32.1 Yeast and what it does

Yeast is a fungus made of one cell. It multiplies by budding, and in the right conditions it ferments. How it turns sugar into alcohol and gas is explained in the fermentation chapter. see chapter 26: Alcohol and gas

People have made risen bread for a very long time. The early Chaldeans, Egyptians, Greeks and Romans all let their doughs ferment, but the results were hard to predict and control. In the 19th century the French scientist Louis Pasteur studied the causes of disease and how to prevent it. That work laid the ground for the yeast that factories grow and sell today.

Kinds of yeast

Wild yeast floats in the air on specks of dust, and it lives in flour and on the skins of fruit. It forms spores more quickly than grown yeast, but it does not behave the same way twice, so it is no good for fermentation that must be controlled.

Compressed yeast, also called fresh yeast, comes from one chosen strain that tests have shown to be strong and steady. It is made in stages.

• It is grown in a sterile place, away from other yeasts, and fed on wort: a mix of cleaned molasses or malt, nitrogen-rich matter and mineral salts.
• When it is mature, machines spin it away from the used wort.
• It is chilled, filtered and pressed into blocks.
• The blocks are cut, wrapped and kept cold.

It is sold in 454 g (1 lb.) blocks, and in 20 kg (45 lb.) bags for large bakeries. It is mostly water, about 68% to 73%.

Active dry yeast comes from a different strain. It is grown the same way, then mixed with starch or another absorbent and dried, down to about 7% moisture. It was first made after the Second World War, for the armed forces, home cooks, and places with no fresh yeast.

• It is dry, but it is still alive. For best results keep it closed and refrigerated below 7°C (45°F). If there is no refrigerator, keep it unopened somewhere cool and dry, and only for a limited time. Let it come up to room temperature slowly before you use it.
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• Soak it before use for about 15 minutes, in water weighing at least four times as much as the yeast, at 42°C to 44°C (108°F to 112°F). Never let that water drop below 30°C (86°F), and never use the yeast until it has fully dissolved.
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• To stand in for 2.2 lb. (1 kg) of compressed yeast, use about 550 g (20 oz.) of active dry. For every kilogram of dry yeast, add another kilogram of water to the mix. Bakers now seldom use it, because instant yeast has taken its place.

Instant dry yeast does not need soaking first. You can stir it into the flour before the water goes in, or add it once the other ingredients have mixed for one minute.

• It can be dissolved if you wish. Some makers say to use five times its weight of water, between 32°C and 38°C (that is, 90°F and 100°F).
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• To replace compressed yeast, most formulas use one part instant to three parts compressed, and some use one to four. In place of each kilogram (2.2 lb.) of compressed, use about 250 g to 300 g (9 to 11 oz.) of instant in bread dough, or about 400 g (14 oz.) in a rich Danish dough. Add a little extra water too, roughly the weight of compressed yeast you left out minus the weight of instant you used. The packet gives exact instructions.
• It holds about 5% moisture and comes in vacuum pouches. Sealed, it keeps about a year at room temperature with no clear loss of gassing power. Once opened it becomes a grainy powder; refrigerate it and use it by the best-before date. It takes up less space and keeps longer than compressed yeast.

Cream yeast is a soft, runny yeast. Only large commercial bakeries use it, pumping it straight into the dough.

Yeast food is not a yeast. It is a blend of mineral salts, such as a calcium or ammonium salt with potassium iodate, used to condition bread dough and make it ferment faster. It tightens the gluten, which helps most where the water is soft, and it improves how the bread looks and tastes. Retail bakers rarely use it.

What yeast does in dough

Yeast has two jobs in a dough. It turns sugar into carbon dioxide gas, which lifts the dough and fills it with air. It also softens and conditions the gluten, so the dough can stretch evenly as the gas builds up and still hold it in. A baked product made with yeast has more volume and better flavor, texture, grain and color.

Flour, water and yeast together hold everything fermentation needs: soluble protein for building new yeast cells, and sugar to feed them. Enzymes in the yeast first change complex sugar into invert sugar, which the cells take in and turn into gas and alcohol. Meanwhile, other enzymes from the yeast and the flour break soluble starch down into malt sugar, and then into sugar the yeast can ferment. That keeps the gas coming.

Handling and storing yeast

Good compressed yeast is moist and firm. It breaks cleanly without crumbling and smells fresh and fruity. As it ages it turns into a sticky mass that smells of cheese. It weakens slowly even in the refrigerator, and you cannot always tell by looking whether it is still strong enough. How well it works depends on its age and on the temperature and humidity it was kept in.

• The best storage temperature is −1°C (30°F). Yeast does not freeze at that temperature and keeps its full strength for up to two months.
• Use the oldest yeast first.
• Leave space between packs on the refrigerator shelf. Yeast is alive and keeps breathing, slowly in the cold and quickly in a warm shop, so packed tightly it can overheat. With no food, breathing yeast starves and dies.
• Yeast that has been frozen and thawed must be used at once. Freezing weakens it, and it cannot be frozen again with success.
• Take out only what you need, and never leave compressed yeast on the bench for more than a few minutes; at room temperature it spoils fast. Some bakeries keep a small cooler on the bench to save trips to the refrigerator.
• Keep it wrapped at all times. Air turns the edges and corners brown, a fault called air-burn.

Getting yeast into the dough

Many bakers crumble compressed yeast straight into the dough. The older way is to dissolve it first in lukewarm water. That water must never go above 50°C (122°F), because heat kills yeast cells. As a rule, keep salt from touching the yeast directly, because salt draws water out of it.

Temperature sets the pace of the yeast.