Grapes

The Complete Grape Growing System

The Complete Grape Growing System

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Grapes (vitis vinifera and vitis lubruscana), especially the dark red varieties, contain generous amounts of flavonoids and relatively high levels also of hydrox-ycinnamates that all exert potent anti-oxidant activities in various assay systems. The antioxidant activity of wines have received much attention owing to their possible physiological benefits. However, several of the phenolics present in fresh grapes and grape juice are also potent antioxidants in various in vitro assays, including several containing biologically relevant lipid substrates, notably human LDL. In fresh grapes and grape juices the polyphenolic compounds are primarily present as glucosides, while the phenolics in wines are principally aglycones. Glycosylation is generally considered to dampen the antioxidant potency of polyphenolics, but the available data on this are conflicting, as the impact of gly-cosylation and in turn antioxidant solubility and partitioning are very system dependent.

Depending on the variety, red grapes may contain about 100-4000 mgkg-1 of anthocyanins, 5-285 mg kg-1 flavonols, mainly rutin, 0-25 mg kg-1 flavanols, 2-25mgkg-1 hydroxycinnamates, very low levels of hydroxybenzoic acids and hardly any vitamin C, E or carotenoids (Table 3.1).31 Except for hydroxycinna-mates, where the content range is approximately the same in white and red grapes, the levels of phenolics in white grapes are about 20-25 times lower than in dark red grapes, and white grapes do not contain anthocyanins.2,31 Certain white grape varieties contain flavonols, notably rutin, at the same levels as found in red grapes. Grape hydroxycinnamates are uniquely esterified to tartaric acid, and caffeoyl-, coumaroyl- and feruloyl-tartrates are generally found in grape pulp.

Flavonoids, free hydroxycinnamates and hydroxybenzoic acids are mainly present in grape skins and seeds, but the levels and composition depend very much on the grape variety. Since the seeds and skin of grapes (especially of red grapes) are particularly rich in phenolic compounds, the extraction method employed for extracting phenols from whole grapes strongly influences the yields of phenolic substances and the antioxidant potency of the extracts. Thus, by using extended solvent contact times (24-165 hours) and crushing the seeds prior to extraction, flavan-3-ols and hydroxybenzoates - i.e. antioxidant compounds of considerable potencies - can be obtained in high levels in extracts of fresh grapes, while they are virtually absent in grape extracts produced during short extraction, for example one minute solvent contact time.2

Red wines, extracts of different types of fresh grapes, 'grape skin extract', American Concord grape juice, as well as European red grape juices, strongly inhibit human LDL oxidation in vitro and this antioxidant activity is associated with the phenolic compounds.2,47-50 Thus, not only has the antioxidant activity of similarly diluted grape samples been shown to be proportional to concentration of total phenols, but in certain cases, the antioxidant potency also correlates to the levels of different classes of compounds. Thus, the relative antioxidant potency towards human LDL oxidation in vitro is strongly correlated to levels of anthocyanins and flavonols for fresh grape extracts; for Concord grape juice and red European grape juice strongly correlated to the level of anthocyanins, and in white grape juice samples the antioxidant potency on LDL correlates to the levels of hydroxycinnamates and flavan-3-ols.2,49,50 Extracts of fresh grapes also inhibit both development of lipid hydroperoxides and their degradation to produce hexanal in lecithin in liposomes, and the relative antioxidant potency is statistically correlated with the total phenols.51 Compared to the data obtained on human LDL oxidation in vitro, the grape extracts exhibiting highest anti-oxidant activity on lecithin liposomes were those of the red table varieties (Red Globe and Emperor) and the white wine grape varieties (Chardonnay and Sauvignon Blanc);51 these extracts had only low antioxidant potency on human LDL oxidation in vitro? The removal of phenolic compounds by polyvinyl-polypyrrolidone stripping abolishes the antioxidant activity of grape juices and a mixture of representative carboxylic acids of red wine do not exert antioxidant activity.16 The presence of ascorbic acid was somewhat surprisingly found not to affect antioxidant activities of Concord grape juices on LDL oxidation in vitro.47

In contrast, equimolar addition of ascorbic acid (5 ||M) to European red grape juice samples significantly increased the antioxidant activities of the red grape juices on human LDL oxidation in vitro (Fig. 3.1).50 The phenolic profile of Concord grape juice is dominated by anthocyanins, levels range from about 300-450 mgL-1,49 where the dominant compound, which is also the major contributor to the dark, purple-bluish colour, is delphinidin-O-3-monoglucoside. In the ORAC antioxidant assay employing b-phycoerythrin as the oxidising substrate, Concord grape juice exerted the highest antioxidant activity among commercial fruit juices followed by grapefruit, tomato, orange and apple juice.52 Phenolic extracts from red grape pomace that remained after red wine production and isolated catechins and procyanidins extracted from grape seeds are all effective inhibitors of human LDL oxidation in vitro.26,33 Grape seed procyanidins also act as free radical oxygen scavengers in aqueous in vitro model systems with pro-cyanidin B2 3-0-gallate being the most potent compound.53 The pronounced

5 |M Ascorbic acid

0

150 Time (min)

Fig. 3.1 Effect of ascorbic acid addition on antioxidant activity of European red grape juice concentrate on human LDL oxidation in vitro.

5 |M Ascorbic acid c

150 Time (min)

Fig. 3.1 Effect of ascorbic acid addition on antioxidant activity of European red grape juice concentrate on human LDL oxidation in vitro.

antioxidant activity of fresh grapes and grape juices is thus attributable to different types of phenolic constituents, but the antioxidant effectiveness in different oxidation systems is correlated to distinct types of phenolics and their relative concentrations in various samples.

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