The Spring 2015 issue of Artisan Spirit Magazine includes an article entitled “Measuring and Calculating Alcohol in Distilled Spirits and Liqueurs.” The article, written by Gary Spedding Ph.D., tackles the challenging act of calculating alcohol content in spirits containing high amounts of extracts. The following is a brief set of definitions to aid in the discussion…
A measure of the amount of alcohol in the beverage. The alcohol content of a distilled spirit typically refers to the amount of ethyl alcohol (as opposed to higher or fusel alcohols). The analysis of the spirit for alcohol content is an important part of distilling laboratory work for quality assurance programs and for legal reporting purposes. Results, however, are subject to appreciable variation and under official methods the analyses are time consuming and expensive.
Apparent and Real Extract
Brewers measure changes in density as sugars are consumed during fermentation and converted into alcohol. Measurements are obscured (the true gravity is “hidden”) by alcohol (of lower density than water, sugar solutions or beer) causing “buoyancy effects” with hydrometers for example. Thus false or apparent readings of gravity are made when instruments measure beer (containing water, sugars and alcohol); hence “apparent extract”. For distillers the same principles are in effect but the extract is from the addition of sugars, proteins and other solids materials from flavor additions, fruit etc. So in cases of high solids content the distiller needs to run laboratory scale distillations to separate the alcohol from the extract (solids) and to determine the alcohol SG in the distillate (largely the alcohol and water). The solids are left behind but can be determined by bringing the distillation pot residuals back up to the initial sample volume and measuring the extract-containing solution (with no alcohol present) to obtain the extract content.
The real extract is a true(r) measure of remaining sugars – and proteins etc. – as determined in the absence of alcohol (removed via distillation or boiling). When distillers measure a sample containing sugars this will be an “apparent” gravity and neither the true alcohol content or extract content can be directly determined. The true or real extract is expressed in grams per 100 grams of sample (a Plato value) and is needed to obtain correct calorie content of a spirit sample along with the alcohol by weight value.
A term we use in our laboratory and might not be discussed in the same way by others. It refers to the solids – any matter left behind in the residual liquid portion of the distillation flask after all the alcohol and some water has been distilled-off. It represents the concentrated sugars, non-sugar carbohydrates, protein and minerals etc. from the spirit, wine or liqueur and needs to be reconstituted back with distilled water to the original or starting volume used for the distillation operation (usually and officially 100 mL) prior to extract determination. For gravimetric distillations the residuals would typically be returned back to 100 grams (+/- 0.1g) before measurement of the extract density or specific gravity. The correct and careful determination of the true extract content of liqueurs etc. is not discussed in this present article and this term is therefore mentioned here only for the sake of completeness in the discussion.
Gravimetric and Volumetric Distillations
Distilled spirits often need to be measured following laboratory-scale distillation to obtain the alcohol from the spirit free of any solids or extract present. The latter are left behind in what this author calls the distilland residuals (see above). Officially approved methods describe the process in detail, however it should be noted that other alcohols and volatiles do distill over and reactions take place in the heat of distillation that can affect the subsequent reading of alcohol; it is not as simple as expected that the final distillate is simply a binary mixture of ethanol and water. Though most alcohol vs. specific gravity tables were generated over 100 years ago by careful distillations of alcohol and water blends; other than quite pure vodka or grain neutral spirits (of high purity!) most distilled spirits cannot be considered in the same way based on solids present from sugars, acids, flavorings, fruit pulp , creams etc. Yet tables, generated from specific gravity determinations of pure alcohol/water mixtures, were rapidly adopted by the alcohol beverage industry and have served well for over 100 years.
Early on the density values and specific gravity values were obtained gravimetrically – the latter values using an instrument known as a pycnometer. Gravimetric distillations (based on weight) giving true alcohol by weight values for a sample of water and ethanol but requiring a correction for alcohol by volume. The density of pure ethanol being 0.7907 (grams/milliliter) meaning volume determination for the same weight of alcohol required a factor of 1.2647 (the inverse of 0.7907) to convert to volume. Under ideal conditions 100 grams of alcohol would exist in a physical volume of 126.47 mL (temperature dependent!). Volumetric distillations (starting typically with exactly 100 mL) would give a true alcohol by volume content (temperature dependent) but would require a compensating correction for weight. The issue today becomes one of accounting for the extract content in sugar rich or flavor rich spirits. The sample specific gravity includes the extracts present and alcohol values need to be corrected for the actual sample specific gravity (SG). A fact lost to many and perhaps due to the fact that traditionally spirits such as whisky, whiskey (Bourbon) rum and tequila and the classic white spirits (vodka, gin) contained little in the way of extracts; only a little possibly derived from any wood aging or minor additions of sugar/citric acid (in vodka) for example. Such spirits carrying SG values significantly below 1.0000. [Several pure vodka’s on the market make excellent calibration standards for checking various measuring instruments and efficiencies of distillations being largely or almost totally free of solids/extracts.]
A typical way to report extract in distilled spirits (sometimes referred to as degrees Brix which are closely similar though not 100% exactly correlated) as weight percent (e.g. 12 Plato is 12 grams/100 grams extract). Plato values may be obtained directly from hydrometers calibrated in degrees Plato, by means of specific gravity measurements as related to standard Plato (or extract) tables and (to varying degrees of approximation) by calculation – some equations seen in the text here. The Plato scale and tables are themselves approximations for complex physical-chemical reasons.
Brewers and winemakers take this as an expression of the sugar content in units of grams of sugar per 100 grams of wort equivalent to % weight/weight (w/w). In the brewing industry this is denoted as “degrees Plato” (°P) and winemakers referring to it as “degrees Brix”. Plato and SG values can be inter-converted using appropriate formulas (as discussed in the text) and this will prove important in working through the equations presented for determining distilling analytical parameters.
While distillers need to be aware of this term and its use here the exact determination and use of the true and complete extract present in spirits and liqueurs it is not considered in detail in the present article. It will be covered in depth in a future note. For purposes of the present discussion it is appropriate to state simply that Plato readings are an alternative expression of specific gravity used when substances other than alcohol, water and congeners are present. Alcohol and water mixtures such as obtained from distillation will be expressed as a specific gravity number less than 1 (e.g., 1.00000) whereas depending on the amount of other components and alcohol higher extract samples (sugars etc.) will yield specific gravity numbers above 1. In such cases Plato hydrometers will give solids or extract values in grams/100grams of sample and these can be converted into their respective and equivalent specific gravity values as discussed in the text.
Proof and Proof Obscuration
Alcohol measured as “degrees Proof” in the US is 2 x the percentage of alcohol by volume expressed at 60°F. Proof obscuration is the “masking” or hiding” of alcohol during measurements by the solids or extract content in the spirit. As discussed in the text a proof obscuration factor is determined for spirits containing between 400-600 mg/100 mL of solids and added to a measured apparent proof to give true proof.
Sample Density or Gravity
The sample density or SG of a distilled spirit is that value measured directly in the spirit containing the alcohol, water, any sugars (protein) or additives (fats in cream-based liqueurs), etc. For brewers the measurements always include the alcohol and extract content of the beer and the latter being related to an expression of sugar content – grams of sugar per 100 grams of wort. [Equivalent to % weight/weight (w/w)]. In the brewing industry this is denoted as “degrees Plato” (°P) and to a first degree of approximation beer responds in the same way as the original solutions of sucrose and water used to derive the Plato scale. Distillers do not usually refer to extract content in their process operations as the usual product of distillation is a mixture of water and ethanol with some congeners added to the mix. With sugars added or flavorings, etc., there will be “extract content” to the spirit and this makes difficult the measurement of the true alcohol content. This is due to obscuration or the “masking” of alcohol by solids in the extract. Instruments cannot measure the true alcohol content with high concentrations of solids present. Distillers need to be aware of this and current regulations and sometimes need to run a test on solids to perform a proof obscuration adjustment to the measured alcohol content. Distillers will measure an apparent alcohol content unless distilling the sample when high solids/extract containing beverages are involved. In cases with very low solids there will be no major issue measuring alcohol but for high extract samples the distiller will need to make some corrections to the evaluations of alcohol content to account for the higher than 1.0000 SG of the sample due to extracts. [See Proof and Proof obscuration.] Sample densities and specific gravities can be inter-converted using equations described in the text. See related terms below.
Specific Gravity (SG) or Relative Density
Is an intensive property of a substance (be it solid or liquid) and is, historically, the ratio of the density of a substance at the temperature under consideration to the density of water at the temperature of its maximum density (4 °C). The actual density in theory – for the purposes of most discussions with respect to alcoholic beverages – is generally 20 °C/20 °C relative to water at unity though, elsewhere in the alcohol beverage world, it is typically expressed as at 20 °C/4 °C. The complexity of the temperature associations could not be presented in depth here – full details may be found in the literature (see the references). An SG value is numerically equal to the density in grams per milliliter (or Kg per liter) but is stated as a pure number (because the division by the water’s own density value leads to a cancellation of the units), while density is stated as mass per unit volume. Water has a specific gravity of 1.0000 at 20°C. Base and derivative density values form mathematical grounding in many of the formulas used in alcohol and extract calculations. The density value 0.998201 will be seen in the literature as pertaining to the formulas and conversions of alcohol density, specific gravity, and alcohol by weight and volume and while understood within this article is also not covered in any depth here.
Two articles by Spedding (2013 and one in press) cite many references used to prepare this and related articles and should provide more material for the reader wishing to go into more depth on this topic. A detailed set of references can be made available to interested readers via contact with the author or from a reading of the two articles listed here. Several web-based links are also provided for further coverage on this topic.
Spedding, G. “Empirically Measuring and Calculating Alcohol and Extract Content in Wort and Beer with a Reasonable Degree of Accuracy and Confidence – Using a Series of Inter-related and Conversion Equations, Algorithms, Tables and an On-line Calculator”. Brewers Digest (online) July-August: 23-34. 2013.
Spedding, G. “Alcohol and its Measurement”. In: Brewing Materials and Processes: A Practical Approach to Beer Excellence. (Edited by C. W. Bamforth.). Elsevier Inc. (Under Review)
Website links (all last accessed November 2014):
The Alcoholometric Tables may be found here: http://www.itecref.com/alcohol-tables.html or http://www.oiml.org/en/files/pdf_r/r022-e75.pdf/view.
Electronic Code of Federal Regulations (Alcohol): http://www.ecfr.gov/cgi-bin/text-idx?SID=37ea73c1ca6d752d6b1bba44b03c1680&tpl=/ecfrbrowse/Title27/27cfrv1_02.tpl#0
Gauging Manual: http://www.ttb.gov/foia/gauging_manual_toc.shtml
Lab Methods for Distillers: http://www.ttb.gov/expo/presentations-black/s14-bw.pdf
Proofing video series: http://www.ttb.gov/spirits/proofing.shtml
The Beverage Alcohol Manual (Spirits): http://www.ttb.gov/spirits/bam.shtml
The TTB Public Guidance document: TTB G: 2014-1. Approval of Alternative Devices for Determination of Proof of Distilled Spirits for Tax Determination Purposes: Guidance and Performance Standards. Alcohol and Tobacco and Tax Trade Bureau. [July 21, 2014] is available here: http://www.ttb.gov/pdf/2014-7-21-device-approval-guidlines.pdf