Polyphenols are a diverse group of chemicals found in plants, some of which may reduce the chance of developing cancer and heart disease if consumed regularly in the diet. Flavonoids are the most widely studied of the polyphenols, but other groups with potential health benefits include stilbenes (e.g. resveratrol), benzoic acids (e.g. gallic acid), the hydroxycinnamic acids (e.g. caffeic acid) and the gallotannins (e.g. pentagalloylglucose). Because polyphenols have a wide variety of structures, different polyphenols can have quite different effects on health. Processing foods can cause changes in the quantities of individual polyphenols present and can also cause chemical alteration to the molecular structures. While processing is generally acknowledged to be detrimental to nutritional content, some interesting polyphenol derived compounds that are beneficial to health have been identified and researched in some foods.
Generally polyphenols are unstable and as a result their concentrations decrease during processing. For example, catechins in rhubarb, broad beans and cooking pears decrease by 28, 58 and 26 % respectively in response to cooking in traditional recipes. Some polyphenols are associated with the seed or peel component of fruits. For example, anthocyanins are found only in the red varieties of apples and grapes, and tend to accumulate in the peel. In grapes, proanthocyanidins are found in high concentrations in the seeds. In apples, hydroxycinnamic acids, flavanols and dihydrochalcones are present in much higher concentrations in the peel than the flesh. Therefore peeling apples or de-seeding grapes, decreases the polyphenol content of that food. Red apple and grape juices will tend to have different polyphenol composition than white juices from the same fruit.
In plants, enzymatic oxidation of the cell occurs as soon as the integrity of the cell is broken. Browning is seen as the end result of this oxidation processing and this can have an influence on the final polyphenol content in processed fruits and vegetables. In some instances oxidation in a necessary component of the final product. For example, oxidation occurs during the lengthy fermentation process that produces black tea. In the case of black tea, the enzyme polyphenol oxidase catalyses the reaction that converts the flavan‑3-ols in the tea leaves into thearubigins and theaflavins, which are responsible for the dark colour of black tea; as well as smaller quantities of theaflavic acids and theasinensins. In contrast, green tea does not undergo this oxidation process and as a result maintains a much high concentration of the original flavan-3-ols.
Chemical reactions involving polyphenols are also important in wine making because they lead to changes in the colour and taste. As the wine ages, the polyphenol content of the wine also changes and this can lead to complex formations of polyphenols not present in the original grape. The polyphenol content of wine has been extensively studied but much is still not known about the chemistry of wine. Grape polyphenols consist of anthocyanins, flavonols, hydroxycinnamic acids, flavan-3-ols and proanthocyanidins. In an extract of two years old wine, these polyphenols represented only 50 % of the content of the wine, with the other 50 % being represented by unknown phenolic acids that originate during the wine making process. As the wine ages, the percentage of the original grape polyphenols present in the wine diminished further.
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