FTIR Spectroscopy Separates Good Green Teas from Also-Rans

Lynn M. Savage

Although tea comes from just one source plant, Camellia sinensis, it can be fermented, processed and brewed to impart a multitude of flavors. Considered one of the most popular drinks in the world, tea is revered especially in Asia, and green tea reigns supreme there. Highly trained tasters in China and Japan, for example, judge the quality of various green teas each year at competitions involving tea producers. Human tea tasters, however, are subjective in their analyses of criteria such as leaf appearance, smell, brew coloration and taste.

Now researchers at Osaka University, at Nara Institute of Science and Technology, and at Nara Prefecture Agricultural Experiment Station, all in Japan, have developed a spectroscopic technique that can help identify high-quality green tea concoctions. According to principal investigator Eiichiro Fukusaki of Osaka University, Nara is one of the most important regions for tea production in Japan because it has a good climate for tea cultivation.

Using 13 kinds of green tea that were presented and ranked at the 2005 competition in Kansai, Japan, the researchers ground 200 mg of dried leaves and mixed them with 600 μl of glycerol to form a paste. Using a Fourier transform infrared (FTIR) spectrometer made by Thermo Electron KK of Kanagawa, Japan, the investigators acquired diffuse reflectance spectra three times for each sample. Spectra were recorded between 10,000 and 4000 cm^–1 with intervals of 3.857 cm^–1. The scientists then analyzed the data using multivariate methods.

They found that the range 5500 to 5200 cm^–1 had a high contribution ratio, leading them to believe that high-quality teas can be fingerprinted by that region. Using this information, the rankings from the Kansai contest and partial least squares analysis, the investigators derived a prediction model for judging tea quality. The model is accurate for predicting low-quality teas but less so for high-quality ones. They believe that this is because the differences among better teas are smaller and less easily discerned.

Fukusaki said that the spectroscopic technique could be useful to tea producers and dealers and that it even could be applied by professional tea tasters, who could use it as a practical tool. The scientists plan to apply their methods to folk medicines and to other foods, such as vegetables.

“The technique is very convenient, reproducible and cheap,” Fukusaki said. “In some cases, it can be more useful than tea tasters.”

Journal of Agricultural and Food Chemistry, Nov. 28, 2007, pp. 9908-9912.