Bread Better: The Fourth Ingredient

A good loaf of bread requires only four ingredients: flour, water, salt, and yeast. Flour, of course, is the sine qua non. Trying to make bread without it would be trying to make a painting without paint, or wine without grapes: an interesting exercise for postmodernists, but impractical otherwise. Water, as discussed in a previous post, turns the flour from an unpalatable powder into a cohesive dough. Salt activates our sense of taste. This leaves today's topic: yeast. Yeast is included to provide leavening – it turns dough from a dense paste into something soft and squishy by fermentation: the conversion of sugars in the dough into alcohol and carbon dioxide.

A scanning electron microscope image of yeast cells. The pock marks are bud scars - places where new cells budded off the mother cell, as in the center foreground. Source: The Genome Analysis Centre

A scanning electron microscope image of yeast cells. The pock marks are bud scars - places where new cells budded off the mother cell, as in the center foreground. Source: The Genome Analysis Centre

Yet there's more to the story than simply the leavening power that yeast provides. The story of yeast is the story of the first, and likely only, fungus that humans have domesticated. The relationship between our two species goes back thousands of years and likely played a key role in the transition from a hunter-gatherer society to a settled one.

The general word “yeast” actually refers to hundreds of species of fungi, many of which are not related to one another. The common feature between them is that they have forsaken the typical multicellular lifestyle of fungi and gone back to being single-celled organisms. However, in cooking, “yeast” almost always refers to one species, Saccharomyces cervisiae. This has to be one of the best Latin names ever assigned – it translates to “sugar fungus of beer.” “Sugar fungus” is an evocative title, as sugar is the molecule that has influenced fungal evolution more than any other. Unlike plants, fungi cannot produce their own sugar, and unlike animals, they can't scuttle, flit, slither, or ooze around until they find some. These limitations have pushed fungi to some creative ways of exploiting potential sources of energy.

In this case, however, the same lust for carbohydrate that drove fungi into many a precarious lifestyle has instead created a lazy, fridge-raiding roommate. Their highly simplified, single-celled lifestyle allows yeasts can stay dormant for a long time, and they can travel easily by drifting on air currents. This combination of features has lead to a pretty simple strategy: yeasts will float around until they land on something, and if its edible, they'll eat it, and if not, they'll wait until it is edible. It's likely most of the fruit you eat already has a thin coating of yeast cells on it, just waiting for the delicious sugars within to become available for them to consume.

It's this proclivity to hang out on fruit that first brought humans and yeasts together. It's likely that we discovered fermentation not, as S. cerevisiae's name suggests, though beer-making, but through wine-making. Crushing grapes, as for juice extraction or storage, would bring into contact all the yeasts waiting on the skin of the fruit with the easily-digested carbohydrate they crave, leading to fermentation.

Yeasts have a problem, though, and that problem is competition. Unlike the fungi who have to work for a living and have thus become very specialized at getting nutrients out of places where they generally aren't available, the simple sugars in grape juice are open to all comers. If the yeasts don't do something to intervene, something else – something bacterial – might come along and scarf up their food supply. However, single-celled organisms have a limited array of options when it comes to dealing with their competitors:

  1. Eat them
  2. Outpopulate them
  3. Chemical warfare

Option number 1 is out, as fungi have developed rigid, chitinous cell walls that prevent them from engulfing food, and they haven't yet figured out mouths, which I think we can all say is a good thing. Option 2 is also a poor choice, since bacteria are generally capable of growing way faster than yeasts. That leaves option 3: scorch the earth and pitch a tent in the smoldering ashes. This strategy is not uncommon in microbes – that's how we ended up with antibiotics and why that tomato in the back of your fridge is starting to smell so weird: it's a fungus's way of saying “I got mine – back off, loser.” Generally this is bad news for the humans who may want to eat something that a microbe has claimed, but in a limited number of scenarios, microbes can actually be beneficial. In this case, yeasts turn some sugar into ethanol, which humans have shown an ability to tolerate* and even appreciate. Many microorganisms, on the other hand, can't hold their liquor and will die off when the concentration gets too high, leaving the yeast and a few yeast-friendly bacteria to enjoy their sugar in peace. The product is something that can be safely consumed by humans and can be stored for long periods of time without spoilage. This feature would prove to be extremely important when agriculture came along.

But what does this have to do with bread? Well, genetic evidence suggests that the Saccharomyces cerivisiae that is used in baking today originated from a wild species known to hang around grapes. As it turns out, yeasts like the carbohydrates in bread just as much as they like those in wine, and they will happily set up shop, breaking down sugars and pumping out alcohol and another metabolic byproduct, carbon dioxide. As discussed in the first part of this series, the main proteins in bread will, when exposed to water, cross-link and form pliable sheets; they will then trap the carbon dioxide and form the air pockets in much the same way a balloon does, leading to bread's spongy texture.

Leavening is not the only service yeasts provide. While munching starch, which humans can't taste, they'll leave behind some simple sugars; they'll also free up nitrogen by breaking down wheat proteins and phosphorus by digesting phytic acid. The longer the dough ferments, the richer it becomes in free amino acids and sugars, which contribute to the distinctive aroma, brown crust (a product of the Maillard reaction), and the savory flavor of bread.

Deliberate inoculation of yeast into dough probably came later than what was likely the original form of bread leavening: sourdough fermentation. Analysis of stone tools suggests that humans have been processing grains into flour for about 20,000 years. The action of grinding, like smashing grapes, brings dormant yeasts (typically Candida and a few other species of Saccharomyces) and bacteria, particularly Lactobacillus (the same genus responsible for turning milk into cheese and yogurt), into contact with their preferred food source. Like yeasts, Lactobacillus will engage in chemical warfare, by pumping out lactic acid and lowering the pH of its environment to between 3 and 4, which is way too low for the kind of molds that will make food fuzzy and gross. The wild yeasts, however, have evolved to tolerate this acidic environment, and their catabolic enzymes are even optimized for these pH levels. Because their metabolisms are slower than the human-selected Saccharomyces, a good rise can take anywhere from 8 to 24 hours, giving the yeast enzymes longer to work and release flavorful substances into the dough. Yeast and bacterial digestion of protein also results in less glutenin, and, therefore, a denser loaf of bread, and, with a lot of caveats, one that may be easier on people suffering from celiac disease.

While it's possible to make bread without yeast, the discovery of yeast fermentation was incredibly important to early societies, and, for better or for worse, played a key role in the development of agriculture by turning a grain we could produce in sizable quantities into a palatable and less perishable product. So next time you nosh some toast or make a toast, be sure to thank all the fungi that played a role in making your food.

*This is likely because of yeast: they are so ubiquitous that any fruit-eating animal is bound to end up with a high-proof snack eventually, making alcohol metabolism a must-have ability.

Selected Sources
Di Cagno R et al. (2004) Sourdough Bread Made from Wheat and Nontoxic Flours and Started with Selected Lactobacilli Is Tolerated in Celiac Sprue Patients. Applied and Environmental Microbiology 70(2): 1088-1096.

LeGras JL et al. (2007) Bread, beer, and wine: Saccharomyces cerevisiae diversity reflects human history. Molecular Ecology 16: 2091-2102.

Revedin A et al. (2014) New technologies for plant food processing in the
Gravettian. Quaternary International. 1-12.

Thiele C, et al. (2001) Contribution of Sourdough Lactobacilli, Yeast, and Cereal Enzymes to the Generation of Amino Acids in Dough Relevant for Bread Flavor. Cereal Chemistry 79(1):45-51.