While looking for a Brix refractometer, I saw several listings for an
alcohol refractometer - some even specifically mentioned as being for
grape wine. Is anyone familiar with these devices? I was under the
impression (from this group) that the only way to directly measure
alcohol during and after fermentation was with an ebulliometer, which
is much more elaborate and expensive than a refractometer.

Thanks,

Stephen

On Mar 17, 11:56 pm, shbailey wrote:
While looking for a Brix refractometer, I saw several listings for an
alcohol refractometer - some even specifically mentioned as being for
grape wine. Is anyone familiar with these devices? I was under the
impression (from this group) that the only way to directly measure
alcohol during and after fermentation was with an ebulliometer, which
is much more elaborate and expensive than a refractometer.

Thanks,

Stephen

You would want to speak to the manufacturer directly but I suspect
they are still reading must but are calibrated in alcohol potential.
You cannot depend on the refractive index for a solution of must and
alcohol. There are many ways to measure alcohol, from the little
vinometer for dry wines, to distilling up to chromatography are all
used depending on the precision required.

I guess you could calibrate a refractometer to measure alcohol but you
would still need to distill it to remove the dissolved solids as I see
it.

Distilling followed by use of a proof and tralle hydrometer is a
relatively inexpensive way to measure precisely.

Joe

Folowing is a summary of alcohol analyses I prepared some years ago
when I was a guest speaker at a conference.
I hope it comes through in a readable form

Bob Molony

MOLAB Ltd.
Consulting Analytical Chemist & Food Technologist.
14 Goldie St., St. Heliers, Auckland, New Zealand. eMail

www.molab.co.nz Phone/Fax (09) 5755478 , Mobile (027) 4523984.


ESTIMATION of ALCOHOL CONTENT in BEVERAGES.

There are number of ways in which the amount of alcohol in a beverage
may be estimated. The actual method used depends both on the product
and the reason for determining the alcohol content.
The methods of estimation fall into two categories.
(A) The separation of the alcohol from the product by distillation
followed by the estimation of the alcohol in the distillate by a
chemical or physical measurement.The only primary standard procedures
belong to this category. GLC of the whole product (not headspace
chromatography ) is a special case as a distillation actually occurs
in the machine.
(B) The measurement of some physical parameter the value of which is
related in some way to the alcohol content of the product. In general
any physical parameter which can be used in this way is influenced
not just by alcohol but by other materials present in solution. Note
in particular the vapour pressure (and boiling point) of alcohol is
affected by the soluble solids present. These procedures are
calibrated by the use of samples which have been previously analysed
by a primary standard procedure.

(A)(1) Standard distillation followed by determination of alcohol in
distillate.
(a) Specific Gravity
(aa) Specific Gravity Bottle **
The traditional standard procedure.
For reasonable accuracy a water bath with
a temperature
stability of 0.2 degrees C or better is
required
(ab) Hydrometer **
Special small hydrometers are available
with integral
thermometer for temperature correction.
These hydrometers
are very expensive and also very fragile

(ac) Electronic Densitometer
An expensive instrument suited to multiple
determinations.
However the same intrument can be used in
other
applications to defray cost.
(b) Refractive Index **
Accurate procedure but requires the use of an
immersion
refractometer and very accurate temperature control
(c) Ebuliometer
Distillation first avoids the use of emperical
correction for
residual extract
(d) Chemical oxidation **
The chemical oxidation procedure as per JAOAC
52,85(1969)
may be carried out directly on an aliquot of
distillate.The
dichromate is less susceptible to some interfering
factors than
the specific gravity procedure.

(2) Steam Distillation and Oxidation
(a) AOAC Method **
Steam distillation followed by Dichromate oxidation as
per JAOAC
52,85(1969).
Typical accuracy 1 part in 200 of the alcohol present.
Critical factors
(1) Preparation of the dichromate reagent.
(2) Water used in preparation of reagents.
(b) Combi method
Dichromate oxidation procedure. There are a number of
objections
to the use of this procedure for accurate work.These are
technical questions related to variations in the
stochiometry of
the reactions used.
Other critical factors as for (2)(a)

(A)(3) GLC
(a) Direct
(a) Without Internal Standard
Accuracy is low due to the problems of accurately
measuring
very small volumes. A standard solution of alcohol must
be
used for comparison

(b) With internal Standard
n-Butyl alcohol is usually used as the standard
Procedure is capable of a reproducability of 2% however
standard solutions are not stable and must be made up
regularly.
Alcohol and Butyl Alcohol are volatile and absorb water
readily. This
can make accurate measurement of these materials
unreliable.

(B)(1) Ebuliometer
The ebuliometer or boiling point procedure at one time was
an
official UK customs procedure. With care results can be
obtained
consistantly to the nearest 0.1 % alcohol by volume.
Individual on site
calibration with known samples is not required as
Temperature vs boiling
point varys little between instruments and a variety of
suitable corrections
for soluble solids are available.
Critical factors are
(1) Correction for any extract present. Tables are
available.
(2) Adequate cool water in the condenser.
(3) Use of a barometer to monitor for the presence of
barometric
pressure change between calibration and
determinations.

(B)(2) Headspace Gas Chromatography
Vapour pressure of alcohol component is affected by non
volatile
components in sample. Equipment must be calibrated against
similar samples of a known composition. The calibration
samples would
normally be analysed by the distillation and specific
gravity or
by distillation (steam or otherwise) and dichromate
oxidation.
Carbon dioxide also has the potential to alter vapour
pressure
of the alcohol present an therefore has the potential to
cause
inaccuracies.
Standard solutions are unstable and must be made up
regularly.
Alcohol and Butyl Alcohol are volatile and absorb water
readily. This can
make accurate measurement unreliable

(B)(3) Headspace gas alcohol meter
(a) Fuel cell
Vapour pressure of alcohol is affected by presence of
non
volatile components.
Equipment must be calibrated against similar samples
of known
composition. The standards must be analysed for
alcohol by
another procedure.


(B)(4) Loss of gravity during fermentation
The traditional method for determining alcohol levels
during
fermentations. Can be quite accurate provided that no
additions are made
to the product in the time between the gravity
determinations.
Care must be taken when degassing sample in order to
prevent alcohol
loss.

(B)(5) Williams Field test
A rapid field test of very limited accuracy used for higher
alcohol
levels in the US. Not suitable for beer or wine.

(B)(6) NIR , Near Infrared Spectrophotometry.
A modern instrument which provides very rapid results.
The results are affected by other components in solution
and the
calibration must be carried out using samples of similar
composition
which have been analysed by other procedures.

** Suitable primary standard procedures.