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Yeast Fermentation: Using Fermentation to Make Alcohol

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  • 0:07 A Cold Bottle of Microbiology
  • 1:39 Fermentation and the…
  • 4:26 Alcohol for Consumption
  • 7:20 Alcohol as a Commodity
  • 7:47 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Without microbiology, you wouldn't be able to enjoy a refreshing adult beverage after work. In this lesson, we will examine the role of yeast in alcohol production.

A Cold Bottle of Microbiology

I'm curious. How many of these microbiology lessons have you listened to? There are over 200, after all. I think we could both use a little break. Let's you and me kick back, crack a beer and relax for a minute. All this microbiology is starting to make my head spin. Or maybe that's the beer. Or both? You do realize that without microbiology, there is no beer, right? No wine or liquor either. I guess as long as we're here, we should learn a little something about microbiology, so we'll stick with the topic literally at hand: alcohol.

Believe it or not, beer containers dating to between 10,000 and 9,000 B.C. have been found in China. Egyptian writings depict alcoholic beverages as early as 4,000 B.C. And of course, wine plays a prominent role in many Bible passages. All of these civilizations drank alcohol, but almost certainly none of them actually understood what was going on at a microscopic level.

It wasn't until 1836 that French physicist Charles Cagniard de la Tour determined that the yeast present in beer was actually a living organism, not simply a chemical compound. He guessed that yeast was responsible for not just the alcohol but also the carbon dioxide that made the beer bubbly. In 1860, Louis Pasteur scientifically proved that the yeast present in wines and alcoholic beverages produced alcohol and carbon dioxide using the process called fermentation.

Fermentation and the Yeast Saccharomyces

So, what is fermentation? Fermentation is the anaerobic catabolism of a single chemical compound using a series of redox transformations with the goal of generating ATP by substrate-level phosphorylation. What? That might be the book definition of fermentation, but it is way too deep for this lesson, especially if you actually did crack a beer during the introduction.

Let's simplify this to be a bit more applicable here. In a solution without oxygen (anaerobic), yeast break down (catabolize) sugar (a single chemical compound) to produce energy (ATP) and waste products (alcohol, specifically ethanol and carbon dioxide). You can see how this definition fits with the jargon-heavy book definition of fermentation, but is a bit easier to swallow.

But fermentation is only one part of this equation. We need to take a closer look at yeast. I'm sure everyone has heard of yeast: microscopic fungi that help bread rise and cause irritating infections in women. In reality, there are around 1,500 characterized species of yeast, but the members of only one genus are responsible for the vast majority of alcohol fermentation. Saccharomyces is one of the most studied organisms in science and the major producer of commercial ethanol.

Saccharomyces is a common member of the microbial population living on plant material, including fruits, vegetables and grains. The alcohol found in ancient historical beverages was a result of these natural yeasts fermenting the sugars naturally found in the foods. Early humans ate these foods or drank beverages made from the fermented foods, noticed the changes the fermentation products imparted to them and began fermenting foods intentionally.

Fast forward thousands of years and the process of alcohol production is basically the same. There is one minor difference: Brewers don't rely on naturally occurring yeast. I mentioned that Saccharomyces was one of the most studied organisms in science, and its role in alcohol production is a major reason.

Once yeast was identified as responsible for alcohol production, brewers spent the next several hundred years manipulating strains of Saccharomyces, succeeding in increasing growth rate, increasing alcohol production and decreasing waste production. Individual species and subspecies of Saccharomyces have been engineered to be the best at what they do, depending on what spirits they are producing. Alcohol producers today completely wipe out the natural yeasts found on the starting plant materials and add in their own carefully chosen yeast, guaranteeing the best possible yield and flavors while eliminating unwanted waste products.

Alcohol for Consumption

Alcoholic beverages can be broken down into three major categories: wine, beer and distilled liquors. Each requires carefully selected yeast strains and specific techniques to produce the characteristics of the desired beverage.

Beer is produced by the fermentation of grains. Beer production is actually a bit more complex than simple fermentation. The yeast involved, usually Saccharomyces cerevisiae (cerevisiae is Latin for beer), is only able to ferment sugar, not the starch-heavy grains. Brewers must add malt, which is a compound made from germinated barley seeds. Inside the seeds is amylase, an enzyme that breaks down the starch into sugar, allowing the yeast to do their thing. Grain fermentation usually produces only about 3-6% alcohol.

Wine is produced by the fermentation of fruit. Grapes are by far the most commonly fermented fruit, but any fruit can be turned into wine. The yeast Saccharomyces cerevisiae Ellipsoideus is the strain most commonly used during wine production. This variety generally produces wine with an alcohol content of 6-14% and produces fermentation products that give different wines distinct flavors.

The interesting and vexing thing about Saccharomyces is that the yeast on their own can only reach 17% alcohol. Above 17%, the toxic ethanol inhibits yeast metabolism. So, how do we get liquors with upwards of 75-90% alcohol? The answer is through distillation. Distillation is the process of separating liquids based on different boiling points.

Simply put, not all liquids evaporate at the same rate. Spill vegetable oil and water on your counter and over time, the water will disappear while the oil remains. Water evaporates much more readily than oil. But spill alcohol and water on your counter and the alcohol is likely to completely evaporate before you can even get a paper towel to sop it up.

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