Tuesday, April 05, 2005

Why brewed green tea sometimes changes color

A customer recently emailed to ask why brewed green tea turns brown when kept in a thermos. We thought others may have experienced the same thing, so we are posting the reply below.

>I have a serious green tea question. When I brewed my green tea
>in my stainless steel thermos it turned a red color. I tried
>letting the tea cool down a little and "breath" before capping
>it and this did not change anything. The flavor was also
>different. Is this because of the heat or a reaction with the
>stainless steel or both? Have you ever heard of this?
>Thank you....

Thanks for your question. I've had the same thing happen to me -- brewed green tea kept in a thermos for too long first turns light brown, then eventually reddish brown. The problem is the constant high temperature at which the tea is kept in a thermos. For instance, a cup of brewed tea left standing won't lose too much "greenness," even overnight. A teapot full of tea, however, will take on a slightly brownish hue. That's because the pot holds the heat in longer than a cup. A thermos, obviously, retains heat much, much better.

Tea that turns too brown has lost much of its antioxidant content. That's why green tea is healthier than oolong or black (English) teas, which are brown because the leaves were dried in the sun after harvesting and then allowed to ferment. Sun-drying allows an enzyme in green tea (called polyphenol oxidase) to oxidize (i.e., combine with oxygen in the air). The oxidized enzyme alters the disease-fighting catechins, which give the tea its green color. Fermentation also causes the catechins to be oxidized. Oxidized catechins give black tea its distinctive aroma and flavor but don't have the health benefits of catechins in their natural state.

Consequently, when green tea is harvested, the grower's first concern is to get the leaves to the steaming or frying vat as quickly as possible. This stops oxidation and fermentation, locking in green tea's distinctive earthy flavor and aroma -- and preserving the catechins.

But green tea's catechins also begin to break down after brewing. (And sustained high temperatures, as in a thermos, only accelerate this process.) This is why I always recommend that people drink their green tea within an hour of brewing.

(And think of how long bottled, ready-to-drink green tea has been sitting around! Without artificial green coloring, it wouldn't be green at all.)

I hope this answers your question. Please drop me another line if you have any other questions or comments.

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