1) Firstly it is necessary to measure the temperature-corrected natural
alcoholic strength of the must in °Oechsle, with a refractometer or
hydrometer. Look up the predicted alcohol %vol in the Wine Standards Board
table (these differ slightly from German tables).
2) Decide on the desired total
alcoholic strength
of the enriched must, according to the type of wine you are making. The maximum enrichment allowed is 3.5 %vol.
For Sparkling wine, I aim for 10.5 %vol,
as the secondary fermentation will add a further 1.5%vol.
For dry white wine, I aim for 10.5 or 11.0 %vol, according to the
vine variety.
For medium-dry or medium white wine, I aim for 11.5
%vol (the maximum permitted for white wine), since this will be reduced later by the addition of Süss-reserve
before bottling.
When enriching any wine
which you think you may need to sweeten (even by a few grams
per litre) the enrichment should be to at least 11.4 %vol. This
is because German Qba Süss-reserve is normally 84 °Oechsle (11.4 %vol
total alcohol), and under European Commission Regulations it would be illegal to
add this to an enriched wine which has less than 11.4 % total alcohol.
For rosé, I aim for
11.5 or 12.0 %vol (the maximum permitted).
For red wine, I aim for 12.0 %vol (the maximum permitted).
3) Calculate the weight of dry sugar required to
achieve the desired total alcoholic strength; water may not be added to
dissolve the sugar. Concentrated grape must or rectified concentrated grape must can also be used, but normally
these are not used in UK.
Beet sugar or cane sugar may be used, beet sugar contains less pectin so is
more likely to give a protein haze. Beet sugar is cheaper and hence is normally used, although cane
sugar has more flavour.
a) For white wine or rosé, Wine Standards Board's
figure of 16.5 grams of sugar per litre of must, to produce 1 % alcohol by
volume, is slightly low; they admit it is conservative. German data is 16.85 g/l, Australia
(according to Bryce Rankine) is 16.95 g/l
and France (according to Peynaud) is 17 g/l.
According to Peynaud, cane sugar was used exclusively for white wines,
although I doubt it is used in France today. The land around Champagne is a
major sugar beet growing area.
Hence, for white wine or
rosé: total grams of sugar to be added =
total litres of must x (desired total alc %vol - natural alc %vol) x 16.85 g/l
Enrichment by 3.5 %vol
(the maximum allowed) is achieved with 58.975 grams
of sugar per litre of must.
It is essential to rouse continuously while sugar is
being added, else the sugar will form a solid lump on the tank bottom.
b) For red wine, which is generally fermented at a
higher temperature, 19 (according to Bryce Rankine) or 20 (according to Peynaud) grams of sugar per litre
is needed to produce 1% alcohol by volume, because of loss of alcohol by
evaporation during pumping over of the warm wine. It is the free-run wine that is
most enriched; the press wine more or less keeps its initial strength.
According to Peynaud, there is no distinction made between using beet or cane
sugar for red wines.
Recent discussion in UK has concluded that about 18.3 grams of sugar per litre of must,
should be added in order to produce 1% alcohol by
volume.
Hence, for red wine: total grams of sugar to be added =
total
litres of mash x forecast %-extraction x (12.0 alc %vol - natural alc %vol) x
18.3
Forecasting the %-extraction (% of must/juice that will eventually be
pressed out from the de-stalked mash) is not easy and relies on experience.
Each variety and vintage year is slightly different, according to the
juiciness of the particular grapes. In my vineyard with the varieties Pinot
Noir, Rondo, Regent, Dornfelder and Dunkelfelder, the overall average has
been 83.3%, but individual wines have varied from 77 % (small dry berries)
to 89 % (large juicy berries):-
Pinot Noir (thin skins): from 82 % to 89 %, average 84.5 %
Rondo is the most consistent: from 81.5 % to 84.5 %, average 83 %
Regent/Dunkelfelder: from 77 % to 86.5 %, average 82.5 %
Dornfelder: from 80 % to 89 %, average 83.5 %
This constitutes a possible error in enrichment of up to +-0.23 %vol.
Because red wine is fermented 'on the skins' it is
not possible to use a rouser in the tank to dissolve the sugar, so an alternative method
has to be used:-
- if an open-top tank is being used, the sugar can be added gradually
day-by-day, simply tipping some on to the top of the mash just before the
pomace cap is pushed down.
- or the technique of 'pumping over' can be employed. The sugar needed is
tipped little by little
into a small container fed by must from the tank. The resulting syrup is
stirred continuously and pumped back to the top of the tank over the pomace
cap.
This is usually done in one step at the beginning of fermentation when the
fermenting must is starting to get warm and the pomace cap is just forming.
Sugar dissolves more easily in warm must.
Other sources of error in the enrichment process:-
1) Measurement of
the volume of must:-
The % error for every 1 mm error in measurement of height in a vertical cylindrical tank = 1
divided by the height in mm; eg. for 1000 mm high must the error is 0.1 %. The diameter of the tank
is less significant.
For tanks that do not have a flat base there is the additional problem of
allowing for this.
The temperature of the must can lead to volume calculation errors.
A must that is not completely settled will contain suspended solids which will
contribute a small volume calculation error, as would sludge in the bottom of the tank.
2) Initial measurement of grape-sugar (natural alcoholic strength):-
There are several methods of measuring the sugar content of a liquid, all
require a representative, homogenous sample.
A hydrometer reading is affected by the temperature of the must and by the
presence of material other than glucose/fructose in the must (which would
increase the apparent must weight). The temperature effect can be eliminated
by the use of a water bath, at 20 °C, to maintain a constant temperature for
the measurements.
A refractometer reading is affected by the temperature of the must and by the
presence of material other than glucose/fructose in the must (which could
either increase or decrease the refractive index of the must and therefore
give an apparent must weight that was higher or lower than the actual). The temperature effect
is compensated for by a adding or subtracting a correction factor according
to the measured temperature. The latest refractometers have built-in
automatic temperature compensation.
Chemical analysis for reducing sugars is affected by other substances that reduce CuII
to CuI (this would have the effect of increasing the apparent
must weight). The analysis technique is straightforward.
Enzymic methods are affected by other substances that react with the enzyme or
interfere with its action (leading to an apparent must weight that was
higher or lower than the actual).
The only really accurate method, of determining sugars in grape must, is liquid
chromatography. Here glucose and fructose are separated from other sugars,
and from other compounds, in a liquid chromatography apparatus. This
apparatus is prohibitively expensive to buy or operate.
3) Sugar addition:-
The scales used to weigh out the sugar may be inaccurate, leading to too little or
too much alcohol being produced.
The sugar may not completely dissolve in the must, settling out in the sludge
at the bottom of the tank, leading to too little alcohol being produced.
The types of sugar used for enrichment (granulated beet sugar, cane
sugar, rectified concentrated grape must) will produce slightly different
amounts of alcohol.
4) Natural variations in the fermentation:-
C6H12O6 = 2 C2H5OH
+ 2 CO2
Glucose/fructose = ethanol (ethyl alcohol) + carbon dioxide
180 g = 2 x 46 g + 2 x 44 g
So, theoretically, 180 grams fructose/glucose = 92 grams alcohol
alcohol has a specific gravity of 0.7893 at 20 °C, therefore, 180 grams sugar gives 116.6 ml
alcohol at 20 °C; therefore, 1.54 g sugar gives 1 ml
alcohol at 20 °C (assuming that the reaction goes to
completion, with no side reactions).
Similarly, 2.04 g sugar gives 1 g carbon dioxide (0.51 litres) at 20
°C.
In practice the conversion of sugar (either
natural grape sugar or added sucrose) to alcohol is not an exact
process. It varies with the glucose/fructose balance
in the grape, the fermentation temperature, yeast
strain etc:-
The reaction sometimes does not go to completion, giving lower than
expected alcohol.
The reaction can proceed by numerous routes to a variety of products, giving
rise to lower than expected alcohol.
Other reactions may produce alcohol, giving rise to higher than expected levels.
The alcohol produced can be physically entrained, from the fermenting medium, by
the action of carbon dioxide evolution and out-gassing. The level of
entrainment will depend of the speed of fermentation and the fermentation
temperature. This entrainment may also remove some water vapour, similarly
affecting the volume of the ferment.
Bryce Rankine writes "100 grams of invert sugar produces between 45 and 48 grams of alcohol
during complete fermentation". This variation for a wine which is
expected to be 11.0 %vol gives a possible 'error' of +-0.35 %vol.
Hence, at the time of enrichment, it is impossible to predict actual
alcohol with accuracy greater than +-0.35 %vol, even without allowing
for the other inaccuracies.
Comparing this practical result with the theoretical complete stoichiometric reaction
you
find that it is 88 % to 94 % of theoretical.
WSB's recommended
Chaptalisation (of 16.5 g/l sugar = 10 ml/l alcohol (1.0 %vol), compared with theoretical of
15.4 g/l = 10 ml/l) assumes a yield of 93.3 % of theoretical, so is always likely to
result in
low enrichment. The German figure of 16.85 g/l predicts a yield of 91.4 % of
theoretical, which is close to the mid-point.
www.winegrowers.info