In some analytical report, you can find the parameter “Obscuration”.
“Obscuration” is the difference between Real alcohol strength (Real A.S.V.) and Gross Alcohol strength (Gross A.S.V.).
Gross A.S.V. is given by an alcoholmeter, directly immersed in the spirit.
Real A.S.V. is given by an alcoholmeter, immersed in the spirit after distillation or by some specific analytical methods.
The more appropriate name might be “% masking volume”.

Obscuration = Real A.S.V.  –  Gross A.S.V.

The maximum Obscuration is limited for many “Appellations”.
Refer to the regulations of spirits and the specifications of these “Appellations”.

Origin of the difference between the Real A.S.V. and the Gross A.S.V.

Some basic concepts

 -When the product only contains water and ethanol, the density of a hydroalcoholic mixture is directly related to the alcohol content.

– The alcoholmeter is an hydrometer apparatus which measures density but calibrated in alcohol.

– The dry extract of a spirit improve the density.

More the density is high, the less the alcoholmeter finds alcohol.

An alcoholmeter immersed directly in the spirit, without first removing its dry extract, will indicate a lower A.S.V. (Gross A.S.V.) than the real A.S.V.

The difference between the Real A.S.V. and the Gross A.S.V. depends on the dry extract.

This helps to better understand the official definition of the real Alcohol by Volume of a spirit (1):
“The real Alcohol by Volume 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” (Collection of International Methods for Spirit Drinks (2)).

Therefore, to analyze the Real A.S.V. of a spirit containing dry extract, it is necessary to distill it or use a method that which does not take into account the dry extract, such as infrared methods (1).

The main compounds that contribute to Obscuration:

The main compounds that increase the Obscuration are primarily sugars and tannins.

1) Influence of sugars on the Obscuration 

There is a relationship between the sugar content (sucrose-glucose-fructose) and the Obscuration.
This relationship also depends on the Gross  A.S.V., specifically the density (2).

Examples:
1% vol. of Obscuration corresponds approximately to
–  4 g/l of sugars, at 40% vol.
–  5 g/l of sugars, at 50% vol.

2) Influence of tannin content associated with aging “in wood” or the addition of wood compounds

Significant amounts of tannins can have an influence on the Obscuration.
However, for amber spirits corresponding to the “Appellations” described in the European regulation, this influence is generally limited to 0.2% vol.

The “Dry extract – Obscuration” Boxette allows the calculation of dry extract from the Real A.S.V. and Gross A.S.V.
Conversely, it allows the estimation of the Real A.S.V. from known sugar concentrations and the measurement of the Gross A.S.V.

Evelyne CHANSON – Quality Control Consultant for Wines and Spirits at EC Consulting

(1) To learn more about methods for analyzing actual and raw alcohol content, read the articles dedicated to this topic in the “Production Control” section:
       – “Measuring the Alcohol Content of a Spirit: Distillation or Infrared.”

       – “Methods for Measuring the Alcohol Content of Spirits.”

(2) O.I.V. Collection of International Methods for Analysis of Spirit Drinks

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