The official method for determining the alcohol content of a spirit requires prior distillation to eliminate residues that could distort the density measurement. However, distillation does not completely eliminate all volatile compounds other than ethanol.
What about measurements made with automatic or semi-automatic devices, very selective for the analysis of ethanol, such as near infrared analyzers?
Official method for analyzing alcohol content of a spirit
The official method of measuring alcohol content derives from the official definition of alcohol content which is as follows:
According to European regulation CE 2870 of 2000, the real alcoholic strength by volume (real A.S.V.) of spirit drinks is equal to the number of liters of ethyl alcohol contained in 100 liters of hydroalcoholic mixture having the same density as the spirit drink after distillation.
-> A hydro-alcoholic mixture at 40% vol. therefore theoretically contains 400ml of ethyl alcohol (Ethanol) in 1 liter at 20°C.
To respect the official definition, it is necessary to measure the Real A.S.V. by densimetry, on a product containing no dry extract.
However, to eliminate the dry extract, it is necessary to go through a distillation step.
For a product containing only water and alcohol, the measurement of the alcohol content can be done directly on the product, with an alcoholometer or other device for measuring by densimetry.
On the other hand, for a product containing dry extract, this must be eliminated by distillation.
The measurement of the alcohol content with an alcoholometer or other device for measuring by densimetry must be done on the distillate.
Alcoholmeters or other densimetry measuring devices measure density, but are calibrated to give the T.A.V. directly.
The official table which gives the correspondence between the measurement of M.V. and the T.V. is Table II of Recommendation No. 22 of the O.I.L.M. (International Organization of Legal Metrology) (1).
Note:
As long as the product does not contain dry extract, measurement by density measurement methods, known as “densimetric”, corresponds to the Real A.S.V.
When the product contains dry extract, the measurement by densimetric method, without prior distillation, corresponds to what is called Gross A.S.V..
Indeed, the dry extract increases the density and therefore generates a reading of A.S.V. weaker.
The difference betwwen the Real A.S.V. and the Gross A.S.V. is called “Obscuration”
For more precision refer to the article “Obscuration” (thema: Alcoholometry)
Whether carried out in the laboratory or in the distillery, distillation, although it makes it possible to get rid of the dry extract, allows a certain number of volatile substances to pass through which can modify the density of the distillate.
Among the volatile compounds which pass in whole or in part during the distillation, there can be aldehydes, esters, higher alcohols and many other compounds in more or less important concentration. The presence of these compounds in the distillate modifies the density so the Gross A.S.V..
The official method to analyze alcohol content by distillation isn’t selective of ethanol content.
Influence of concentrations of volatile substances on the measurement of A.S.V. by the official method
Based on analyzes carried out by gas chromatography – flame ionization detector (FID), of all types of spirits, an internal study was carried out by EC Consulting (2) to calculate the influence of the profile of the main volatile compounds on the density measurement.
The summary table below gives examples of chromatographic profiles for a number of spirits.
– The selected spirits are all free of dry extract.
– The gross A.S.V, measured directly on these spirits, is indicated in the 2nd column.
– The concentrations of different compounds analyzed in GC are in mg/l.
– The real A.S.V. was recalculated from the density that would be obtained without the effect of these compounds.
– The difference between the A.S.V. measured and the A.S.V. recalculated is reported in the last column.
(a) Aldehydes = Acetaldehyde + Acetal expressed in acetaldehyde
(b) Superiors alcohols = Butanol-1 + Butanol-2 + Propanol + Isobutanol + Isoamyl alcohols
(c) Fatty acid esters = Ethyl caprylate + caprate + laurate
The conclusions of this study, based on the profiles studied, are as follows:
- In the case of a density measurement whether after distillation or directly in a spirit containing no dry extract, the measurement of the A.S.V. by densimetric method can be influenced by some volatile compounds.
- The most influential compounds are ethyl acetate and higher alcohols.
- For most of the spirits in this study, the discrepancy found between the measurement of A.S.V. by densimetry and the calculation of the density is between 0.2 and 0.3% vol.
- For spirits even more loaded with higher alcohols and/or ethyl acetate, the deviations can exceed 0.3% vol.
What about methods of measuring real A.S.V. by Infrared apparatus which are very selective in the determination of ethanol pourcentage ?
- Near or medium infrared analyzers can be used to analyze a certain number of compounds in spirits.
- They require little volume and little sample preparation. In general, a simple filtration is enough.
- Depending on the type of device, they can be very selective in measuring ethanol. In this case, these devices measure the A.S.V. without being influenced by other volatile compounds.
Conclusions:
When the volatile compounds are in such a concentration that they significantly influence the measurement of the A.S.V. by official method, the comparison with the result obtained with these devices can give deviations greater than the measurement uncertainty of the official method (which is approximately 0.2% vol.).
Consequences:
It is not recommended to use this type of device, when the measurement of A.S.V. must be comparable to that of the official method and that the chromatographic profile of the main volatile compounds of the product analyzed is not known.
On the other hand, these devices are entirely recommended for internal production control, if the chromatographic profile of the product analyzed is known or if the product corresponds to a range whose profiles vary little.
However, it is necessary to ensure that the field of application of the device covers the range of A.S.V. to measure.
With Infrared device, the analyzes are fast, automatable, consume little sample and the reproducibility is generally quite sufficient given the uncertainties required for this analysis.
The accuracy of the result and its uncertainty will essentially depend on the quality of the calibration.
Can we calibrate the near infrared analyzers to find the same A.S.V. than the official method?
It is possible to obtain results comparable to the official method by following one of these 2 methods:
1st method: Calibration with a range of the same type of product, products for which the reference value of the A.S.V has been established with the official method by an accredited laboratory. Depending on the product ranges, these can be more or less loaded with volatile compounds. It may therefore be necessary to carry out several different calibrations.
2nd method: Calibration with water-ethanol mixtures whose reference values of A.S.V. are established by densimetric measurement. Analyze a range of products of the same type, for which the reference value has been established with the official method by an accredited laboratory. Then determine the correction factor for this range by calculating the average of the differences obtained between the infrared analyzer and the official method. The correction factor will depend on the volatile compound load.
Calibration with hydroalcoholic mixtures is easier to perform.
Indeed, when the device is coupled to an electronic densimeter, the values returned by this device are very reliable and allow a calibration of the near infrared analyzer that is more precise and easier to control.
Given the influence of volatile compounds on the measurement of A.S.V. by densimetry, what about the precision of the alcohol level measurements that are made during distillation for the production of eaux-de-vie ?
Measurements of A.S.V. in the distillery are used to adjust certain distillation parameters or to calculate the distillation yield.
These measurements are all carried out by densimetry (alcoholometers, electronic densimeters, mass flowmeter).
However, certain distillation fractions can be highly loaded with volatile compounds, in particular ethyl acetate in the heads.
Conclusion:
Only in-depth studies carried out from the chromatographic profiles of the distillation fractions would make it possible to assess the influence of A.S.V. measurements by densimetry, on the adjustment of the apparatus to be distilled or on the calculations of distillation yield.
Evelyne CHANSON – Quality Control Wines and Spirits Consultant at EC Consulting
- Recommandation O.I.M.L. R022 -f75 – Official alcohol tables
- EC Consulting internal study, carried out from known chromatographic profiles according to different types of spirits.
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