Grapes are one of the oldest cultivated plants. They are classified as true berries because the fruit wall or pericarp is fleshy all the way through. The cultivation of grapes dates back more than 5,000 years in Egypt, and they were highly developed by the Greeks and Romans. Today there are nearly 200 cultivated varieties. Modern cultivars have all been derived from two main species, the European (Mediterranean) Vitis vinifera (a tight-skin grape with wine-like flavor) and the North American V. labrusca (a slip-skin grape with Concord-type flavor). In the European tight-skins, which are used for wines, the skin does not separate readily from the pulp. North American slip-skin grapes are generally more hardy than the European. The fruit is round with a more watery flesh and a thin skin that slips off very easily. The North American V. labrusca is also called the fox grape and is the source of the famous cultivar discovered in Concord, Massachusetts. Concord grapes are the most important American grape for juices, jellies and preserves. They are also used for certain wines. Some of the best wines and popular eating grapes, such as ‘Thompson Seedless’ and ‘Red Seedless’ are cultivars of V. vinifera. Sterile, triploid cultivars have been developed that do not produce seeds because of synaptic failure during Meiosis I resulting in non-viable gametes. Several varieties of grapes are dried and used for raisins. The best raisin grapes are selected for flavor, reduced stickiness and soft texture. In the United States, most raisins are produced in California’s Central Valley.
Concord grapes are used for jellies, jams and juices. Jellies are made from fruit juice, pectin and sugar. Jams contain the actual crushed fruit.
The fermentation of grapes is brought about through the action of wild yeasts which are present on the skins of the fruit (whitish powder). The maximum alcoholic content of natural wines is about 12 to 16% (24 to 32 proof). Higher alcoholic content will kill the yeast cells. Brandy is made from distilled wines and has a much higher alcoholic content (up to 140 proof). Red wines are made from grapes with colored skins (with anthocyanin), while white wines are made from white grapes (or red grapes with skins removed). In dry wines the sugar is almost completely fermented. In sweet wines fermentation is stopped before all the sugar is converted. Viticulture (the cultivation of grapes) and enology (the study of wine making) are enormous topics beyond the scope of this section of WAYNE’S WORD. They are discussed in more detail in the required textbook for Plants and People (Botany 115).
Two popular varieties of seedless grapes in California: ‘Thompson Seedless’ (left) and delicious ‘Red Flame’ (right). Grapes are considered a true berry because the entire pericarp (fruit wall) is fleshy.
A native California wild grape (Vitis girdiana) that grows in canyon bottoms and along streams in southern California. This species often forms massive vines that drape over large trees such as coast live oak (Quercus agrifolia). It intergrades with the very similar V. californica of central and northern California. Unlike the tight-skin V. vinifera of Europe, this is a slip-skin grape in which the skin readily slips off of the juicy, seed-bearing pulp (see arrow).
For years it has been known that people in France who consume red wines on a regular basis have a reduced risk of coronary heart disease compared with the United States. This data is paradoxical considering that the French also consume a lot of fatty foods, such as pastries. A phenolic compound in the grape skins called resveratrol was discovered that seems to inhibit the plaque build-up or clogging of arteries (atherosclerosis) by increasing the level of high density lipoproteins (HDLs) in the blood. Beneficial HDLs carry cholesterol away from the arteries so that it doesn’t form plaque deposits in the arterial walls. Resveratrol also reduces blood platelet aggregation or clotting (thromboses) within blood vessels. Resveratrol belongs to a class of plant chemicals called phytoalexins. They are used by plants as a defense mechanism in response to attacks by fungi and insects. One interesting phytoalexin called psoralen comes from the leguminous herb Psoralea. It has a chemical structure similar to coumarin. Psoralen has been used in the treatment of certain cancers, including T-cell lymphomas in AIDS patients.
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