Agriculture Reference
In-Depth Information
loss from bulbs. Skins split and slough off if onion bulbs are handled roughly at
harvest or during removal from storage, grading and delivery to market.
There may be between one and three dry skins on an onion depending on
conditions. These are 0.02-0.10 mm thick depending on variety and position
on the bulb (Tanaka
et al.
, 1985c; Tanaka, 1991; Hole
et al.
, 2002). The skins
are thinnest at the widest part of the bulb and on the side of the bulb opposite
to the former leaf blade. The dry skins consist of tightly compact layers of cells
with a shiny outer cuticle.
THE CURING OF BULB SKINS
As bulbs ripen before and after harvest, the drying
of the outer scales is accompanied by biochemical changes that give rise to
dark pigments and antifungal substances in the dry skins and to changes in
structural carbohydrate composition of the cell walls; this is the process of
curing. It is known that temperatures > 21°C and high humidity round the
bulbs during curing promote a dark-coloured skin (Isenberg and Ang, 1963).
In bulk stores it has been found that the depth of colour in onion skins
corresponds to the number of day-degrees above a base of 21°C that the
onions receive during curing (Bleasdale
et al.
, 1970). Warmth might favour
rapidity in the reactions leading to browning, and high humidity may slow
drying out and therefore prolong the period during which both the enzymic
and auto-oxidative reactions described next can occur.
During the cell senescence and drying of onion outer scales as they are
curing, di- and mono-glucosides of quercetin (see Fig. 8.9) are converted to free
quercetin by glucosidase enzymes, particularly in the still moist but drying
areas of the scale. In this zone the level of the enzyme peroxidase is high,
and this can oxidize free, but not glucoside-linked, quercetin to produce
3,4-dihydroxybenzoic acid (protocatechuic acid) and 2,4,6-trihydroxybenzoic
acid in reactions involving hydrogen peroxide (Takahama and Hirota, 2000).
The hydrogen peroxide formed in the senescing cells, coupled with the decrease
in ascorbic acid (vitamin C) that occurs in ageing onion scales, are probably
factors favouring the peroxidase-dependent oxidation of quercetin to phenols
like 3, 4-dihydroxybenzoic acid. In non-senescent tissue, ascorbic acid counter-
acts the oxidizing action of hydrogen peroxide. In the presence of hydrogen
peroxide, the phenolic substances can undergo further autooxidation reactions
to form quinones and hydroquinones, and these can participate in cross-linking
reactions to form dark-coloured polymers, the exact structures of which are not
yet known (Takahama, 2004). This process is similar to lignin formation.
Protocatechuic acid has long been known to confer resistance to the onion
smudge fungus,
Colletotrichum circinans
(see Table 5.7; Link
et al.
, 1929) and to
be present in coloured onion skins but not in white onion skins. White cultivars
are susceptible to smudge because they do not produce quercetin (see Fig. 3.3)
and therefore cannot form protocatechuic acid. In the presence of water the
brown substances in onion skins produce hydrogen peroxide, which is strongly
antimicrobial and probably has some protective value (Takahama
et al.
, 2001).
