Thursday, June 18, 2020
NMR and Food Chemistry - Rhubarb
Many gardeners here in Canada are currently harvesting their rhubarb to make pies and other desserts. Like cranberries, rhubarb is very sour due to the presence of organic acids. The leaves of the rhubarb plant contain so much oxalic acid that they are toxic. The stalks of the plant contain less oxalic acid and are not toxic (although I would argue that they taste awful). Oxalic acid is used in many cleaning products, so if you are like me and hate the taste of rhubarb, you can still make use of it as a cleaning agent. If you have a very dirty stainless steel pot, you can clean it by simply stewing rhubarb in it. The figure below shows the 1H and 13C NMR spectra of an aqueous extract of rhubarb stocks. The major constituents are organic acids with smaller quantities of sugars. Some of the 13C signals from oxalic, malic and citric acids are labelled.
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2 comments:
I disklie the tase of rhubar, too. Also it attacks my teeth, they feel strange after eating it. Maybe bc of the oxalic acid?
Here a question regarding 13C NMR referencing in CDCl3: Conventional wisdom has it that the CDCl3 peak should be set to δC 77.16 ppm (±0.06, found in Gottlieb et al JOC 1997, 62, 7512; ). However, whenever I reference to TMS, I find the CDCl3 peak in my samples to be closer to δC 77.02–77.05. In fact, some people reference to δC = 77.0, and that would be closer to what I find than the Gottlieb value.
Is this dependent on the machines or why is my value lower than the one by gottlieb et al?
Unknown,
The 13C chemical shift of CDCl3 with respect to TMS at 0 ppm should not be instrument dependant. It may however depend on the concentration of a solute (if one is present), temperature and digital resolution (points/Hz) in the spectrum.
Glenn
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