The Science of Fermentation

Fermentation is one of the oldest technologies in human history, though for most of that history no one understood how it worked. Long before anyone had heard of microorganisms, people were making bread rise, turning grape juice into wine and preserving vegetables in brine. What these processes share is the activity of microscopic living things, chiefly yeasts and bacteria, that transform the sugars and starches in food into new substances. In doing so they change the flavour, texture and keeping quality of what we eat, and they do it without heat or modern machinery. For thousands of years these transformations must have seemed almost magical, and many traditional recipes were passed down as fixed rituals whose steps were followed exactly even though the reasons behind them were unknown. Only in the last few centuries, with the invention of the microscope and the study of microbial life, did the true nature of fermentation begin to be understood.

At its simplest, fermentation is the breaking down of sugars by microorganisms in the absence of oxygen, or with very little of it. Yeasts, which are single-celled fungi, are especially important in baking and brewing. When a yeast cell consumes sugar without enough oxygen, it produces two main products: alcohol and carbon dioxide gas. In brewing and winemaking the alcohol is the prized result. In bread making it is the gas that matters, because the bubbles of carbon dioxide become trapped in the stretchy network of the dough and cause it to swell. Much of the alcohol produced in dough later evaporates during baking in the heat of the oven.

The stretchy quality of bread dough comes from gluten, a network of proteins that forms when wheat flour is mixed with water and worked by kneading. Gluten gives dough both strength and elasticity, allowing it to stretch around the expanding gas bubbles without tearing, rather like the skin of a balloon. Without this protein network the gas would simply escape and the bread would stay flat and dense. This is why breads made from flours low in gluten, such as rye, tend to be heavier, and why they are often more difficult to work with.

Sourdough bread relies on a particularly interesting partnership. Instead of using commercial yeast bought from a shop, a baker keeps a mixture of flour and water known as a starter, which is left to attract and cultivate wild yeasts and bacteria from the flour and the surrounding air. The starter is home to a community of microorganisms in which wild yeasts and lactic acid bacteria live together. The yeasts generate the gas that makes the loaf rise, while the bacteria produce acids that give sourdough its characteristic tangy flavour. A well-tended starter can be kept alive for years, being fed regularly with fresh flour and water to maintain the balance of its inhabitants.

The acids created by the bacteria do more than add flavour. By making the dough more acidic, they help preserve the bread and slow the growth of unwanted moulds, so a sourdough loaf often keeps longer than one made with commercial yeast alone. The acids also change the texture and can make some of the minerals in the flour easier for the body to absorb. Many people who find ordinary bread difficult to digest report that they tolerate slowly fermented sourdough more comfortably, although this varies from person to person and sourdough is not free of gluten.

Temperature and time are the baker's main tools for controlling fermentation. Microorganisms are more active when they are warm, so dough left in a warm place will rise faster than the same dough kept cool. Many bakers deliberately slow the process down by placing dough in a refrigerator overnight, a technique that allows flavour to develop while the rise is gentle and controlled. Rushing fermentation with too much warmth can produce gas quickly but leaves little time for the complex flavours to form. Patience, in other words, is rewarded with taste.

Fermentation reaches well beyond bread. The same basic principles explain how cabbage becomes sauerkraut, how milk becomes yoghurt and cheese, and how tea leaves and soybeans are transformed into a range of familiar foods around the world. In each case, particular microorganisms are encouraged while harmful ones are held back, often by salt, acidity or the exclusion of air. What began as a practical way of preserving food long before refrigeration existed has become, in modern kitchens, a craft valued as much for the flavours it creates as for the food it saves from spoiling.