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Fermentation

During fermentation, yeast, a living micro-organism, consumes the sugar in the beer wort and excretes alcohol, carbon dioxide gas and flavour compounds, thus turning our sweet wort into beer.

For a successful fermentation, the temperature of the wort is vitally important, and needs to be somewhere between 19 º C and 24 º C for ale yeast. However different yeast strains have different ideal temperatures, and the same strain can produce different results under different temperature conditions (higher temperatures generally cause yeast to produce a beer with a more 'fruity' character, lower temperatures a cleaner flavour).

Generally, a steady rather than a high temperature is best for fermentation, and many brewers create fermentation chambers out of old fridges to allow better temperature control. A hot airing cupboard (the traditional home brewing space) is probably the worst place imaginable for your beer.

Pitching

If you thought starting fermentation was as easy as sprinkling the contents of a pack of dried yeast on your wort, you are partly correct; however this is only one way of starting fermentation, and not the best way. In order to get the best performance out of your yeast (which will allow it to produce the best possible beer) you need to give it the best possible start. Different methods of doing so are explained below.

Sprinkling Dried Yeast

The simplest and most straightforward method. Just cut the top of the packet of yeast and sprinkle on the surface of the wort - a stir is optional but beneficial.

While this is often recommended on beer kit instructions and works OK, the lag time (time for the yeast to get going) can be greater than with other methods, leaving your wort open to infection for a longer period before the yeast takes hold properly.

Re-hydrating Dried Yeast

This is a very common method for more experienced home brewers, and is usually the method recommended on the instructions that come with proprietary dried yeast packs. A small amount of warm water (usually 30 - 35 º C) is prepared in a sterile jug or jar and the yeast added to that and left to stand for about 15 minutes. The whole mixture is then added to the prepared wort and stirred in.

Yeast Starter

This is the most advanced method and probably the best. A yeast starter is made a day or two before brew day by adding either liquid yeast or re-hydrated dried yeast to a small quantity of wort (typically half a pint/250mls, and usually made by dissolving some spraymalt in water). The yeast cells multiply rapidly under these conditions and when they are added to the main batch, fermentation takes off very quickly and the yeast remains at its peak of health.

A starter is pretty much mandatory when using packs of liquid yeast, since the number of yeast cells in a typical pack can be insufficient to start a healthy fermentation.

Pitching Rate

This is a measure of the number of active yeast cells you add to the wort, and getting this right matters if you are to get a reasonably short lag time and a healthy fermentation.

 If you brew 5 gallon batches of normal strength beer and use proprietary packs of home brew yeast, you don't need to worry as the packs you buy are designed to give the correct pitching rate for 5 gallons. Likewise, if you brew 10 gallon batches you're OK as well because you can just use 2 packs.

However, if you are brewing much stronger beers, or are either splitting yeast starters or cultivating yeast from slants, it's useful to have at least a basic understanding of pitching rate. Mr Malty's website has a pitching rate calculator that will allow you to estimate how many cells you have and how many you need for a particular batch of wort.

Wort Aeration

During the lag or 'growth' phase that happens just after yeast has been pitched, the yeast requires oxygen which it uses to build the newly created cells. To help the yeast through this phase, it's a good idea to aerate the wort. Some ways of doing this are: -

  • Allow the wort to drip from a height into the fermenter during cooling (e.g. through a sieve)
  • Stir the wort vigorously in the fermenter (some brewers use a brew paddle fixed into an electric drill chuck)
  • Oxygenate using an air-stone and a pump (some brewers use pure oxygen for this)

While aeration is less important for well-prepared dried yeast (because all the necessary nutrients for cell growth are added to the pack during manufacture) it is still beneficial and it is pretty well essential for liquid yeast.

Before moving on to 'Racking' let's think a little more about yeast characteristics and controlling the fermentation temperature.......

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