Phenohc flavonols

Phenolic compounds are present separately from the substrates, predominantly located in the cell vacuoles. Particularly prominent substrates are caffeic acid and its esters, as well as some flavonoid substances, of which monomeric flavan-3-ols (catechins) are the most important. Other groups of phenolic compounds, such as condensed forms of flavan-3-ols and flavan-3,4-diols (tannins), flavonols, flavones, flavans, chalcones, dihydrochalcones and anthocyanins, are only partly oxidised. One of the reasons for this is probably the steric hindrance caused by the corresponding glycosides and substrate specificity of polyphenol oxidases. The content of phenohc compounds depends on genetic factors (on plant species and varieties), the degree of maturity and external (environmental) factors (hght, temperature, nutrients, use of pesticides and so on). The only substrate in animal tissues is the amino acid tyrosine. 

The main substrates of polyphenol oxidases in apples are hydrox-ycinnamic acid derivatives, especially caffeic add depsides, such as chlorogenic acid, and some groups of flavonoid compounds, such as flavan-3-ols, flavonols, dihydrochalcones and (in red-skinned apple varieties) also anthocyanins. In the flesh of apples, the main phenohcs are chlorogenic (3-caffeoylquinic) add, flavan-3-ol epicatechm, and procyanidin B2 representing condensed tannins. These three compounds represent more than 90% of the phenolic compounds present. The main substrates in apple peel are flavan-3-ols and flavonols (quercetin glycosides) and to a lesser extent derivatives of hydroxydnnamic acids are present (chlorogenic add and some other depsides). 

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