Alcohol measurement in liquors using Alcolyzer Spirits 1
Chapter 94
Alcohol measurement in liquors using Alcolyzer Spirits
Roman Beneš, Christina Reichart and Peter Brugger
Anton Paar GmbH, Anton-Paar Straße 20, 8054 Graz, Austria
Introduction
A well-defined alcohol content is an obligatory
parameter in the production of alcoholic beverages both
for the need of a constant quality in industrial production
and from the point of view of governmental and taxation
regulations. There are several common technologies
for the measurement of the alcohol concentration, for
example distillation followed by density measurement,
gas chromatography (GC), high-pressure liquid
chromatography (HPLC) or (near) infrared spectroscopy.
Near infrared (NIR) spectroscopy has been established in
the last tens of years as an attractive alternative because
it is very rapid and no or little sample preparation
is needed. Anton Paar utilizes this technology in its
Alcolyzer series to measure the ethanol concentration
in beers, wines, and distilled spirits [1, 2] but – at least
so far - not in liquors.
Alcolyzer Spirits, designed for the determination
of alcohol in distilled spirits like whisky, cognac or
brandy, measures the alcohol concentration with
a reproducibility of 0.1 %v/v in the range between
35 %v/v and 65 %v/v and the extract concentration up to
20 g/L with the specified reproducibility. This measuring
range correlates with the extract in distilled spirits.
However, the analysis of alcohol in liquors with
Alcolyzer Spirits was not possible until now because
the extract concentration in liquors is much higher than
20 g/L, sometimes even up to 300 g/L to 500 g/L. The
extract consists mainly of sugars like saccharose, fructose
and glucose; organic acids and diverse aromas. Typically,
the concentration of organic acids and aromas does not
exceed 10 g/L.
In this publication, Anton Paar presents the way to
overcome these limitations making the analysis of liquor
with Alcolyzer Spirits possible.
Theoretical background
Alcolyzer Spirits measures the NIR spectrum of the
sample in the range of the 2nd overtone of the C-H
stretching vibration near 1200 nm. It utilizes the
symmetrical vibration of the methyl group because its
pronounced and distinct shape is little influenced by
the matrix. A water and ethanol spectrum are shown
in Figure 1.
10
Absorbance [cm-1]
Water
Ethanol
1
Figure 1. Near infrared spectra of ethanol and water
between 850 and 1600 nm
0,1
0,01
900
1000
1100
1200
1300
1400
1500
1600
wave length [nm]
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2 R. Beneš, C. Reichart and P. Brugger
were set in relation to the water spectrum, i.e. the
absorbance difference Asample- Awater was determined.
In this investigation, about 80 different liquor
samples, purchased in supermarkets and special
liquor stores, with alcohol concentrations between
11 %v/v to 42 %v/v and extract concentrations up to
570 g/L were investigated. The alcohol concentration
in liquors within this study was measured:
1. by classical distillation followed by density
determination as the reference method, and
2. with Alcolyzer Spirits. Alcolyzer Spirits was
adjusted in a special way to minimize the
influence of fructose and glucose.
Water molecules build clusters via the weak
H-bonds. The water clustering is strongly dependent
on the temperature. The H-bond clustering influences
the NIR spectrum of water [3, 4] exactly in the region
used for the ethanol measurement by Alcolyzer
Spirits. To overcome this influence, Alcolyzer Spirits
stabilises the sample temperature. In addition, the
presence of polar or non-polar molecules or ions
influences the equilibrium in water clustering at a
constant temperature. As a consequence, the NIR
spectrum of water will change even if these molecules
or ions are NIR - inactive.
More complex aqueous solutions can also cause
interferences, especially when they contain molecules
with groups that can build H-bonds. Ternary mixtures
of water with ethanol and sugars are good examples
for such systems.
The concentration of the reducible sugars (sum of
fructose and glucose) was determined by iodometric
titration and subtracted from the total extract. The
remaining extract is attributed to saccharose which is
a sufficiently correct approximation for the purpose
of this study.
Measurements and results
General considerations
Ternary solutions
Demineralized water and p.a. grade chemicals
were used for the preparation of binary and ternary
mixtures of water, ethanol and fructose or saccharose.
Two sets of four solutions each, one set with an
ethanol concentration of 30 %v/v and different sugar
concentrations of 0, 10, 20, and 30 %w/w, and one
set of 40 %v/v and different sugar concentrations were
prepared by volumetric dilution of stock solutions at
a constant temperature of 20°C. The NIR spectra of
the solutions were measured with an Alcolyzer Spirits
in the range from 1177 nm to 1191 nm. The spectra
The absorbance spectra were measured in the
region of the second overtone of the symmetrical
C-H stretching of the methyl group. The vibration
of a methyl group reacts sensitively to structural
changes, in particular to weak H-bonds between this
group and other components. The absorbance was
zeroed by subtracting a base-line between 1177 nm
and 1191 nm. The baseline spectra of ethanol
solutions with 30 %v/v and 40 %v/v with variable
concentrations of saccharose are presented in Figure 2.
Absorbance related to base line [a.u.]
0,040
0,035
0,030
0,025
0,020
Figure 2. Baseline spectra of aqueous solutions
of ethanol and of saccharose with various
concentrations
0,015
0,010
Ethanol 40 %v/v
Ethanol 40%v/v, Saccharose 10% w/w
Ethanol 40%v/v, Saccharose 20% w/w
Ethanol 40%v/v, Saccharose 30% w/w
Ethanol 30 %v/v
Ethanol 30%v/v, Saccharose 10% w/w
Ethanol 30%v/v, Saccharose 20% w/w
Ethanol 30%v/v, Saccharose 30% w/w
0,005
0,000
-0,005
1176
1178
1180
1182
1184
1186
1188
1190
1192
wave length [nm]
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Alcohol measurement in liquors using Alcolyzer Spirits 3
The absorbance is obviously dependent on the
ethanol concentration. Consequently, the absorbance
peak of the solutions with 40 %v/v ethanol is higher
compared to the absorbance peak of the solutions
with 30 %v/v. According to Lambert-Beer’s law, the
absorbance is a linear function of the concentration.
This is only the case at 1183 nm where the
absorbance of the pure aqueous ethanol solution
and ternary mixtures with saccharose are identical.
This point is called isosbestic point. Outside the
isosbestic point the system behaves non-linearly due
to the mutual interactions of the components in the
mixture. To make the non-linearity clearly visible,
difference spectra between ternary solutions and the
corresponding aqueous ethanol solution with exactly
the same ethanol concentration were computed.
The difference spectra of the two sets of ethanol and
saccharose solutions are presented in Figure 3 .
The isosbestic point can be clearly seen at the
wavelength 1183 nm where the difference spectra
go through zero. The absorbance difference is
positive at wavelengths higher than the isosbestic
point which means that the ternary mixture absorbs
more light than the binary mixture in this wavelength
region. Below the isosbestic point the ternary mixture
absorbs less than the binary mixture. It is extremely
important to note that the difference spectra are
not only a function of the sugar concentration,
but also, simultaneously, a function of the ethanol
concentration: the higher the ethanol concentration,
the higher the magnitude of the difference spectrum
for the same sugar concentration. The absorbance
spectrum of the aqueous saccharose solution with a
concentration of 10 %w/w is also plotted in Figure 3
(plain curve without circles). This spectrum has no
isosbestic point and exhibits no change in the sign.
The absorbance is relatively small as saccharose
has no methyl groups. It has only CH2 groups with
an absorbance peak at about 1210 nm, i.e. out of
the spectral range covered by Alcolyzer Spirits. It is
evident that the interactions between components
with polar groups and the methyl group of ethanol
are responsible for the non-linear changes of the NIR
spectra. In our model system, all components, i.e.
ethanol, water and saccharose, have polar groups
with oxygen. Hence, changing the water and sugar
concentrations by keeping the ethanol concentration
constant will change the equilibrium in the clustering
of the molecules via H-bonds. As a result, the NIR
spectra manifest this change in the clustering.
To demonstrate the effect more clearly, ternary
solutions with fructose instead of saccharose were
prepared. The concentration of ethanol and sugar
remained the same. The difference spectra for the
ethanol concentration of 30 %v/v and different
sugar types are plotted in Figure 4. The spectra look
similar, but the isosbestic point has shifted. This
can be explained by the fact that disaccharides and
monosaccharides have a different number of polar
groups with oxygen and therefore influence the
clustering of the molecules in a different way. The
same experiment was carried out with glucose and
the difference spectra were identical to those with
fructose since both monosaccharides have the same
number of polar groups.
0,0035
Absorbance difference [a.u.]
0,0030
isosbestic point
0,0025
0,0020
0,0015
0,0010
0,0005
0,0000
Ethanol 40%v/v, Saccharose 10% w/w
Ethanol 40%v/v, Saccharose 20% w/w
Ethanol 40%v/v, Saccharose 30% w/w
Ethanol 30%v/v, Saccharose 10% w/w
Ethanol 30%v/v, Saccharose 20% w/w
Ethanol 30%v/v, Saccharose 30% w/w
Saccharose 10% w/w
-0,0005
-0,0010
-0,0015
-0,0020
1176
1178
1180
1182
1184
1186
1188
1190
Figure 3. Difference spectra of ternary (Ethanol,
Saccharose, and Water) and binary (Ethanol, Water)
solutions with ethanol concentrations of 30 %v/v
and 40%v/v
1192
wave length [nm]
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4 R. Beneš, C. Reichart and P. Brugger
0,0025
Absorbance difference [a.u.]
0,0020
0,0015
Fructose: isosbestic point
Saccharose: isosbestic point
0,0010
Figure 4. Difference spectra between ternary
solutions of water, ethanol, saccharose or fructose
and an aqueous ethanol solution with the same
ethanol concentration
0,0005
0,0000
-0,0005
Ethanol 30%v/v, Saccharose 10% w/w
Ethanol 30%v/v, Saccharose 20% w/w
Ethanol 30%v/v, Saccharose 30% w/w
Ethanol 30%v/v, Fructose 10% w/w
Ethanol 30%v/v, Fructose 20% w/w
Ethanol 30%v/v, Fructose 30% w/w
-0,0010
-0,0015
1176
1178
1180
1182
1184
1186
1188
1190
1192
wave length [nm]
Real samples
Apart from ethanol and sugar flavours, liquors contain
organic acids, glycerol, higher alcohols and etheric
oils. The sum of these components varies and can be
up to approx. 20 g/L. The dominant components in
liquors remain ethanol, water and sugars, whereas
the sugars may be composed of saccharose and/or
glucose and/or fructose. Moreover, the liquors are
mainly acid with a pH value under 4. Under this
condition the saccharose will slowly invert to fructose
and glucose.
The analysis of the ternary system with sugars
(see Ternary solutions section) has a practical impact
on the applicability of Alcolyzer Spirits for the
measurement of the alcohol content in liquors:
1. Alcolyzer Spirits has a linear response on the
ethanol content only at the isosbestic point
2. Alcolyzer Spirits can be adjusted in a way that the
influence of saccharose or inverted sugar (sum of
fructose and glucose) can be eliminated.
3. The adjustment of Alcolyzer Spirits to exclude the
influence of inverted sugar has an advantage over
excluding the saccharose because saccharose can
be inverted chemically.
For the next investigation, Alcolyzer Spirits was
adjusted to exclude the influence of inverted sugar.
Using the adjusted Alcolyzer Spirits, 80 different
liquors with an alcohol concentration between
11 %v/v to 42 %v/v and extract concentrations up to
570 g/L were measured. The alcohol concentration
was determined by density measurement after
distillation. The samples were neutralised prior to
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distillation to exclude the influence of volatile acids on
the density measurement of the distillate. The results of
Alcolyzer Spirits were compared to the density results
after distillation. In general, Alcolyzer Spirits measures
an apparently higher concentration of up to approx.
1 %v/v. The difference between Alcolyzer Spirits and
the reference method is a function of the saccharose
concentration in the sample. The differences scatter
along the linear fit in the interval of +/- 0.2 %v/v. The
reason for this is the presence of other components
in the extract apart from sugars that influence the
NIR spectra either due to their own absorbance or
interactions. To get rid of the error due to saccharose,
we inverted the saccharose chemically to fructose and
glucose. We have developed a quick procedure of
the inversion of saccharose to glucose and fructose.
It is based on the addition of 6N hydrochloric acid
to the sample and heating to 70°C in a closed
vessel. The full inversion takes approx. 5 minutes.
Figure 5 demonstrates the results of this approach
on 15 selected liquors with different saccharose
concentrations and, consequently, with different
errors of Alcolyzer Spirits compared to the reference
method. After inversion, the error decreases. The
accuracy of the ethanol measurement for liquors with
chemical inversion can be estimated as +/- 0.2 %v/v
on the 95 % confidence level.
Conclusion
Samples with high ethanol and extract concentrations
like liquors form highly complex systems in which
the components build clusters using weak H-bonds.
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Alcohol measurement in liquors using Alcolyzer Spirits 5
1.00
original sample
Alcolyzer - distillation [%v/v]
0.80
inverted sample
0.60
Figure 5. Difference between alcohol concentrations
measured by Alcolyzer Spirits and density measurement
after distillation (reference) for samples with and without
sugar inversion
0.40
0.20
0.00
-0.20
-0.40
0
10
20
30
40
Ethanol concentration [%v/v]
These clusters can be easily reorganized by changing
the sample composition. This has an immediate
impact on the NIR spectra that reflects very sensitively
the intra-molecular interactions. NIR spectroscopy for
measuring the ethanol concentration in such samples
is not straightforward and needs a deep knowledge
of the sample.
A new, special adjustment of Alcolyzer Spirits was
developed to extend the method for cognacs to cope
with extract contents up to 500 g/L of inverted sugar,
i.e. mixtures of fructose and glucose. The modified
Alcolyzer Spirits was found to have a reproducibility
of 0.2 %v/v. The only limitation is the presence of
saccharose in the extract which increases the apparent
ethanol concentration up to approx. 1.5 %v/v for
saccharose concentrations of 400 g/L. Such an error
is too high. For this reason, a quick procedure of
inversion of saccharose to glucose and fructose was
developed. It is based on addition of hydrochloric
acid and sample heating to 70°C in a closed vessel.
The full inversion takes approx. 5 min and results in
an accuracy of 0.2 %v/v ethanol.
94-Benes.indd 5
Literature
Beneš, R., et al., Verfahren zur spektroskopischen
Bestimmung der Konzentration von Alkoholen mit 1
bis 5 Kohlenstoffatomen, Austrian patent AT-406711
B, Joanneum Research (1999), Austria.
Zanker, G. and Beneš, R., Analytical Device for Measuring
the Ethanol Concentration in Beer (Based on NIR
Absorption), in Mitteilungen Österreichisches
Getränkeinstitut 9/10, OEGI (2001), Wien
Leitner, M., Temperaturabhängigkeit des spektralen
Absorptionskoeffizienten von Wasser im Nahen Infrarot
zwischen 10 und 40 Grad Celsius, Master thesis,
Technische Universität Graz (1996), Graz.
Del Bianco, A., Beneš, R., Trinkel, M., Reininger, F. and
Leitner, M,: Temperature influence on the 1.2 µm band
of water, in Near Infrared Spectroscopy: Proceedings of
the 9th International Conference (Davies, Giangiacomo
ed.), NIR Publications (2000), ISBN 0-9528666-1-7.
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