Homebrew tips: Is it ok to use Expired Yeast?

During the Covid-19 lockdown, we have been asked by many customers about using old kits. The conversation goes something like this: I stopped brewing a while ago, and have found an old kit in my garage. It's out of date, can I still use it? And the answer is always: yes it's probably fine but it may not be ok to use expired yeast.

Liquid malt extract and hops lasts years and years. Yeast however is a living organism. There is always a 'best before' date on a packet of yeast, but even if that date is passed, it doesn't mean you can't use it. 

Is it ok to use Expired Yeast?

The answer is: possibly, but only if it's still alive. Modern yeasts are usually well packaged and sealed, and can last well beyond their 'best before' date. However what do you really don't want to do is start making any home brew without knowing whether your old yeast has died. Because if the yeast fails, you risk the whole batch spoiling before you've been able to make another attempt with fresh yeast.

I recently got a tub of Young's All Purpose Yeast out of the cupboard to make something, only to realise that the best before date had lapsed 2 months previously. Now we run a Home Brew Shop so, luckily for me, all I have to do is sell myself another one. However we've been in lockdown, and some of our products have been in short supply. So I didn't want to take another yeast from the shop if I didn't need to. Besides, why waste the yeast if it works?

Using Expired Yeast

The good news is: there is a simple way to test old yeast. Sometimes known as proofing yeast, all you are going to do is attempt to activate the yeast by giving it a liquid to work in and sugars to feed on.

How to Test whether it is ok to use Expired Yeast

Testing old yeast

You will need:

  • half a teaspoon of sugar
  • half a cup of luke warm water (ie you can feel it's slightly warm but no where near hot, just blood temperature)
  • one teaspoon of the expired yeast

and

a small bowl.

Method

1. Put your luke warm water into the small bowl.

2. Add the sugar.

3. Stir until the sugar has dissolved.

4. Add the teaspoon of expired yeast.

How to test expired yeast

5. Go away for 20 minutes.

How will I know whether my Yeast is still Active?

If your yeast is still alive, you'll start seeing signs very quickly.

This is how my yeast, sugar and water looked when first mixed together:

Testing expired yeast

This is how it looked after 10 minutes:

Is my yeast dead

Yeast works by eating sugar, and then excreting carbon dioxide and alcohol as byproducts. 

So the foam forming on the top is a clear indicator that this yeast is very much alive. And as I watched the area where the foam hadn't yet reached, I could see some tiny explosions under the surface where the yeast was working away.

So now I know that this yeast is fine to be used. However, I kept going. Just for fun :)

This is how it looked after 20 minutes.

Testing whether yeast is dead

 

You can clearly see bubbles in the foam, which has completely covered the surface.

And this is how it looked after 30 minutes.

Active Yeast Testing

If you look carefully, you may be able to tell that the surface of the foam now has a bulge in it, where the foam is building.

Eventually this yeast will run out of sugar and stop producing gases, at which point it will stop working.

What if my Yeast doesn't do anything?

If after half an hour or so there is no sign of life in your yeast, then your yeast is dead. So don't be tempted to use the rest of it, discard it instead.

You can compost old yeast. If it is in any way active, it will speed up the composting process. If it is completely dead it will still decompose.

When someone asks us if they can use an old beer kit with a 'best before' date which has passed, we usually say yes. But we advise them to purchase another packet of yeast so that, if their yeast is no longer active, they can make the kit with a fresh yeast rather than discarding the whole kit. 

In fact, it's useful to always have a spare sachet of yeast around when you're brewing. Yeasts that are in-date very rarely fail, but I like to have spare yeast around. That means I'm prepared if that ever were to happen, rather than risk spoiling a whole batch whilst I try to source another packet of yeast.

And finally: never chuck out your old yeast before you've tested it first. You just never know!

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 Proofing old yeast

 

 

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