The first study looked at several catechins — the antioxidants in green tea that are known to be beneficial in fighting viruses and bacteria and in combating cancer cell growth. The team, headed by Mendel Friedman at the Western Health Sciences Center in California, looked at eight different varieties of green tea, sold in stores in the U.S., Korea, and Japan. They shelved the teas in areas without moisture and light, still in the packaging in which they came, for one week, one month, two months, four months, and six months.
[A]fter six months of storage, antioxidants had dropped to 40% of their original levels.
The team then analyzed the catechin make-up of the teas, and found that after dropping off somewhat in the early stages, the levels had plummeted to an average of 32% of initial levels by the six-month mark. At this time, the most common catechin, EGCG, had dropped by about 28%; this compound is also thought to be the most potent of the antioxidants in tea. The next most common catechin, ECG, had dropped by 51% after six months of storage.
Friedman says that more research will be needed to determine fully how the degradation of catechins works over time; most likely, various teas differ in how they react as time passes, so the question will be worth addressing in the future. In the meantime, it may be beneficial for the consumer to be made aware of the "storage history" of teas sold in stores, as current packaging often gives no indication of how long a product has been shelved.
In another study, Italian researchers at University of Foggia followed extra-virgin olive oil from the time it was pressed to six months after it was shelved. In comparison to preliminary measurements around the time of production, the level of antioxidants was virtually the same after three months. But after six months of storage, antioxidants had dropped to 40% of their original levels. The antioxidants in olive oil are known to be beneficial in heart disease, stroke, and cancer prevention.
Head researcher Antonella Baiano and her team recommend that olive oil be switched from its original container and placed into small glass bottles, and stored in a cool, dark location. Critics note that if the reason for the purchase of certain foods is antioxidants in particular, it may be worth buying smaller containers. Though many people try to buy in bulk for financial reasons, this may not actually be the best method in terms of nutritional value, given the loss of the key compounds over time.