Lawrence Brewers Guild

I have read this before but cannot remember where, so I have lost the information I bet that one of you remember how

Assuming 4 gallons of beer that has been forced carbonated with CO2 gas in a corny keg to 2 volumes of CO2.

How much corn sugar would be required to raise the CO2 volume from 2 volumes to 4 volumes?

Way back in the day I gave a seminar on priming sugar: amount\per vol\per temp=desired level of co2. it was broken down into grams and was pretty darn accurate for priming those little 5L mini kegs that I used to use a lot... I think I still have that graph down in the basement in a box somewhere... I'll see if I can't dig it out this weekend and post it.

If You Really Need To Know
by Rob Lauriston

Many brewers are perfectly happy to start with the three-quarters-cup-sugar-per-five-gallons-of-beer rule of thumb and then experiment until the right level of carbonation is reached. But if you need to be more precise, here is the scientific side to choosing the right amount of priming sugar.

Carbon dioxide has a density at STP (Standard Temperature and Pressure) of 0.122 pounds per cubic foot, or 1.94 grams per liter.

Sometimes the level of carbonation of beer is given by weight rather than by volume. The conversion factor from volumes to CO2 by weight is 0.194. For example: 2.5 volumes x 0.194 = 0.485 g/l. To convert to volumes, multiply by 5.147 (the inverse of 1.94).

Carbon dioxide has a molecular weight of 44.01 and glucose has a molecular weight of 180.16. During fermentation each molecule of glucose creates two molecules of ethyl alcohol and two molecules of CO2. A mole is the number of molecules that has a weight in grams equal to the molecular weight of the molecule. That means that one mole of glucose weighs 180.16 grams and ferments to produce two moles of carbon dioxide weighing 2 x 44.01g = 88.02 g.

Say you want your five gallons (18.93 liters) of beer to have 2.5 volumes of CO2. That’s 18.93 liters x 2.5 volumes = 47.32 liters of CO2.

Since a mole of CO2 occupies 22.414 liters (STP), we need 47.32 / 22.414 = 2.11 moles of CO2. This will be produced by half that many moles of glucose, or 1.056 moles. Multiply by the molecular weight of glucose and you find that you need 190.2 grams of glucose or 6.71 ounces.

If your fermentation ended at 50° F, you might have 1.2 volumes of CO2 in your beer already. That means that you need 1.3 volumes more CO2 to end up with 2.5 volumes. You could get that by adding (1.3 / 2.5) x 190.2 = 98.9 grams of glucose, or 3.5 ounces.

You can make similar adjustments if you have a different amount of beer or if you’re aiming for a different level of carbonation. If you want to carbonate to 2.8 volumes instead of 2.5, multiply by 2.8 / 2.5. If you have four gallons of beer instead of five, multiply by 4 / 5.

For example if you have 10 gallons of ale that you figure has 0.9 volumes of CO2 already and you’re aiming for 2.0 volumes, you need 1.1 volumes more. You calculate: (190.2 g glucose)(10 gal./ 5 gal.)(1.1 vol./ 2.5 vol.) = 167.4 g glucose (or 5.9 oz.).

When you’re adjusting to add more sugar (e.g. more beer), put the big number on top; when you’re adjusting for less sugar (e.g. less carbonation) put the smaller
number on top.