Beer

With many people celebrating St. Patrick’s Day this week, let’s look at the chemistry of beer and why people refer to beer as a solution - chemically speaking of course.

Beer is a common sight for St. Patrick’s Day, along with people donning green outfits and shamrocks. St. Patrick’s Day is meant to honour the Irish Saint Patrick and commemorate him bringing Christianity to Ireland. Nowadays, most spend the day celebrating Irish culture with a green beer or one of the greatest Irish inventions - Guinness. So in the spirit of St. Patrick’s day, grab a pint of your favourite beer and let’s learn about the chemistry behind it.

Chemically speaking, beer is solution: a homogenous mixture of water and alcohol. To make beer you need water, barley, hops and yeast. First, the barley and water are mixed together to form a slurry and heated. At higher temperatures (~ 60 °C) enzymes start to break down the starches in barley into sugars. Enzymes are a type of biological catalyst. They are naturally occurring in living things and promote a specific biochemical reaction to occur. In the barley slurry, enzymes will modify starch molecules which causes starch to react with the water to form sugar. Enzymes are catalysts because they are not consumed in the chemical reaction; they increase the rate at which the reaction occurs.

Once the enzymes have converted most of the starch into sugar, the solution is filtered and hops are added. Hops give beer that “grapefruit flavour” but also act as a natural preservative and form solid complexes with any unreacted starch which can be filtered out. Hops are important for both the taste and stability of the beer.

Next, yeast is added to the sugary solution. Yeast are tiny fungi which essentially eat the sugar and form carbon dioxide (CO₂) and alcohol (ethanol). This process is called anaerobic (meaning without oxygen) fermentation. The formation of CO₂ in solution is what makes beer fizzy. The brewmaster will allow this reaction to occur for ~ 1 week or until the desired alcohol concentration is reached. The end product is filtered and bottled and voila, that is the chemistry of beer.

But the chemistry doesn’t stop there! You may have noticed beer bottles come in brown, green or clear bottles - and for good reason. Hops contain compounds called alpha acids. When exposed to sunlight, UV light will cleave bonds in alpha acids which can react with sulphur compounds in the beer to form 3-methyl-2-butene-1-thiol (MBT). MBT smells awfully similar to a skunks’ odor and beer that has undergone this reaction is called “skunked” or “light-struck.” Brown beer bottles can effectively filter out UV light and prevent the MBT reaction from occurring. So if a beer has a high hop concentration, they are often found in brown bottles to avoid this skunky reaction. Clear and green bottles are equally ineffective at filtering out UV light - so they are just for looks and branding.

Hopefully now you’ve enjoyed your beer more knowing about all the fascinating chemistry that goes into a bottle of beer.