Technological Importance of Phenolic Compounds

The taste of fruits is influenced by phenolic compounds. The presence of tannins yields an astringent, harsh taste, similar to an unripe apple (or an apple variety suitable only for processing). Table quality apples are low in phenolic compounds. Flavanones (naringin, neohesperidin) are the bitter compounds of citrus fruits. 

Phenolic compounds are substrates for polyphenol oxidases. These enzymes hydroxylate monophenols to o-diphenols and also oxidize o-diphenols to o-quinones (cf. 2.3.3.2). o-Quinones can enter into a number of other reactions, thus giving the undesired brown discoloration of fruits and fruit products. Protective measures against discoloration include inactivation of enzymes by heat treatment, use of reductive agents such as SO2 or ascorbic acid, or removal of available oxygen. 

Polyvalent phenols form colored complexes with metal ions. For example, at pH 4, Fe + forms 

The compounds responsible for the typical aromas of different grape varieties has not been clarified in each case. Esters contribute to the fruity notes. The flowery-fruity aroma note of American grapes (Vitis labrusca) is based on 2-aminobenzoic acid methylester (methyl anthranilate), which is not found in European varieties, 2-Isobutyl-3-methoxypyrazine is responsible for the green paprika-hke aroma note of Cabernet Sauvignon grapes. 

Aroma compounds contribute significantly to the importance of fruits in human nutrition. The aroma substances of selected fruits will be outlined below in more detail. The structures and synthesis pathways of common aroma substances are explained in Chapter 5. 

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