How to Test Yeast Fermentation

Yeast fermentation is perhaps the most important aspect of brewing in the entire industry. Without yeast fermentation, we would have no beer. Period. No wine. No leavened bread. No kombucha. And while it is critical to understand how yeast ferments and that it will ferment, it is also helpful to understand the strength of the yeast you’re working with. So, figuring out how to test yeast fermentation is a fundamental part of brewing.

Importance of Yeast Fermentation

It is no understatement to say that yeast is the most powerful ingredient in brewing or any other fermentation industry. Yeast is the living organism responsible for converting sugar to alcohol, and it is the only material, living or otherwise, that can do it.

It is a process that has been going on for millions, if not billions, of years.

Yeast, one of the oldest forms of life on earth, has a single goal, to stay alive. In order to do that, it needs energy. The source of energy for yeast is sugar. We call them fermentable sugars because some “sugars” like xylitol, cannot be broken down by yeast.

Test Your Yeast


But anything with glucose, maltose, fructose, lactose, or dextrose can be consumed by yeast, broken down, and digested.

Because yeast is such a simple organism – just a single cell with a single function – the digestive process is also simple.

It consumes sugar for energy, and then it produces carbon dioxide, water, and alcohol as “waste.”

Lucky for anyone who loves beer, wine, liquor, leavened bread, or anything else that requires alcoholic fermentation.

Why Is It Necessary to Test the Yeast Before Fermenting?

For thousands of years, humans have been fermenting with little trouble. Indeed, the first few times fermentation took place under human observation were likely accidents.

There’s a reason the Egyptians used to call wine the elixir of the gods.

It seems like a miraculous event.

After all, to make wine, you really just have to crush grapes and leave them out.

Within a couple of weeks, wild yeast will settle in, ferment the sugars in the grapes, and produce alcohol in the grape juice. Et voila! Now you have wine.

However, once industrialization kicked in, corporations commercialized yeast and sold it in mass quantities on the market.

Now, you can go to pretty much any market and find a little packet of dry active yeast on the shelf.

And that packet came in a large box with dozens of other packets.

Even brewers who buy liquid yeast from manufacturers are typically buying that yeast from commercial producers who cultivate and sell the yeast in large quantities.

This industrialized reality means that when your yeast arrives, while you might think it’s fresh, it might not be.

Thus, it is critical you test, or “proof” your yeast before you use it.

How to Test Yeast Fermentation

The easiest way to check to see if your yeast is alive is to feed it.

Place a packet of dried yeast, usually 2 ¼ teaspoons, or the same amount of liquid yeast into ¼ cup of warm water with 1 teaspoon of sugar.

Wait 10 minutes. If you notice the water fuzzing and bubbling, your yeast is alive and well and eating that sugar voraciously. You can pitch it into your wort with no problems.

How Do You Test Yeast Without Sugar?

If you want to properly test the viability of your yeast, rather than just checking to see if it is still alive, you can perform the dye test.

You’ll need:

  • Pasteur pipettes
  • Yeast
  • Cell counting chambers
  • Methylene blue or purple dye
  • Glass vials
  • Microscope

You’ll need to:

Mix 0.1 grams of your blue or purple dye with 100 mL of distilled water in a glass vial and mix well.

Now, place the equal parts of the solution and the yeast into a glass vial and mix well.

Using a pipette, place some of the dyed yeast solution onto the cell counting chambers.

Under a microscope, count how many of the cells have absorbed the dye and how many have not.

The cells that have absorbed the dye, and so appear blue or purple, are dead. Living (viable) yeast cells will process and eliminate the dye.

To find the percentage, simply add the total number of cells and divide the number of undyed cells by the total number of cells.

For example, if you have 10 total cells and 8 of them are not dyed, then you have 8/10, which is 80% viability.

Viability is a critical factor in fermentation and brewing as a yeast culture with low viability can stagnate and even die off during fermentation.

Thus, while your yeast might look alive and well and feeding on your warm cup of sugar, it may not perform so well in a large batch of wort.

Instead, it could get overwhelmed and stress.

And even if it does perform, it may underperform and produce off flavors that are difficult to get rid of.

In general, brewers like to see a minimum of 85% viability with a preference for well over 95%.

In the end, you can proof your yeast with the warm water test, but if you are preparing to pitch into wort, it is always a safe bet to run the viability test.


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