Kathy Pikolas, age 11, of Lancaster, Pennsylvania, for her question:
How do they make yeast?
Yeast, believe it or not, is alive, and the only way to get more of it is to coax it to multiply. A wad of yeast is a mass of single celled plants, too small for our eyes to see individually. They teem everywhere in nature, in air, in soil and moisture. There are numerous different species, most of them shaped like short fat sausages or round balls. Some types live as parasites in animals and other plants. But a few species perform wonders to our bread and bakery foods.
Yeasts are single celled cousins of the fungus plants and our useful species are cultured, much as a farmer cultivates his plant crops. Like mushrooms, the tiny cells need warmth and moisture, shade and a very rich diet. They also need air to digest their food, to grow and multiply. When all the conditions are just right, one pound of yeast may become 30 pounds of yeast in about 12 hours.
Most commercial yeasts are cultured from types that are suitable for bread making and bakery duties. As a rule, the culture is made in a vat big enough to hold more than 300 gallons. It is fitted with a pipe to add air to the culture and more pipes to keep it warm. The food for the thriving yeast cells must be rich in sugars with smaller quantities of nitrogen. In yeast culture, this soupy mash is called wont.
As a rule, a culture is begun by pouring 20,000 gallons of wort into the vat. The main ingredient may be sugary molasses with a helping of ammonium salts to provide the nitrogen. Yeasts cannot digest starches, which are cheaper than sugars. But sometimes corn is cooked, mashed and mixed with barley, malt and rye. When sprouted barley is added, the starchy cereal converts into malt sugar. And malt sugar supplies the yeast with a balanced diet that needs no added ammonium salts.
The first helping of food is very thin and watery. Into the mixture goes a 300 pound wad of milky white yeast. The tiny oval cells gorge themselves, thrive and grow. In about two hours, each one sprouts a bud that becomes a duplicate daughter cell. It remains attached to the mother cell and the original yeast doubles. The culture is ready for more food.
The second feeding is about 800 gallons of more concentrated wort. Air is piped in and the culture kept warm. In about two hours the mothers and daughters sprout new cells. As they continue to multiply, smaller helpings of food are added in more concentrated form. After 10 or 12 hours, all the food in the vat in converted into yeast cells. These are filtered from the liquid, drained and packaged. The dry yeast stops multiplying because it has no food or moisture.
The dry yeast wakes up when we soak it in warm, sugary water and it continues to grow in warm, unbaked bread. As it thrives, it converts the food into alcohol and carbon dioxide, filling the dough with spongy bubbles. In the oven, the alcohol disappears and the baked yeast cells add their rich protein ingredients to the bakery goods.