How Does Sugar Affect Yeast Fermentation?

Anyone in brewing has wondered about yeast and sugar and how they coexist. You find yourself wondering, exactly how does sugar affect yeast fermentation? After all, you need both in order to brew, but how? Why? What exactly happens?

Sugar Fermentation by Yeast

First, it is important to understand that the only way to get yeast to ferment is with sugar.

Yeast is a living organism with an enclosed nucleus, and it has been around for millions, and more likely billions of years.

Scientists have theorized that yeast actually evolved from bacteria in order to better find energy sources and continue to reproduce.

From its earliest origins, yeast has fed on sugar.

Yeast cells will hover in the environment, anywhere that does not have extreme heat or cold, like the Antarctic or a volcano. It will seek out any source of fermentable sugar. It will consume that sugar, and then it can continue to thrive and reproduce.

These sugars are found in abundance in nature, whether from fruits and vegetables, starches and grains, or even honey.

The process of yeast consuming sugar for energy and converting that sugar into either alcohol or water, and carbon dioxide, is called fermentation.

Truly, fermentation just refers to the transformation of one chemical to another through an enzymatic process.

To Summarize the Process of Alcohol Fermentation in Yeast

In terms of alcohol fermentation, it is important to note, first, that whenever yeast consumes sugar in the absence of oxygen, it converts that sugar to alcohol and carbon dioxide as waste products. In the presence of oxygen, it converts the sugar to water and carbon dioxide.

It is also interesting to note that the fermentation process itself creates a kind of oxygen barrier or cap around itself, so it will almost always convert to alcohol.

For example, when beer or wine are made, the brewer can brew in an open vessel and still end up with an alcoholic beverage.


Well, the beginning processes are pretty similar.

Grapes are crushed and sediment is filtered out to make wine. Grains are ground and boiled and the grains are filtered out of the water to make beer.

The must (from the grapes) and the wort (from the grains) are both then mixed with yeast.

Both liquids are rich in sugars, either from the grapes or the grains, and the yeast will be hungry for those sugars and get right to work.

In earlier times, the liquids would simply be left out and yeast in the environment would be attracted to these sugar-rich beverages.

Today, most brewers or winemakers add their own yeast.

Either way, the yeast gets into the liquid and begins consuming the sugars almost immediately. As it does so, a film of carbon dioxide is produced and lies over the liquid like a cap. You can witness the foaming and bubbling as fermentation takes place.

That cap traps whatever oxygen is already inside and blocks any more oxygen from entering. It is a perfectly natural way of ensuring that the yeast cells can do their work undisturbed.

Then, when fermentation is nearing completion and the alcohol has been created, the cap begins to dissolve. In fact, you know fermentation is complete when the fizzing and bubbling stop and the “krausen,” that foamy cap, dissolves entirely.

How Does the Type of Sugar Affect the Rate of Yeast Fermentation?

While our early ancestors all worked with fermentable sugars, not all fermentable sugars interacted with yeast in the same way.

Fructose and glucose are highly fermentable sugars while maltose is harder to break down and convert for yeast cells.

This concept explains why we see much higher alcohol percentages in wine than in beer.

Wines are rich in the fructose and glucose found in grapes, which are also extremely high in those sugars.

In contrast, grains are not rich in sugars, and the sugars they do hold are called maltose. Indeed, brewers often germinate those grains in order to get some of the maltose in the grains to convert to glucose and make it easier to ferment for yeast.

For this reason, wine has a much higher alcohol percentage than beer.

More recently, brewers, winemakers, and others in the fermentation industry are wondering if they can use alternative sweeteners to ferment.

The answer is, sadly, no.

Stevia, Splenda, Xylitol, Erythritol, and Lactose are all non-fermentable sweeteners that can be added after fermentation is complete to sweeten an alcoholic beverage, but they cannot be fermented by yeast.

How Does the Amount of Sugar Affect Yeast Fermentation?

Finally, the amount of sugar absolutely affects yeast fermentation. Essentially, the more sugar you have, the more alcohol you will produce.

At the same time, it is important to understand that yeast can be overwhelmed by too much sugar.

In nature, yeast cells know how to manage their intake. A certain amount of yeast will descend on a piece of ripening fruit on the vine. Likewise, a certain amount of yeast will descend on a fermentation vessel left out in the open air.

However, if a brewer or winemaker is attempting to control the environment and makes a miscalculation, stagnant fermentation may happen.

If the brewer adds too much sugar, yeast could get overwhelmed, stressed, and lie dormant.

In contrast, if the brewer adds too much yeast, with too little sugar for the yeast to consume, the yeast may produce at a too rapid rate that can result in excessive amounts of carbon dioxide, which creates bursting bottles and blocked airways. This accident can also produce off-flavors.

Ultimately, yeast knows exactly what it needs in nature. It is up to humans to explore and experiment to find that perfect balance.


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