The particular peppery taste of good fresh olive oil comes from the alkaloids contained in it, in the same way that an unripe apple tastes different from a mature one so the immature olives of central Italy produce an oil with a taste much more pronounced than those left to ripen and fall.
Though many other olive producing areas will be justly proud of their product it is universally accepted that a lower level of acidity is an indication of better oil. It is the acidity caused by the oxidization of oil, turning it into a fatty acid, that kills off the alkaloid and hence the flavor (the same way lemon juice or vinegar will tone down the heat of chili pepper). In fact olive oil, to be classed as a virgin has to have an acidity of below 2%, to be classed as extra virgin less than 1%.
The best Tuscan extra virgin oils have acidity of less than 0.01%, almost unheard of elsewhere. This makes them particularly good for use in Bruschetta, on green vegetables, and of course on the rich meat and bean dishes of the area. (It has to be said that for delicate fish dishes, a lighter condiment is probably more appropriate.)
The much prized new olive oils, normally unfiltered, have a perfume and flavor much more intense than when kept for some months. Though storing out of the light and at a constant cool temperature will help to keep it fresh, there is an inevitable decline in fruitiness over time.
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