Preparing the yeast

Most brewing instructions, particularly those that come with cans of concentrate, advocate sprinking dried yeast on top of the wort when all the ingredients are in the fermenter and it's topped up with water. While this can certainly be and is done by homebrewers the world over, it's beneficial to rehydrate the yeast first.

For those cultivating a yeast from a bottle-conditioned beer, making a yeast starter is essential to multiply the yeast so that enough cells are added to the wort. Not adding enough yeast — known as underpitching — can lead to problems such as ferrmentation ceasing before the yeast has consumed all the sugar, resulting in an overly sweet beer.

Some packets of liquid yeast claim to have enough cells for the correct pitching rate. However, many homebrewers still make a yeast starter to increase the amount of yeast that's pitched, particularly if the yeast is approaching its use-by date because yeast cells start dying pretty much as soon as they are packaged.

And it's common practice among brewers to reuse some yeast from a previous brew. Liquid yeast is a little more expensive than dried yeast and reusing the yeast from a previous brew helps defray the initial cost. It's also common practice to take a cupful of yeast slurry from the bottom of the fermenter after bottling, kegging or racking, then simply adding that yeast into a fresh wort. Indeed, some homebrewers simply tip their wort onto the yeast cake once their fermenter is empty. Of course, this relies on the yeast being appropriate for the style. Better, though, is to "farm" the yeast by washing it first to remove impurities.

This page will show you how to reydrate yeast, make a yeast starter and reculture yeast from a brew.

Rehydrating yeast

If using dried yeast, rehydrating it before pitching it into the wort will get your brew off to the best possible start. As yeast rehydrates by letting liquid through its cell wall, some cells die. The cells cannot regulate what comes through its walls during rehydration and pitching dried yeast directly into the wort will kill much more yeast than rehydrating the yeast in sanitised water then pitching that mixture. Here is how to rehydrate yeast.

1. Add about 250ml of water to a cup, cover it with cling wrap and boil it in a microwave oven. If you don't have a microwave, boil some water in a kettle, add about 250ml to a cup and cover it with cling wrap.
2. Allow the water to cool to room temperature.
3. Sprinkle the dried yeast on top of the water and re-cover it.
4. After 10 minutes stir the mixture with a sanitised spoon.
5. Let stand for another 20 minutes, during which time it should foam up.
6. Stir vigorously and pitch into the wort. There is no need to mix it in.

Starting liquid yeasts

Making a starter involves propogating the yeast that you're going to use in your beer before you add it to the wort. A yeast starter is not essential, but will make sure your beer starts off with a vigorous fermentation and ensures any rogue yeast doesn't get hold and ruin the brew. As well, a starter will allow you to make sure the yeast is viable and ensure there are sufficient yeast cells (pitching too little yeast can stress the yeast and lead to inferior beer).

Ideally — and if you plan ahead — you will begin with a small volume of starter and "step up" to 2 litres or so for an ale or 5 litres for a lager that is to be brewed at low temperatures. Stepping up a starter will take up to seven days. See Re-culuring yeast from a bottle, below, for instructions on stepping up to larger volumes.

1. Wyeast yeasts come in what is called an activator pack, which contains a small bag of yeast nutrients and wort floating in liquid yeast. This needs to be "smacked" (instructions are on the packet) to break the bag of nutrients and allow the yeast to begin multiplying. Smack the pack and wait for it to swell. While a swollen pack isn't necessary, it is an indication that the yeast is viable and has started multiplying.
2. Boil 2 litres of cold water with 200g of malt extract (liquid or dried) for 10 or so minutes.
3. Cool to 25-30C and add the liquid to a sanitised vessel (a large bottle will do).
4. Add the yeast and to aerate it, put the lid on the bottle and shake it until your arm is about to drop off.
5. Put it on a stir plate (if you're a boffin) or shake the container every time you pass it.
6. Wait until fermentation is complete, or close to.
7. Put the bottle in the coldest part of your fridge overnight to cause the yeast to drop out of suspension.
8. Pour off the liquid and discard it, leaving the yeast cake behind.
9. If you want to retain some of the yeast to use in a future brew, tip about ¼ of the yeast cake into a sanitised container. Seal the bottle and keep it in the fridge. When you're ready to use it, repeat this process for making a new starter.
10. Pitch the remaining yeast slurry into wort.

This process can also be used to make a starter for dried yeast, for instance if brewing a strong beer that requires more yeast than is in a single sachet.

Re-culturing yeast from a bottle

If culturing yeast from bottle of commercial brew it is essential to make a starter because the quantity of yeast in the bottle is minuscule and far too small to ensure a healthy fermentation. As well, a starter ensures that the yeast is still alive, not infected and not producing any off smells. The process is similar to making a yeast starter for liquid yeasts.

1. Get the freshest bottles you can, because the older the beer the fewer live yeast cells will be in it.
2. Chill the bottles of bottle-conditioned beer (such as Coopers Sparkling Ale, Best Extra Stout or Pale Ale) for a few days to make sure the yeast settles to the bottom of the bottles.
3. Gently pour the beer into a glass, leaving the sediment of yeast behind. Drink the beer.
4. You can begin your starter in the bottle the beer came in. However, if you're going to use a stir plate you will have to transfer the beer to a sanitised flask.
5. Make about 250mL of wort by boiling 25g of light dried malt extract with 250mL of water, cover and allow to cool.
6. Add this to the yeast and shake it until your arm is about to drop off.
7. Add an airlock to the bottle or cover it with with plastic food wrap and a rubber band.
8. Every time you walk past the starter give it a good shake (using plastic wrap makes this easier than using an air lock).
9. Don't be concerned if it takes 24 to 48 hours to show any signs of fermentation.
10. You need to "step up" this starter to a larger volume, and it's best to do this at high krausen (when the foaming of the starter is at its greatest) rather than letting the fermentation finish. It's not really an issue if you let it ferment out as sometimes the first step won't show much of a krausen.
11. Make a 2-litre starter by boiling 200g of light dried malt extract and 2 litres of water, covering it and allowing it to cool.
12. Add this to a larger sanitised flask or bottle (at least three litres).
13. Give the 250mL starter a shake and pour it into the larger vessel.
14. Let this ferment out completely.
15. Crash chill by putting it into the coldest part of the fridge overnight to make the yeast drop out of suspension.
16. Pour off the liquid beer and pitch only the slurry that is on the bottom of the bottle.

Yeast farming

Yeast farming is the harvesting of yeast from a batch of beer so that you can reuse the yeast in another batch. There are limits to how many times you can reuse a yeast before it starts to mutate and produce different flavours to those it should. Some yeast manufacturrs recommend yeast should only be harvested two or three times, although many homebrewers have done it a large number of times without reporting any ill-effects. Here's how to harvest your yeast.

1. Rack your beer (transfer it to another fermenter) once fermentation is complete, or bottle or keg straight from the fermenter.
2. Add about 1.5 litres of cooled, boiled water to the yeast cake and shake it, then pour it into sanitised container.
3. Alternatively, scoop about a cup of the yeast cake into a sanitised container (using a sanitised spoon, of course), add about a litre of cooled, boiled water and shake it.
4. Let it sit for 5 minutes then pour off the milky suspension into a sanitised container, leaving the solids on the bottom of the container behind (the solid left behind is cold break and contains a small amount of yeast but not enough to worry about).
5. Discard the solids left behind.
6. Cover the container of milky liquid with cling wrap, then put it in the fridge and wait until the yeast forms a layer on the bottom of the container.
7. Pour off the liquid and add more cooled boiled water.
8. Repeat the last two steps until the water is pretty much colourless.
9. Split the yeast slurry into several batches to store it for future use, pitch the whole lot into your next beer or pitch half into a fresh wort and split the rest. You can safely store the farmed yeast in the fridge for a week or so before use.