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Peggy O'Leary, age 10, of San Francisco, California, for her question:     

How do they make packages of yeast?

The yeast we use to make bread may come wrapped in a neat, air tight package. In the refrigerator, it stays this way for a while. But when we open the package and mix the yeast with the bread dough, it starts to multiply at a great rate. As it thrives, it tenderizes the heavy dough with numerous spongy bubbles.

We share our world with goodness knows how many wild yeasts. A few types have been domesticated to make breads, other foods and various beverages. All the yeasts are single celled plants of the fungus family. Like mushrooms and other fungus plants, they have no green chlorophyll to manufacture their own basic sugary food.

This lack of chlorophyll governs the life style of all the fungi. It determines what the yeasts must do to survive. It also gives us a chance to store the useful types in packages. The single cells are shaped like miniature footballs or baseballs, too small to be seen. In the wild, they thrive by absorbing nectars and malts, plus a varied menu of other organic sugars and starches.

Their nourishment must be served in liquid form and most yeasts do best in hot summery weather.. Then they gorge and multiply. In most cases, a well fed yeast cell sprouts a bud that becomes a daughter cell. Some types produce spores that become more cells. A few multiply when a cell divides into a pair of identical twins. When food is plentiful and other conditions are suitable, all yeast cells keep increasing at a great rate.

However, these thriving fungi are prepared for periods of famine. This occurs when there is no food or moisture or perhaps the air and temperature are not just so. Then the yeast cells become dormant and do nothing    nothing at all. When conditions improve, they resume their busy activities. Yeast manufacturers simply arrange for the cells to become dormant    then wrap them up in moisture proof packages.

Through the ages, bakers have selected the best yeasts for the job. A smallish sample is started in a huge vat of warm liquid food and air. The basic food may be molasses. The exploding yeast population may be skimmed off or whirled away from the soupy mixture.

When this newly grown yeast is washed and dried, it is deprived of food and other things it needs to keep multiplying. It promptly becomes dormant. In this stage it is pressed into cakes and sealed into packages. These yeast cells may sleep quietly for several weeks. But when we mix them with warm moist dough, they wake up and continue to feast and multiply.

Packaged yeast stays useable for a while if we keep it chilled. Some bakers prefer to use loose granules of dry yeast because it keeps longer. The batch of yeast is made in the same way. But the cells are mixed with corn meal. As usual, the cells are reactivated when mixed with warmish water, flour and maybe a little sugar.

 

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