Chaptalization

In the previous post I mentioned that sugar levels of fruit from our fungus effected vines was too low (10.0° Baumé) to make reasonable wine ie. the grapes were not sugar ripe.
Sugar content is related to alcohol content so a must (juice) with 13.0° Baumé (23° Brix) will produce a wine with an alcohol content of around 13%.
While there is a trend towards lower alcohol wines, our opinion is that any red wine under 13% is 'thin'. This is probably due to being brought up on wines from a hot sunny climate that generally produces grapes with a potential high alcohol.
A number of people asked why we simply don't add sugar (sucrose) to increase the level.
To increase alcohol by 1% around 19g/L sugar needs to be added to the juice. 
Adding sugar to grape juice is called chaptalization.
While this is a legal practice in many countries eg. France, it is not permitted in Australia.
What is legal here, to increase sugar, is adding grape juice concentrate. This can be made by freezing the grape juice and removing the water component as ice. Grape juice is 70-80% water. The liquid remaining is a concentrated sugar (glucose and fructose) solution. It is also commercially available in both red and white forms at around 38.0° Baumé. Of course in our non commercial operation the legal situation would not be a concern.
But there are other reasons not to do it.

Grape ripeness has a number of definitions. There is sugar ripeness, discussed above, and then there is flavour ripeness.
Apart from sugar and water, grape juice contains:
-organic acids
-phenolic compounds
-nitrogenous compounds
-aroma compounds
-minerals
-pectic substances
As grapes ripen many of the above components change. Unripe grapes with undeveloped aroma and flavor compounds also make unsatisfactory wine so simply 'sugaring' their juice does not always produce a satisfactory result.
The possible problems associated with under ripe red grapes are:
-pH and acid imbalances
-under ripe tannins
-decreased fruity aromas
-higher risk of herbaceous and bitter characters
-low pigment potential
In some years we have added a little sugar to increase Baumé by 0.5° (from 12.5° to 13.0°) without any noticeable problems but this year would have meant an increase from 10.0° to 13.0° for the Tempranillo which is just too big a gap.