Stability of green tea catechins
1997
Zhu, Q.Y. | Zhang, A. | Tsang, D. | Huang, Y. | Chen, Z.Y.
Green tea catechins (GTCs), which include (-)-epicatechin (EC), (-)-epicatechin gallate (ECG), (-)-epigallocatechin (EGC) and (-)-epigallocatechin gallate (EGCG), possess a variety of biological activities. We have previously studied the effect of dietary GTCs as a mixture on membrane oxidation of red blood cells and found that GTCs were partially absorbed and detected in the blood of rats given an oral ingestion of 100 mg of GTCs. To explain the partial absorption of GTCs and their varying free-radical scavenging capacity at different pH, the present paper was to study further the pH stability of these GTC isomers because there is a sharp increase in pH from the acidic stomach to the slightly alkaline intestine. Longjing GTCs as a mixture in alkaline solutions (pH > 8) were extremely unstable and degraded almost completely in a few minutes, whereas in acidic solutions (pH < 4) they were very stable. For the pH between 4 and 8, the stability of GTCs was pH-dependent, i.e., the lower the pH, the greater the stability. Four epicatechin isomers examined demonstrated varying stability in alkaline solutions with EGCG and EGC being equally instable, and EC and ECG being relatively stable. The present results suggest that part of the mechanism by which GTCs were partially absorbed may be attributed to instability of EGCG and EGC in the intestine where the pH is neutral or alkaline.
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