Physicochemical characteristics of onion (Allium cepa L.) tissues

The structure and mechanical properties of onions are important factors aff ecting their textural quality. The onion bulb consists of several layers of pigmented, papery scales surrounding fleshy storage scales that comprise a n upper epidermis, an intermediate parenchyma tissue, and a lower epidermis . The purpose of this study was to examine the chemical composition of cell walls from the papery scales and outer fleshy scales of onion (Allium cepa L. cv. Sturon) in relation to their mechanical properties. Cell-wall mater ial (CWM) was prepared from the component tissues and analyzed for its carb ohydrate and phenolic composition. The CWMs were rich in uronic acid and gl ucose, with smaller quantities of arabinose, galactose, and xylose. In the fleshy scales, the lower epidermis contained relatively more galactose-rich pectic polysaccharides, whereas the upper epidermis and the papery scales contained virtually no galactose. Analysis of mechanical properties showed that the order of strength of the tissues was papery scales > fleshy scales , which were in the order lower epidermis > upper epidermis > intermediate parenchyma. The upper epidermis of fleshy scales was stronger in the vertic al than the horizontal direction, and both orientations showed negligible n otch sensitivity. Cyclohexane-trans-1,2-diaminetetraacetate-induced vortex- induced cell separation of the intermediate layer of fleshy scales indicate d that calcium cross-linking may play an important role in cell-cell adhesi on. A small but significant amount of ferulic acid was found in the walls, predominantly in the thick cuticle of the lower epidermis of fleshy scales. Alkali-labile wall-bound flavonoids were also detected.