Byproducts of Ethanol Fermentation
While most of us in brewing don’t at first realize it, alcoholic fermentation, scientifically known as ethanol fermentation, has many byproducts that go beyond alcohol and carbon dioxide. The process is at once simple and complex, both a naturally occurring phenomenon and one that has myriad implications for the world within brewing and beyond.
What Is Ethanol Fermentation?
Ethanol fermentation is the name we give to the process that involves yeast consuming sugar and converting it to alcohol and carbon dioxide.
We differentiate it from lactic acid fermentation, which is where bacteria convert sugars into lactic acid, which we see when probiotic foods like yogurt and kefir are produced. It is the same process human muscle cells undergo when we run out of breath. As we are exerting energy, usually during exercise, our breathing speeds up and the oxygen to our cells decreases. Our cells are then triggered to undergo lactic acid fermentation, converting the carbs in our body to ATP, which is what provides us with energy.
Acetic acid fermentation, which is the third type of fermentation, occurs when yeast and bacteria convert sugars and starches from fruits and grains into vinegar and other sour tasting condiments.
In the case of ethanol fermentation, bacteria is not required, but it is often included. In the case of lactic acid fermentation, yeast is not required, but it is often included.
Alcoholic fermentation requires only two ingredients – a sugar source and yeast.
Yeast, a single celled, living organism is present virtually everywhere on earth and is in a constant state of seeking sugar as its source of energy. When it finds sugar, it consumes it and expels alcohol and carbon dioxide as byproducts.
The primary strain used for alcoholic fermentation, in the case of wine, beer, and even the biofuel ethanol, is Saccharomyces cerevisiae, which will, in general, produce alcohol at room temperature and in the absence of oxygen. In fact, those interested in fermenting don’t even have to keep their products in airtight containers as the yeast creates a foam or crust over its sugary course that helps keep oxygen out.
In the case of beer, we see this during krausening. After a few days, a thick, foamy crust will form, which creates an airtight seal over the liquid. Then, yeast can switch from aerobic fermentation, which produces more water than alcohol, to anaerobic fermentation, which produces only alcohol and no water.
Yeast also reproduce asexually, so these cells do not need to mate. The mother simply grows a daughter cell from her body which extends outward and continues to grow while feeding from the mother. Once the daughter cell has reached more than 50% volume of the mother, it will split off and begin its own sugar seeking journey.
Thus, alcoholic fermentation is a naturally occurring phenomenon that can be found in any forest, field, jungle, or desert floor, among other natural locations. Humans have adapted the process to fit our love of beer and wine and our desire for environmentally friendly fuel.
Chemical Process of Ethanol Fermentation
The chemical process of ethanol fermentation is simple enough. The first step is glycolysis, during which the yeast breaks down the glucose in a fermentable sugar source into 2 pyruvate molecules.
From there, those pyruvate molecules convert to 2 molecules of ethanol and 2 carbon dioxide molecules.
Byproducts of Ethanol Fermentation
Thus, the primary byproducts of ethanol fermentation are obvious and have been stated numerous times in brewing discussions.
Alcoholic fermentation produces alcohol and carbon dioxide. However, there are hundreds of secondary byproducts, or metabolites, produced during alcoholic fermentation, and it is those that give beer and wine their flavor and aroma that make it stand out from mere grain water or grape juice.
Secondary Byproducts of Ethanol Fermentation
Those secondary metabolites are largely esters and polyphenols, which give beer and wine their flavor, aroma, and nutrients. While it is true that polyphenols are already present in plants before fermentation, the act of fermentation significantly increases the effects of those polyphenols in the end product.
Other sub-products of ethanol fermentation include acetic acid, which is the souring effect that leads to vinegar. Acetic acid has been shown to lower blood sugar and blood pressure as well as kill off harmful bacteria and decrease inflammation.
Diacetyl is another byproduct produced during alcoholic fermentation, which can create off flavors like buttered popcorn, but which are usually “cleaned up” during secondary fermentation.
Another off flavor common to alcoholic fermentation is acetaldehyde, which can produce a strong, fruity flavor and odor.
Finally, there’s also succinic acid as a byproduct of ethanol fermentation, which produces an acidic taste.
What Completes Ethanol Fermentation
We typically know that primary fermentation is complete when the krausening dies down and the yeast flocculate, or gather in clumps, either on the top of the fermented product or at the bottom of the vessel.
Sometimes, fermentation can stagnate, or stall, in which case you might need to activate the yeast by stirring the liquid a bit, or raise the room temperature. Sometimes, the yeast gets too cold and cannot produce the heat it needs to keep fermenting.
Once fermentation is complete, it is critical to allow the beverage to undergo secondary fermentation, which involves moving the beer or wine to a secondary vessel after straining off the trub, which will activate the remaining yeast cells and allow them to “mop up” the secondary byproducts that can cause unwanted off-flavors.
Bio-Products Produced During Ethanol Fermentation
It is also important to differentiate byproducts of ethanol fermentation from bio-products, which are the additional products created during alcoholic fermentation, specifically in the production of beer.
Brewers have found ways to utilize the spent grain by selling or donating it to ranchers to feed livestock. These bio-products include distillers grains, gluten meal, and gluten feed, often called BSG, or brewer’s spent grain.
Cheers!
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