Hydrometers and densitometers are well-established tools to accurately measure the ethanol content of distilled spirits—but there are other ways.
As scientific knowledge has steadily grown, researchers and distillers have created myriad ways to measure and report ethanol content, each with its own advantages and disadvantages.
Here, we explore four newer methods that have begun to grow in popularity.
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Gas Chromatography
The first method gaining acceptance is gas chromatography (GC).
GC is a relatively modern technique used in analytical chemistry to separate and identify compounds. It works by using a carrier gas to volatilize compounds within a sample and push them through a very thin, tightly wound column. These columns, often made from metal or glass, use capillary action to separate compounds based on their unique physical and chemical properties, allowing each compound to be analyzed individually using a detector. There are numerous types of detectors that can be used in GC, each optimized to measure specific compounds. For determining alcohol content, the two main options are flame ionization detection (GC/FID) and mass spectrometry (GC/MS).
GC/FID is particularly effective for measuring the ethanol content of samples whose makeup is already known. It involves placing a small hydrogen flame at the exit of a GC column, then measuring the number of ions formed from the combustion of compounds as they leave the column. Electrodes monitor the differential current and then correlate it to the mass and amount of each individual compound. Producers around the world have used GC/FID effectively to measure the ethanol content of beer, wine, and spirits, although it’s less effective at detecting inorganic or highly oxygenated compounds. It also sometimes struggles to identify unknown compounds, which can make it unsuitable for analyzing complex or new products without a known composition. Nevertheless, GC/FID has become a standard method for many distilleries that produce clear spirits at scale.
GC/MS is the most popular type of GC detection method when dealing with unknown compounds. It can accurately differentiate between similar compounds by measuring the mass-to-change ratio of individual ions. That means it can accurately analyze the ethanol content in products that have high concentrations of similarly structured alcohols or other compounds that might otherwise obscure the ethanol reading on a standard GC/FID. That property makes GC/MS important for distillers who want to fully understand the chemical makeup of their spirits.
Although GC/MS is considered the gold standard for compound analysis, it is expensive and time-consuming. Understanding the chromatograms produced by GC/MS is a complex task that takes years of practice. While only larger distilleries can invest the time and money to rely on GC/MC, it has become an invaluable tool in providing a more holistic view of their spirits.
Overall, gas chromatography in multiple forms can be effective for measuring alcohol content in spirits. The largest drawback to using GC is that the devices require careful calibration to be accurate. That, coupled with the costs of buying and operating a gas chromatograph, make it difficult for smaller distillers to adopt them as tools.
Another method used to determine ethanol content is high-performance liquid chromatography (HPLC).
Like GC, HPLC uses capillary action through a column to separate compounds based on their physical and chemical properties. The main difference is that HPLC measures the liquid phase of a compound, not the gaseous one. That gives HPLC the edge when measuring nonvolatilized compounds, such as sugars.
As with GC, HPLC requires the use of a secondary detector to properly measure the amount of ethanol in a solution. Most (but not all) HPLCs use either a UV or refractive index detector. You place the samples into a port on the chromatograph, which pumps a solvent solution—known as the mobile phase—through the sample into a column. The columns are packed with solid materials, such as silica, that impede the flow of the solution, separating it into its constituent components. The detector unit then measures these components individually as they exit the column, determining the total composition of the sample.
One advantage of HPLC columns is that they can be packed with different materials depending on the sample to be analyzed, allowing the user to tap into different separating principles, such as selective adsorption or ion-exchange. That significantly increases the number of compounds that HPLC can measure, making it a powerful tool for many different analyses.
The only downside to HPLC when measuring alcohol content is that its ability to measure ethanol accurately depends on the type of column being used. Unfortunately, columns that are good at reading sugars and organic acids are less accurate at reading ethanol—and vice versa—hindering HPLC’s usefulness when measuring high-sugar finished products. That’s why many larger distillers prefer to use HPLCs to monitor fermentation health rather than finished products—during fermentation, the exact accuracy of the ethanol content is less important.
Infrared Spectroscopy
A third way of measuring the ethanol content of spirits that is only beginning to grow in popularity is the use of infrared spectroscopy (IR).
IR is a diagnostic approach to measuring concentrations of compounds. It’s become popular in many adjacent food and beverage industries because it’s easy to use and requires little sample preparation.
IR technology operates on the principle that, when exposed to infrared-light radiation, molecules will absorb certain frequencies of energy and begin vibrating. Each molecule absorbs frequencies in a way unique to its structure and quantity, giving those molecules a unique fingerprint by which they can be measured. A technician can then collate those fingerprints into a database, referencing them to identify and quantify a specific compound within a sample.
For spirits, mid-infrared and near-infrared bandwidths (MIR and NIR, respectively) appear to be the most effective for measuring ethanol content. That has led to the creation of several commercial devices that use NIR methods to rapidly measure ethanol content, in-line, without the need to take a sample.
Although IR tech can look like a magic bullet—and its use is growing—it does come with an important caveat: To measure the ethanol content of a spirit, NIR machines typically rely on reference methods that have already been created. Those reference methods typically require a density measure to be calculated effectively, and those methods may not have been based on the same substrates being measured.
Seemingly small differences in the makeup of a sample can sometimes cause wild variations in readings between two products, so distillers may have to create new reference methods for each product. That can be especially frustrating to distillers whose products change with each batch, requiring constant recalibration to match them.
Enzymes
Finally, another less common method for measuring ethanol content in spirits is with enzymes.
Currently, enzymatic determination is not very common for beverage producers—but it has grown in popularity recently, and there are even companies selling kits for this purpose.
The most common method involves breaking down a sample’s ethanol into acetic acid, then spectroscopically measuring the amount of an enzyme called nicotinamide adenine dinucleotide (NADH). That is accomplished by first exposing the solution to alcohol-dehydrogenase and NADH, which oxidizes the ethanol into acetaldehyde and NADH. The aldehyde-dehydrogenase then further breaks down the resulting acetaldehyde to produce more NADH and acetic acid. A UV-visible spectrometer can then measure the concentration of NADH at a wavelength of about 340 nanometers, making it possible to calculate the initial amount of ethanol present in the sample.
Enzymatic determination of alcohol content can be extremely accurate, especially at lower alcohol concentrations. That’s why some producers use it to confirm low- or no-alcohol claims in beverages. For distillers who want to measure the ethanol of their spirits enzymatically, it may be necessary to dilute their products before completing the test, thus adding another layer of complexity to the calculations.
Nonetheless, enzymatic determination can be an effective and reliable method for determining ABV.
Methods of the Future?
When compared with the centuries-old hydrometer, the methods described above are still relatively new to the distilling world. Furthermore, the TTB has not approved as acceptable any of these methods for reporting ethanol contents in finished spirits.
Nonetheless, they’re all effective and are growing in popularity among distillers. There may come a day when most of us measure the ethanol of our spirits using one or more of these approaches.
