What Is Fermentation in Food?

In broad terms, fermentation is microbes breaking down carbohydrates without needing much oxygen, producing acids, alcohol or gases as by products. Those by products are what preserve and flavour the food.

The preservation works because most spoilage organisms cannot survive the conditions the fermenting microbes create. Lactic acid bacteria drop the pH until competitors die off. Yeasts produce alcohol, which does the same job. The food becomes an environment only the useful organisms tolerate.

Most food fermentation falls into a few categories:

• •Lactic acid fermentation, giving yogurt, sauerkraut, kimchi and sourdough
• •Alcoholic fermentation, giving beer, wine and cider
• •Acetic acid fermentation, turning alcohol into vinegar
• •Mould fermentation, used in soy sauce, miso and some cheeses

Fermentation does more than preserve. Microbes break large molecules into smaller ones, and small molecules are what our tongues and noses can detect.

Proteins are broken into amino acids, including glutamate, which is the source of savoury umami taste. That is why aged cheese, soy sauce and fish sauce taste so intense. Starches become sugars, and fats become aromatic compounds. Fermented food is essentially pre digested, which makes it more flavourful and often easier to absorb.

Traditional fermentation relies on microbes already present on the ingredients or in the environment. Sauerkraut needs nothing but cabbage, salt and time, because the right bacteria are already on the leaves and salt gives them the advantage.

Modern production usually adds a specific starter culture for consistency. The trade off is reliability against complexity. Wild ferments vary between batches and locations, which is prized in sourdough and natural wine and unhelpful in industrial yogurt.

Fermentation is generally safe when the acid or alcohol level rises quickly enough, which is why recipes are strict about salt levels and keeping vegetables submerged. Home canning of low acid foods carries genuine risk and needs proper method.

On health claims, be careful. Live fermented foods do contain bacteria that may benefit gut health, and research on this is active and promising. But many fermented foods are pasteurised or baked, which kills the microbes, and specific health claims are often stronger in marketing than in evidence.

Sauerkraut is the standard first project because it needs only cabbage and salt. The technique matters more than the recipe, and a few rules cover most failures:

• •Use around two percent salt by weight of the vegetables
• •Keep everything submerged below the brine, since mould grows on exposed pieces
• •Ferment at cool room temperature rather than in a warm spot
• •Expect bubbling and cloudiness, which are signs it is working
• •Discard anything with fuzzy mould rather than trying to rescue it

People brewed, baked and pickled for thousands of years without knowing that microbes were involved. In the nineteenth century, many chemists thought fermentation was a purely chemical breakdown, with yeast perhaps a by-product rather than a cause. The French scientist Louis Pasteur showed through careful experiments in the 1850s and 1860s that fermentation is carried out by living microorganisms, and that different microbes produce different results. He also showed that unwanted microbes could spoil wine and beer, work that led to the gentle heating process now called pasteurisation.

The story took another turn in 1897. The German chemist Eduard Buchner ground up yeast cells and pressed out a juice containing no living cells, yet this juice could still turn sugar into alcohol and carbon dioxide. That showed the chemical work was done by substances inside the cells, which we now call enzymes, rather than by some mysterious life force. Buchner received a Nobel Prize for the discovery. Together these findings connected everyday foods such as bread and cheese to biochemistry.

Chocolate depends on a fermentation that most people never hear about. Cocoa beans grow inside pods filled with a sweet, white pulp. After harvest, farmers pile the beans and pulp in heaps or boxes, often covered with banana leaves, and leave them for several days. Yeasts feed on the sugary pulp first, making alcohol. Bacteria then turn that alcohol into acids, and the activity of the microbes warms the heap. The pulp drains away and the beans inside begin to change.

The heat and acid kill the seed so it cannot sprout, and they break down cell walls inside the bean. This lets enzymes in the bean produce the small sugars and amino acids that later react during roasting to create chocolate flavour. Beans that are poorly fermented tend to taste bitter and astringent, whatever happens to them afterwards. Many coffee beans go through a related step. In the washed method, the sticky layer around the bean is loosened by fermentation in tanks of water before drying, which affects the final taste of the cup.

Yeast carrying out alcoholic fermentation turns each molecule of glucose into two molecules of ethanol and two of carbon dioxide, which chemists write as C6H12O6 becoming 2C2H5OH plus 2CO2. Brewers and bakers use exactly the same reaction but keep different products. A brewer wants the alcohol, and in many beers some of the carbon dioxide is kept for fizz. A baker wants the gas, which forms bubbles that stretch the gluten network and make the dough rise. The small amount of alcohol in bread dough mostly evaporates in the heat of the oven and contributes to the smell of baking.

Your own body can ferment too. When you sprint hard, your muscle cells need energy faster than oxygen can arrive, so they briefly rely on lactic acid fermentation, the same basic chemistry used by the bacteria in yogurt. A popular belief says this lactic acid lingers in your muscles and causes the soreness you feel a day or two after hard exercise. It does not. The lactate is cleared within roughly an hour, much of it reused as fuel by the heart and liver. Delayed soreness is linked instead to small-scale muscle damage and the repair that follows, especially after unfamiliar exercise.

A sourdough starter is a small, stable community of microbes living in flour and water. The bacteria, mostly lactic acid bacteria, usually far outnumber the yeasts. One well studied bacterium, first described from San Francisco sourdough and now named Fructilactobacillus sanfranciscensis, feeds mainly on maltose, a sugar released as enzymes break down the starch in flour. The yeast often found with it, Kazachstania humilis, cannot use maltose, so the two partners avoid competing for the same food. The bacteria make lactic and acetic acids, which give the bread its tang, while the yeasts make most of the carbon dioxide that raises it.

A new starter goes through a messy succession before it settles. In the first days, many kinds of microbes, mainly from the flour, multiply, and the mixture may smell odd or bubble wildly. As acid builds up, species that cannot cope with it die back, and within a week or two an acid-tolerant community usually takes over. Regular feeding with fresh flour and water keeps it active. The acidity has practical benefits for the bread too. It slows staling and the growth of moulds, which is one reason sourdough loaves often keep longer than bread made with baker's yeast alone.

Fermentation is the controlled use of microbes to preserve and transform food, working by creating acid or alcohol that blocks spoilage while breaking large molecules into the small ones we taste as flavour.