Non-structural carbohydrates in processed soft fried onion ( Allium cepa L.)

Abstract

The effects of curing, removal of top and root, peeling, storage of raw and peeled onion on the content of non-structural carbohydrates and the ability of onion to develop a brown colour were studied. Glucose, fructose, sucrose, 1-kestose, neokestose, nystose, 1F-β-d-fructofuranosylnystose and four unknown oligofructosides were quantified using high performance anion exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD). Field drying affected the concentration of fructose only. Glucose, fructose and an unknown fructan decreased in concentration while the concentration of, e.g. 1-kestose, neokestose and nystose increased by removal of root and top of the onions when stored at 15 °C for one week. Peeling had minor influence on changes in non-structural carbohydrates. Storage of non-peeled (raw) onions at 5 °C and storage of mechanically damaged and peeled onion at 20 °C promoted fructan hydrolysis and browning of soft fried onion. Similar changes were produced by long-term storage of cultivars of common onion. Dry matter and fructose increased during storage of peeled onions at both 5 and 20 °C. The increase in fructose content during storage was due to a significantly decrease in the content of 1-kestose, neokestose, 1F-β-d-fructofuranosylnystose, an unknown fructan and total fructans. It was concluded that the missing ability of industrially processed onions to develop a brown colour may be overcome by storage of the peeled onions for a few hours or even overnight at 5 °C.