On Alcohol, Alcoholic Strength and Measurements
-by Gary Spedding, Alcohol Beverage Chemist-
Alcoholic strength is the term used to denote the measure of the amount of ethyl alcohol (ethanol) in solutions such as beer, cider, kombucha, malt-beverages, RTD cocktail mixes, wines and spirits/liqueurs. It may be reported as percent by mass of solution (% weight /weight – w/w or mass/mass - m/m) or as percent by volume (% volume/volume – v/v). These values are expressed at either 60 °F (15.56 °C) or at 20 °C (68 °F) depending on country, regulatory authority or other local requirements.
Over 180 years of research, laboratory practice, and a number of established verified tables of data and extensive algorithms help the alcohol beverage chemist best determine alcohol content in their samples. Many of the methods used to determine alcohol (once only available in well-equipped laboratories) have evolved through new technological developments, with modern instrumentation now simplifying the measurement of alcohol by beverage producers in their own facilities. That said, the grounding principles and theories behind alcohol determinations remain largely unmodified since their discovery, as will be demonstrated.
The Properties of Alcohol and Its Measurement
Ethanol is miscible with water in all proportions, with ethanol molecules fitting within the “spaces” in the three-dimensional structure of water. This “space filling” property means that the final volume of any mixture of water and alcohol is less than the sum of their individual volumes, i.e. there is a volume contraction upon mixing. The volume contraction has to be accounted for when determining alcohol by volume in aqueous mixtures. Solution volumes are temperature dependent and so temperature compensation also has to be allowed for in measurements. More sophisticated instrumentation compensates for temperatures of measurement and can deliver reportable data at specified temperatures. In the US , alcohol by volume is sometimes still reported at 60°F (15.56°C) though more frequently 20°C (68°F) is the required temperature. The temperature volume change is, however, effectively so small between these two temperatures that the difference in values is a little redundant for the tolerances allowed for most alcoholic beverages. Most modern instruments and methods are in fact now measuring and reporting values at 20°C (68°F) and this will be the value largely discussed here.
Ethanol (CH3CH2OH : C2H5OH ) has, at 1 standard atmosphere pressure, a boiling point of 78.29°C (172.92°F), an ignition or flash point of between 9-11°C (48.2-51.8°F), a freezing (or melting) point of -114.14°C (-173.45°F), a density (d ² ⁰4) of 0.78934 g/ml (or 789.34 kg/L at 20°C), a density (d20/20) of 0.78927g/mL at 20°C and a refractive index (n20/D: 20°C measured at a wavelength of 589 nm using the Sodium D-line) of 1.3611.
These properties may be used to determine the concentration of ethanol as outlined below. Density values may be expressed as relative density or specific gravity values as defined elsewhere and will be important numbers expressed throughout this note. Specific gravity values are unitless numbers as they are defined relative to a standard substance, usually water, and so the unit terms of density g/mL (or kg/L) cancel. For equations expressed herein it is noted that 0.7907 as a unit- less number, or the rounded values of 0.790 or 0.791 are the specific gravity values most commonly used in brewing circles for pure alcohol. The number is derived from the density value of alcohol at 20°C (68°F) and the density of water also at 20°C (68°F): 0.998203. [These listed properties of ethanol will be employed in methods for its quantitation as noted throughout this article series.]
Outline of Methods to Measure Alcohol
Over the years many physical and chemical methods have been used to determine the amount of alcohol present in solution. These methods relied on measuring some physical or chemical property of ethanol and they evolved in sophistication along with micro-chip technology and micro scale developments. In general, for the purposes of this article, only the more modern and sophisticated (officially accepted or approved) methods of alcohol determination are covered. As such alcohol may be accurately determined via:
• Chromatography – Gas (GC) and Liquid Methods (Including High Performance or High Pressure Liquid Chromatography (HPLC))
• Density and or Specific Gravity Measurement
• Enzymatically - [Highly specific biochemical tests for analytes]
• Spectroscopy – Near- infrared (NIR ) & Nuclear Magnetic Spectroscopy (NMR )
Before detailing the methods listed above, it is to be noted that alcohol was traditionally measured following its separation from samples by distillation. During distillation alcohol is boiled out of the aqueous solution and collected by condensation. Along with alcohol and water some other volatile components of the beverage will be distilled over, sometimes impinging upon the specificity of detection. Such issues if known can be compensated for with careful design of test protocols or by separate measurement of the suspected co-distilling compounds causing such interferences. While often dealt with in the past these issues are largely ignored today or are of only very minor impact on final results. Ultimately the concentration of alcohol in the distillate is estimated and then converted by calculation into alcohol content of the beverage.
Stay tuned for Part II of this informative series, to be continued next month.