How to Proof with Precision

Measuring the ethanol content of a spirit is one of the most nerve-racking things a distiller must do. Simple errors, such as misreading a hydrometer, can have serious consequences—think expensive product recalls and costly fines.

Yet with a strong grasp on the basics of how ethanol acts in solution, how to accurately measure it, and how federal institutions want it to be measured, you can go a long way toward reducing the heartburn and headaches.

[PAYWALL]

The Properties of Ethanol and Water

Before talking about measuring alcohol, let’s talk about ethanol’s interactions with water. Ethanol-and-water solutions have several important properties that every distiller should understand when measuring concentrations.

The first of these properties is that both ethanol and water are perfectly miscible. This means that when combined in a solution, they create a homogenous mixture that doesn’t stratify or separate, regardless of the concentration of either.

Miscibility is an essential property for distillers. Without it, adding ethanol to water (or vice versa) would be like trying to mix oil and water. No matter how much you stirred, the two liquids would eventually stratify, leaving a layer of ethanol and a layer of water. That would make measuring and adjusting the strength of spirits impossible, as there would be a constantly moving gradient within the solution. (It also probably would not be pretty in a bottle.) The fact that ethanol and water are perfectly miscible is the basis for the ability to measure ethanol concentration in spirits.

The next interesting property of ethanol-water solutions is that they exhibit a characteristic known as volume contraction. As the name implies, volume contraction means that when ethanol and water are mixed, the resulting solution has a volume slightly lower than the sum of the initial two volumes. This occurs because, in their liquid form, water molecules have a crystalline structure that allows for many gaps and empty spaces. When you introduce ethanol to a water solution, the ethanol molecules naturally fit into the gaps produced by water’s crystalline structure, creating a homogenous mixture with a volume slightly less than expected.

The best way to see this is to perform an experiment by mixing different concentrations of ethanol and water, and then measuring the resulting volume change. For example, if you were to mix 250 ml of pure ethanol with 250 ml of pure water, the resulting solution would be roughly 480 ml, not 500 ml. This property is especially important for distillers when trying to determine how much water or ethanol you need to add to reach a certain concentration.

Finally, another important property of ethanol-water solutions is that they exhibit temperature-dependent densities. Like most other liquids, an ethanol-water solution’s density is negatively correlated with increasing temperatures—that is, the density of an ethanol-water solution decreases as the temperature increases. In addition, the process of mixing ethanol and water is exothermic: It produces heat because the energy released when ethanol and water molecules bond is greater than the energy it takes to separate ethanol molecules from themselves.

In the previous example, you should see a temperature increase of roughly 14°F (8°C) in the final 480 ml solution. That property is significant because temperature plays an important role in measuring density for government reporting. Measuring the density of a solution at the wrong temperature can greatly affect the accuracy of a chosen method.

TTB Proof and OIML

Now that we have a better understanding of how ethanol and water interact, it’s time to consider how to measure the concentration of ethanol in solution.

Historically, there have been several methods and procedures developed to measure ethanol, but modern distillers rely primarily on two: TTB Proof and OIML (International Organization of Legal Metrology). These two systems primarily use the density of a solution to determine the amount of ethanol it contains, and for the most part they’re almost identical. Where they differ is the temperature at which the final solution’s concentration is measured—and, because density varies with temperature, it can sometimes be difficult to convert the results.

In the United States, TTB Proof is the prevailing method, and it reports ethanol concentrations at 60°F (15.5°C). Outside the United States, OIML is more popular and is reported at 68°F (20°C). Knowing which system you should use on your spirits is important for staying within compliance.

Approved Methods

Regardless whether you’re using TTB Proof or OIML, both systems have the same approved methods for measuring density and, therefore, ethanol content. These methods are time-tested and accurate, and knowledge of the two most common is important for all distillers.

Hydrometers

The first commonly approved method for measuring the alcohol content of a spirit is one with which all brewers-turned-distillers should be familiar: the hydrometer.

The use of hydrometers for measuring alcohol has been around for more than 200 years, with the modern hydrometer design often credited to German inventor Johann Georg Tralles in the late 1700s. (That is up for debate, however, as there are other claimed inventors around that time.)

In 1816, the British government adopted the Sikes hydrometer as the only official means for measuring alcohol content—a standard that many British colonies also adopted, and one that lasted until 1980. These days hydrometers come in all shapes and sizes, and you can buy ones that have markings for both TTB proof and OIML.

Hydrometers operate using the principle of buoyancy—that is, the denser a liquid, the more upward force it exerts on a solid placed into it. When you place a hydrometer into a liquid, you then use the degree to which it floats (or doesn’t) to measure the density of that liquid. Most hydrometers consist of a long tube with graduated markings and a weight on one end to ensure stability and easy reading. Because ethanol-water solutions tend not to be very dense, those hydrometers are commonly made from glass. Some modern hydrometers even have thermometers added to their base for ease of use.

Using a hydrometer is relatively easy, though it does take some specialized knowhow to consistently get an accurate reading. The first step is to acquire an appropriate container to hold the hydrometer and sample. Most distillers prefer graduated cylinders, though any container that is deep enough to accommodate the hydrometer is acceptable. Although it may be tempting to use the original container the liquid is in—such as the 300-gallon tank full of whiskey you need to gauge—that is not good practice. For one, the hydrometer could sink past your ability to recapture it. Plus, glass hydrometers are notoriously easy to break. No distiller wants to spend their afternoon filtering out tiny glass particles that had no business being there in the first place. It’s much better to use a sample container.

After you have the container, fill it with a sample of the spirit. The sample should be free of gas or air bubbles, as these may affect the results from the hydrometer. You slowly lower the hydrometer into the container, with many distillers adding a small twirl when letting go to ensure that the hydrometer is floating freely. Allow a few minutes to let the hydrometer settle, then take a reading.

When you take the reading, follow the manufacturer’s recommendations carefully: Some hydrometers require you to read from the bottom of the meniscus while others read from the top of the liquid line. Depending on the temperature of the sample, you may also need to add or subtract a correction factor, which you can do by measuring the temperature of the sample with a thermometer and then consulting the relevant correction tables.

Once you’ve read the hydrometer and made any temperature adjustments, finding the ethanol concentration is as simple as looking it up in a table. The TTB and many other alcohol-control authorities have tables that correlate ethanol content to the correct density, and they publish them publicly on their websites.

Densitometry

The second commonly approved method for measuring the strength of ethanol in solution is densitometry. Densitometry is a term used in many fields to refer to various concepts relating to density. In spirits, it specifically refers to the measurement of a liquid’s density through use of an “oscillating U-tube.”

Oscillating U-tubes measure density through electron excitation principles. They are extremely complicated, but their operation is surprisingly simple. To take a density measurement, you simply place a sample into the U-tube, which has a known capacity and oscillation, and then the tube is electronically excited into oscillation. This oscillation is measured and used to determine the density of the sample, which can then be correlated to the amount of ethanol.

Oscillating U-tubes are extremely accurate and regularly give density values up to six decimal places. However, they are also sometimes finnicky, and there are some important steps you need to take to get valid results.

The first factor you need to consider when using an oscillating U-tube is how recently the U-tube has been calibrated. Like other scientific instruments, U-tube calibrations tend to drift over time and with repeated use. Regularly recalibrating U-tube readings is vital to maintaining consistent results.

Then, before you place a sample into a U-tube, you must ensure that the U-tube is completely clean. Any residue from past samples or other contaminants will affect the oscillation, giving an invalid result.

Finally, when you fill the U-tube with the sample, it’s important that there are no bubbles or large particles in the sample, as they may prevent the U-tube from oscillating. This is of special concern for carbonated products, such as RTDs, which will sometimes shed carbonation during the oscillation process, creating bubbles and invalidating readings. For this reason, it is considered best practice to thoroughly sonicate carbonated samples before testing.

Currently, there are several companies that sell oscillating U-tube densitometers for the alcohol industry. Perhaps the most popular is Anton-Paar, whose benchtop DMA 4500 and 5000 lines the TTB has preapproved for measuring spirits. There are also a few handheld units that operate using oscillating U-tube technology. Although they are smaller and easier to wield, they lack the accuracy to makes them acceptable for final gauging by most federal agencies.

Obscuration

Although the above methods are approved for measuring the ethanol content of spirits, they come with an important caveat: Both hydrometer and densitometer readings can be affected by something known as obscuration.

In simple terms, obscuration is anything that falsely alters the measurement of ethanol in a solution when testing under standard conditions. If a solution contains a chemical or compound that causes the measurement of ethanol by hydrometer or densitometer to read untruly, then that spirit is obscured.

There are many compounds and chemicals that can cause obscuration, though the most common one in distilled spirits is probably sugar. Like ethanol, sugar is miscible in water. Also like ethanol, when sugar dissolves into water, its molecules fit into the gaps produced by water’s crystalline structure. However, unlike ethanol, because of its size and structure, sugar increases the density of water. As a result, when sugar is present in sufficient quantities in an ethanolic solution, the density of the solution appears higher than it would normally. That makes hydrometers and densitometry inaccurate. The same principles apply in reverse for lighter chemicals such as methanol, which can lower the apparent density of a solution, though this is much less common.

Correctly measuring the alcohol content of obscured spirits can be frustrating. Luckily, there are two relatively easy methods that the TTB has developed to solve the problem.

The first is the evaporation method. It involves simply taking a set amount of spirit and evaporating all the liquid off, so that only solid compounds, such as sugar, are left behind. These solids are then weighed, and an obscuration factor is calculated. (See the TTB website for exact instructions.) You can then add this obscuration factor to any traditional measurements obtained by hydrometer or densitometer and the true alcohol calculated. Unfortunately, this method is only effective for products that have just a small amount of obscuration; it doesn’t work for heavily obscured products.

The second method for determining the ethanol content of obscured spirits is effective for all levels of obscuration, but it can be more time- and labor-intensive. It involves distilling a set amount of the obscured spirit, to separate the ethanol from any possible obscurants. You then measure the distilled ethanol as well as the original solution. This method can be incredibly accurate, but the amount of time and investment often make it difficult to accomplish regularly.

Precision Matters

Regardless of what type of spirit you’re making, it is vitally important that you know and understand what obscuration is and whether your spirits are affected by it. When measuring obscuration, accurate and consistent methodology is key. There are many distillers who’ve had to recall products simply because they forgot to account for obscuration.

Correctly measuring the amount of ethanol in a solution can be daunting, but it’s an important skill to have. Successful distillers should be able to use their understanding of the interactions between water and ethanol as well as their knowledge of the methods and systems of measurement to consistently and correctly produce excellent spirit with the right amount of alcohol.

Failure to do so can sometimes be the difference between a successful product launch and a business-crushing fine.