Well, not all grapes are sour but many are...as are strawberries, blueberries, raspberries, blackcurrants, peaches, pears, oranges, plums, cherries, pineapples, mangoes and any other five a day friend you care to buy. Why? They mostly say 'home maturing", or some such rubbish. What this means is leave them in a bowl to "mature" and you end up with a bowl of green slime: a rotted mess of expensive fruits. It was different in 'the old days.' Then fruit came from our countryside, not the other side of the world. Supermarts have a lot to answer for!