Regardless of their refining, oils in general are sensitive to heat, light and air (oxygen). These exposures can make oil go rancid. Rancid oil has an unpleasant aroma and a sharp smell, while its nutrients are either lost or severely decreased.

Rancidity of oils is caused by a reaction that creates a specifically unpleasant smell that is formed as a result of lipolytic or oxidative degradation. The oxidation of plant oils is a process of autoxidation of the fatty acids and it occurs in several stages. In its first stage, initiation, the free radical (mainly reactive oxygen molecules) from the environment reacts with the hydrogen atom separating it from the fatty acid molecule, which in turn makes the actual molecule very reactive as well. In the second stage the fatty acids react with the oxygen molecules from the surrounding air creating peroxy radicals that are also highly reactive and continue to react with the other molecules of fatty acids, creating hydro peroxides and more free radicals as a byproduct. This creates a chain reaction that can make all of the molecules of the fatty acids from the oil rancid, until final degradation products such as ketones, alcohols and aldehydes (that give the specific unpleasant smell of the rancid oil) are created.

Oils that are exposed to oxygen and are additionally heated for prolonged time are subjected to the last stage of the process of oxidation, called oil polymerization. This process is specific to the cooking method of deep oil frying. We should note that the cold pressed oils can also withstand heat and some cooking for a short period of time without losing a substantial part of their nutritional and health properties. However, the unsaturated oils of any kind, both cold pressed and processed/refined, heated repeatedly for an extended time at temperatures above 170 ˚C (frying of the oil) create polymers, molecules with high molecular weight, that cause foaming and increased viscosity of the oil and have a carcinogenic effect on the human health.

As we have already mentioned above, leaving or storing oils exposed to higher temperature, light and air can make them go rancid, which particularly effects cold pressed oils which predominately contain unsaturated fatty acids that are easily subjected to the process of autoxidation. For that reason, regardless of the degree of processing and refining, all oils in general and cold pressed oils in particular, should be stored closely tight in a dry, cold and dark space in their original packaging (bottle). Storing oils in such condition (refrigerator) can make the oil ticker, which easily softens and tins again once put back on a room temperature. One of the advantages cold pressed oils have is the large concentration of antioxidants they contain, such as vitamin E, which prevents them from oxidation and going rancid for some period of time. However, if we want to optimally utilize all nutritional and health benefits of the cold pressed oils we have to follow the recommendations for proper and save ways of storing always!