## Day 384

In *Brewing Calculations*, Matt Howell is ready to take over, so Kevin Somerville handed him the chalk and eraser. Today Matt did a review of last week’s hop calculations, then led us on to yeast calculations, specifically, how to calculate the pitching rate and the total number of yeast cells needed for a particular batch of wort.

Having the proper number of yeast cells working on your wort is important–too few can mean the yeast are quite comfortable living on the oxygen you so generously provided, so they may never be forced to resort to anaerobic fermentation to survive. But too many yeast cells may result in a lack of resources for the yeast cells–sort of like planting too many flowers in your garden without enough fertilizer.

As we learned last year in *Intro to Brewing*, we normally want 1.1 million yeast cells per millilitre of wort per degree of Plato. That is, at 1°P, you want 1.1 million cells per millitre; at 2°P, you want 2.2 million cells per millilitre, at 3°P, you want 3.3 million cells per millilitre; and so on.

(This makes sense–higher gravity is an indication that there is more sugar in your wort, so obviously you would want more yeast to gobble up the extra sugar and make tasty tasty alcohol.)

The formula is a two-step process:

Step I: r = Y x °P

where

- r = the pitching rate of yeast cells/mL needed
- Y = the constant of 1.1 million yeast cells/mL (or in scentific notation: 1.1 x 10
^{6}cells/mL) - °P = specific gravity of the wort, expressed in degrees Plato

Once you have arrived at the pitching rate per millilitre of wort, you then simply multiply by the number of millilitres of wort you have to arrive at how many yeast cells in total you need for the entire batch:

Step II: n = r x V

where

- n = total number of yeast cells needed
- r = the pitching rate derived from Part I of the formula
- V = the cooled volume of the wort, expressed in millilitres

Once you’ve done this a few times, you can combined the formulae into

n = (Y x °P) x V

For instance, the pitching rate for a 70 hL batch of wort (70 hL = 7000L = 7,000,000 mL or 7.0 x 10^{6} mL) with a specific gravity of 12°P would be calculated as

n = (1.1 x 10^{6} cells/mL x 12°P) x 7.0 x 10^{6} mL

= 1.32 x 10^{7} x 7.0 x 10^{6}

= 9.24 x 10^{13} (or 9,240,000,000,000,000) total yeast cells needed

The next step–which we will cover next week–will be to take a cell count of your yeast slurry to determine how many viable yeast cells per mL of slurry you have, then calculate how much yeast slurry you need to add to your wort.

On to *History of Beer*, where Bill White covered the Age of Empire Building: the story of great conquerors, brave explorers and beer. Julius Caesar toasted his legions with beer, Martin Frobisher sought the Northwest Passage in 1576 fortified by 84 tons of beer (and was forced to return to England when the beer ran out), and the first thing Spanish explorers did after conquering the Aztecs was set up a brewery.

The sheer scope of beer history almost makes one pause whilst lifting a beer to one’s lips… Almost.

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**Tags:** Bill White, Brewhouse Calculations, History of Beer, Matt Howell, yeast count

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