Isolation and Structural Analysis In Vivo of Newly Synthesized Fructooligosaccharides in Onion Bulbs Tissues (Allium cepa L.) during Storage

Fructooligosaccharides are involved in physiological activities and quality attributes of onion bulbs. This work describes structures of newly synthesized oligosaccharides formed by fructose moieties in onion bulb tissues during storage. Onion bulbs were stored for four weeks at 10C°. HPAEC-PAD analysis showed that saccharide 1 was eluted after 1-kestose while saccharide 2 was eluted after nystose (4a). Saccharides 1 and 2 have R-sucrose values of 1.55 and 2.15 by HPAEC, a reducing terminal, a reducing sugar-to-fructose ratio of 0.5 and 0.3, and a degree of polymerization of 2 and 3 by TOF-MS, respectively. GLC analysis of the methyl derivatives and NMR measurement of the saccharides confirmed the presence of two different structures: the structure of saccharide 1 is composed by two fructose moieties and linked by β(2→1) linkage and was identified as inulobiose [β-D-fructofuranosyl-(2→1)-β-D-fructopyranose]. The structure of saccharide 2 consists of three units of fructose linked by β(2→1) linkage and was identified as inulotriose [β-D-fructofuranosyl-(2→1)-β-D-fructofuranosyl-(2→1)-β-D-fructopyranose]. The spectra also showed that 70 to 80% of the terminal fructose residue of the two saccharides is of pyranosyl form, while 20 to 30% is of furanosyl form. This finding demonstrated that these newly produced saccharides, catalyzed by onion-purified 6G-FFT, were synthesized by the action of 1-FFT fructosyltransfer from 1-kestose to free fructopyranose yielding inulobiose and sucrose, while elongation of fructofuranosyl units occurs at this transferred fructofuranosyl residue to produce inulooligosaccharide having an additional unit of fructofuranose.