Chemical Composition

The composition of any colorant containing anthocyanins will reflect the composition of the raw material. At present it is grapes and red cabbage, but other sources may become available in the future. About 275 anthocyanins are known currently and are part of about 5000 flavonoid compounds of similar chemical structure (4). The anthocyanins are composed of an aglycone (anthocyanidin), sugar, and perhaps organic acids. Twenty-two aglycones are known, of which 18 occur naturally. Only 6 are important in foods, and the structure of these is shown in Figure 1. Free aglycones occur very rarely in plants and are nearly always combined with sugars. One reason for this is that the sugars stabilize the molecule. In order of relative abundance, the sugars are glucose, rhamnose, galactose, xylose, arab-inose, and glucuronic acid. Anthocyanins may also be acylated, which adds a third component to the molecule. One or more molecules of p-coumaric, ferulic, caffeic, malonic, or acetic acids may be esterified to the sugar molecule. The aglycones in grapes are cyanidin, peonidin, mal-vidin, petunidin, and delphinidin with the organic acids acetic, coumaric, and caffeic (5). The only sugar present is glucose. Grapes usually have a very complex anthocyanin profile with the Concord variety having 31, the greatest number in any single variety. The 15 anthocyanins in red cabbage contain only cyanidin and glucose with the acids ferulic and coumaric.

Plants that contain anthocyanins also invariably contain flavonoids, and the distribution of flavonoids is more widespread than the anthocyanins. In view of the large number of flavonoids (5000), the pigment profile is usually characteristic of the plant family. A number of other poly-phenolic compounds are also present in grapes (6). The polyphenolic compounds in grapes, in view of their importance in wines, have received much more research attention than those in red cabbage but the latter can be assumed to also have a complex polyphenol profile. A typical compound from each group of compounds is shown in Figure 2. The composition of grape extracts is further complicated by polymers produced by association with catechins and flavonoids with or without the interaction with acet-aldehyde. Actually most of the tinctorial power in eno-cyanin preparations is due to the complexed anthocyanins. The overall conclusion is that it is impossible to specify accurately the composition of enocyanin preparations.

Figure 1. The anthocyanidin nucleus. Pelargonidin (4' = OH); cyanidin (3',4' = OH); delphinidin (3',4',5' = OH); peonidin (4' = OH, 3' = OMe); petunidin (4',5' = OH, 3' = OMe); malvidin (4' = OH, 3',5' = OMe); all other substitution positions = H.

Figure 2. Typical components of grape colorants extracts, (a) A red anthocyanin, mal-vidin-3,5-diglucoside; (b) a yellow flavonoid, quercetin-3-rhamnoglucoside, (rutin); (c) a component of tannin catechin; (d) a phenolic acid, caffeic acid; (e) a stilbene phytoalexin, resveritol.

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