Why Is Glucose the Best for Yeast Fermentation?

If you’re relatively new to the brewing, winemaking, or fermentation worlds, you may be wondering why the sugar matters. You probably know already that yeast cells need sugar to survive, but why does the type of sugar matter? And why is glucose the best for yeast fermentation? To understand the answer to this question, it helps to begin at the beginning.

The Internal Mechanisms of Yeast

Yeast is a living organism that is thought to be billions of years old. It apparently evolved from bacteria, developing its own enclosed nucleus and becoming, presumably, the first eukaryote, as opposed to the prokaryotes which had previously inhabited the earth alone.

This evolutionary design allowed yeast and then other eukaryotes to compete for food in new ways, giving them advantages over prokaryotes.

Since that time so long ago, yeast has been on the search for its one energy source – sugar.

In nature, yeast cells can find sugar in fruits, vegetables, and even starches, and, thanks to French scientist Louis Pasteur, we discovered that yeast is responsible for what we came to understand as fermentation, all thanks to yeast’s fondness for sugar.

The Role of Yeast in Fermentation

For as long as there have been yeast cells, there have been sugars in nature, and yeast have taken advantage of that persistent truth.

Consuming sugar allows yeast to continue to grow and reproduce, typically asexually. As waste, yeast release alcohol, water, and carbon dioxide. This act of consuming sugar and “converting it” to alcohol and carbon dioxide is what we now call alcoholic fermentation.

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Fermentation simply refers to the chemical transformation that takes place when enzymes convert one chemical to another.

Thus, alcoholic fermentation has been taking place since the beginning of the lifespan of yeast, completely independent of humans.

Fruit grows on trees and vines, ripens, drops to the forest floor, and yeast lands on the fruit and gets to consuming.

You could ostensibly walk into the forest, select overripe fruit that has undergone fermentation, and get a buzz from the alcohols in the fruit.

Animals do it all the time.

Because of this natural process, humans likely discovered fermentation entirely by accident. A barrel of grapes could have been sitting for a few weeks, the crushed grapes at the bottom were fermented by the local wild yeast cells, and the grape farmer would have discovered wine.

The winemaking process is not much different today than it was hundreds of thousands of years ago when humans first began mixing this lovely elixir.

When it comes to beer, we begin to understand the importance of the types of sugars a bit better.

While fruit like grapes is rich in fructose and glucose, grains are rich in another kind of sugar – maltose.

Glucose is the easiest sugar for yeast cells to break down. Fructose comes next in terms of ease of consumption. Maltose is harder for yeast cells to break down.

For this reason, beer in the early days had a much lower alcohol content than it does today. Most families drank beer brewed specifically brewed for the family, which may have contained close to 1% ABV.

Overtime, humans caught to the fact that germinating the grains encouraged an enzymatic process. The enzyme maltase converts the maltose into glucose during germination, which is really just a way of tricking nature.

Grains are soaked and then dried, mimicking a kind of photosynthesis, making the grains sweeter and more attractive to yeast.

Fermentable Sugars

Now, why is glucose so much easier for yeast cells to break down?

It’s all about monosaccharides versus disaccharides.

Also read: Why Does Yeast Ferment Glucose Faster than Fructose?

The building blocks of all sugars are monosaccharides and disaccharides. Monosaccharides are composed of one simple sugar and disaccharides are composed of two monosaccharides. Glucose is a monosaccharide, and maltose is a disaccharide.

So you see, when yeast is looking for food, for an energy source, the simpler the sugars, the easier the consumption process will be. The harder it has to work to break down sugars, the more energy it expends, and it can end up stressed, overwhelmed, and even lie dormant or die.

Yeast need to make sure they take in more energy than they expel.

Thus, when making wine, beer, or anything else we hope to ferment, it is in our best interest to ensure the sugars we are feeding to the yeast are easy to break down, or rich in glucose and fructose, both monosaccharides.

When we have maltose in grain, we can facilitate a better brewing process by ensuring we germinate the grains so as many of those maltose sugars convert to glucose as possible.

Of course, you can play around with leaving some of the grains ungerminated, which could add more texture and, at the same time, less alcohol.

Brewing and fermentation are all about experimentation, exploration, and figuring out what works for you and your loyal customers.

Cheers!

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