Where does bread get its fluffiness? Swiss cheese its holes? And what makes vinegar so sour? These foods may taste completely different, but all of these phenomena come from tiny organisms chowing down on sugar and belching up some culinary byproducts. Let’s start with yeast. Yeast are single-celled fungi used to make bread, beer, and wine, among other products. Yeast break down carbohydrates, like sugar, to get energy and the molecules they need to function. They have two different ways to do this: the oxygen-dependent, or aerobic, pathway, and the oxygen-independent, anaerobic pathway, which is also called fermentation.
When you bake bread, yeast can use both pathways, but they normally prefer to start with the anaerobic process of fermentation. In this process, ethanol is produced in addition to CO2. No, bread isn’t alcoholic. Small amounts of alcohol that are secreted evaporate during baking. In the aerobic, or oxygen-dependent pathway, the yeast consume some of the sugar and produce carbon dioxide gas, or CO2, and water. In both processes, the CO2 accumulates and creates tiny bubbles.
These bubbles get trapped by gluten and create a sponge-like structure that gives the bread its soft texture. Wine also relies on yeast. But a wine-making set-up keeps the oxygen levels low so that yeast consume sugar using fermentation, the anaerobic pathway. The process often starts with wild yeasts already hanging out on the grapes. But to get consistent results, most winemakers also add carefully selected strains of yeast that can tolerate high levels of alcohol. The yeast consume the sugar in the grape juice, and as the sugar level drops, the alcohol level rises. This doesn’t necessarily mean that sweeter wines have less alcohol.
Different types of grapes start with different amounts of sugar, and sugar can also be added. What happens to the carbon dioxide? It just bubbles away through a vent. In carbonated alcoholic beverages, like champagne and beer, sealed containers are used in primary or secondary fermentation to keep the carbon dioxide in the bottle. Wine also introduces us to our second type of food-producing microorganism: bacteria. A special strain of bacteria turns a tart compound in grape juice into softer tasting ones that are responsible for some of the flavors in red wines and chardonnays. Another type of bacteria, called acetic acid bacteria, isn’t so desirable in wine, but they have their function, too.
If there’s oxygen around, these bacteria convert the ethanol in wine into, well, acetic acid. Let this process continue and you’ll eventually get vinegar. Bacteria are the key for cheese, too. To make cheese, milk is inoculated with bacteria. The bacteria gobble up the lactose, a kind of sugar, and produce lactic acid, along with many other chemicals. As the milk gets more and more acidic, its proteins start to aggregate and curdle. That’s why spoiled milk is clumpy. Cheesemakers usually add an enzyme called rennet, naturally found inside of cows, goats, and some other mammals to help this process along.
Eventually, those little curdles turn into bigger curds, which are pressed to squeeze out the water, and create a firm cheese. Different strains of bacteria make different kinds of cheese. For example, a species of bacteria that emits carbon dioxide is what gives swiss cheese its characteristic holes. Some cheeses, brie and camembert, use another kind of microorganism, too: mold. So your kitchen functions as a sort of biotechnology lab manned by microorganisms that culture your cuisine. Yogurt, soy sauce, sour cream, sauerkraut, kefir, kimchi, kombucha, cheddar, challah, pita, and naan. But maybe not all at the same dinner.