The production of good quality oil does not automatically ensure that it will remain such until the moment it is consumed. It is necessary to shed some light on the chemical reactions causing its deterioration, so as to minimize their impact.
The first factor that should be taken into account is the quality of the raw material: it is difficult to produce low quality oil from good, healthy olives. The oil thus obtained may not appeal to every consumer, but usually, this is for reasons independent of its quality, such as personal eating habits, lack of familiarity with olive oil, available budget, and so forth.
The quality of olive oils tends to decrease in time due to oxidation and hydrolytic reactions. Hydrolysis releases fatty acids from the triglycerides contained in dietary oils and fats. This reaction, which cannot be perceived by our senses, is triggered by acids, including the fatty acids that it produces. Hydrolysis at first progresses slowly, but at a rate that is proportional to the acidity of the environment, hence as time goes by, it steadily increases.
Oxidation occurs in the presence of oxygen and is influenced by a number of factors, such as the presence of light, storage and cooking temperatures, oil age and acidity levels.
It is difficult to eliminate oxygen completely, but it is advisable to purge the headspace of the vessels containing olive oil, be they vats or bottles, with inert gas, so as to prolong its shelf life. The antioxidants that are naturally present in oil should protect it from any residual oxidation processes.
The temperature should be as low as possible, but not so low as to cause the oil to solidify, else its antioxidants would be incapable of protecting it from spoilage. The ideal storage temperature ranges between 10 and 12 °C, and should never fall below 8 °C.
Generally speaking, time only becomes a critical factor after the bottle is opened, because only then does the air, and therefore oxygen, come in contact with the oil. More oxygen mixes with oil every time we pour some out from its vessel, and for this reason we should always purchase bottles of appropriate size, proportional to the rate at which we consume oil, so that it will all be used before it has time to oxidize and turn rancid. Once opened, the bottle should be finished within a month: if it takes two months to finish it, then we should consider buying bottles half the size.
When exposed to the light, oxidation rates increase dramatically, up to thirty thousand times: it is therefore always best to store oil in the dark, or even better, in vessels that are not transparent to the light. Nowadays, the bottles of oil that are on the market are always well sealed, and guarantee the longest shelf life possible.
Olive processing techniques too affect the shelf life of the oil, because they influence the content of antioxidants present in the olives, the level of which can also depend on the cultivars employed.
Another factor that plays a major role, although we often tend to neglect it because it is beyond our control, is the transportation method by which oil reaches the retailer: the distance and means of transportation employed may accelerate its degradation, especially if during this period, the oil is subjected to sudden changes in temperature.
Photo by Luigi Caricato