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Structural and chemical differences in the cell wall regions in relation to scale firmness of three onion (Allium cepa L.) selections at harvest and during storage

Coolong, Timothy W., Randle, William M., Wicker, Louise
Journal of the science of food and agriculture 2008 v.88 no.7 pp. 1277-1286
onions, cultivars, lines, storage quality, food storage, storage time, firmness, food composition, carbohydrate content, polygalacturonase, pectinesterase, enzyme activity, uronic acids, cell walls, pectins
BACKGROUND: Firmness in vegetables is an important textural attribute affecting consumer attitudes toward freshness and quality. Firmness, structural carbohydrates, polygalacturonase (PG), and pectin methylesterase (PME) activity were measured in three onion (Allium cepa L.) lines at harvest and after 4, 8, and 12 weeks of storage.RESULTS: The high-dry-matter onion, MBL87-WOPL, had the firmest bulbs at harvest and delayed softening during storage. MBL87-WOPL had the thickest cell wall/middle lamella region, and highest levels of dry matter and total uronic acid. Furthermore, MBL87-WOPL had the lowest levels of PG and PME activity during storage. Pegasus, a poor-storing cultivar, had the softest bulbs at harvest, lowest levels of uronic acid, and thinnest cell wall/middle lamella. A good storing, moderately firm onion cultivar (MSU4535B) presented intermediate levels of firmness and total uronic acid content. Differences in uronic acid in water-soluble pectin accounted for much of the difference in total uronic acid among lines. Cellulose concentrations were similar among all lines at harvest. In addition, cellulose concentrations decreased in all lines during storage. Transmission electron microscopy performed on bulbs at harvest and after 12 weeks of storage indicated that degradation of the middle lamella had occurred during storage, leading to cell separation.CONCLUSION: Our results suggest that differences in onion scale firmness at harvest may be due to differences in water-soluble pectin uronic acid concentrations. Furthermore, the rate of bulb softening during storage at 6.6 °C was greater in onion lines with higher levels of PME and PG activity in storage.