Tomomi Ujihara

Evaluation of the umami taste intensity of green tea by a taste sensor.

A method for evaluating the umami taste intensity of green tea by a taste sensor system was established. Interference in the measurement from catechins was solved by removing the catechins from sample solutions with poly(vinylpolypyrrolidone). A 5.00 mM aqueous solution of glutamic acid monosodium salt was used as the standard solution. Sensor outputs were converted into EIT uma (estimated intensity of taste concerning umami) values. One unit on the EIT uma scale was defined as the amount of the sensor output corresponding to a difference in 1.2 times the concentration of the standard substance (glutamic acid monosodium salt). The umami taste intensity of green tea was classified into six grades on the EIT uma scale. Sensory tests proved that the EIT uma value had a high correlation to the human gustatory sense.

Rapid determination of caffeine in tea leaves.

For the purpose of efficient screening of low-caffeine tea shoots, a method for the rapid determination of caffeine was developed using high-performance liquid chromatography. Polyvinylpolypyrrolidone was packed in a pre-column and used to remove polyphenols from tea extracts on-line. The concentrations of caffeine extracted from powdered tea leaves at 50 degrees C during 1 day could be analyzed in 2-5 min intervals. The pre-column and the analytical column could be used for the analysis of more than 2000 samples.

Techniques for Universal Evaluation of Astringency of Green Tea Infusion by the Use of a Taste Sensor System

A practical method for universal evaluation of the astringency of green tea infusion by a taste sensor system was established. The use of EGCg aqueous solution as a standard enabled analysis with high accuracy and reproducibility. The sensor output was converted into taste-intensity on the basis of Weber’s and Weber-Fechner laws, which was named the “EIT ast ” value (“EIT” and “ast” are abbreviations for “Estimated Intensity of Taste” and “astringency” respectively). It was clarified that green tea infusion is to be classified into eight grades on the EIT ast scale. Furthermore, the high correlation of the EIT ast value with the human gustatory sense and the high stability of the taste sensor were proved.

Reduction of catechin astringency by the complexation of gallate-type catechins with pectin

The reductive effect of pectin on tea catechin astringency was investigated by using a taste sensor system and 1H-NMR spectroscopy. The sensor analysis revealed that the astringency of gallate-type catechins (EGCg and ECg) was reduced by the addition of pectin, whereas that of non-gallate-type catechins (EGC and EC) hardly changed. Changes in the 1H-NMR chemical shifts of the catechins and pectin in mixed solutions showed that the gallate-type catechins formed complexes with pectin more closely than the non-gallate-type catechins. These results demonstrate that complexation between the gallate-type catechins and pectin is a factor for reducing catechin astringency.

Binding Energy of Tea Catechin/Caffeine Complexes in Water Evaluated by Titration Experiments with 1H-NMR

Evaluating the binding energy of a catechin/caffeine complex in water is important in order to elucidate the ability for molecular recognition of tea catechins. The results of this study revealed that the stoichiometric ratio of the complexation between tea chatechins (EGCg, ECg, EGC, and EC) and caffeine was 1:1 at least up to a concentration of 5.0 mM. The free energy (−ΔG) values for binding in water at 301 K were evaluated to be 2.7, 2.6, 2.2, and 2.0 kcal/mol for EGCg, ECg, EGC, and EC, respectively, by the titration method with 1H-NMR. An investigation of the 1H-NMR chemical shift change and NOESY spectra in the catechin/caffeine solutions showed the participation of the A-rings of the catechins in complexation, as well as that of the galloyl groups or B-rings.

Construction of a high-density reference linkage map of tea (Camellia sinensis)

A few linkage maps of tea have been constructed using pseudo-testcross theory based on dominant marker systems. However, dominant markers are not suitable as landmark markers across a wide range of materials. Therefore, we developed co-dominant SSR markers from genomic DNA and ESTs and constructed a reference map using these co-dominant markers as landmarks. A population of 54 F1 clones derived from reciprocal crosses between ‘Sayamakaori’ and ‘Kana-Ck17’ was used for the linkage analysis. Maps of both parents were constructed from the F1 population that was taken for BC1 population. The order of most of the dominant markers in the parental maps was consistent. We constructed a core map by merging the linkage data for markers that detected polymorphisms in both parents. The core map contains 15 linkage groups, which corresponds to the basic chromosome number of tea. The total length of the core map is 1218 cM. Here, we present the reference map as a central core map sandwiched between the parental maps for each linkage group; the combined maps contain 441 SSRs, 7 CAPS, 2 STS and 674 RAPDs. This newly constructed linkage map can be used as a basic reference linkage map of tea.

β-Cyclodextrin/Surface Plasmon Resonance Detection System for Sensing Bitter-Astringent Taste Intensity of Green Tea Catechins

To develop a methodology for creating a sensor with a receptor for specific taste substances, we focused on constructing a sensing system for the bitter-astringent taste intensity of green tea catechins: (-)-epigallocatechin-3-O-gallate (EGCg), (-)-epicatechin-3-O-gallate (ECg), (-)-epigallocatechin (EGC), and (-)-epicatechin (EC). (1)H NMR titration experiments revealed that beta-cyclodextrin was an adequate receptor for sensing the bitter-astringent taste intensity of catechins. A surface plasmon resonance (SPR) system immobilized beta-cyclodextrin indicated larger responses for the gallate-type catechins in comparison to the non-gallate-type catechins. These responses corresponded to the tendency of the bitter-astringent taste intensity of the catechins felt by humans. Furthermore, the SPR system detected the larger stability of the complex between the gallate-type catechins and beta-cyclodextrin, which was interpreted as the aftertaste produced in humans by the gallate-type catechins. These results demonstrate that the beta-cyclodextrin/SPR system can sense the bitter-astringent taste intensity of the green tea catechins similar to human gustation. The methodology presented in this study can be used as a basic strategy for developing taste sensors with specific receptor functions.

A Water-Soluble Acyclic Phane Receptor Recognizing 2,3-trans-Gallate-Type Catechins

To recognize gallate-type catechins in aqueous solution, two water-soluble acyclic phane receptors containing three aromatic rings were synthesized. The binding of these receptors to eight catechin analogues was investigated with (1)H NMR spectroscopy. The stoichiometric ratios of the complexes between the receptors and the catechins were 1:1 in every case. The binding abilities were estimated by (1)H NMR titration. The meta-substituted receptor showed excellent binding ability for the 2,3-trans-gallate-type catechins. This study revealed that a simple acyclic phane receptor can distinguish differences in the structures of the catechin analogues.

Evaluation of the Astringency of Black Tea by a Taste Sensor System: Scope and Limitation

Grading the astringency of black tea by a taste sensor system was studied. The black tea samples manufactured in India and Sri Lanka were classified into ten steps on the basis of two standard solutions (0.65 mM and 0.26 mM EGCg aqueous solutions). An organoleptic test demonstrated that the sensor output was correlative to the human gustatory sense.

Identification of Japanese and chinese green tea cultivars by using simple sequence repeat markers to encourage proper labeling.

To identify commercial Japanese monovarietal green tea and imported green tea samples, leading Japanese cultivars were fingerprinted by using six simple sequence repeat markers analyzed by a capillary sequencer. Two well-authenticated imported Chinese monovarietal green tea samples were also fingerprinted by the same markers, one of which, was Fuyun, was a clonally propagated cultivar, and the other, Jiukengzhong, was seed-propagated. At least three markers used in this study identified 16 leading Japanese cultivars and Fuyun. Although Jiukengzhong was a mixed population with diverse genotypes, some individuals had a unique allele in one simple sequence repeat marker that was not detected in the 16 leading Japanese cultivars, an additional 39 cultivars, and Fuyun. This allele was effective as a detection marker for Jiukengzhong. These results support the use of simple sequence repeat markers for the identification of Japanese monovarietal green tea and also of imported green tea made from foreign cultivars.