This is a part of the What is … Wednesdays feature, where we take scientific topics that might seem convoluted to some and make them more understandable.

Fermentation has been used to make foods and drinks for thousands of years. This is despite the fact that the science behind the process was only recently discovered. Common fermented products include chocolate, coffee, soy sauce, and yogurt, among others. The applied science of fermentation is called zymology

An approximate definition for fermentation is the extraction of energy from carbohydrates without oxygen. A number of bacteria and yeasts are involved in fermentation. There is a predictable series of dominant genera (the plural of genus, the level above a species in taxonomy) at different times as fermentation goes on. 

The history of fermented foods

Evidence of fermentation in China dates back to 7000-660 BCE – the Neolithic era. The first documented fermented product was an alcoholic beverage consisting of fruit, honey, and rice. Cheese was made in Europe as early as the sixth millennium BCE. Ancient Egyptians of the Old Kingdom (2686-2181 BCE) made alcohol and bread

To many demographics, fermented foods have a special significance. For example, in Judaism, leavening agents (known as chametz) are forbidden during Passover, according to the Torah. Andean societies discovered by Spanish colonists seemingly encouraged drunkenness. French culture involves frequent social drinking, usually of wine and sometimes to the point of intoxication. 

For some time in the scientific world, it was thought that fermentation was simply a chemical reaction. Prominent scientists including Antoine Lavoisier held this view

During the mid-nineteenth century, Louis Pasteur proved that fermentation involves microorganisms through an experiment with tartaric acid. Later, he found that bacteria were responsible for milk souring. That breakthrough led to his invention of pasteurization, which uses heat to kill microbes in foods. 

Eduard Buchner discovered that enzymes are responsible for fermentation in 1897, a finding that won him the 1907 Nobel Prize in Chemistry. By extracting a liquid from live yeast, Buchner observed that even dead cells can ferment sugar into alcohol, meaning that proteins produced by cells and not cells themselves are responsible for fermentation. 

The Danish Carlsberg Lab focuses its research on barley, yeast, and brewing technologies. The lab has been fundamental in refining commercial fermentation practices, and it has also contributed much to molecular biology and biochemistry in general. 

Modern fermentation as done to make beer and bread uses carefully formulated starter cultures of microorganisms. These cultures contain species known to demonstrate certain behaviors, such as high production of lactic acid or the production of compounds that contribute to flavor. 

The biology and chemistry behind fermentation

Ecological succession is the gradual change in prominent species in a community. Fermentation is a process with a predictable ecological succession. 

The exact composition of organisms in any instance of fermentation depends on its timing as well as the substance being fermented, among other variables. Note that the only species present during fermentation are those initially on the food to be fermented. This goes along with the law of the conservation of mass.

Changes in dominant species occur due to changes in the environment. For an example of ecological succession in fermentation, let’s look at the process of making cheese. 

Fresh, unprocessed milk is warm and has a neutral pH – ideal conditions for Lactococcus bacteria to flourish. Lactococci produces lactic acid from the lactose already in milk, lowering the pH and setting the scene for lactobacilli to become more dominant. As the pH is lowered even more, yeasts become prominent. 

Although that is the general ecological succession, particular additives are put into cheese cultures to alter the flavor and texture. Controlled temperature and salt levels are also crucial to a succession going as planned. 

Unlike food fermentation, alcohol fermentation is largely done by yeasts. Samples of the “same” wine from separate batches or years taste different because of the varying compositions of yeasts present. 

While you can include seasonings, sugar, or vinegar in a fermentation, those additions are not necessary. The most basic fermentation only requires a grain or vegetable, salt, and water. 

Make your own pickled vegetables at home by packing veggies of your choice in a glass jar, pouring a solution of salt water over them, adding spices if desired, and tightly shutting the jar. Then, wait a few weeks before enjoying your fermented treat!

Alternatively, you could make bread. While it’s rising, check on it periodically to see how it grows.

For a fun, non-edible way to see fermentation in action, gather an empty water bottle, about two ounces of water, one teaspoon of sugar, and a latex balloon. Put the water in the bottle, add the yeast, then recap the bottle and shake. Add the sugar to the solution next, and place the balloon over the neck of the bottle. Sit back and watch the balloon be blown up by the products of yeast fermentation! 

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