Molecular Interactions of Phenolic Compounds in Relation to the Colour of Fruit and Vegetables

Abstract

Three large families of pigments give colour to plants: chlorophylls, carotenoids and flavonoids (Britton, 1983). The two latter are seen by humans on the green background provided by the chlorophylls. Carotenoids and flavonoids are widespread in fruit, flower and vegetable tissues. Flavonoids belong to a larger group of natural organic plant compounds, the polyphenols. Good recent introductions to the biochemistry of polyphenols may be found in Scalbert (1993) and in Brouillard et al.(1995). About ten thousand or so natural plant polyphenols have been presently identified, half of them probably being of the flavonoid type (Harbome, 1993). Coloured flavonoids are best seen in flower and fruit epidermal tissues. Such pigments are synthesized by almost all flowering vegetables and can be found in all plant organs. They are also found both unaltered or chemically or enzymatically modified in food products of vegetable origin and they constitute a regular component in human and animal diets. Every day about 10 g are consumed by an adult person. Among their many biological activities (Cody et al., 1986; 1988), only their prominent role in producing the most vivid colours to be found in plants will be taken into consideration. Almost all polyphenols strongly absorb light in the ultraviolet range but only a few members of this huge family do so in the visible range. In fact, the anthocyanins are the only meaningful subgroup of polyphenols visible to the human eye. Anthocyanins gradually appear in fruit, flower and other plant tissues when chlorophylls are fading away. Anthocyanins are stored, if not biosynthesized, within the vacuoles of mature …