Antioxidant-Rich Foods Lose Nutritional Luster Over Time
Studies found levels of good compounds dropped after 6 months
THURSDAY, April 2, 2009 (HealthDay News) -- For those who swear by antioxidant-rich food and drink, two new studies show those health benefits can wane if the products are stored for too long a time.
The findings focus specifically on how well antioxidant activity holds up in commercially available green tea bags and olive oil when stored unopened and unexposed to light or moisture. And, in each case, the research revealed that steep drops in antioxidant activity take place within the first six months.
"The whole general concept driven by both of these studies is that if we want to maximize the nutritional value of the foods we eat, we really should buy only what we can use in a short period of time," observed Connie Diekman, a registered dietitian and director of nutrition at Washington University in St. Louis.
Diekman was not involved with either study, both of which are published in the March issue of the Journal of Food Science.
One study focused on the organic compounds found in green tea leaves that are known as catechins.
When consumed in tea, these antioxidant compounds are thought to have a bacterial and virus-fighting capacity, as well as the ability to inhibit cancer cell activity.
However, given that commercial green tea does not spoil and can be shelved for extended periods of time, Mendel Friedman and colleagues from the Albany, Calif.-based Western Regional Research Center of the U.S. Department of Agriculture set out to explore the stability of catechins during long-term storage in homes, restaurants, commercial warehouses, and/or stores.
The team chose eight teas sold commercially in tea-bag form in the United States, Korea and Japan.
The tea bags were kept stored in their original packaging in dark rooms heated to 68 degrees Fahrenheit for one of five different lengths of time: one week, one month, two months, four months and six months. Undesirable moisture exposure was not a factor.
After each period, the teas were ground into powder and mixed with boiling water, before being cooled and analyzed.
"We found that among the teas we looked at there seems to be a progressive decrease in the amount of antioxidants as a function of time," lead author Friedman said.
The team found at least some drop-off in catechin antioxidant content early on in the storage process, and went on to observe that by the end of six months catechin concentrations had plummeted among all eight teas by an average of 32 percent -- a figure the authors characterized as "highly significant."
Specifically, the most prevalent form of catechin (EGCG) decreased by 28 percent after six months of storage, while the second most common catechin (ECG) dropped by 51 percent in the same timeframe.
Friedman described his work as preliminary, and expressed the hope that the findings would prompt more research into the storage-antioxidant question, given the large variety of teas on the market and the strong probability that not all teas would experience nutrient degradation in exactly the same way or pace.
For their part, the authors of the Italian olive oil study noted that to be considered "extra-virgin," olive oil must be sourced directly from olive tree fruit through a process confined solely to washing, decanting, filtration and high-speed mixing.
The final product is known to be rich in a specific blend of fatty acids and phenolic compounds, the latter acting as antioxidants. Consuming olive oil has long been considered beneficial with respect to lowering the risk for heart disease, stroke and several kinds of cancer.
To explore the durability of antioxidants found in extra-virgin olive oil, Antonella Baiano and colleagues at the University of Foggia in Italy looked at several varieties of the oil that had been produced within 24 hours of having been plucked as olives from two groves located in the Apulia region of Italy.
After analyzing the oils during both production and packaging, Baianos team found that antioxidant activity remained unchanged throughout the first three months of storage. However, by the six-month mark, most of the oils had lost about 40 percent of their antioxidant properties.
Diekman expressed little surprise with the findings.
"Although this might surprise a lot of people who would expect that if something stays sealed it will not lose nutritional value, antioxidants are very fragile," she noted. "And, of course, in general, it is well known that when we look at plant foods as a whole, the nutritional value is best the fresher it is."
"So, the message here is that when we go shopping, we need to think about the quantity of produce and fresh foods that we buy," Diekman advised. "This is true whether we're talking about the actual food -- for example, olives -- or whether it is the olive oil derived from the food. The question should be: 'Can you use it within a reasonable period of time?' Because from a nutritional standpoint, big quantities may not be a real dollar savings, when you look at the loss of nutritional value that will occur over time."
Diekman also suggested that consumers should generally favor tinted containers over clear ones, to protect antioxidant, vitamins and minerals from exposure to the sun. In that regard, the authors of the Italian study specifically noted that extra virgin olive oil should ideally be stored in small glass bottles placed in a dark setting at room temperatures ranging from 68 degrees to 77 degrees Fahrenheit.
For more on antioxidants, visit the U.S. National Library of Medicine.