This chapter will explain how the sugar and alcohol
content of a must or wine can be measured, calculated and adjusted.
The sugar content is usually expressed in grammes of sugar per liter (g/l) of must or wine.
Sugar present in the must and the final alcohol content in the wine are correllated.
Measuring and adjusting the sugar and alcohol content of a wine or must is better than just adding the amounts of sugar stated in a recipe. The amounts of sugar in a recipe can differ much from the amount needed to get the same final alcohol content than in the recipe. This is because the other ingredients in the must also contain sugars, and that content is never the same. It depends on a lot of factors like the amount of sun the fruit got that year, the location where grown etc.
The sugar content of a wine or must cannot be measured directly. The specific gravity (SG) is measured and is expressed in grammes per liter (g/l). Another scale is degrees Oëchsle (°Oë) which is equivalent with SG-SGwater, thus SG-1000.
The SG of a wine or must is roughly equivalent to the sugar content of a must, so that the sugar content can be calculated.
The alcohol content is expressed in percents by volume (%vol). Most commercial wines have an alcohol content between 9 and 12 %vol.
Measuring the Specific Gravity
Calculating sugar and alcohol before fermentation
|Specific gravity (S.G.)||Potential %vol alcohol||Grammes sugar / litre|
1 °Oë = 2.7 g sugar/l 1 % alcohol = 19 g sugar/l SG water = 1000 kg/m3 1 % acid = 10 g/l
We want to get 10 litres of wine with 12.5 %vol alcohol content.|
The must has an SG of 1035.
When fermentation has stopped, the final alcohol content can be calculated using the starting and final SG readings. The following formula can be used:
Alcohol content = (Starting SG - Final SG) / 7.36
ExampleThe wine started with a SG of 1080, and ended with a SG of 992. The alcohol content of the wine should be:
(1080 - 992) / 7.36 = 12.0 %vol
Of course, if you've taken proper hydrometer readings, you can calculate the amount of alcohol formed in your wine. But if you haven't got one or want to check your calculations, you can use a vinometer. But there is one mayor restriction: A vinometer can only produce accurate results in a dry wine (containing little or no residual sugar). The vinometer works on the principle of capillary action, so it actually measures viscosity, which is dependant on the alcohol/water ratio. It has a scale of alcohol content marked on it.
The procedure goes as folllows (note the image on the right):
The trick of making sweet wines is to prevent yeast cells from fermenting the sugar you'll add to make the wine sweet. So the yeast cells must be gone or the time the sugar is present should be so short, that the yeast cells haven't got time to make alcohol from it.
So it's not that easy to make a sweet wine. It takes longer than a dry wine and there's always a risk of refermentation in the bottle.
Basically there are four ways to make sweet wine:
Adding sugar just before you drink itJust before serving, add a few teaspoons of sugar to your wine. A sugar syrup of invert-sugar works best.
This way you've always got dry wine which you can turn into sweet wine when necessary. Sweet wine cannot be made dry again. So you can make 2 wines from 1 bottle. The greatest advantage of this method is that you do not run the risk of refermentation.
Adding liquorBefore the wine has completely fermented out, pure alcohol or another high-alcohol liquor is added to the wine exceeding the level that the yeast can tolerate. This is about 16 %vol. The blending ratio can be calculated just like explained in the Acids chapter. Port and sherry wines are made using this method.
FilteringThe wine can also be filtered after it has fermented out and it is stable. A very fine filter is required to get all the yeast cells out. This method is usually the way commercial sweet wines are made. But you really need a good filter. The wine can then be sweetened.
Using potassium sorbateFirst ferment to dryness, let the wine clear and rack it. It must be brilliantly clear and no yeast may be present any more. Often filtering is done to speed up the process. The wine is then aged. The wine must be absolutely stable. Then sugar can be added to taste, sulphite and potassium sorbate must be added. Potassium sorbate inhibits the last few yeast cells from reproducing. So it doesn't stop the yeast from fermenting, so really very few yeast cell can be allowed to be present at that time. Also, when using sorbate, sulphite level must be high enough, or you can get a geranium-like smell, produced by bacteria in the wine in the presence of sorbate. The normal dose of potassium sorbate is 200 to 250 milligrams per liter of wine. This about one level teaspoon per 10 liters of wine.
The amount of sugar to dissolve in the wine of course depends on your own taste, but here's a guideline:
The amount to be added can be calculated using the hydrometer tables or the formulas from the Calculating sugar and alcohol before fermentation paragraph.
ExampleWe'll use the formulas from above. We want to sweeten 10 liters of a wine that has SG=995 and we want it medium sweet.
Looking at the table above, let's make it SG=1015.
So we'll have to add:
(1015 - 995) * 2.7 * 10 = 540 g