Measuring the actual alcohol content (%vol) of wine
Alcohol in wine is derived from the natural grape sugars and any sugar
enrichment.
There are several methods of measuring the alcohol content of a liquid,
all require a representative, homogenous sample.
The principal
methods are:-
Density and Refractive Index method
The refractive index of alcohol is different from water, so can be used
as an indicator of alcohol content. It is the quickest
method, taking only a few minutes. It is prone to interference by other
compounds in the wine, but the accuracy is dependent mainly
on correcting for the temperature of the wine, a sophisticated refractometer will have a
temperature compensation facility, the accuracy can be at best +-0.5 %vol.
Distillation
followed by hydrometry/refractometry method
The alcohol must first be separated from the wine by distillation with
special equipment. The specific gravity of the distilled alcohol is then
determined with a hydrometer which is calibrated directly in %vol at 20°C. If the distillate temperature varies from 20°C then a
correction needs to be applied, since the volume of the liquid changes with
temperature. 0.1 %vol is added for each 0.5°C below
20°C, or subtracted
for each 0.5°C above 20°C, so this is very significant.
This is a complex and time consuming procedure, the accuracy can be affected
by the work of the technician as there are various possible causes of error,
such as interference from co-distilled compounds, when
there are ethyl esters (cleaved to ethanol during the analysis) and
other alkali-stable steam volatile compounds present.
A much superior alternative to the hydrometer: The MISCO Palm Abbe
VINO5 Digital Wine-Alcohol
Refractometer from USA ($525, www.misco.com) is engineered specifically for testing the
potential alcohol content of grape must, as well as the actual Alcohol by
Volume in the finished wine. Measurements are made with precision comparable
to mid-range bench-top refractometers costing thousands of dollars more. The
actual Alcohol by Volume of a particular finished wine can be established
following procedures and methods recognized by international bodies. After
distilling a small volume of wine, the ethanol distillate is measured on the
Palm Abbe VINO5 Digital Wine/Alcohol Refractometer and the actual alcoholic
strength (%vol) or specific gravity is displayed. Protection against
inaccurate readings due to temperature differences (a major concern in
refractive index measurement) is assured with non-linear temperature
compensation specific to grape juices. Temperature compensation is automatic
for fluids read between 0 and 50°C. The accuracy is claimed to be +-0.25 %vol, twice the accuracy of any competing instrument.
Ebulliometer method
The ebulliometer is a measuring device that is designed to evaluate the
boiling point of different types of liquids. It's use in the wine industry is
based on the fact that alcohol boils
at (78.4°C) a lower temperature than water, so the boiling point of
alcohol-water mixtures changes as a function of their concentration. A
precision thermometer is involved, to determine the boiling temperature of
the wine within 0.02°C. The boiling point of any
liquid depends on the atmospheric pressure, so the zero point has to be set
against the boiling point of pure water prior to starting each test.
The method is fairly quick and sufficiently accurate (+-0.5
%vol) for general purposes.
It is affected by atmospheric pressure changes, a barometric change of 4 mm
(during a test) would cause an error of 0.5 %vol. A simple ebulliometer is less accurate than
the Distillation method.
There are two modern
types of the device that are currently in use. The Swietoslawski ebulliometer
relies on an isobaric method. This form of ebulliometer contains a boiler,
Cotterell pumps, a condenser, and a thermowell.
The measurements obtained with this type of ebulliometer are considered to
be very exact. An isobaric ebulliometer provides measurements of such factors
as the exact temperature needed to reach a boiling point, solvent purities
within the properties of the wine sample, and the molecular weight of the
substance. The use of a resistance temperature device helps to create the
accurate readings on the vapor-liquid equilibrium of the wine, which is one
factor that makes the isobaric approach the most accurate reading possible
with this type of device.
An isothermal ebulliometer contains similar components as the isobaric
type, but usually involves the presence of a stirring mechanism that is
operated by a small pump. The stirring takes place during the process of
boiling the mixture and is thought to increase the chances for a more accurate
reading of such gases within the mixture as methanol.
While not as prominently in use as the isobaric type, there are a some
wineries that prefer this method.
Gas chromatography (GC) method
The equipment is expensive to buy so the analysis is
normally carried out by contract analytical chemists. It is considered to be the
most accurate means of ascertaining the alcohol content of wines and is used
by HM Customs & Excise.
Essentially, the gas chromatograph works by analyzing the mixture of
organic compounds found in the wine. The gas chromatograph has a series of
filters made from porous materials. Samples taken from the wine batch are
gathered into a syringe and then injected into an ejector port on the device.
The temperature of the injector port must be in excess of the boiling point
for the sample in order accurate readings to occur. This allows the components
of the wine to convert into gas, which then is then pushed into the filters by
way of helium or a similar carrier gas.
As the wine gases pass through the filters, the compounds are identified by
electronic equipment and the alcohol content is determined. While some models
of the gas chromatograph feature a printer that creates a graph of the
progress of the wine sample, new models utilize a terminal display, making it
possible to view the results without having to decipher a graph.
Modern gas chromatographs are smaller and less expensive to purchase
and maintain compared to equipment from years ago. Using a gas chromatograph for alcohol measurement has
the extra benefit of providing the winemaker with
additional information on the nature of the wine.
There are a number of manufacturers around the world that provide state of
the art gas chromatograph equipment to the wine industry. Research
and development departments at these manufacturers routinely find ways to
fine tune the efficiency of their products, which makes it possible for the
process of accurately gauging the alcohol content of wine more accurate every
year.
Spectroscopy (infra red) method
The Alcolyzer Wine from Anton Paar which uses a patented
method based on Near Infrared spectroscopy to determine the alcohol content
in a highly alcohol-specific range. For this reason, the other constituents
of the beverage do not influence the result. This means, the determination
of white or red wine, sweet or dry wine, can all be done with one
adjustment.
A highly alcohol-specific range of the spectrum was
identified between 1150 and 1200 nm. The evaluation method uses the
significant alcohol peak in this area and two spectral points very close to
it for defining the baseline. Extensive investigations showed that the
alcohol results based on this type of evaluation are virtually free of
influences from other known wine constituents. This allows adjustments to be
done simply with water for the zero point and one binary ethanol/water
mixture.The Alcolyzer Wine utilizes an optical set-up without any moving
parts. The instrument consists of a Near Infrared Light Emitting Diode, a
condenser lens, a sample cell, a collimator lens to focus the parallel beam
and a grating spectrometer with a detector array. The absorption information
read by the detector array is used to determine the alcohol content of the
sample. A measurement is carried out in less than one
minute. The Alcolyzer Wine has a built-in Peltier thermostat to ensure
accurate and automatic control of the temperature in best time.
Consequently, there is no need for manual temperature adjustment and
correction.
Methods such as
the combined density and refractive index method or Ebulliometer
determination tend to be inaccurate because the underlying measuring
properties are non-specific to alcohol. Traditional analysis methods such as distillation
are time consuming and require experienced operators. The accuracy of the above mentioned
methods can be difficult to maintain and repeatability is often at
unacceptable levels. Compared to distillation, an acknowledged reference
method for alcohol determination, the Alcolyzer Wine achieves
accuracies of ± 0.1 %vol and repeatabilities of ± 0.01 %vol.
More recently a Mid Infrared wine analyzer has been released that has
similar accuracies to the Anton Parr Near Infrared instrument with a cheaper
price tag. If you want information about it contact: Wilks Enterprise,
Inc. www.wilksIR.com
www.winegrowers.info