How to Measure the Rate of Fermentation in Yeast

If you’re wondering how fast your yeast is fermenting, you’re not alone. There are a couple of reasons for this question. First, you might just be taking notes and making calculations with a specific yeast, in case you might want to use it again, you’ll know what to expect. Second, you may just be interested to know what steps your fermentation process is undergoing without your intervention. Either way, the process is an easy one to understand.

The Role of Yeast in Fermentation

Yeast plays a primary role in fermentation as it is the sole ingredient you cannot ferment without. That’s right. You can work with pretty much any grain or fruit, starch or vegetable — anything with sugar. You can add cane sugar, honey, or agave syrup. You can add hops and other herbs.

But if you don’t have yeast, you won’t have wine, beer, or any kind of alcoholic beverage.

Fortunately, the earth is abundant with this single-celled organism so eager to consume your sugary beverage and turn it into a frothy beer or tannic wine.

Yeast is a simple life-form. Like most cells, it feeds on glucose for energy and it undergoes a process called cellular respiration, where it essentially excretes the waste from the sugar it took in.

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You see, yeast will not use up all of its energy at once. It will consume the sugars it encounters and then use bits of energy at a time. Like all cells and all organisms, it must then get rid of what it no longer needs.

That waste, for yeast, comes in the form of water, alcohol, and carbon dioxide.

When oxygen is present, yeast will excrete water and CO2. When oxygen is absent, it will excrete alcohol and CO2. We have come to call this latter process fermentation.

Interestingly enough, yeast seems to prefer anaerobic fermentation, as you’ll notice when it is consuming sugars on any food or beverage, it creates its own airtight bubble so that it is forced into anaerobic fermentation and the creation of alcohol.

The Steps of Fermentation

Which brings us to the steps of fermentation.

When you make any alcoholic beverage, the key is to first prepare your beverage so that your yeast has maximum and easy access to the sugars inside.

For this reason, brewers will often ensure their grains have been malted, or germinated, which means the disaccharides, or complex sugars, are broken down into monosaccharides, or simple sugars. The grains are soaked until they sprout and then dried. The next step is to toast the grains and grind them so the sugars inside are exposed. Finally, the grains are boiled and steeped in water to flood the water with those grain sugars.

With wine, the grapes are harvested at their peak ripeness to ensure the most sugars are exposed. The grapes are then crushed to create a juice.

Now, the first step in fermentation has already begun. You see, the grapes have yeast on the skins and yeast is everywhere in the atmosphere. Leaving either of these liquids exposed to open air for even a few minutes will allow yeast to enter the liquid and begin fermentation.

Of course, in today’s world, most brewers and winemakers will add their own yeast.

Once you lock the fermenter, it is typically a matter of minutes and no more than 72 hours before you will notice signs of yeast activity in your wort or must.

These signs of fermentation are the bubbling and fizzing that creates the foam on top of your fermentation vessel — this is the CO2 created.

This CO2 will then form a crusty cap on top of the liquid, often called krausen. For this reason, this part of the fermentation process is called krausening. This is the part of fermentation when yeast protects the liquid with its own airtight bubble and reaches the height of fermentation where it consumes the most sugars and excretes the most alcohol.

Finally, you will notice the krausen begins to die down and fade away, at which point you can be pretty sure primary fermentation is ending.

Once the foam is gone and you don’t see visible signs of fermentation, you can guess primary fermentation is complete.

How to Measure the Rate of Fermentation in Yeast

Thus, there are a few ways you can measure the rate of fermentation.

You can time it from beginning to end. Calculate how many days it took for the yeast from the time you put it in the liquid to the time the last bit of foam disappears. This will give you a rough estimate of time.

You can also measure the CO2 volume in your fermenter by checking how high the flow of carbon dioxide rises over time and calculate your rate of fermentation that way.

Finally, and most typically, you can take a specific gravity measurement before fermentation begins and once it has completed.

Gravity of a liquid tells you how it compares to water, which has a gravity of 1.0.

When you add sugar, which is much denser than water, your specific gravity will be higher than 1.0. By the time yeast has consumed the sugars, however, you will have mostly alcohol, which is less dense than water.

So, your OG, or original gravity reading, could be something like 1.050, and your final gravity, or FG, will be closer to 1.010.

When you take these readings throughout the fermentation process, this offers another way to measure the fermentation rate of your specific yeast.

The best approach when taking calculations and recordings for posterity is to do all three.

Then, you’ll be able to report that it took this particular strain of yeast, for example, 7 days to go from 1.050 to 1.010, and the CO2 levels rose by this much and dropped by that much.

The best purpose for keeping these recordings is just so you know what to expect next time you use this yeast.


Are you still pitching fresh yeast every time? By reusing your yeast, you can save up to hundreds of thousands of dollars per year on just yeast alone!

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