Beer & Wine the Science of Fermentation

Over thousands of years, humans have harnessed the natural functions of yeast cells to produce fermented foods and beverages. While various cultures have developed many distinct fermentation methods, one of the most widely practiced fermentation processes is alcohol fermentation. In this type of fermentation, the activity of yeast cells produces ethanol, the toxic compound most widely known as alcohol. Since yeast can proliferate in various substances, provided that they have access to sugars, alcohol fermentation may make use of a wide variety of ingredients. For this reason, there exist such a range of alcoholic beverages. In essence, grapes or other fruits are used to “feed” the yeast cells in wine making. In beer making, the fermentation relies on grains instead of fruits. Beyond this basic difference, each process involves multiple critical steps, which have been developed over centuries of “trial and error.”

To understand alcoholic fermentation, you must first understand what yeast is. Classified as a type of fungi, yeast is a living, single-celled organism. Like any other living thing, a yeast cell requires sustenance, which it finds in the form of sugars. For this reason, whether you are baking bread or brewing beer, the critical first step in fermentation is to combine yeast cells with some form of sugar. As grains and fruits both contain large quantities of sugars, they are the predominantly used ingredients in alcoholic fermentation. When the conditions are favorable for the yeast cells, they metabolize these sugars and, in the process, they give off two by-products: alcohol and carbon dioxide.

The basic fermentation process transforms the sugars present in various foodstuffs into alcohol and carbon dioxide. This release of carbon dioxide is why you notice a bubbling or frothing in fermenting foods and beverages. This is also the origin of the term “fermentation,” which can be traced to the Latin word fervere, meaning “to boil.” The alcohol, or ethanol, that is produced is what gives spirits their intoxicating capacity. Alcohol is a toxic substance to living organisms, which is why its effects on the human system are described as “intoxication.” The drinker’s metabolic system must respond to the intake of intoxicating beverages much as it would respond to any other class of poison, in order to maintain the organism’s overall health. Likewise, once the fermentation process produces a certain quantity of alcohol, the toxic by-product will kill off the original yeast population. Usually, yeast can survive in a solution with a content of up to about 18 percent alcohol. For this reason, any alcoholic beverage produced through fermentation alone will contain no more than 18 percent alcohol. Varieties of liquor with higher alcohol content make use of a secondary process, called distillation, following fermentation.

In the case of both beer and wine, the beverage is produced through fermentation alone. Wine is essentially fermented fruit juice, and most often, fermented grape juice. To produce wine, grapes are harvested and crushed, producing juice. This juice is allowed to ferment, relying on the naturally present yeast organisms. In modern winemaking, sulfur dioxide is typically added at this stage to remove any unwanted bacteria. When white wines are produced, the grape skins and stems are removed from the fermenting juice solution. For red wines, the whole grape is left in throughout fermentation. Therefore, despite widespread misunderstanding, both red and white wines are often produced from red grapes. Depending on the type of wine desired, this fermentation process may take just a couple weeks. In general, the longer that fermentation goes on, the more sugar is converted into alcohol, resulting in a less sweet (or “drier”) and more alcoholic beverage.

To produce beer, various grains are used instead of grapes as the source of sugars. Barley is among the most common grains used, though wheat, rye and other grains have also been used throughout history in various regions. Whereas grapes are simply pressed to provide grape juice for wine, the grain used for beer must first be dried. This drying process converts the starches present in grain into sugars. The dried grains are known as malt, and constitute the first ingredient in beer. The second key ingredient is hops. A cluster of flowers from the Humulus lupulus plant, hops slightly change the flavor and aroma of beer. In addition, hops give beer its clear, sparkling appearance and leach bacteria from the fermenting solution, reducing the risk of spoilage. The third ingredient of beer is water, which enables the yeast, malt and hops to mix and for fermentation to take place.