Fermentation is arguably the most important part of the whole beer making process. This is where the beer is really made – the yeast are doing their work to make the alcohol and all the flavour compounds that make beer, beer. Happy yeast will mean happy beer drinkers. Yeast that are not in their comfort zone will mean unexpected/undesired results. Also, different yeast can drastically change your beer, selecting the right one is important, but can also be a fun place to experiment.
So, what then will make yeast happy? In addition to the sugary wort, temperature, a lot of its friends, and oxygen.
Temperature can be easy to control, but it also can easily get out of control. All yeast have a range of temperatures they like best. In that range the yeast will preform normal, but there will be flavour differences from one end to the other. This is due to the different stresses on the yeast creating different compounds. That’s not to say this is a bad thing, some beers do well with some of these flavour and are actually desired. For example, English beers are fruitier than American styles—these flavours are ester compounds that are formed by yeast at a bit higher temperature and can be subdued by lowering the ferment temperature.
Yeast selection will, of course, have a barring on flavour, but the same yeast at different temperatures will have a different outcome. Also, different yeast are happy at different temperature, the best example of this is the difference is lager yeast that are happy at 8-13 C (46-55 F) and ale yeasts that are happy at 17-21 C (63-70 F),
or some Belgian strains that are happy up to 28 C (82 F).
If the temperature is too low the yeast will get sleepy. The yeast may decided to finish fermenting early and fall out. This can leave beer that is overly sweet and possibly with some off flavours that the yeast didn’t clean up before they went to sleep.
Numbers, Health, and Oxygen
The health of the yeast that goes in the wort is important. If dying yeast go in, how can they do their job? They will die and decay (call autolysis), rupturing and releasing off flavours into the beer. That wouldn’t be good.
When yeast first get in the wort it takes up the available oxygen to help aid in building up the cells and reproduce (a process called budding). There has to be available oxygen in the wort, if not the yeast will be sluggish and very stressed out – this will create flavours that aren’t desirable and the yeast may not be able to complete their job to ferment the wort properly. The result can be overly sweet, bad tasting beer. The most common and easiest way to get oxygen into the wort is to shake the carboy or bucket. A few minutes is probably enough. Shaking only gets so much oxygen in. If the beer is a strong one the only way to get enough oxygen dissolved in is to use pure oxygen—using a stainless steel diffusion stone, a flow of about one liter/minute for about a minute or so is a good place to start.
Note: the only time that adding oxygen to the wort/beer is advised it before fermentation has started. Once there fermentation starts, adding oxygen will cause staling or promote the growth of bacteria that can make the beer change to vinegar or worse. When oxygen is added before fermentation the yeast clean it all out before bad things happen in the wort.
The right amount of (healthy) yeast is important too. If there aren’t enough cells then the yeast will struggle to ferment the wort. This will result is stress in the yeast and off flavours in the beer. If there is too much yeast then the yeast may not produce the flavours expected, making a very clean and one-dimensional beer. It is most common to under pitch (to pitch is to add the yeast) rather than over pitch, which is a bit harder to do.
A lot of study has gone into determining the right amount of yeast, and it varies with the strength of the beer being made. The simple math looks like this:
0.75 million cells x millilitres of wort x degrees Plato
(degrees Plato is an expression of wort gravity and 1P is equal to 1.004 SG, so 15 P = 1.060)
for example: 0.75 million cells x 19,000 mL x 15 P = 214 billion cells
For Lagers (due to the cold ferment, they need more cells, and a clean profile is desired)
1.5 million cells x millilitres of wort x degrees Plato
for example: 1.5 million cells x 19,000 mL x 15 P = 428 billion cells
Now, that sounds like a lot of yeast. The right amount is important, but there is a bit of leeway.
The two major manufactures of liquid yeast in North America (Wyeast and White Labs) both have about 100 billion cells in their most popular packs. Both claim to be directly patchable, but really that would be for an ale wort that’s below 1.040 or so, above that and a yeast starter should be used.
A yeast starter is like an small, unhopped beer. Usually made with dry malt extract (DME) to a gravity of about 1.030-1.040, it is best to use continuous aeration, easily achieved with a magnetic stir-plate (using a flask with a magnetic bar in the wort, it’s placed over another spinning magnet that causes the bar in the wort to spin and stir the wort).