Brewing Science - Glycerin

Glycerin - Glycerin

Our topic is "Glycerin."

This might be another dry topic, but since it has a huge impact on the flavor of alcohol, I hope you'll read on if you're interested!

What is Glycerin?

Glycerin (also known as glycerol) is a colorless, odorless, viscous liquid. Chemically, it is a type of compound called a "trihydric alcohol."

It is originally produced when fats and oils from animals and plants are broken down, and it is sometimes used as a food additive for its properties as a sweetener and moisturizer.

It is also the third most abundant byproduct of yeast fermentation, following ethanol and carbon dioxide.

Because of its viscous and sweet characteristics, it is said to have a positive effect on the body and richness of alcoholic beverages.

Aside

Glycerin plays two important roles in yeast fermentation.

1. Osmotic Pressure Regulation

When yeast is exposed to osmotic stress (such as high concentrations of sugar or salt), moisture inside the yeast cells rapidly moves outside the cells. To compensate for this osmotic difference, the yeast accumulates glycerin to adjust the osmotic balance of the cell membrane and maintain survival.

2. Maintaining Redox Balance

When yeast undergoes fermentation, a substance called NADH is produced during the process of breaking down glucose to obtain energy. Normally, in an environment with oxygen, mitochondria "oxidize" NADH to create energy; however, in an oxygen-free environment, this mechanism cannot be used, so a method to handle NADH is necessary.

Therefore, yeast consumes NADH by synthesizing glycerin to regenerate NAD⁺ (the oxidized form of NADH). In other words, glycerin synthesis plays the role of consuming the excess NADH generated by fermentation and supplying the NAD⁺ necessary to continue the fermentation process.

The Effect of Glycerin on Mead

The effects of glycerin are:

• Providing a sense of sweetness

• Smoothing the mouthfeel

These are the two most significant elements.

Let's look at them in more detail.

・Providing a sense of sweetness
Although glycerin is not a sugar, humans perceive it as being about 60% as sweet as table sugar. This is said to be because the molecular structure of glycerin binds easily to cells that perceive sweetness. Consequently, in wines with high glycerin content, one may perceive sweetness even if there is no sugar present.

・Smoothing the mouthfeel
Because glycerin is viscous and has high density, it works in tandem with the sweetness to soften the harshness of alcohol and astringency, creating an effect of "smoothness" and "roundness."

In one study, it was confirmed that adding 0.3% to 2.0% of glycerin to non-alcoholic beer improves its body. However, there are also research results suggesting that in high-sugar wines like ice wine, which already possess sufficient body and mouthfeel, the influence of glycerin is harder to perceive.

In mead as well, it could be said that the influence of glycerin varies significantly depending on whether it is dry or sweet, and whether it has low or high alcohol content.

Should Glycerin be Added?

The amount of glycerin in typical alcoholic beverages is said to be 1–3 g/L for beer, and 4–9 g/L for wine and mead.

Research suggests that an appropriate amount of glycerin is desirable, with some studies indicating 7.8 g/L as optimal for mead.

On the other hand, excessive glycerin can make the mouthfeel feel too heavy or unnatural, and some people dislike the fact that it was not produced naturally through fermentation.

Pros of Glycerin

• Smoother mouthfeel, making it easier to drink

• Provides a sense of sweetness and improves body

• Softens astringency and bitterness

Cons of Glycerin

• Too much can lead to an unnatural taste

• Some people dislike the addition of glycerin

I personally conducted an experiment in the past by adding 2g/L, 4g/L, and 6g/L of glycerin to a semi-dry mead, and I found the 2g/L version to be the most delicious.

I did not measure the original glycerin content, but the 4g and 6g versions tasted quite sweet, and the 6g version also had a noticeable viscosity. Ultimately, I felt that 2g was the best for balancing the sweetness, acidity, and mouthfeel.

In other words, when using glycerin, it is necessary to judge the appropriate amount to add by considering the alcohol content, sugar content, pH, aroma, and flavor in a comprehensive manner.

Aside: Ethanol and Glycerin

Glycerin is a byproduct of fermentation and has a strong correlation with ethanol production. During yeast fermentation, glucose bonds with phosphate to create two compounds:

・Dihydroxyacetone phosphate (DHAP)

・Glyceraldehyde-3-phosphate (GAP)

These two are created in a 1:1 ratio, but because yeast requires a large amount of GAP to produce ethanol, most of the DHAP is converted into GAP. As a result, only a portion of the DHAP is converted into glycerol. This means that, fundamentally, the higher the amount of ethanol, the higher the amount of glycerin.

How to Increase Glycerin

Beyond adding glycerin, there are ways to increase it during the fermentation process itself.

・Yeast Selection

Yeast is the most significant factor influencing glycerin production, and it fluctuates depending on yeast growth and environmental factors.

Glycerin production volume varies based on the type of yeast used, sugar content, and raw materials. It is also affected by the available nitrogen source, particularly the type of amino acids. In nitrogen-deficient conditions, it is said that adding nutrients can increase glycerin by 0.5 to 1.5 g/L depending on the yeast strain.

The yeast strain Lalvin S6U is said to have an extremely high glycerin production yield.

I'd love to try it, but it doesn't seem to be available in Japan...

・Sugar Content and Temperature

The higher the sugar content of the must (the liquid before fermentation), the higher the glycerin production. This is because cells produce more glycerin to withstand osmotic pressure. It has also been confirmed that higher temperatures increase glycerin levels. Some research results show that exposing it to heat at 30–50°C for 20–120 minutes increased glycerin by about two times.

According to research on optimal production conditions for mead, fermenting at 24°C results in a mead with a moderate and well-balanced glycerin concentration.

Also, high temperatures can have drawbacks, such as an increase in acetic acid, so blindly fermenting at high temperatures is not necessarily beneficial.

There are other methods, such as manipulating the yeast's genes, which are beyond the reach of an amateur, so it's probably best not to overthink it and just try out various things.

***

I took a deep dive into glycerin.

Honestly, it is difficult to determine whether it has been added to a mead or wine if it is within an appropriate range.

However, I think it is worth utilizing as a technique to make beverages more delicious!