How many times have you wondered about why some olive oils come in clear bottles where you can see their color and some come in dark glass bottles?

The following information has been provided by Italian research scientists concerning the components of olive oil that provide great health benefits, are not found in the same abundance in other oils, contribute to its organoleptic qualities and enduring storage capacity and can be lost if subjected to light which makes proper storage of olive oil an important issue.

Olive oil is a very complex mixture of components which affect its stability. Research has shown that while polyphenols are important, tocopherols, phytosterols, and particularly avenasterol contribute to the
olive oil's antioxidant activity.  While the polyphenols have been shown to be antioxidants, some polyphenols are better antioxidants than others, so the antioxidant effects do not simply reflect the total amount of polyphenols but rather the levels of those with the more potent antioxidant effects. It is possible that some olive oils may have lower amounts of these more potent antioxidants even though the total polyphenol levels are high. Some antioxidants can, under certain conditions, act as prooxidants thus
increasing oxidation. This is why you should avoid clear olive oil bottles and purchase olive oil in dark glass bottles instead, and you should store your olive oil away from light and high temperatures. Many experts believe that olive oil sold in clear bottles and stored on the top shelf of a supermarket under strong lights lose some of their profound health benefits and flavor after being subjected to these conditions for a period of time.

Olive oils with a high polyphenols rate have a long preservation capacity. Polyphenols are antioxidant components and they are recognized as protective substances. Many experts believe that polyphenol contents in olive oil can indicate its quality and its real value.

"In olive oil, a high polyphenols rate is most important to preserve the integrity and benefits of the oil and to prevent ranking [deterioration/rancid quality]. We can say that phenolic substances, and not tocopherols, are the right inhibitors in this fat matter, and found more in olive oil rather than in seed oils. In fact, it has been determined that there exists a positive correlation between the polyphenols rate and the oxydation stability of virgin olive oils. It has also been noticed that a positive correlation exists also between polyphenols rate (especially phenolic acids) and organoleptic characters of the oil."
(Vitagliano M. -  Industrie Agrarie, UTET, Torino, 1982, pag. 664)

"The phenolic patrimony is the most precious characteristic of virgin olive oil, which is the only vegetal fat in which polyphenols are abundant. Such substances, which contribute to its typical 'fruttato' aroma as well as its 'piccante' and bitter taste, have a great antioxydant power. The extra virgin oil, thanks to these components, is the most preservable fat and the one with the highest biological value. For these reasons, in olive oils the polyphenols level determination represents one of the most significant analyses aiming to the determination of quality parameters." 
(AA.VV. - Extravergine, Manuale per conoscere l'olio di oliva, Slow Food Editore, Bra, 2000, pag. 94)