Thursday 19 May 2022

Should I Rehydrate Dry Yeast Before Pitching?

There are a number of processes and factors to consider when brewing your own beer. Everyone wants to make the best beer they possibly can, but often want to keep things as simple as possible in the process of doing so. A common question that brewers will ask themselves is "Do I need to rehydrate my dry yeast before pitching it into my wort?".

The answer to this question can be quite contentious, and will often come down to personal preference and experience. Let's dive deeper and take a look.

First of all - what is rehydrating yeast? As the name suggests, it involves converting your dry yeast into liquid yeast by adding it to water. One of the most popular yeast manufacturers, Lallemand, recommend sprinkling the dry yeast onto the surface of 10 times its own weight in clean, sterilised water at 30-35C (86-95F), then leave it for 15 minutes before gently stirring and adjusting the temperature before adding to your wort. (Check out Lallemand's Best Practices for Rehydration for more information on this practice).

The alternative to rehydration is the simple process of sprinkling the dry yeast onto the wort after it has been added to your fermenter - or alternatively, adding the dry yeast to your fermenter whilst it is being filled with wort.

So what is the problem with doing it this way? It's argued that simply adding the yeast to your wort directly will lead to a percentage of the viable yeast cells being killed off, thus reducing the number of viable yeast cells available to ferment your wort. This kill off rate has been reported to be as high as 50%. This in turn can lead to an increased workload for the remaining  yeast, which can cause the yeast to become 'stressed' and impart undesirable off flavours and aromas into your beer throughout the fermentation process.

Unfortunately there isn't much hard, scientific evidence to backup any of these claims (at least that we were able to find) - which is why we previously mentioned that for most brewers this decision will come down to personal preference and their own experience.

One of the most popular manufacturers of yeast, Lallemand, state that "rehydration is recommended (but not essential)" in their Best Practices for Rehydration guide, as it "reduc[es] the osmotic stress and enhanc[es] a homogenous disperson".

One of the most popular brewing yeasts - Safale US-05

On the flip side, Fermentis state under the technical features for what is arguably one of the most popular yeasts used in brewing, US-05, that direct pitching can done by following these steps;

"Pitch the yeast directly in the fermentation vessel on the surface of the wort at or above the fermentation temperature.

Progressively sprinkle the dry yeast into the wort ensuring the yeast covers all the surface of wort available to avoid clumps. Ideally, the yeast will be added during the first part of the filling of the vessel; in which case hydration can be done at wort temperature higher than fermentation temperature, the fermenter being then filled with wort at lower temperature to bring the entire wort temperature at fermentation temperature."

Another popular yeast manufacturer, Mangrove Jacks, state that with regards to rehydrating yeast "No, you don't have to. You can sprinkle the yeast as is."

If you are going to pitch dry, it's important that you follow the yeast manufacturers guide in terms of dosage to ensure you are pitching a sufficient amount of dry yeast for the gravity of wort you are fermenting. This will of course vary between yeast strains and manufacturers.

Temperature is another potential factor - with thermal shock being cited as a potential reason for dry yeast cells being killed off as a result of pitching dry yeast directly. Dry yeast should typically be stored in a fridge to  help preserve shelf life and yeast health, but it's important the yeast is removed from the fridge and allowed to warm up to room temperature before being used. Sounds pretty obvious but taking your yeast out of the fridge that is likely set to around 5 degrees Celsius and pitching it directly into your wort at around 20 degrees is a bad idea and will likely lead to poor results.

Popular homebrewing blog site, Brulosophy, have run a couple of exbeeriments (experiments) directly comparing the results of two otherwise identical beers made using dry pitched and rehydrated yeast. The first experiment from back in 2014 had 5/13 people correctly identify the odd beer out in a blind triangle test of the two beers which were a Munich Dunkel made with Nottingham yeast. Of the 5 who did accurately identify, the majority went on to report essentially no difference between them.

Their second experiment, run 4 years later in 2018, yielded a similar result when brewing the same recipe using the same yeast. In this experiment, only 11 out of 24 people were able to identify the unique beer in a series of blind triangle tests. The author even admits he was guessing when performing the tests.

A point made in the authors discussion of the second experiment seemed to resonate with us - and that is that it's possible that only a small percentage of batches made by directly pitching yeast may come out "bad" or have a noticeable off flavour or other problem. But as a yeast manufacturer, you wouldn't want to risk any batches coming out bad, which is why most manufacturers still recommend rehydrating yeast, but don't insist on brewers doing so. Perhaps it's just a matter of risk, and perhaps an element of luck? After all, once the yeast has been pitched, as brewers nearly everything aside from fermentation temperature is out of our control.

Dry yeast sprinkled on top of our wort

For us, we've always directly pitched our dry yeast by sprinkling it on top of our wort after adding it to the fermenter, and we've made some awesome beers doing it this way. To be fair, we've never rehydrated yeast so haven't done a like for like comparison, but certainly the argument of imparting off flavours from stressed yeast isn't true in our case. We've been dry pitching using dry yeast from Lallemand, Mangrove Jacks and Fermentis. We've always had our yeast warm up to room temperature before pitching, and in most cases have pitched it while the wort is a little bit warmer than our target fermentation temperature (eg. 22-24C with a target fermentation temperature of 20C) - but this is more due to our impatience with chilling our wort after the boil than a deliberate effort to help with yeast health.

Will we try rehydrating our dry yeast in the future? Potentially, but for the time being we just don't see a compelling need to, as we're perfectly happy with the results we're getting with dry pitching. There are definitely some use cases for yeast starters, such as when re-using yeast you've top cropped or captured from a previous fermentation, or from a commercially available beer, but that's a whole other conversation. When it comes to dry yeast, we'll keep on pitching dry, for now.

Of course there will be those who always rehydrate their dry yeast before pitching and that's also perfectly fine. As stated at the beginning of this post, for most brewers this decision will come down to experience and personal preference. If you've always rehydrated your yeast and get excellent results doing it this way, why change it?

If you're undecided or on the fence, then see for yourself. Try pitching it dry first, especially if you're a beginner - as this is the simplest method. Once you're more comfortable/familiar with your brewing process, try rehydrating and see if you can notice any difference.

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  1. Good survey of the available info, but you've mis-characterised rehydration. As the name implies, rehydration is absorption of water, not wort, by the dried yeast granules.

    1. Thanks Jonathan - you are absolutely correct. I've adjusted the wording in that section accordingly. Cheers!