Well, steviosides or better steviol glycosides as they are known are not sugars. They are diterpenes. There are about 11 different steviol glycosides with prevailing Reb-A and Stevioside accumulated in the leaves of Stevia rebaudiana Bertoni. In order to measure the concentration of these substances in the extract, you need to separate them first. This process is achieved in the chromatographic column. This brings us to the conclusion that the way to measure the concentration of the steviol glycosides is a HPLC-method. By the way, the measuring process itself requires the wavelength of 210 nm, which puts most UV-Vis photometers out of question.
Dr. Shevchenko is correct. You cannot measure just the two using UV-vis; besides, the working wavelength of 210 nm gives you very iffy results even with HPLC. The only effective way to measure them is with HPLC-MS-MS; there are several methods published for that. By the way, it is a very, very difficult analysis in natural products. We run it, but there is nothing easy about it.
I am myself not very familiar with such techniques but I advise you to contact Prof Jan Geuns ([email protected]), prof at KU Leuven. He is specialized in Stevia and its metabolytes.
Have you tried the TLC densitometry method? Anyone can suggest me for the TLC method for stevioside from Stevia?
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