Yeast

How does yeast work?

First, what is it? Yeast is a “thallophyte,” a complete, smaller-than-we-can-see one-celled fungi. Each cell is the same, i.e. as they grow, they don’t become differentiated to form more complex organisms like the plants that we can see. The name for all strains of bakers’ yeast (with the exception of sourdough or wild yeast) is Saccharomyces cerevisiae.

The tiny pellets you see in a container of dried yeast are not single yeast cells, but agglomerations of many cells. It takes about 25 billion of them to make a gram (1/28 of an ounce). One yeast cell may be tiny, but in huge numbers, they can certainly make their presence known — primarily by their waste products, which are very valuable to the baker. The most important ones are carbon dioxide, which leavens bread; and alcohol, which creates its flavor. Yeast definitely has its likes and dislikes, which have a large impact on how it grows and reproduces. By knowing a little bit about these, we can help or hinder yeast. But even in the worst of conditions, it takes a lot to kill it.

A mixture of flour and water creates one of the most favorable of growing conditions for yeast. It provides a good source of the carbohydrates and other minerals that yeast needs to grow. But beyond this, yeast also needs enough oxygen, the right temperature, and an appropriate pH. What are the optimum living conditions for yeast? The temperature should be somewhere in the range of 75°F to 90°F, although a dough can stray somewhat above or below these temperatures without undue effect. In fact, many people like to proof their doughs at temperatures that are cooler than 75°F, as it creates a bread with fuller flavor (even if it isn’t an optimum temperature for the yeast).

If actively fermenting yeast is too cold for too long, the yeast cells slowly die. Thus if you freeze a dough, it will remain viable only for a couple of months. On the other hand, yeast that is dormant, or not actively fermenting, can remain in the freezer (above 0°F, and not self-defrosting) for years at a time.

At the other extreme, yeast cannot tolerate temperatures in excess of 140°F (e.g., yeast does not survive the baking process, as the internal temperature of a fully baked loaf of bread is 190°F). People who feel they are sensitive to yeasts need not be concerned about the yeast in bread, as it is no longer viable after the bread is baked.

Yeast also needs access to the sugars and minerals in dough, and while it ultimately has the ability to break these down itself, bakers sometimes add a small amount of sugar to start the process. In addition, the pH of the liquid plays an important role. “Soft” (alkaline) water is relatively free of minerals. Because yeast has its own characteristic mineral content, it wants a growing medium that’s similar. So it doesn’t like soft water. When that’s all it has, it grows very slowly and attacks the protein in the flour, looking for what it needs, which produces a limp, slack dough. Hard (acidic) water, on the other hand, contains lots of minerals and yeast will grow very quickly when it has access to such abundance — up to a certain point, where it overdoses and comes to a grinding halt.

The home baker clearly has the ability to create an environment that any yeast cell will love. The only “iffy” one is pH. A small amount of ascorbic acid (vitamin C) can help correct water that is too soft. Slightly more yeast can help overcome water that is too hard.

Yeast exploré

Domestic yeast is one that has been developed from wild yeast, but is 200 times stronger. How do manufacturers change a wild yeast into a domestic yeast? Just as it’s done with plant and animal genetics and breeding. They identify certain characteristics that they decide are desirable; isolate them, and then replicate them. The resulting yeast is given a “training” diet to make it replicate, then the cells, now in high density, are filtered, dried, measured into appropriate sizes, packaged, and sent off to the market.

There are several types of yeast available to home bakers today. Here’s how to make sense of the ones most of us have probably heard of.

Cake or compressed yeast

This was the original “domestic yeast,” moist, mushroom-colored, clay-like in texture and reasonably perishable. It’s what our mothers and grandmothers used, and what most commercial bakers use today. Cake yeast will keep, refrigerated, in an airtight container for about a week. If your recipe calls for cake or compressed yeast, you may substitute 1/4 ounce (2 1/4 teaspoons) dry yeast for every ounce (cake) of compressed yeast.

Active dry yeast is live yeast that’s been dried, a process that kills up to 70 percent of the yeast cells. These dead cells surround the live cells, acting as a cocoon to protect them. For this reason, you must “proof” active dry yeast — dissolve it in water, to expose the live cells — before baking with it. Yeast in this form was developed just before World War II to provide fresh bread for American troops: fresh yeast wouldn’t survive long-distance shipping and remain viable. Active dry yeast is much more stable than cake yeast and will keep, in an airtight container, almost indefinitely in the freezer (above 0°F), or for several weeks refrigerated. To substitute for cake yeast, use about 40 percent by weight.

Instant yeast is also live yeast, but it’s been dried at a much lower temperature, and using a different process. Only about 30 percent of the cells are dead, and therefore it begins to work much faster than active dry yeast (though active dry yeast will eventually catch up over the course of several hours’ fermentation). Mix instant yeast with your dry ingredients; there’s no need to proof it first. While the instructions say you can use less, we never bother to make the conversion, just use whatever amount the recipe calls for. Instant yeast is particularly favored by bread machine users and is often identified as “bread machine yeast.” It, too, can be kept for long periods in the freezer. Following are three types of instant yeast.

Cream yeast

This is a fairly new form of yeast that’s developed a following at the wholesale level, because it’s easy to handle, easy to weigh, and is highly active. It’s not available to home bakers. But if you can cadge some from a bakery, you need to use about 1 2/3 as much cream yeast as compressed yeast.

Yeast: active dry vs. instant

You may substitute active dry yeast for the instant yeast called for in our recipes without making any changes in the amount; if the recipe calls for 2 teaspoons of instant yeast, use 2 teaspoons of active dry yeast. (The official word on instant yeast is that you can use 75% the amount of instant yeast to replace active dry yeast in a recipe, but we never bother with the conversion; we find it all tends to even out in the end.)

To “proof” active dry yeast, dissolve it in a few tablespoons of the liquid in your recipe, along with a half teaspoon or so of sugar, or a tablespoon of flour. Wait 10 to 15 minutes; if you don’t see any activity (small bubbles forming), try some newer yeast. If the yeast is active and producing bubbles, add the liquid mixture along with the other liquid ingredients.

Active dry yeast is a little bit slower off the mark than instant, as far as dough rising goes; but in a long (2- to 3-hour) rise, the active dry yeast catches up. If a recipe using instant yeast calls for the dough to “double in size, about 1 hour,” you may want to mentally add 15 to 20 minutes on to this time if you’re using active dry yeast. When dough is rising, you need to judge it by how much it’s risen, not how long it takes; cold weather, low barometric pressure, how often you bake, and a host of other factors affect dough rising times, so use them as a guide, not an unbreakable rule. Remember, bread-baking involves living things (yeast), your own personal touch in kneading technique, and the atmosphere of your kitchen; there are so many variables that it’s impossible to say that “Dough X will double in size in 60 minutes.” Baking with yeast is a combination of art, science and a bit of magic; stay flexible, and your bread (and you!) will be just fine.

My yeast isn’t working!

What makes yeast “misbehave”? There are a couple of potential explanations for what’s happening when yeast doesn’t produce the desired effect. The first is pretty easy. In spite of the fact that yeast is newly purchased, it may not have been stored or rotated correctly prior to your purchasing it so that it isn’t, in fact, as new as you think it is. A vacuum-sealed bag of yeast stored at room temperature will remain fresh indefinitely. Once the seal is broken, it should go into the freezer for optimum shelf life. A vacuum-sealed bag of yeast stored at high temperatures, however—e.g., in a hot kitchen over the summer, or in a hot warehouse before delivery—will fairly quickly lose its effectiveness. After awhile, if stored improperly, yeast cells will slowly become inactive (die). If you aren’t using your yeast fairly quickly (or even if you are), it’s a good idea to keep it in an airtight container in the freezer. It will keep for quite long periods of time that way (years in the freezer, if your freezer isn’t self-defrosting). If you think you got some bum yeast, take it back to the store and ask for a new batch. Any business worth its salt will certainly replace it.

The second reason may be this: When new yeast is dried after production, there’s a certain mortality among yeast cells. Regular active dry yeast contains a much higher percentage of inactive (dead) yeast cells than does instant yeast. If you’ve been using instant yeast and have gone back to active dry, you may feel that the yeast is “slow,” when in fact it’s just acting as active dry normally does.

In either case, there’s certainly no harm in using more yeast, if that’s what it takes to get what you think is the appropriate response.

Up-sizing

Unlike many baking recipes, you can increase the size of most bread recipes simply by doubling, tripling, etc. all of the ingredients. The exception is the yeast; if you increase the amount of yeast at the same rate you increase everything else, you may find yourself with a lot of dough on your hands and not enough time to deal with it. For example, by the time you’ve shaped the eighth loaf, the first may be well on its way to doubled in size; eventually you’ll start feeling like the Sorcerer’s Apprentice!

Most home bread bakers prefer to stick with 1 tablespoon of yeast, for up to eight loaves, and just giving the bread a longer, slower rise. Not only does this improve the flavor, it slows down the rising dough so that you can work with it more effectively.

Fitting bread to your schedule

The amount of yeast you use in your bread dough has a significant bearing on how quickly it’ll rise, and thus on your own schedule. By reducing the yeast, you insure a long, slow rise rather than a series of quick rises and resultant falls. The more yeast in a recipe initially, the quicker it reproduces, and the more alcohol it produces (alcohol is a byproduct of yeast growth). The alcohol, being acidic, weakens the gluten in the dough, and eventually the dough “poops out” and won’t rise, or won’t rise very well. By starting with a smaller amount of yeast, you slow down the cycle of reproduction, thus lowering the amount of alcohol produced, thus insuring the gluten remains strong and the bread rises well.

You can reduce the yeast in most types of bread recipes (sweet breads being the exception) to produce a dough that will rise slowly over a long period of time, rather than one which rises for an hour in the bowl and half an hour in the pan before baking. This long rise is often much more convenient than the regular, short-rise method. (Isn’t it easier to mix up a batch of slow-rise dough in the morning and let it rest most of the day, while you go about your business, rather than having to “baby-sit” a batch of short-rise dough for the 1 1/2 or 2 hours it usually takes to go from bowl to oven?) In recipes calling for a packet of yeast (2 1/4 teaspoons), we recommend cutting the amount back to 1/2 to 1 teaspoon of instant yeast, depending on how long you want to let the dough ferment before the final shape-rise-bake process; 1/2 teaspoon would give you lots of flexibility, such as letting the dough “rest” for 16 to 20 hours; 1 teaspoon would be a good amount for an all-day or overnight rise (10 hours or so, at cool room temperature). If you’re using active dry yeast, which isn’t as vigorous as instant yeast, we’d up the range to 3/4 to 1 1/2 teaspoons.

Whole-grain doughs are naturally slow rising, due to the bran in the grain, which interferes with gluten development. If you’d like to slow down a familiar whole-grain recipe, then do cut back on the yeast; but if you’re making a particular whole-grain recipe for the first time, we recommend using the amount of yeast indicated, and seeing just how long it takes the dough to rise fully. Often it takes longer than the directions say, and there’s probably no need to slow things down even more.

Basic flour-water-yeast-salt doughs (which may also contain a bit of oil and/or sugar), such as those for baguettes, focaccia and pizza, are the best candidates for an all-day countertop rise. Keep in mind, however, the vagaries of your own kitchen. If you bake bread all the time, your kitchen is full of wild yeast and any dough you make there will rise vigorously. If you seldom bake bread, or are just beginning, your kitchen will be quite “sterile;” your dough won’t be aided by wild yeast, and will rise more slowly than it would in a more “active” kitchen. We’ve found that here in our King Arthur kitchen, where we bake bread every day, we can cut the yeast back to 1/16 teaspoon in a 3-cup-of-flour recipe and get a good overnight rise. In a kitchen where bread is seldom baked, we needed 1/2 teaspoon of yeast to get the same effect. Use your judgment in rating your own kitchen as to “yeast friendliness.”

Keep in mind, also, that this slow rise usually extends to the shaped loaf, as well as dough in the bowl. Once you’ve shaped your loaf, covered it, and set it aside to rise again, it may take 2 hours or more, rather than the usual 1 to 1 1/2, to rise fully and be ready for the oven.

There is no hard and fast rule for the amount of yeast you should use in any particular recipe. It depends on how slow (or fast) you want the dough to rise; the composition of the dough itself (whole-grain, sweet/dairy, or “straight”); and your kitchen. Be flexible, and experiment; you’ll soon discover the formula that’ll work just right for you, producing a ready-to-shape dough when you’re ready to shape it.