All About Leavening
PUT SIMPLY, leavening is the difference between a brick and a bread, cake, or pastry (cookies are a different story entirely). By way of background, the word itself comes from the Latin libero, which means "to raise," and is the ancestor of words like levain and levadura (French and Spanish respectively for leavening), "levitate," "alleviate," "lift," "libation" (raise the cup) and "liberty." Which makes perfect sense when you consider that leavening is both the agent and the process that elevates (also a libero descendant) the dough.
Although there nominally are three ways to leaven baked goods, all depend, at the end of the day, on steam, which at its standard boiling point of 212°F/100°C expands to almost 1,700 times the volume of the water that produced it.
Two of the three methods - microbial and chemical - use leavening ingredients that generate CO2 and form bubbles in the dough or batter. Mechanical leavening, on the other hand, is based on either incorporating a great deal of air into a batter, as in classic sponge and pound cakes; or creating thin, flexible layers of dough, separated by equally thin layers of fat, forming expandable pockets that can capture the steam, as in puff pastry and pie crusts.
In practice, most of the steam generated during baking will escape through broken bubbles and gaps between laminations, but enough will stay in the dough or batter long enough for the heat to set the proteins, hardening them into a light, open textured matrix, even after the product has cooled. For a substance that can, under pressure, pull 100-car freight trains and power entire factories; raising a loaf of bread, croissant or apple turnover is - pardon the expression - a piece of cake.
As noted above, there are two ways of creating the chambers that capture the steam: whipping and folding.
- Whipping is the process of incorporating air into either fat or eggs, and involves one of two techniques:
- Creaming refers to whipping together sugar and a solid fat, like butter or vegetable shortening As the two combine, the fat breaks down and coats the sugar grains, which then dissolve in the fat, producing a hollow bubble of sugar and fat. The eggs and dry ingredients are then quickly folded in, the batter is poured into the appropriate pan, and put into the oven as quickly and gently as possible.
- The sponge method starts by blending the dry ingredients, egg yolks and any liquids into a thick batter, which then is folded into a stiff meringue - a thick, heavily aerated roam of of egg whites, cream of tartar and sugar.
- Lamination is a process in which dough and fat are layered together, rolled out, and folded. This process is repeated until there are hundreds of ultra-thin layers of dough, each separated by an equally thin layer of fat. Although this may sound daunting, in fact, it's not particularly complicated.
For example, Danish dough calls for a sandwich of two layers of fat between three layers of dough. This is then rolled out and folded into thirds, yielding 9 layers, and then into fourths, for 36 layers. After a chilling, the process is repeated, rolling the dough thin, folding it in thirds, yielding 36x3=108 layers, and then into fourths, for a total of 432 layers. If you're feeling really ambitious, one more three-fold will produce almost 1,300 layers.
If done properly, the process of chilling, folding and rolling not only distributes the fat through the dough, but also encourages gluten formation, making the dough both flexible and elastic, so that it will literally inflate when steam is released during baking.
Microbial (Biological) Leavening
Microbial leavening refers to the activity of a single-celled fungus - yeast - that lives all around us. Yeast spends its days and nights eating sugar and excreting CO2 and alcohol, a process documented at least as far back as the breweries of ancient Egypt.
Indeed, for most of recorded history, bakers relied on the foam skimmed from the brewers' vats to leaven their breads; or they propagated the wild yeasts that live on whole grains and some fruites into starters around which they'd build their doughs.
It's hard to say exactly when commercial yeast first appeared on the scene. No doubt the brewers and bakers who parted with their leavens did so for cash in hand. While most of the breads of Jewish Eastern Europe were rye-based and, therefore, relied mainly on wild yeast sourdoughs, the local rabbi often held a monopoly, granted by the community as part of his livelihood, on the yeast needed for baking sweet Shabbes challah.
On the most basic level, all that yeast, eating all that sugar, produces CO2, which in turn creates millions of tiny gluten balloons in the dough that inflate with steam during baking.
However, the actual chemistry of microbial leavening is somewhat more complicated than just yeast at lunch. Fermenting dough supports a host of other microbes and enzymes - many present in the flour itself or in additives like barley malt - that break down complex starch molecules into simple sugars that in turn affects the action of the yeast. They can also produce acids and other substances that give a properly made loaf of bread its ineffable flavor and aroma.
Interestingly enough, although yeast thrives on sugar, too much sugar will actually slow its action considerably. For that reason, most sweet yeast doughs contain high percentages of yeast. Where a typical bread formula may call for 2% fresh yeast as a percentage of total flour, a super-rich dough like babka dough not only calls for 10% yeast, it also boosts the yeast further by combining the yeast, water and part of the flour into a sponge that is allowed to ferment for 30-60 minutes before adding the sugar and fat.
To address this problem, Lesaffre, a leading yeast manufacturer, developed an "osmotolerant" yeast designed for sugar and fat concentrations in excess of 10% of total flour weight, which it markets as SAF Gold Label Instant Yeast.
Yeast is commercially available to home bakers in three forms:
- Fresh compressed yeast, also called cake yeast, is made up of living yeast cells and growing medium, usually barley malt and wheat flour, compressed into a crumbly, slightly moist light tan cake. Compressed yeast has the highest concentration of live yeast cells per ounce/gram, but is highly perishable, with a shelf life of about two months, under refrigeration. Before the advent of dry yeast, fresh yeast was the only thing available to home bakers. However, it has since largely disappeared from the shelves of grocery stores.
- Active dry yeast, which Fleischmann's developed for the U.S. Army during World War 2, consists of small cylindrical particles of active yeast cells surrounded by a hard shell of dead cells. Although less efficient than cake yeast, active dry's long shelf life - it remains usable for as much as five years - and greater temperature tolerance fueled explosive commercial growth in the second half of the 20th century.
- Instant yeast was developed by Lesaffre - manufacturer of SAF and Red Star yeasts - in the 1980s, and has revolutionized the commercial yeast market. Like active dry yeast, it comes in dry powdered form and has a shelf life of several years; however, it contains a far higher percentage of live yeast cells than active dry, and can be added directly to a dough, without having to rehydrate. Because high-sugar and high-fat doughs can inhibit yeast activity, SAF offers both regular (Red Label) and osmotolerant (Gold Label) yeast for use in pastry doughs.
To use cake yeast, simply crumble it into your other ingredients or dissolve it in water.
Active dry is the most widely available form of baker's yeast, and is available at most supermarkets in ¼oz/7g envelopes and 4oz/114g jars, and in 2-lb/908g packages at warehouse stores like Sam's Club and Costco, as well as on the Internet.
To use active dry yeast, most authorities recommend rehydrating it first in hot (105-120°F/40-50°C) water, to which a small amount of sugar or honey has been added.
Instant yeast is available in most U.S. supermarkets in ¼oz/7g envelopes, and in 1-lb/454g packages via mail order on the Internet.
It should also be noted that some brands of instant yeast, often identified as rapid-rise or bread-machine yeast, contain added ascorbic acid (Vitamin C), a naturally occurring dough conditioner that promotes gluten formation and stimulates yeast activity. While this may be a time-saver, it also cuts down the amount of time that enzymes and bacteria have to unlock the deeper flavor of the dough.
Yeast Conversion Factors
To find weight equivalents among fresh, active dry and instant yeast, multiply by the factors in the table below:
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Chemical leavening is based on one simple bit of chemistry: that an acid like vinegar, buttermilk, or cream of tartar, combined with an alkaline compound like lye or baking soda, produces CO2, which in turn creates the bubbles in a dough or batter that inflate with steam during baking, thereby leavening the bread, pastry or cake.
The first written reference to chemical leavening appeared in 1796, in American Cookery, by Amelia Simmons - generally regarded as the first American cookbook. In it, the author calls for the addition of pearlash, a mild form of lye derived from wood ashes, to gingerbread dough as a way of lightening the crumb.
News of this American discovery spread quickly across the Atlantic, and by the mid-19th century bakers everywhere were leavening cakes and pastries with some combination of acid and alkaline.
Chemical leavening really didn't figure prominently in traditional Jewish baking, but only entered the baker's repertoire late in the 19th century, when the pastries and cakes of cities like Vienna, Berlin, Budapest, and Warsaw began filtering back to the shtetls; and more importantly, in America, where improving economic conditions, availability of ingredients and the vigorous cultural ferment of the Melting Pot created a demand for products that, for most Jewish immigrants, were near-mythic, if not totally unknown, back home.
Baking Powder, which was invented by British pharmacist Alfred Bird, appeared in 1843. Half a century later, a German pharmacist, August Oetker, developed and aggressively marketed a baking powder named Backin that's still sold today. (Incidentally, both Bird's and Dr. Oetker flourished over the years and have become leading European consumer packaged goods brands.
Baking powder can be either single- or double-acting. Single-acting baking powder, which contains only acid and alkaline, reacts in the presence of water, and even without it; heat is not necessary to get it going. Thereform, it's critical that batters made with single-acting get into the oven before the fizz dies off.
Double-acting baking powder, on the other hand, contains both the acid-alkaline combination of its single-acting cousin, plus a second slow-acting acid that only activates when heated. This gives batters made with double-acting baking powder a more forgiving nature and what the bakers call a longer bench life.
Baking Soda, also known as bicarbonate of soda and sodium bicarbonate, is an alkaline compound that reacts with acid to produce CO2. Unlike baking powder, baking soda doesn't contain any acid, so its leavening action depends on other acidic ingredients in the batter, such as chocolate, natural cocoa, fruit juice, honey, molasses and sour cream.
Baking Ammonia is a mixture of ammonium carbonate, ammonium bicarbonate and ammonium carbamate that decomposes rapidly when heated to form CO2, ammonia gas and water. Because baker's ammonia acts quickly and needs to be thoroughly heated to drive off all of the ammonia gas, it's most often used in small products that are baked until dry, such as cream puffs, cookies and egg kichel. Practically impossible to find at retail, baker's ammonia is a specialized product that's used almost exclusively in commercial bakeries.