Fructose and inulin: Behaviour under analytical pyrolysis

EGA-MS (evolved gas analysis-mass spectrometry) and Py(HMDS)-GC/MS (pyrolysis-gas chromatography coupled with mass spectrometry with in situ derivatisation using hexamethyldisilazane as a silylating agent) were used to study the behaviour under pyrolytic conditions of fructose, inulin and topinambour (Jerusalem artichoke), a tuber, in which inulin coexists with other organic and inorganic species. The aim was to acquire a complete picture of the chemical characteristics and reactivity of fructose and its polymers (fructans). In fact, fructans constitute the reserve carbohydrates of several botanical species and are important substrates for obtaining high value-added products. EGA-MS of inulin and topinambour provided information on their different thermal and chemical complexities. Despite tuber being constituted mostly by inulin, its thermogram was much more complex than obtained for inulin alone. The EGA curve of topinambour extended for a wider temperature range and provided mass spectra containing several peaks related to the fragmentation of compounds different from those obtained in the analysis of inulin. The evolution of levoglucosan clearly indicates the presence of glucose units in the tuber. The pyrolysis of fructose and inulin carried out by Py-GC/MS generated a high number of pyrolysis products, the main ones being dihydroxyacetone, 5-hydroxymethyl-2-furaldehyde, and 2,6-anhydrofructofuranose. The similarity between the two pyrograms suggests that under pyrolysis conditions, one of the first reactions of inulin is the cleavage of the glycosidic bond. An important difference was obtained in the pyrolysis of inulin alone, which led to the formation of di-fructose dianhydrides. A different quali-quantitative distribution of the pyrolysis products was obtained for topinambour likely due to the different monosaccharides in the topinambour composition from fructose, as well as to the presence of significant amounts of inorganics.