4.7: Alcohol
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Alcohol is a fermentation product of carbohydrate. Yeast converts sugar to alcohol, using enzymes for this conversion. The first enzymes were, in fact, discovered in yeast, and the word enzyme comes from the Greek words meaning in yeast. Alcohol has an energy value of about 200 calories per ounce (7 calories/gram).
Yeast use sugars—not starch—in fermentation. For this reason, sweet liquids, such as the juice of sweet fruits (including wine grapes) are good starters for making alcohol.
Starch must be first broken down to the sugar maltose or glucose before the yeast can use it. One way to do this is by a process appropriately called malting: Grain is allowed to germinate (sprout) for a few days; this produces enzymes that convert starch (the plant’s stored form of energy) to maltose and glucose (providing the fuel for the seedling).
There are other ways of converting starch to sugars for alcohol production. One particularly interesting method was used in Peru in the 16th century:3 Corn was ground and soaked in a pot of water. People then chewed this soaked corn, thereby breaking it into smaller pieces and mixing it with a digestive enzyme in saliva that breaks the cornstarch into maltose. Rather than swallowing the mixture, it was spit into a pot, where the action of the enzyme continued. Then, the entire mixture was boiled for several hours. This killed microbes from the saliva and concentrated the sugars by evaporating the water. The mixture was then filtered, providing a clear liquid rich in sugars, ready for fermentation.
In wine, grape juice typically provides the sugar that’s converted to alcohol; in beer, malted barley commonly provides the sugar. Such wine and beer thus have some of the nutrients that are in grapes and barley.
Wine and beer also have a limited alcohol content because yeast can’t grow once the alcohol reaches about 15-20% by volume. Wine is about 12% alcohol, beer about 4%.
In order to make high-alcohol liquors (hard liquor), the alcohol must be distilled (vaporized and condensed). Alcohol vaporizes at a lower temperature than water, so when a mixture of alcohol and water is heated, the alcohol will vaporize first, separating and concentrating it.
Alcohol content of distilled liquors is designated by proof. Doubling the % alcohol in the liquor gives its proof, e.g., liquor designated as 100 proof is 50% alcohol by volume. Brandy (~80 proof) is distilled from wine; whiskey (~90 proof) from beer; and rum (~90 proof) from fermented molasses.
Distilled liquors are not only a concentrated source of calories because of their high alcohol content, but are essentially devoid of nutrients. The small amount of nutrients in the original fermented product is left behind in the distillation process. For some people, more than half their caloric intake comes from alcohol. It isn’t surprising that people who drink a lot of alcohol are susceptible to nutrient-deficiency diseases, as well as the toxic effects of alcohol itself.
Part of our corn crop is being used to produce ethanol to partially replace gasoline (see Fig. 4-8). The U.S. is the world’s leading producer of ethanol. Brazil is second, and uses sugar cane to produce most of its ethanol. Making ethanol from sugar cane is much more energy-efficient (uses less energy) than making it from corn.
There’s much interest in developing efficient methods of making ethanol from cellulose, e.g., from corn cobs and husks. A big advantage is that it uses inedible crops (e.g., switchgrass), but the process is much harder and not yet commercially viable.