Saturday, June 26, 2010

An explanation of carbonation

Carbonation is the dissolving of a gaseous carbon dioxide (CO2) in a water based solution, the most important application of this is beer. In beer there are two types of carbonation, natural and forced, both will be covered here. Before delving into carbonation's application in beer products it is important to understand the science behind the process.

Henry's Law

William Henry was an English chemist that lived in the late 18th century. His work with gas analysis lead to the development of what would come to be known as Henry's Law.

HENRY'S LAW

p=khC
Where:
p=partial pressure of the gas above the solution (atm)
c= the concentration of the solute (mol/L)
kh=Henry's Constant, : 29.4 for CO2 at 289K (60.53°F) (atm/mol)

Note: kh is actually a changing coefficient that changes rather than a constant. It changes with respect to temperature using this equation: kh = kh,ref Tref.exp(-C.((1/T) - (1/Tref))) This is important when carbonating beer because it will determine how much carbon dioxide get absorbed into the beer.

This relationship tells us that for higher pressures of gas you will get higher concentrations of gas in your solution. This is how the CO2 gets into the beer. Now where does the CO2 come from? There are two different sources for carbon dioxide in beer.

Natural Carbonation

When yeast is put in a solution containing sugar it begins to "eat" causing a chemical reaction producing carbon dioxide and ethanol, two key ingredients in beer creation.
REACTION: C6H12O62 CO2 + 2 C2H5OH
Carbon dioxide is an odorless colorless gas that you exhale when you breathe out. After beer has sat in the secondary and is ready to be transferred to bottles a small amount of priming sugar is added to the solution to help carbonate the beer. During primary and secondary fermentation the CO2 is released through the airlock and into the surrounding air. When bottling there is no place for the CO2 to go so it builds up pressure. And as explained previously when a gas is under pressure over a liquid it is absorbed into said liquid and in our case carbonates the beer.

Forced Carbonation
The process of forced carbonation is very similar to natural but instead of the yeast creating the and the pressurized CO2, a bottle of liquefied gas is used to create the artificial pressure and carbonate the beer.

Dry Ice Carbonation
I have never actually heard of someone doing this with beer but I thought I would put it on here anyway. You can drop a small brick of dry ice (Frozen carbon dioxide) into a sealed container and have it pressurize that way NOT RECOMMENDED because the amount of pressure you get harder to predict and could cause an exploding bottle/keg.

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