The acidity of an olive oil is a parameter that measures the amount of free oleic acid that exists in the oil with respect to the total triglycerides.
In the pure oil that resides inside the olive, all the molecules of oleic acid are united to the three alcoholic radicals that present the molecules of glycerin or glycerol. The complete structure of fatty acids (not all of which are oleic acid but more than 80%), linked to glycerine is known as triglyceride, and consequently the oil is a set of triglycerides, which is the fundamental molecule. Along with these can be found other compounds, especially vitamins and natural antioxidants, and eventually serum compounds, other fats, often from the skin of the olive and contact with the leaves.
When we observe that an oil presents a percentage of free fatty acids, we must think that they can only come from the breakage of some triglicéridos, and the same one only takes place by means of oxidation by competition of the oxygen. In this sense, the measurement of acidity serves to evaluate the quality of the collection, transport, storage and extraction treatment of olive oil, because the better it is, the less fatty acids it will present.
An oil with a very high acidity indicates that either the olive has been damaged during harvesting or during transport, causing wounds through which oxygen penetrates and oxidises the oil. It also indicates too long a storage, in which the weight of the olives themselves squeezes them and releases oil that oxidizes. Finally, a slow extraction with high temperature pressing, which favours oxidation, also increases the acidity of the resulting oil.
In short, the acidity indicates the purity of the whole process from the moment the olive is beached on the tree until it is pressed and stored in the vats. It even tells us what to do before beating, because olives bitten by insects or hit by hail, for example, will increase acidity. In fact the acidity serves to mark differences between virgin olive oils and a normal or refined oil.
However, between virgin olive oil and extra virgin olive oil other parameters influence, the acidity also has its weight. Specifically, an extra virgin olive oil must have equal or less than 0.8º of acidity. On the other hand, in a virgin olive oil the acidity must be equal to or less than 2º, and any oil that exceeds 2º of acidity cannot be considered virgin or extra virgin olive oil, given that so much acidity indicates bad quality of the process.
In spite of the general belief, which attributes to acidity whether an oil scrapes or bites in the mouth, this parameter does not affect either the texture or the flavour. Fatty acids are insipid, and therefore will not give the oil a bad taste. However, an oil with high acidity, which indicates poor processing and extraction, may have bad smells from parallel fermentation in storage or from organic remains of contaminations. But such oils do not reach the market without chemical treatments.