Do You Stir Mash After Adding Yeast? 4 Things To Know

Brewing is like a chemistry experiment. You need to set the right conditions and procedures before you start. You have to know when to stir, or else the whole batch could be ruined. 

It’s not recommended to stir the mash after adding the yeast, especially after fermentation has begun. There are risks of contamination by bacteria or oxygen. It could also cause the yeast to clump together, and the beer would not ferment properly. 

Throughout this article, I’ll discuss yeast’s role in brewing drinks and the conditions it needs to work correctly. Keep reading to learn more about it!

Things To Know About Adding Yeast To Mash

Making beer is the process of turning grains into alcohol. The addition of yeast makes this transformation process possible. 

Brewing begins with mashing. In this step, the starch in grains is turned into sugars, which the yeast will use later to create alcohol.  

1. Fermentation Begins When Yeast Is Added To Mash

The next step in brewing is the fermentation of mash or wort by yeasts. Yeasts use sugars such as glucose and maltose and turn them into alcohol and obtain energy for metabolic purposes. 

For fermentation, the yeast can’t use pure starch or sugar. The glucose must come from grains that underwent mashing, which provides optimal nutrients as well as sugar. 

While you can’t use just pure or plain sugar as the only base material for beer brewing, you can use it as an additive to make the beer taste stronger. Learn more about it in my article about adding sugar to beer.

If the yeast cannot grow or multiply properly before the actual fermentation, it may not be able to convert sugar to alcohol efficiently. 

2. Yeast Converts Sugar to Alcohol Under Anaerobic Conditions

Fermentation is an anaerobic process. It can only proceed if there is no oxygen in the mash or wort being processed by the yeast. 

Should oxygen be introduced into the mash or wort during fermentation, it may be possible for the yeast to stop fermenting and proceed with aerobic ways of producing energy. 

This is why it’s essential to maintain anaerobic conditions once fermentation has begun.

Fermentation Formula 

If the sugar used by the yeast were glucose, the fermentation formula would be this: 

C6H12O6 (glucose)⟶2C2H5OH (ethanol) + CO2 (carbon dioxide)

Besides alcohol or ethanol, carbon dioxide is the only other byproduct of this natural fermentation. It makes the beer have a fizzy texture. 

Although carbon dioxide is a natural fermentation product, it can also be added manually. Here’s an article that explains how carbonation in beer works.

3. Stirring Mash Is Not Recommended After Adding Yeast

As I’ve mentioned, adding oxygen once fermentation has begun would risk affecting the brewing process. Stirring is one way of introducing oxygen and contaminants into a fermenting mash and it could ruin the whole brew.

Stirring May Introduce Oxygen Into the Mash

Stirring mash after adding yeast may allow oxygen to enter the mash or wort, especially when stirring from the top. This is something you don’t want to happen. Fermentation is an anaerobic process, which means it needs to be free of oxygen to proceed without disruption.

Stirring May Introduce Other Contaminants 

Besides introducing oxygen, you may also allow other contaminants, such as bacteria, to enter through stirring. This could result in infected beer.

Technically, infected beer is drinkable—that is, there aren’t harmful effects to drinking one. However, because of the sour and unpalatable taste, you would not want to do so. You’d end up with wasted beer.

You Can Stir When Fermentation Has Stalled

Although stirring, especially during fermentation, is highly discouraged, there are certain situations when it may be necessary. One of them is when the fermentation process has been stalled. 

Measuring the gravity is a way to check if the fermentation has stalled. If it’s dropping too fast or too slow, it has stalled. You can also use temperature to assess this. Ideally, it should be around room temperature. 

Once you’ve determined that fermentation has stalled, you can stir it for a bit to introduce oxygen. This oxygen would allow the yeast to thrive for a moment, kickstarting the fermentation process. 

Make sure that the spoon you’re using reaches the bottom and that the container is sealed immediately after stirring. 

4. Aerate the Mash or Wort Before Fermentation, Not After 

While it may seem like oxygen is the worst thing you can introduce to your brewing setup, it’s actually quite crucial. 

Aeration or adding oxygen to the mash or wort before fermentation allows the yeast to thrive and multiply. However, it should be done strictly before the process and not anytime after.

The yeast wouldn’t be able to multiply if the mash or wort wasn’t appropriately aerated. It would ferment less efficiently and at a much slower pace.

If the yeast does not multiply enough, there is a chance that your mash or wort will not have enough yeast for fermentation. They might all die off before the process is done, and you’d be left with beer with a weird taste.  

Late Aeration Affects Beer Flavor 

After the aeration and multiplication phase, the yeast should enter the fermentation or anaerobic process. At this point, oxygen should not be introduced unless the process has stalled.

Aerating too late could lead to the beer having a cardboard-like or leathery taste. This may be off-putting for many and lead to beer going to waste. 

Optimal Conditions for Yeast Are Crucial for Beer-Brewing

By this point, you probably understand why said brewing is like chemistry.

You need to know when you should and shouldn’t add oxygen, as well as when to stir and when not to and how much yeast you should add.

Ultimately, it all boils down to the fact that optimal conditions for yeast are necessary for producing a great-tasting beer. Slight changes could lead to drastic effects on flavor. 

Final Thoughts

Stirring the mash after adding the yeast is not a good idea. You risk disrupting the fermentation process that turns sugar into alcohol. Instead, make sure your mash has the optimal conditions for the yeast to thrive. 

About HomeBrewAdvice

Hello, my name is Simon. Together with a group of writers I write about brewing beer and making wine. We all share a passion for the great things in life, such as making stuff from scratch.

The business of HomeBrewAdvice is to bring you great information, stories and product reviews from brewing at home, and making wine


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