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|Winemaking - Technical Aspects|
The basic facts are simple - every 100 grams of sugar fermented in a 5 litre demijohn ends up raising the alcohol level by 1%. IE for 5 litres of a typical 13.5% alcohol wine you need 1350 grams of sugar. This is not only simple to remember but also a very accurate approximation and it continues to surprise me that this almost exact correspondence is not pointed out in any of the books I know - perhaps because winemakers still work in ounces and gallons.
But what about the sugar in the fruit and the effects of racking? Again there are some good working assumptions which one can make which again do not seem to be brought out in the books. Most fruit juice seems to have about 100 gms of sugar in every litre - grape juice is much higher which is why it is used to make wine without added sugar and apple juice is a bit higher so can make cider of 6 or even 7% - but if you look at the side of packets of fruit juice on a supermarket shelf you will find most are close to 100 grams per litre. We do not use pure fruit juice or pulped fruit to make home wines - they would be too tasty. Typically one uses a couple of kilos of pulp/juice for 5 litres which potentially adds a couple of hundred grams of sugar. In the case of flower wines or wines such as elderberry one normally adds sultanas or raisins and again they contribute about 200 grams of sugar - all very convenient!
However we lose some of the sugar/alcohol when we take away the remaining pulp after the initial fermentation in a bucket and a bit more when we rack off the lees (dead yeast) after a few weeks. I usually weigh the pulp after I have strained and squeezed it dry and it comes to about 500 - 600 grams. The strained juice typically comes to about 4 litres so one is wasting about 12.5% of the sugar (by now alcohol) in the Must at this time. The fruit has contributed 200 grams and I normally only add 500 grams sugar to the initial fermentation so we have lost a little under 100 grams. By the time we have racked off the lees, to a good approximation, we will have lost 100 grams. So the end result is that we can assume an extra 100 grams sugar from the other ingredients and even in extreme cases (such as apple and raisin) we are unlikely to be more than plus or minus 50 grams or 0.5% alcohol out.
To put this into perspective the difference between an abrasively dry wine and a sickly sweet wine is about 300 grams in a 5 litre demijohn so, assuming we know the alcohol level to which the yeast will ferment, we have a good chance of making an acceptable wine. To be absolutely sure it is it is worth using a hydrometer to measure the specific gravity and hence derive the sugar content towards the end of the fermentation. The information needed is that 100 grams of sugar raises the gravity by .0075 (often refered to as an increase of 7.5 by winemakers).
The specific gravity range we are aiming for will be from 0.990 with no residual sugar to 1.015 for a very sweet dessert wine. The lower figure is under 1.000 because of the alcohol present at the end of the fermentation. The best way to proceed is to add what you expect to be the final sugar in 50 gram batches as the gravity falls below 1.000 - that way you will never end up with a sweeter wine than you want. You can always add more sugar but it difficult to remove! I have several hydrometers (thanks to the local Help the Aged who are also good for demijohns) and I tend to keep one floating in a demijohn from each batch so I can judge when to add sugar.They are not expensive to buy, even for the good glass ones, and are worthwhile investment if you take winemaking seriously.
Peter and Pauline Curtis
Most recent significant revision: 1 st July, 2010