest amount of fruits and vegetables have a 20% lower risk for coronary heart disease, and the lowest risk were seen in people who ate more fruits rich in polyphenolic compounds and vitamins.

Red, blue, and purple fruits (such as apple, blackberry, blueberry, blood orange, cranberry, grape, nectarine, peach, plum, prune, pomegranate, raspberry, and strawberry) are good sources of flavonoids and other phenolic compounds that are positively correlated with antioxidant capacity of the fruit.

Orange-flesh fruits (such as apricot, cantaloupe, mango, nectarine, orange, papaya, peach, persimmon and pineapple) and some red flesh fruits (such as tomato, watermelon and pink grapefruit) are good sources of carotenoids.

Lycopene, an antioxidant available in tomatoes, has a low availability in tomato juice, but cooking tomatoes in an oil-based medium substantially enhances intestinal absorption.
Antioxidants in fruit

Vitamins as antioxidants in processed foods

Oxidation, a series of chemical reactions yielding undesirable and products (off odors, colors, and flavors), may occur in many fruits and vegetables and foods high in fat and oil during exposure to air, light, heat, heavy metals, certain pigments or alkaline conditions.

Enzymatic browning may occur in some fruits and vegetables, particularly apples, banana, peaches, pear, and potatoes, which contain phenolase enzymes. Enzymatic browning is the rapid, enzyme-mediated conversion of plant phenolic compounds into dark polyphenolic polymers.

When these fruits and vegetables are cut or sliced and exposed to air, the phenolases catalyze oxidation of phenolics compounds to ortho-quinone compounds, which then polymerize, forming brown pigments.

Fruits and vegetables must be blanched and pretreated prior to pressing, in order to inactivate the enzyme. 

Oxidation in lipids (autoxidation) and in fat and oil containing foods, on the other hand, occurs as a result of the susceptibility of fatty acids (building blocks of fats and oils) to oxidations and subsequent formation of reactive compounds referred to as “free radicals”.

The free radicals promote the development of a series of chemical reactions which lead to the production of off-flavors, colors, odors, and rancidity.

Some oxidations have more than one function. For example, ascorbic acids may function as a free-radical chain terminator, and oxygen scavenger, or a metal chelator. Under certain conditions, it may act as a promoter for oxidation.

Due to increasing concern over the potential toxicity and safety of some phenolic antioxidants, the replacement of synthetic antioxidants by ‘safe, natural’ antioxidants such as vitamins E and C flavonoids and other plant phenolic has received considerable attention.

Vitamin E compounds are reported to effectively inhibit lipid oxidation in foods and biological systems.
Vitamins as antioxidants in processed foods