All fruit requires acidity, be it an apple, lemon, mango or grape, the most important ingredient in our wines! Acidity is what gives fruit its refreshing, flavoursome sensation. Without it fruit would by super sweet and cloying, think of that episode of the Simpsons in Brazil when Homer ordered the sweet fruit juice and had to eat dirt to counteract the sweetness. Or on a practical level; the sugary fruit syrup in which some canned fruits are kept. Acidity is what keeps the sugars in balance. And just like fruit and fruit juices, wine, also requires acidity.
Too little, and it will seem dull, flabby or perhaps cloying, particularly if it is a desert wine. Too much, and the wine will be sharp, harsh and undrinkable like vinegar. Acidity can be detected by the sharpness of the wine in the mouth, particularly around the edges of the tongue near the front.
Some acids, such as acetic acid (the kind in vinegar), are known as volatile acids, and in small amounts these can really lift the flavours in the wine. Too much, and the wine begins to resemble furniture polish, acetone (nail-polish remover) or even vinegar. Higher acidity denotes a wine from a cooler region (more sun=more sugar, less sun=more acidty), such as Northern France, Italy, Germany or New Zealand. Low acid wines come from areas with warmer weather, such as the Rhone Valley, Spain and Australia, where acidity in the harvested grapes is often low.
Acidity is more important in white wines than in reds as it provides the whites with structure that reds gain else where, but is certainly not to be feared, in fact those searching for crisp dry whites should look on acidity as a good friend willing to serve.