Most tea drinkers don't give much thought to how tea leaves are processed prior to arriving in a tin or tea bag. However, a Korean team of food scientists has. Realizing that many people are trying to consume more of certain tea-derived antioxidants—especially compounds called catechins—the researchers wondered whether concentrations of those chemicals might be affected by tea-leaf handling. They now report data confirming that indeed, concentrations of beneficial chemicals can vary dramatically, depending on how newly-harvested leaves are processed.
Seung-Cheol Lee and his colleagues at Kyungnam University focused on green tea, a Korean favorite. It's made from the same plant as black and oolong teas are, but it's processed differently. Leaves that will become green tea are first roasted to inactivate their enzymes, and then rolled and dried. Lee's group compared leaves that were roasted and dried using conventional oven heating with leaves heated by far-infrared (FIR) irradiation in some of the processing steps. The internal temperatures of the leaves are identical during FIR and conventional heating, Lee says.
What's FIR? A red-hot piece of metal or some other material radiates some of its heat in wavelengths outside the visible spectrum, a range known as far-infrared wavelengths.
The Korean team has shown for the first time that a tea's concentrations of antioxidants and a host of other desirable chemicals are higher when roasting is done with FIR. Drying leaves with FIR, however, tended to diminish those chemicals.
Lee's team concludes that FIR roasting—which isn't yet in use commercially—"could be a useful method for increasing the health-promoting properties of green tea."
Letting the cats out
Catechins, sometimes called CATS, belong to a family of antioxidants known as polyphenols. The Korean scientists measured polyphenols and other chemicals released from processed tea leaves during simulated brewing. The sampled tea leaves had been processed with FIR heating in the roasting stage, during leaf drying, in a final extra heating step at the end of leaf processing, or in combinations of all three.
For instance, in one test, the researchers used FIR to roast tea leaves at 90°C for 10 minutes and then applied an extra 10-minute FIR treatment after the leaves had been conventionally dried. When these leaves were later steeped in water, the brew picked up 76 percent more polyphenols than did tea from leaves that had been roasted conventionally at 200 to 300 °C for 3 to 4 minutes and dried conventionally.
When FIR heating was substituted only for conventional oven drying, polyphenol concentrations weren't elevated. In fact, the lowest polyphenol concentration after brewing was from leaves that had been conventionally roasted and then FIR dried. The data collected by Lee's team were reported in the Dec. 27 Journal of Agricultural and Food Chemistry.
Similar trends were found for just the catechins component of these polyphenols, including epigallocatechin gallate (EGCG). That catechin has been linked with reduced risk of a number of diseases, most notably cancers of the breast and prostate.
The maximum value for EGCG—15 milligrams per gram of tea leaves—occurred when the green-tea leaves underwent FIR heating during both roasting and the extra final heating. That value was 70 percent higher than the EGCG concentration in conventionally processed leaves and 56 percent higher than in tea leaves only dried using FIR.
Lee says his team was surprised at what a diverse impact FIR roasting and the final heat treatment had. For instance, the chemists observed the highest vitamin C concentrations in tea from leaves that were FIR roasted and then conventionally dried. Similar to its effect on other compounds, FIR drying of the leaves dropped the vitamin C values to below those in conventionally processed green tea. The same pattern emerged with caffeine.
Lee says his team is trying to understand why FIR heating during certain stages of tea-leaf processing alters a brew so dramatically. At present, he suspects that "FIR radiation might cleave bound polyphenols to free forms" and might have a similar effect on caffeine as well. "However," he told Science News Online, "too high an [FIR-induced] temperature might destroy the polyphenol compounds."
Because black and oolong teas are processed in ways different from that of green tea, Lee says he's not sure whether using FIR with those teas would boost desirable constituents.
Currently, Lee's group is conducting tests to identify whether antioxidant increases in green tea from FIR roasting and end-stage heating are big enough to potentially affect tea drinkers' health. Lee and his colleagues say they're hoping for the best, seeing how catechins concentrations from conventionally processed teas have already been linked to many health benefits.
Department of Food Science and Biotechnology
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