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Ultimate Guide to Yeast Starters for Homebrew

Written by Josh Charig

2 August 2022

I’ve said it before, I’ll say it again: fermentation is the most important part of brewing, and good yeast health and pitch rate will result in a good fermentation. Yeast health and cell count can be drastically improved through making starters. Plus, knowing how to make starters and treat yeast will open many new doors to the homebrewer.

Through making starters, it’s possible to culture up out of date yeast, use yeast from bottle conditioned beers, build up a cell count for a high gravity beer, and possibly many other uses. It means there’s less reliance on getting healthy liquid yeast arriving within a couple of days of brewing, using solely dry yeast, and opens up the possibility of storing yeasts in a yeast bank.

There are a few caveats to doing this:

Sanitisation is VERY important! Whilst this is important at any cold side stage of brewing, it becomes more vital when dealing with yeast. We don’t want to accidentally culture up a wild yeast strain which will ruin the beer.

When culturing up from a bottle, be aware of what you’re growing, some breweries filter out the primary fermentation yeast and add a yeast specific for bottle conditioning. These types of yeasts won’t make a good beer so where possible try to find out. Similarly, I’ve previously cultured yeast from a can of saison not knowing it was mix culture, and had to wait months for the brett to ferment out.

The example I’m using for these instructions is Kernel’s Bier De Saison. It’s a mixed culture saison (it said this on the bottle so no surprises this time) and contains brettanomyces as well as saccharomyces, but unfortunately there’s no more information on specific strains.

I use a stirplate for my starters. They can be bought for quite cheap on eBay, but if you’re handy at all they can be built for <£10. I have two borosilicate glass erlenmeyer flasks which can go on a kitchen hob. It’s possible to not use a stir plate, but the process will take a bit longer.

Start here if culturing yeast from a bottle, a very old yeast pack or you know there’s a very low cell count.

Step 1: Cold Crash

Once you’ve found a bottle/can of beer containing yeast you want to grow, place it in the fridge for several days. This will help the sediment stick to the bottom, make the bottle easier to pour and there’ll be less yeast loss. After about three weeks in the fridge, you can see on this bottle of Bier De Saison there’s actually a fair amount of yeast for a commercial bottle of beer.

Ideally use the sediment from two or more bottles where possible. Yeast packs don’t need to be cold crashed but should be stored in the fridge until they’re ready to use.

Step 2: Make a Small Starter

This step requires DME, water, erlenmeyer flask, foil, scales, measuring jug.

This process will involve making a few starters which grow in size. The idea is to gently increase the cell count and viability until it’s big and healthy enough to munch away at 25 litres of wort. Underpitching, or pitching unhealthy yeast, will lead to off flavours in the final beer.

The idea of this first starter is to bring the sleeping yeast back to life and increase viability before really growing it. Make a starter using:

  • 200ml water
  • 10g light DME

Mix both together and boil on the stove for 10 minutes to sterilise. As you can see I put the erlenmeyer flask directly on the stove with the stir bar inside. This sterilises everything at once. Just make sure your erlenmeyer flask is suitable to put directly on a stove!

Just before the 10 minutes are up, create a cold water bath for the erlenmeyer flask.

Once boiled for 10 minutes, using oven gloves, remove from the hob and cover the top with sanitised foil (I spray it with Chemsan). Swirl the wort in the flask for a minute and plunge into the cold water bath. You might find it helpful to run cold water over the flask whilst swirling to cool down quicker. Again, make sure your flask can handle such drastic temperature changes without shattering.

With such a small amount of liquid it won’t take long to cool. Leave it in the bath and every couple of minutes or so come back and give it a swirl. Once at room temperature, it’s time for the fun bit: drinking beer!

Step 3: Getting the Yeast

This step requires a glass big enough for the bottle contents, the beer, sanitiser spray and the starter.

With the yeast nicely settled, get your Chemsan spray and spray the lid and neck of the bottle/ the whole top half of the can. If using a bottle, spray the bottle opener. If in doubt, spray it!

Once everything is sanitised, open the bottle and carefully pour into the glass, making sure not to glug the beer. Leave about a centimetre of beer in the bottle. Some yeast might make its way into the glass, as long as the majority stays in the bottle it doesn’t matter.

Spray the top of the (almost) empty bottle with sanitiser. Swirl it around to mix the sediment into the remaining dregs. Take the foil off the erlenmeyer flask and spray the edge. Pour the bottle dregs into the flask, spray the foil and cover up again.

Alternatively, if you’re drinking the beer and don’t have a starter to pitch into yet like me, pour the dregs into a clean and sanitised jar. This can then go in the fridge until you’re ready to use.

Step 4: Get Started

This step requires starter, stir plate.

Place the starter on the stir plate. It might help to move the starter around the stir plate until you hear the magnets engage with each other.

Leave it for a few days. This can take anywhere from two to six days, sometimes more. It really depends on how much yeast is available, the strain, and the health of the yeast. It can be difficult to know when it’s done as this stage doesn’t always krausen, and rarely does. Usually when the bubbles have died down is a good indication.

This stage is necessary to get the yeast into a healthy state again.

Here’s what my starter looked like after 4 days:

Whilst no krausen, it’s turned cloudy which is the increased amount of yeast in suspension. Once it looks like this, it’s time to grow the starter.

Step 5: Grow

This step requires DME, scales, water, measuring jug, a pan, the existing starter, sanitiser spray.

Measure out 25g of DME and place into a saucepan. Add 250ml water and stir until dissolved, then boil for 10 minutes to sterilise. Cool as quickly as possible (detailed in step 2) and add to the starter.

The erlenmeyer flask is dangerously full, but again a krausen probably won’t happen. The idea at this stage is to gently increase cell count for the next stage. After a day or two, mine did get a small krausen appearing but am leaving it a bit longer to finish.

I decided to cold crash my starter before going onto the next step. As you can see, the yeast has settled to the bottom and is looking nice and healthy. You don’t need to do this but I’m in no rush to use this yeast.

Start here if you have a liquid yeast pack you want to revive or isn’t too old, or you just want to grow the cell count.

Step 6: Full Growth

This step requires DME, scales, water, measuring jug, starter or yeast pack, a pan/larger erlenmeyer flask, sanitiser spray

By now, you should have enough healthy yeast to put into a large starter or a suitable pack of liquid yeast.

The majority of homebrew size batches will only need this step, but in some circumstances another step will be necessary, for example an imperial strength pilsner. The size of the starter will differ, for a clean yeast character (e.g. pilsner or West Coast IPA) a larger starter is recommended.

However, a 500ml starter is usually enough for most ale types and maybe even some lagers, so that’s what I’m going for here. I have a 3L erlenmeyer flask in which I measured out 50g of DME and 500ml water. I mixed them together and boiled for 10 minutes to sterilise. This can be done in a separate pan if you don’t have multiple erlenmeyer flasks.

Once cooled down, pour the yeast and the starter into the erlenmeyer flask and place on the stirplate. After a few days or less you should get a krausen forming. Once it dies down the yeast is ready to pitch.

Step 7: More Growth (optional)

Clean beers – West Coast IPAs, lagers etc – or imperial strength beers will benefit from a higher cell count and therefore should go through this extra step. Some yeast forward styles, like certain wheat beers or saisons sometimes benefit from being slightly under pitched as this increased the phenol and ester production. If in doubt, do this step.

Like previous steps, boil 200g of DME in 2 litres of water. Once cool add to starter and let ferment out.

Step 8: Pitch

This step requires 20-25 litres wort, the starter.

Once your brewday is complete, pitch the starter into your wort and ferment the beer. Try not to pour the stir bar into the wort.

If you’re not pitching straight away, it can go into sterilised jars and stay in the fridge for up to 2 weeks. After this time it can still be stored, it will need a starter before pitching into wort however.

Other Things to Note

Starters can smell pretty bad. The one I have now smells like nail varnish remover which I’ve experienced before. Cider is quite common as is any acidic smell, however the starter is probably fine. Remember, you’re making a small beer which will make a lot of different smells, but there are no hops to mask it. The wort in a starter is being constantly oxygenated which of course is not good for a “real” beer, as it creates off flavours. If in doubt cold crash to remove the bad smelling wort, which brings us onto the next point.

If time is available, cold crash the starter between steps. This involves sticking it in a fridge for a couple of days so the yeast settles at the bottom leaving the starter beer on top. Pour off most of the beer, trying not to spill the yeast. This is especially useful if you don’t have containers big enough to keep building starters and need to make space. As always, be extra careful with sanitisation when handling yeast and spray everything with sanitiser.

I Don’t Have a Stir Plate

Go through the same steps using a 5 litre demijohn or suitable  container with foil over the top. It will need to be manually shaken every so often, so put it somewhere you walk past often, and give it a shake whenever you walk past. Some steps might take a bit longer, but you’ll get the same results.

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