Physical and chemical properties of onions and onionskins

Bulb firmness may be partly related to the adhesion of cell-wall fibrils to one another within the fleshy scales, due to the presence of non-uronide carbohydrates and the strength of the middle lamella (Mann et al., 1986). Changes in carbohydrate metabolism or damage during storage may have bad effects on firmness and onion quality.

Ha et al. (1997) used nuclear magnetic resonance techniques to establish that, in dry onion cell walls, cellulose/xyloglucan microfibrils acted as solid rods while dry pectins were in a glassy state. On hydration, the pectins became gel-like but the microfib-rils continued to provide rigidity. Lancaster et al. (2001) studied onion firmness in relation to S nutrition and found that lack of adequate S gave smaller and softer onions with a smaller proportion of DM in the cellwall material. The authors deduced that the S composition of the cellular components (including ACSOs) is maintained at the expense of bulb growth.

The mechanical properties of onion skins in relation to humidity were studied by Hole et al. (2000). They discovered that, following exposure to air at 95% RH, the damp skins were much more elastic in several directions and consequently could resist stretching better than very dry skins, which were brittle. Therefore, controlled RH of the air during post-storage conditioning can be manipulated to slightly dampen the outer skins, with a target of 75% RH to enhance skin retention (Brice, 1994).

Considerable research on onion physico-chemical properties is being done at Norwich, UK (Ng et al., 1998, 2000), in connection with possible uses for skins from brown onions. Cell-wall materials from skins of cv. 'Sturon' included several phenolic compounds, such as protocatechuic acid (the most abundant), vanillic acid and ^-hydroxy-benzoic acid. In the outer epidermis of adjacent fleshy scales, the most abundant phenolics were trans-ferulic, trans-coumaric and vanillic acids. Flavonoids were also present and it is postulated that these may be involved in peroxidative cross-linking in the cell walls of dry onion skins.

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