Study of total fructan and fructooligosaccharide content in different onion tissues

The objective of this work was to determine the fructan and fructooligosaccharide (FOS) content of different onion tissues in order to evaluate the potential use of onion by‐products from the food industry as a source of FOS and fructans. Assays with two methods were carried out to optimise the extraction procedure. The main FOS, namely kestose (GF₂), nystose (GF₃) and fructofuranosylnystose (GF₄), were measured directly using standard sugars. The method for total fructans was based on enzymatic treatment (Novozym 230) of ethanolic/aqueous extract followed by determination of released fructose and glucose by HPLC. Data showed a clear predominance of GF₂ in every onion tissue and no occurrence of highly polymerised fructans. The tissues richest in fructans were the fleshy layers, so that the outer two fleshy layers turn out to be the best onion by‐product as a possible fructan source. © 2000 Society of Chemical Industry