Hop Talk

Brewers measure the amount of alcohol in beer by comparing the density of the wort (liquid before fermentation) to the density of the beer (liquid after fermentation). The difference between the two is the result of the creation of CO₂ and alcohol during the fermentation process.

To measure density, brewers use a measurement known as “specific gravity.” Wikipedia defines specific gravity well, as:

Specific gravity (SG) is a special case of relative density defined as the ratio of the density of a given substance, to the density of water when both substances are at the same temperature. Substances with a specific gravity greater than 1 are more dense than water, and those with a specific gravity of less than 1 are less dense than water.

To measure this, brewers use a hydrometer. A hydrometer, which contains a weight to float upright, works by floating and displacing some of the liquid it its container (usually a cylinder). The level of the surface of the liquid is noted where it falls on the scale of the hydrometer.

A hydrometer in water should read about 1.000, depending on temperature and mineral content. A hydrometer in wort will read higher because of the increased density due to all of the dissolved sugars. A hydrometer in beer, will be higher than 1.000, but lower than the initial reading (original gravity) because some of the sugar has been turned into ethyl alcohol (final gravity).

How Stuff Works goes on to explain the details of how to calculate the amount of alcohol present by weight. I will let you read all about molecular weight and chemical equations of sugar getting split into alcohol and CO₂ on your own. For the brewer, the hydrometer not only has the scale for specific gravity, but also includes the scale for percent of potential alcohol by volume, already calculated out for you. (the conversion of alcohol by mass to alcohol by volume is already considered on the potential alcohol scale of a hydrometer) Potential alcohol is the amount of alcohol that would be produced if all of the sugar present in the wort is fermented into alcohol, but not all will be.

For an example using approximations with a custom photo of my hydrometer, a wort with an original specific gravity of 1.069 has the potential alcohol of about 9%. If the beer then has a final gravity of 1.032, measuring a potential alcohol of about 4%, the difference gives us a result of 5% alcohol by volume.

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Back in Packs a Punch, I withheld from my friend that my Guinness was less potent in alcohol than his Bud Light. If I didn’t, he would then have asked me how can that be? … No, actually he wouldn’t care, or wouldn’t believe me. But why is it that the darker, thicker, sweeter, stronger tasting beer has less alcohol?

To understand how alcohol is made in beer, you have to understand a little bit about yeast. I ask you… how much do you currently know about yeast? Try this yeast trivia questionnaire by A.B. first and see how you do…

Well, you don’t have to know all of the details about how yeast works, as yeast is a very complicated critter. Basically, yeast eats sugars, divides (and thus multiplies), and produces in turn carbon dioxide and ethanol (alcohol).

So, the amount of alcohol content in beer is dependent on the amount of fermentable sugars available in the wort. Those sugars are present from the mash process of the malted barely during brewing. But wait, there’s more! There are other factors, too, which make it more complicated than just that. For example, the more roasted a malt is, the less fermentable sugars it will produce during the mash. Plus, the environment needed for fermentation to take place has to be just right. For instance, there are enzymes needed as a catalyst for the process. Temperature, oxygen, and other things contribute to how efficient the yeast works.

Fermentable sugars are key. Remember back in Part 3 that specialty grains do not contain all, or as much, of the enzymes needed for fermentation. Also, specialty grains do not contain as much potentially fermentable sugars, but they certainly bring a lot of flavor and color to the party. (ok, I admit, I’ve been watching too much Good Eats)

Bud Light uses rice (genetically engineered) to create more fermentable sugars without adding any taste, but still provide alcohol. Guinness uses lots of roasted malts for taste and color, much of which does not ferment and, thus, produces less alcohol while still packing a punch of flavor.

More from How Stuff Works…
How Beer works
Fermentaion

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