To measure the alcohol content in a spirit, the official method by distillation is still the most used method because it is the most universal and the least expensive in investment.
There are different types of distillation devices (direct distillation or steam entrainment), more or less automated, but which all have a maximum field of application for measuring the alcohol level.
This article explains how to use the device even if the alcohol level is above its use limit, while obtaining accurate results using appropriate methodology.
Reminder of the definition of alcohol content and the principle of measurement by distillation
The “Alcohol level” or “Alcohol degree” of a drink corresponds to the percentage by volume of ethanol (% vol.). It is officially named “Alcoholometric strength by volume” or A.S.V. in % vol.
The A.S.V. is directly correlated to the density in a mixture, when it consists only of water and ethanol (1). Thus, in an alcoholic beverage, to know precisely the A.S.V. by measuring density, it is necessary to get rid of “intruders” which increase the density.
Distillation is one of the methods used to analyze alcohol content.
It eliminates the dry extract that these beverages may contain, recovering a distillate composed mainly of water and alcohol.
The density of the distillate can be measured using an alcoholmeter or an electronic densimeter, and matched to the A.S.V.. Refer to this Blog article : “Alcohol Content Measurement – Part 1: Principle, Equipment, Selection Criteria“
The measurement scope of the A.S.V. with distillation devices are limited.
- For direct distillation devices, the range can be up to 55 % vol.
- For devices distilled by steam distillation, the range is limited to a maximum of 25% vol., or even a little less depending on the temperature of the refrigeration water.
Steam distillation devices are preferred when the drink contains too much sugar. In fact, the presence of sugars risks caramelizing the flask of direct distillation devices. Refer to this Blog article « Alcohol Content Measurement – Part 1: Principle, Equipment, Selection Criteria ».
DUJARDIN-SALLERON specializes in the manufacturing and marketing of direct distillation and steam distillation devices.
Here are some questions they are regularly asked :
- Is it possible to dilute my sample before distillation to stay within the measuring range of the device ?
- Given the contraction phenomena when diluting with water, how to proceed ?
- What is the measurement accuracy of A.S.V. after dilution ?
To answer this, we must look at the behavior of ethanol during dilution. This behavior is described and explained in the article on this Blog entitled : “Assemblage d’eaux-de-vie, un phénomène de contraction bien étrange“.
In summary : It has been shown that when a hydroalcoholic mixture is diluted with water, everything happens as if the volume occupied by the ethanol does not change.
Therefore, to dilute the sample, not knowing a priori the volume contraction factor that will occur, it is essential to start from a very precise volume of product (Vi) and to obtain a volume of distillate also very precise (Vd). After measuring the % vol. of alcohol in the distillate (A.S.V.d), the % vol. of alcohol of the distilled product (A.S.V.i), will correspond exactly to the following calculation : A.S.V.i = A.S.V.d * (Vd/Vi).
To obtain a fairly accurate result, it is necessary to follow the correct dilution protocol :
– The volume to be distilled must be measured using a volumetric flask (Fi).
– The distillate must also be collected in a volumetric flask whose volume will correspond to the desired dilution (Fd).
Practical case of a dilution by 2 and a volume to be distilled of 200ml :
– Pour 100ml of product into a Fi volumetric flask.
– Transfer these 100ml into the distillation flask by rinsing the Fi flask several times with distilled water which is also to be transferred into the distillation flask. The rinsing volume must not exceed 120ml.
– Start the distillation, recovering the distillate in the Fd flask.
– If necessary, degas the distillate in its flask.
– Adjust to the gauge mark with distilled water when the temperature has returned as close as possible to the initial temperature.
– Measure the alcohol content of the distillate (A.S.V.d).
Be careful, these manipulations are delicate.
In order to recover all of the starting alcohol, carefully take into account the recommendations of the distillation equipment supplier and in particular by taking into account the following critical points:
– Volumetric flasks must be class A, well maintained. the F1 vial must be very dry.
– The manipulations must be carried out with products whose temperature is brought as close as possible to 20°C (+/- 1°C), without loss of product when transferring the F1 flask into a distillation flask.
– The recovery of the first drops of distillate, very loaded with alcohol, must be done in a few ml of cold water.
– For certain products containing natural surfactants, the distillate may need to be degassed before adjustment to the gauge mark. In this case, use a specific ultrasonic tank.
It is therefore recommended to carry out some preliminary tests to ensure the proper functioning of the distillation apparatus and good handling. For that :
1) Starting from a hydroalcoholic mixture, with a known alcohol level, close to the desired measurement range and not exceeding the range of use of the device, distill it. Check that you find the initial value of the alcohol level at +/- 0.05% vol.
2) Start with a hydroalcoholic mixture, with a known alcohol level and close to the desired measuring range, but exceeding the measuring range of the device. Distill it by diluting it by 2 (example 100 ml recovered in 200ml). After taking into account the dilution, check that you find the initial value of the alcohol level at +/- 0.1% vol.
A simultaneous measurement of the alcohol content of the initial product and the distillate with the same measuring device makes it possible to overcome the measurement uncertainty of this device.
Calculation of the alcohol content of the distilled product
According to the principle stated at the beginning of this article, the volume occupied by the ethanol in the distillate will be considered as being the same as in the initial 100ml. The % volume of alcohol measured in the distillate will therefore have to be multiplied by the dilution, to obtain the % initial volume of alcohol, in the practical case described above by 2.
Demonstration by calculation : Principle of reasoning and example
Starting alcohol at 40% vol. – 100ml to distill and 200ml recovered (i.e. a dilution by 2)
All volumes, alcohol levels and densities to be taken into account in these calculations are at 20°C.
- Calculation of the mass of ethanol which will be transferred to the distillate: Table II of Recommendation R22 of O.I.M.L. (1) corresponds to a density of 948.05 g/l at 40% vol. of alcohol.
- Table II of Recommendation R22 of O.I.M.L. (1) corresponds to this density of 948.05 g/l, an ethanol mass % of 33.29%.
- The mass of ethanol is therefore 948.05 * 0.3329 or 31.56 g, which will be completely transferred into the 200ml of distillate.
- The density of ethanol being 789.24 g/l (Table II of Recommendation R22 of O.I.M.L. (1)), the volume of ethanol will be 63.14/789.24, or 0.0400 l, or 40.0 ml.
- The % volume in the distillate will therefore be 40/200 or 20.0 % vol.
By multiplying by the dilution rate which is 2, we find 40% vol. initials.
Conclusions
This article follows the blog article “Assemblage d’eaux-de-vie, un phénomène de contraction bien étrange” shows that it is possible to analyze the alcohol level of a spirit by dilution, when this level exceeds the range of use of the distilling apparatus. However, it is necessary to carefully follow the handling recommendations.
The precision of the measurement is based on mastery of the handling and performance of the distillation apparatus.
It will also depend on the uncertainty of the device for measuring the alcohol level in the distillate, whether it is an alcoholometer or another measuring device.
Evelyne CHANSON – Consultant in quality control of Wines and Spirits of EC Consulting
Antoine GRUAU – Oenologist and Sales Manager France for DUJARDIN-SALLERON
- Official alcohol table : Table II () of the Annex to the document “International Alcoholometric Tables” published by l’OIML Recommendation R022 -f75
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