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Food Lipids: Chemistry, Nutrition, and Biotechnology

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ically found as a constituent of polymers or glycosidically linked as a galloyl ester.<br />

Gallate esters are often used as food antioxidants. Gallo- <strong>and</strong> ellagitannins are found<br />

in the leaves, fruits, pods, <strong>and</strong> galls of dicotyledonous plants [52]. These are often<br />

called hydrolyzable tannins <strong>and</strong> range in structure from simple esters to polyesters.<br />

Phenylpropanoids are derived from cinnamic acid <strong>and</strong> p-coumaric acid or their derivatives.<br />

Phenylpropanoids include lignins, lignans, neolignans, monolignols, coumarins,<br />

<strong>and</strong> hydroxycinnamic-acids <strong>and</strong> their derivatives [52]. These compounds are<br />

ubiquitous in plants. Lignans are the most abundant phenylpropanoids.<br />

a. Flavonoids <strong>and</strong> Phenolic Acids. Flavonoids (Fig. 5) are secondary products<br />

of plant metabolism <strong>and</strong> consist of anthocyanins, catechins, flavones, flavonols, isoflavones,<br />

<strong>and</strong> proanthocyanidins. Several of the flavonoids have antioxidant activity<br />

related to their ability to chelate metals. Polyvalent phenol structures in flavonoids<br />

can form complexes with metal ions. Flavonoids also act as primary antioxidants<br />

<strong>and</strong> superoxide anion scavengers [6]. These compounds are responsible for the antioxidant<br />

activity reported in many plant <strong>and</strong> spice extracts. Phenolic acids are structurally<br />

related to flavonoids <strong>and</strong> serve as precursors of their biosynthesis. Phenolic<br />

acids such as hydroxycinammic (caffeic, p-coumaric, ferulic, <strong>and</strong> sinapic acids), hy-<br />

Figure 5 Structures of some flavonoids.<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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