How do they know how much alcohol the beverages contains when companies make alcoholic beverages?

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For example how do they know/measure that a beer contains 4.5% alcohol?

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Alcohol is made by the fermentation of sugar by yeast. Essentially the yeast eats the sugar and poops out alcohol.

A solution of water and sugar is more dense than a solution of water and alcohol. So the way that you figure out how much alcohol was made is by taking the density of the product before fermentation has started, and the density of the product after fermentation is complete. Then you use a formula that computes the difference in density into alcohol percentage.

People who make wine and beer at home do this with a device called a hydrometer, which is a glass bulb that kind of looks like a thermometer. You fill a test tube with your liquid and you float the hydrometer in it, and based on how high it floats, that tells you the density of the liquid.

We use a tool called a hydrometer. You put it into a sample of the beverage and it floats. The degree to which foods depends on the alcohol concentration, which is measured on graduations inside the tool

A very primitive and simple method which is fairly accurate is to measure the density of the wort in order to figure out how much sugar is in it and then measure the density again after fermentation to find out how much sugar have been converted to alcohol. Sugar water is denser then beer so it is fairly simple to calculate this. You can also make similar measurements but with the refractive index of the liquid instead of density. This is a fairly accurate technique for home brewers and even microbreweries. However for big industrial breweries which have tighter quality control requirements and often is much closer to the regulatory requirements for alcohol content they need some better tools.

Firstly they can control the brewing process even tighter and use various techniques to accurately measure the levels of the different sugars in the wort. This can be done either chemically by testing it with various chemicals or by analyzing its spectrum output in high detail. We roughly know how good various yeast is at converting the various sugars into alcohol so based on the various sugar content you can calculate the alcohol content of the final drink. Then during the fermentation you can measure how much gas is coming out. This is carbon dioxide which is another byproduct of the fermentation other then alcohol. By measuring how much carbon dioxide is being released you know how much alcohol is being produced. And lastly there are a number of different tools to measure the alcohol content of the finished drink. There are spectrum analyzers to measure the spectral lines that alcohol have. Calorometers heat the beer up slowly until the alcohol starts burning and then measure how much energy is produced at that temperature. You can also destill the beer to measure the amount of alcohol directly. You can also use chemicals which changes color when reacting with alcohol to find out exactly how much alcohol there is by measuring how much of the chemical you can try to react with it.

Distillation, and refractometry, and HPLC.

Distillation: Beer and other fermented beverages contain lots of stuff that makes measurement difficult. It’s liquid food, so to speak, so there’s sugars, starches, proteins, all the usual suspects. This makes simple measurements like density inaccurate (like hydrometer readings). Since water and alcohol boil easily, and all the other stuff doesn’t, distillation makes measurement easier. You can test distilled alcohol like vodka directly.

Refractometry: Water and alcohol bend light by different amounts. So, if you shine light through an alcohol-water mixture at an angle, you can measure the alcohol content by how much that angle changes when the light comes out the other side. Sugar solutions also bend light by their own amount, so this only works well with alcohol that’s already been distilled.

HPLC (high-performance liquid chromatography): A separate technique that’s even more accurate than distillation and refractometry, HPLC uses the “stickieness” of molecules to separate them, before measuring how much is there. A small sample is mixed with a solvent. This sample is pushed through a really long tube with more of that same solvent. But that tube is filled with something solid, kinda like sand. The alcohol and water molecules will occasionally stick to the surface of the “sand”, slowing it down. Different chemicals stick more or less, so one thing will come out first, then a delay, then the other thing. Then you just have to figure out how to measure how much not-solvent comes out at any moment. As long as you can figure out which blip of not-solvent is the alcohol, the percentage is as simple as dividing how much came out in that blip by how much total you added. This has the advantage of distinguishing ethanol from all the other alcohols like methanol and 1-propanol that are present in boose in small amounts.

> How do they know how much alcohol the beverages contains when **companies** make alcoholic beverages?

Note that companies will use hydrometers during brewing, but in most countries, that’s not sufficient. The percentage on the label is required to be more accurate than hydrometers can muster.