yeast is a living microorganism, a single-celled fungus. While there are more than 1,000 known species of yeast, bakers use domesticated strains of the species Saccharomyces cerevisiae—the name roughly translates to “the sugar fungus found on beer.” Baker’s yeast, also called brewer’s yeast, is a microscopic rod-shaped organism. In the wild, Saccharomyces cerevisiae is principally found on fruit skins, including grapes. A mature yeast cell is about 4 micrometers in length; a grain of salt is about a thousand times larger, and even skin cells are nearly 10 times the size of a typical baker’s yeast cell.
Yeast is added to a wide variety of baked goods as a biological leavening agent. Leaven has its roots in the Latin levare, meaning “to lighten” or “to lift.” Leavening makes a baked good less dense by incorporating many gas pockets into the structure. The 400 grams of wheat flour in a loaf of bread already contain about 120,000 wild yeast cells before any additional yeast is added; however, not all of these belong to strains that would raise a wheat-flour dough, and in any case, this amount is not enough to lift the weight of flour to make an acceptable loaf of bread. It requires billions of yeast cells to leaven a dough made with a kilogram of wheat flour.
In the past, households brewed not only in order to have alcoholic beverages on hand, but also to produce the barm that rose to the top of a fermenting mash, which would be used to make bread rise. Bakers and brewers were experienced microbiologists, carefully (and sometimes jealously) cultivating their yeast colonies. Yeast’s mechanism of action was a mystery for millennia, finally solved by the invention of the microscope, which allowed the living yeasts in barm to be seen, and by Louis Pasteur’s work, which firmly established that yeast is the organism responsible for fermentation in both baked goods and alcoholic beverages.
Yeast is a saprophyte, which means it is an organism that feeds on decomposing plant or animal matter. The mechanism by which yeast leavens baked goods is anaerobic, requiring no oxygen. Dormant when dry, yeast becomes active when wet. Yeast converts the simple sugars glucose and fructose into ethanol and carbon dioxide (CO2). See fructose and glucose. The carbon dioxide produced by the decomposition of the sugars remains trapped within the tangles of proteins (gluten) that give structure to bread and other baked goods, forming small pockets of gas. The resulting porous material is much less dense than it would be otherwise.
The process of fermentation has two basic steps: (1) the breakdown of carbohydrates to simple one-unit sugars, and (2) the conversion of these monosaccharides to gaseous carbon dioxide and ethanol. Yeasts in a wet flour mixture feed first on the simple sugars sucrose, glucose, fructose, and maltose that are naturally present in small amounts in wheat. (About 1 percent of the total weight of wheat flour is actually sugars.) Once these sugars are consumed, the yeasts feed in earnest on the starches. See starch. Enzymes in the flour, released when the wheat is ground, cut apart the starches in the wheat into smaller sugar units, which can then be digested by the yeast. Enzymes in the yeast, principally invertase and maltase, break down the resulting sucrose and maltose into their component sugars, glucose and fructose—the feed stocks for fermentation.
The fermentation reaction enzymatically converts glucose (or fructose) to ethanol, a process that provides energy for the yeast and produces carbon dioxide and ethanol as waste products. The rate of fermentation, and thus the production of carbon dioxide, increases with temperature—much as the respiration rates of humans go up when they have a fever—so warm dough will rise faster. The heat of baking causes the trapped gas to expand, further lightening the texture of the material, and also sets the structure around the bubbles. The alcohol (a scant tenth of a milliliter total in a loaf of bread) evaporates during baking, but technically this is the same fermentation process that produces beer or ale, just in a different matrix and stopped at a much earlier stage. The high heat also kills off most of the yeast cells, which do not tolerate temperatures much above 130°F (54°C).
Yeast-leavened sweet doughs have difficulty rising, not because the sugar inactivates the yeast, but because the sugar reacts chemically with precursors to gluten, limiting the amount of gluten that can be formed. See breads, sweet. Without the structure provided by gluten, sweet doughs are fragile and require longer rise times. Strains of yeast have been developed that produce more carbon dioxide than the regular strains and are more suited to use with low-gluten doughs. Adding spices to the dough, such as cinnamon, cardamom, nutmeg, or ginger, has the same effect, doubling or more the amount of carbon dioxide produced. Fats in the dough also reduce the formation of gluten by coating the individual protein strands before they can interact.
Yeast for baking can be obtained in many forms. Cream yeast, the fresh yeast once begged from local bakers or brewers or, as the name suggests, skimmed like cream from milk off the household’s own fermenting alcohol, is a suspension of live yeast cells. Yeast is a natural product, and it was often difficult to gauge how much of a yeast suspension to use. Recipes routinely called for “proofing” yeast; that is, giving it some flour and water to feed on to check its viability.
It is only in the last hundred years or so that reliable, shelf-stable forms of yeast have been available. Billions of kilograms of yeast are now grown commercially every year. Pressed cakes of fresh yeast can be kept for one to two weeks in the refrigerator. Dried granules of yeast will keep for months on the shelf. Two forms are popular: the coarsely granulated active dry yeast, which must be rehydrated before using; and the more finely granulated instant yeasts, which do not require hydration before incorporation into a dough. And, of course, many bakers keep on hand their own culture, usually as poolish—a starter dough.
See also chemical leaveners.
yogurt is a fermented milk product obtained through the action of certain bacteria. The prehistoric spread of dairying across an area extending from the Balkans to the Indian subcontinent and the western fringes of China went hand in hand with the spread of milk soured to produce what we now call by the Turkish name yogurt. Throughout these regions, hot temperatures during summer milking season have favored the action of particular lactic acid bacteria known as “thermophilic” because they act best at warm temperatures (between about 105° and 115°F, or 40° to 46°C). Such organisms rapidly colonize milk exposed to the open air, fermenting most of the lactose (milk sugar) to lactic acid and other flavorful byproducts. The result is a creamy, refreshingly tart substance useful for both sweet and savory purposes.
In the absence of commercial laboratory cultures, unstandardized clusters of wild lactic acid bacteria figure in the process, to varying effect. Historically speaking, nobody expected fixed uniformity in yogurt from different regions, or indeed from different households. The situation changed after 1900, when the two budding fields of dairy microbiology and gastroenterology began to intersect.
Partly drawing on earlier researchers’ work and partly following his own erratic opinions, the Russian-born Nobel Prize winner Elie Metchnikoff—not himself a bacteriologist—announced in 1907 that yogurt contained certain beneficial organisms able to destroy pathogens in the colon and greatly prolong the human life span. Though his idea that yogurt bacteria could survive in the gut would be substantially disproved, his claims rapidly raised yogurt to the popular status of edible medicine.
Other researchers (especially Stamen Grigorov and Felix Lohnis) had already done pioneering work with lactic acid bacteria. Microbiologists soon were able to isolate and identify two organisms chiefly responsible for the characteristic flavor and texture of yogurt. Industrial laboratories began propagating and selling pure strains of both. Today they are usually known as Streptococcus thermophilus and Lactobacillus bulgaricus. When they are used together to inoculate milk, S. thermophilus rapidly initiates lactic acid production at a temperature of about 110° to 115°F (43° to 46°C) before L. bulgaricus kicks in to produce more noticeable acidity at a slightly lower temperature over several hours. Their joint action converts the casein in milk to a fragile, slightly tart gel that never becomes a fully formed curd.
Yogurt remained a minor specialty until a dramatic marketing makeover after World War II. U.S. manufacturers began masking its natural tartness—unpalatable to American consumers—with sweetened fruit preserves or syrup, and selling it in disposable individual containers from which it was meant to be directly eaten. They also claimed a new health-benefit angle by replacing whole milk with reduced-fat or nonfat milk, in line with the latest low-fat dietary agendas. To offset the loss of body, they often added dried nonfat milk, along with starch, pectin, and/or gelatin.
In effect, promoters had turned an ancient form of cultured milk into a pre-packaged sweet—advertised, however, not as a sweet but as a healthful dieters’ snack or small meal. The two main forms were “Swiss-style,” with sweetening and other flavorings mixed through the whole, and “sundae-style,” with fruit preserves on the bottom to be stirred into the rest of the yogurt by consumers. Both were resounding triumphs. American domestic production of yogurt—virtually all sweetened—went from less than 300 million pounds in 1950 to about 6 billion pounds in 2010, with the most spectacular gains occurring between 1960 and 1990.
Another avenue opened up in the 1970s: frozen yogurt, marketed as a low-fat alternative to ice cream or frozen custard. After some fluctuations in popularity, several highly publicized new retail franchises (Pinkberry, Menchies, the Korean-based Red Mango, and more) featuring self-serve machines and choices of toppings kicked it into a growth spurt around 2005.
Meanwhile, plain unflavored yogurt is enjoying a modest renaissance after long neglect. Small producers using milk from small dairy herds (sheep or goats as well as cows) have begun making yogurt for an informed clientele. The market has expanded with increased immigration from India, Turkey, the Caucasus, Iran, Central Asia, Greece, Russia, and the Balkans. In those regions, plain yogurt has long been served together with honey, jaggery-type sugar, or poached dried fruit (especially apricots). India has produced the most inspired yogurt-sugar marriages: sweetened versions of the drink called lassi (sometimes with pureed fruit); the saffron-perfumed Gujarati dessert shrikhand; and combinations of yogurt with sweetened concentrated milk in the Bengali bhapa doi and mishti doi. Some of these specialties have become known to other American cooks in recent decades.
In the last few years, two new developments have galvanized the mainstream commercial yogurt market.
Shortly before Metchnikoff’s pro-yogurt efforts, the gastroenterologist Ernst Moro had isolated an organism that naturally occurs not in food but in the human colon, where it seems to have some of the pathogen-fighting properties that Metchnikoff had wrongly ascribed to yogurt. It was eventually named Lactobacillus acidophilus, for its resistance to acidity strong enough to kill other lactic acid bacteria. By about 1920 it was being experimentally added to milk, to create a harshly sour second cousin of yogurt. “Acidophilus milk” was eaten by only a small fan club of health-food zealots until the late 1990s. At that point many natural-foods adherents became convinced that foods inoculated with “probiotics,” or “good” bacteria capable of colonizing the colon, could rid the body of pathogens and bacterial toxins. A huge market emerged for yogurt containing not just the usual organisms but L. acidophilus along with clusters of other reportedly therapeutic bacteria. Today, it is impossible to buy yogurt manufactured without L. acidophilus and four or five other supposedly probiotic organisms. Consequently, formerly mellow versions of yogurt—even from artisanal producers—have acquired a sharper acidity that may require adjustments by cooks.
At about the same time, manufacturers in Greece began exploring the U.S. market for yogurt that is pre-thickened by draining off most of the whey. Unaware that this is the normal manner in which most people eat yogurt from Central Asia to the Balkans, American consumers greeted this “Greek yogurt” as an exciting new discovery. No federal standard of identity covers the term. Other manufacturers both here and abroad were thus free to exploit its rising popularity by producing supposed “Greek” versions of what various immigrant groups already knew as labne (Arabic) or suzme (Turkish) yogurt. Drained whole-milk yogurt is far better for cooking purposes—including most sweets—than previous mass-market U.S. versions. However, producers here soon reinvented “Greek yogurt” in a multitude of low-fat and nonfat forms with the sugar, flavorings, and added thickeners that Americans have come to associate with yogurt.