How the early methods of testing alcohol content were completely absurd (and deadly).

If you’ve ever purchased alcohol in the US, you’ve probably seen the ‘proof’ or ‘ABV’ labeled on these beverages. ABV is the easier one to understand: it means “alcohol by volume” and tells you how alcoholic a drink is.

Alcohol, more formally known as ethanol, is what gives booze its intoxicating effects. ABV gives you the percentage — that is, the number of mL of pure ethanol in 100 mL of the drink.

Proof is a bit more complicated. In the US, it’s equal to double the ABV — that is, ABV multiplied by two. But if you go back to 19th-century England, 100 proof is equal to 57.15% ABV — which is oddly specific, and seems kind of random.

So, where did the number 57.15% come from? The word ‘proof’ provides a clue.

You’re down in the cramped galley, taking a few moments’ respite from a hard day’s work. As you relax with your pipe, one of your fellow sailors brings out the drinks. Gratefully, you take a sip…and then spit it out immediately.

Drinks, you say? Who was the merchant who sold us these bottles? This is little better than dishwater!

It was to avoid incidents of this kind that sailors started testing alcohol before buying it — or so one of the stories goes. Another says that it was farmers, not merchants, who used to dilute the alcohol with water, and it was the government who wanted to make sure it wasn’t cheated.

However the story goes, most alcohol sellers today are legally required to give the ‘proof’ of their products.

Historically, proof wasn’t associated with a specific value or alcohol content. Instead, it simply meant that a distiller had to provide proof that their beverages had alcohol in them. That they weren’t some cheap knock off. Or, at least enough alcohol was present to get a positive response to whatever ramshackle analysis was done.

This isn’t to slight alcohol manufacturers: brewing is a tricky business. Even if they’re honest at heart, there is a lot that could go wrong, and sometimes you end up with a failed batch.

The ethanol that the sailors wanted doesn’t magically appear in beverages to make them alcoholic. Once brewers have combined all the ingredients in a tank, they get to sit back and relax. But inside the vessel, a little microorganism called yeast has just begun the heavy lifting.

The word ‘yeast’ may make you think of bakers and bread, but there are many different types of yeast, each involved in a different kind of fermentation (and some in none at all!). Humans have been working side by side with yeast for thousands of years: in fact, it’s argued that yeast was the first organism to be domesticated by humans.

To produce any type of alcoholic beverage, yeast must be present, since they are the sole producers of ethanol.

In essence, yeast eat the sugars provided by the grains the brewer adds to the fermentation tank. For example, bourbon is traditionally made from corn and beer from barley.

The specific sugars yeast are able to digest are called fermentable sugars. And as the yeast are chomping down on the fermentable sugars, like us, they produce waste after eating a meal. Unlike us, yeast waste consists of carbon dioxide and ethanol.

Typically, fermentation takes place in an open vat, so the carbon dioxide leaves the beverage. The ethanol, however, remains. (This means alcoholic drinks are sort of like drinking yeast’s toilet water, which is an amusing tidbit to share when drinking your next cocktail).

For much of human history, brewers simply hoped that yeast were present in their brew. This is known as wild fermentation. Which means, maybe the beverage got fermented, or maybe it didn’t. Today, most brewers can easily purchase pure cultures of yeast to toss into their fermentation tank, which nearly guarantees ethanol production. But, this is a modern day luxury.

This problem of whether or not fermentation actually occurred, is what led to people conducting their own analysis of “alcoholic” beverages — and boy, did people come up with absurd ways to prove a beverage’s quality! Anything from striking fires to dousing gunpowder in their drink. Their creativity is really admirable.

Dating back to the 1500s, the English were looking for ways to verify that alcoholic beverages contained, well, alcohol. At the time, scientific methods weren’t available that could accurately test for alcohol content. Instead, people came up with their own makeshift method of testing for purity — the burn or no burn test.

This was a simple test. Ignite the beverage with a flame and see if it caught on fire.

If the beverage ignited, it was labeled above proof. If it didn’t catch fire, it was called under proof. The results not only verified quality to the consumer, but also impacted how heavily each beverage was taxed. Drinks labeled above proof were taxed more heavily than drinks under proof.

The problem was, the test results were quite variable depending on the temperature, which made it easy to manipulate. Depending on the test conditions, the lower limit for above proof could vary anywhere 40–90% ABV.

And for something as important to humans as alcohol, this was unacceptable. No doubt, a better test was needed.

It’s unclear how exactly the gunpowder method originated. Guns and alcohol are a pretty lethal combination, so perhaps the poor soul didn’t survive to tell the story.

The gunpowder method is straightforward. Place a pellet of gunpowder into the beverage you want to test for alcohol proof, then see if the wet powder will still burn. If it does, the spirit is labeled above proof. If it doesn’t, the beverage is under proof.

From a scientific aspect, the potassium nitrate in the gunpowder should dissolve in water, but not in ethanol (alcohol). At high ethanol contents, none of the potassium nitrate should be lost to the beverage and the gunpowder remains active. However, if the beverage contains more water, the potassium nitrate prefers to move into the water phase and leave the gunpowder.

Although this test couldn’t be manipulated with temperature, it did have its flaws. The particle size of the gunpowder could influence above or under proof. Not to mention, the longer the gunpowder was soaked in the spirit, the more time allowed for potassium nitrate to migrate into any water present. Altering the specific conditions of the gunpowder test could still allow falsification of proof.

In 1816, England began moving away from the flawed burn or no burn and gunpowder tests to more scientifically precise measurements. New methods based on density and gravity were used to verify alcohol content. For many years, the threshold to be labeled above proof was set at 57.06% ABV. So, a bottle labeled 100 proof in the UK was equivalent to saying 57.06% ABV, which is quite confusing for Americans.

But starting in the 1980s, England adopted the practice of simply labeling their beverages with the percent ABV to align with most other European countries.

Beginning in 1848, the US also began using the proof system based on ABV. It was slightly different from the UK’s version. To make a 50% ABV beverage, you would combine 50 liters of pure alcohol and dilute it to 100 liters with water. Since proof is simply two times the ABV, this beverage could be labeled 50% ABV or 100 proof.

Yet in France, a different labeling system emerged using a scale called degrees Gay-Lussac. Luckily, it’s a simple 1:1 ratio to convert between ABV and degrees Gay-Lussac. This means 50% ABV is the same as 50 degrees Gay-Lussac. Easy.

With all the technological advances, even home brewers have a pretty consistent method of measuring alcohol content. The trick is to measure the amount of sugars before and after fermentation. The difference between these two values will tell you how much sugar the yeast consumed and therefore how much ethanol was produced.

All this measurement requires is a rather simple tool called a hydrometer, which measures the “specific gravity” of liquids.

Specific gravity is just a comparison of the density of a liquid in relation to the density of water. Sugars are known for increasing the density of liquids, so we expect a brew to be more dense before fermentation than after. As fermentation proceeds, the liquid should become less and less dense as the yeast consumes the sugars to produce ethanol.

Incidentally, in the late 19th century, the UK officially measured alcohol content using this very method. “Proof spirit” was a spirit with 12/13 the specific gravity of water — which comes to about 57.15% ABV, the magic number from the beginning of this article!

Today, home brewers can find a hydrometer at a local store or online for as low as $10 USD. A fun tool that’s probably worth the investment.

No matter the century, humans have always desired high-quality alcohol.

Even when the scientific methods weren’t all that accurate, makeshift and rather dodgy solutions were created to verify alcohol content. Whether it be the burn of no burn test or gunpowder method, people wanted to be assured they weren’t being swindled.

Luckily, the laws of present day don’t require us to perform some outdated experimental procedures to ensure a drink has alcohol in it. Instead, distillers are legally required to label their beverages with proof or an equivalent measurement.

And this, I believe, is a luxury of living in the modern day. You don’t have to set your drink on fire. (Only the other way round).