WIthout yeast there is no beer. With this in mind, it’s important for a brewer to ensure that this vital ingredient is treated and used in the correct way.

One variable of importance when using yeast is how much. Using too much yeast can limit the amount of yeast growth and prevent a desired yeast character from developing. Not using enough yeast can create a myriad of issues, namely an unfermented beer and/or undesirable characteristics from the stressed out yeast cells.

So, how does a brewer control this? Two pieces of equipment most commonly used in combination are a microscope, and a haemocytometer. With this, the number of yeast cells in a representative sample can be physically counted, and then this can be extrapolated up. Depending on the yeast and the style of beer being made, and the conditions, the window for the ideal amount of yeast can change. This information is supplied by the company supplying the yeast, however it isn’t unusual for brewers to intentionally stray outside these parameters to experiment with different effects.

Now for some maths to demonstrate how this works. If you are making a batch of beer that is 1000L, and you are using a yeast strain that needs 10,000,000 cells per ml, then you need:

1000 x 10,000,000 x 1000 = 10,000,000,000,000  or 1013 cells

A haemocytometer is essentially a glass slide with a tiny grid on it (see pic). You place a liquid sample on this grid, and then place a glass slip over the top. The liquid within this square trapped under the glass is a set volume, and is thin enough to count yeast cells with a microscope easily. Once you know how many cells are in the grid, then you can multiply this number up by a factor based on the size of the haemocytometer, and see if you’ve hit your target number of cells. 

This simple way of counting yeast is used by breweries of all sizes, and helps to give brewers a great deal of oversight over a variable that it is vital to control closely, for the sake of making good beer.