CROP STORAGE AND DORMANCY - Onions and other Vegetable Alliums - page 324

Cell wall material extracted from whole onion bulbs consisted of microfibrils
of cellulose and xyloglucan in a crystalline state embedded in a matrix of pectin
that was glassy when dry and which swelled to become gel-like when hydrated.
The microfibrils behave like solid rods, but they are probably cross-linked by
xyloglucan groups (Ha et al. , 1997). The tensile strength of the dry skins is much
greater than the underlying fleshy scales, and this is thought to result from high
levels of calcium cross-linked with pectic carbohydrates (Ng et al. , 2000). The dry
skins have a much higher ratio of uronic acid-type carbohydrates to neutral
sugars (galactose plus arabinose) than fleshy scales. The lack of long galactose
side-chains probably increases the degree of calcium cross-linking between
uronic acid groups in the pectin molecules, giving greater tensile strength.

WATER VAPOUR ABSORPTION AND TRANSMISSION BY ONION SKIN Onion skins can
gain or lose water to their surroundings and tend to equilibrate with the water
vapour pressure in their surroundings. Skin water content depends primarily on
the relative humidity (RH) of the surrounding air and, to a small extent, on
temperature at a given RH. Water contents range from around 4% in equilibrium
with air at 16% RH to 38% at 95% RH, and 67% when in contact with water
(see Fig. 7.3a; Thamizharasi and Narasimham, 1991; de Matos et al. , 1997; Hole
et al. , 2000). Isolated skins reach hygroscopic equilibrium in about 5 days. The
water content of skin in equilibrium with a given RH is higher if water is
evaporating from the wet skin than if a dry skin is absorbing water from the air to
reach equilibrium, and so the water content of an onion skin in equilibrium with

Fig. 7.3. The relationship between the air humidity and onion skin water content,
and also the rate of water loss from onion bulbs. (a) The equilibrium water content
of the external skins of onion bulbs as determined by the relative humidity of the
surrounding air (from de Matos et al ., 1997. Courtesy of Pesquisa Agropecuaria
Brasileira ; the data shown as (

) are from the results of Hole et al ., 2000). (b) The
rate of loss of fresh weight of stored onion bulbs as determined by temperature and
relative humidity of the surrounding still air (from de Matos et al ., 1997. Courtesy of