The acidity of the juice, and also its sugar content need to be determined, as this gives an indication of the strength and volume of the alcohol that will be produced once the juice will have been fermented. Next some sulphur dioxide is added to the juice, which prevents oxidation of the juice and also prevents it from going off because it kills the micro organisms that may cause this.
Now a pure wine yeast starter culture is added to the juice. Here is the major difference between the two types of wine: in the case of white wine the seeds and skins were filtered out, and the starter culture is added to the juice alone, whilst the starter culture is added to the juice on the skins and seeds in the case of red. The skins are kept so the yeast can extract some of the red pigments, thus creating the red colour. For white wine only the clear juice is fermented to prevent this.
The yeast respires anaerobically by the process of fermentation, producing sugars and carbon dioxide. The alcohol is removed from the dead yeast (they poison themselves with their poisonous product and/or run out of food) and again the sulphur dioxide level is altered to prevent oxidation of the wine into its carboxylic acids, making it very sour.
Red wines are now traditionally aged in wooden conditions, this gives the best quality wine, and as the wine critics say, this gives the flavour more complexity. White wines are not aged in wooden conditions- they are stored in metal and are drunk quite young so the fruity flavours considered to be necessary in a good white are retained.
Wine composition is a vast subject to explore, in that nearly 1000 components have been discovered. The simplest and biggest part is that of water- nearly 90%. This is an even bigger proportion than water is in our own bodies which is considered to be normally around 70%. There is a lot of acid, some resulting from the fermentation, and some acids are organic like tartaric acid, that naturally occurs in the grapes themselves. As far as alcoholic content is concerned, the wines are normally around 12-14% alcohol to volume, the limit of concentration to which yeast can survive, as the alcohol is poisonous to them. But wines can vary in different ways, usually due to the type of grapes used, ie the differences in natural acidity and sweetness. Often, extra sugar is added after the fermentation process.
Fortified wines, such as port and sherry, are simply wines with extra pure alcohol added (remembering the limit for yeast.)
So there you have it, a brief summary of the wine making process. Oenology is a vast science, one that started, as primitive as it was, many many years ago and people are still pursuing this field.
