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Coffee is one of the richest dietary sources of antioxidants - natural compounds that help protect cells from premature aging.

However, researchers have been unable to determine if one type of brew is better than other. Studies have produced conflicting results.

Now food scientists at the University of British Columbia think they know why it's been so hard to nail down this seemingly simple question. Their research shows that the roasting process alters the antioxidant content of the coffee beans, destroying some antioxidants while creating others.

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"Coffee contains thousands of compounds. It's very complicated," said Yazheng Liu, who did the lab work as part of her master's thesis.

David Kitts, the senior author of the paper, noted that previous studies have suggested the antioxidants in coffee can be traced to caffeine or the chlorogenic acid found in green coffee beans. But, he added, "we found that coffee beans lose 90 per cent of the chlorogenic acid during the roasting process."

Yet, at the same time, their study also revealed that new antioxidants are generated by the interaction of the bean's carbohydrates, sugars and proteins in the presence of heat.

"This is a dynamic process, with some antioxidants decreasing and some increasing," said Ms. Liu. The end result depends on the bean's original antioxidant content as well as the time and temperature of the roasting. In other words, there's lots of room for variability. So don't expect scientists to announce the name of the top brew any time soon. Just enjoy your coffee, content in knowing it could be considered a health drink. (So long as it's not a double double.)

The study will be published in the journal Food Research International.

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