The International Olive Oil Council (IOOC) evaluates olive oil based on three main factors: acidity, oxidization and organoleptic features (i.e. smell, color, taste).
Acidity is the most important criterion for professionals and consumers. It determines the quality classification, the gradation, and the price. The fatty acids of the oil are either free or bound with glycerol. Free fatty acids form the acidity. The given acidity is usually indicated by a percentage: the greater the percentage, the more free fatty acids in the oil. The degree of growth results from several factors that influence the fruit from a very early stage. For example, dakos is a specific disease affecting olive trees, hurting the fruit during gathering, storage, and final crushing.
Oxidation occurs when olive oil becomes rancid. This results from exposure to unsuitable conditions after its extraction from the oil press. Oxidization is defined by laboratory measurements, including the number of hyperoxides.
The color range of olive oil usually varies from dark or light green to dark or light yellow. The color is determined by the prevalent coloring substances of the fruit at the time of gathering. For green olive oil, the fruit was gathered at an early stage, when it was still unripe or semi-ripe; this oil is usually bitter. Olive oil may also have a vivid green color if leaves or small twigs are included when grinding. For an unripe or semi-ripe fruit, a purple-blue chlorophyli results. Yellow olive oil was gathered when the fruit was fully ripe and still on the tree; a vivid yellow oil may also indicate oxidization. When the color is dark brown or black, then the oil comes from fruit that was fallen on the ground. These kinds of oil have a milder, sweeter taste. The system of crushing the olive crop and extracting the oil also plays an important part in the determining of the color.