Yeast
Yeasts are microscopic one-celled organisms that are classified in the kingdom Fungi. They lack chlorophyll and hence cannot make their own food as plants do. However, they have cell walls of cellulose and thus were formerly classified as plants. The individual cells multiply rapidly by the process of budding, in which a new cell begins as a small bulge along the cell wall of a parent cell. Soon huge populations of yeast cells aggregate, especially in the presence of an abundant food source. The cells often appear as long chains, with newly formed cells still attached to their parent cells, owing to the short budding time of just two hours. Yeasts are among the few living things that are able to get their energy in the absence of oxygen. During such anaerobic conditions, compounds produced by the yeast break down carbohydrates--starches and sugars--to form alcohol and carbon dioxide gas. This process is known as fermentation, and it has been used for centuries in the production of certain foods and beverages.
Yeasts were first discovered by a French scientist, Charles Cagniard de Latour, in 1857. He realized that these tiny structures were actually living cells, which grow in number by budding and cause fermentation. Further experiments on yeast fermentation were performed by French scientist Louis Pasteur, who showed that fermentation could take place only in the presence of living yeast cells. He also deduced that anaerobic conditions were necessary for proper fermentation of wine and beer.
In the presence of oxygen, yeasts convert alcohol to acetic acid—vinegar. The fermentation process of yeast is caused by enzymes, catalysts in chemical reactions similar to the digestive enzymes in the human body. (In fact, the word enzyme means "in yeast.") Certain yeast enzymes act on starch to break down the long chain-like molecules into smaller units of sugar. Then other yeast enzymes convert one kind of sugar molecule to another. Still other enzyme reactions break apart the sugar molecule (composed of carbon, hydrogen, and oxygen) into ethyl alcohol and carbon dioxide gas. The series of reactions provides the yeast cells with the energy necessary for their own growth and division.
The production of carbon dioxide and alcohol are merely by-products of processes necessary for the yeast to survive. But these metabolic castoffs have been used in human enterprise for centuries. Yeast is traditionally added to liquids derived from grains and fruits to brew beer and wine. The natural starches and sugars provide food for the yeast, and during fermentation, the desired alcohol is released. Commonly the yeast Saccharomyces cerevisiae is added to produce alcohol in wine and beer.
Another variety of the same yeast is added to a dough made from the starchy portion of ground grains, such as wheat or rye flour. When this mixture is allowed to stand for a few hours, the yeast enzymes break down some of the starch and sugar, producing carbon dioxide. The carbon dioxide bubbles through the mixture, forming many air holes and causes the bread to rise. Since oxygen is present, no alcohol is produced when bread is rising. When the dough is baked, the air holes give the baked bread a lighter texture. In the days before yeasts were made commercially, bread makers allowed yeasts from the air to furnish the enzymes necessary for the leavening of bread. Today two kinds of yeasts are available commercially, compressed and dry. Compressed yeast contains starch, yeast, and enough moisture to start the fermentation process and thus needs refrigeration. Dry yeast is made of a mixture of yeast cells and corn meal that has been dried. It can be stored for long periods of time, giving it the advantage of a longer shelf life. Commercial yeast is made of ground grains mixed with filtered water. To this is added sprouted grain or malt, which contains enzymes that convert starch to sugar. Then bacteria are added and the combination serves as food for live yeast cells. As the yeast ferments in the mixture, it is skimmed off, pressed and made into yeast cakes.
In recent times, yeasts have been used to aid in the production of alternative energy sources. Yeasts are placed in huge vats of corn or other organic material. When fermentation takes place, the yeast convert the organic material into ethanol fuel. Geneticists are working on developing yeast strains that will convert even larger organic biomasses into ethanol more efficiently.
In 1996, scientists published the genetic blueprint of the yeast Saccharomyces cerevisiae. This marked the first time that scientists had sequenced all the genes of an organism with cells similar to human cells. The researchers found that the yeast genome contained more than 12 million nucleotide base pairs, the chemical building blocks of DNA. These bases were distributed among some 6,000 genes found on 16 chromosomes.
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