Agriculture Reference
In-Depth Information
a process which normally causes deterioration at the time
of sprout growth initiation. So although MH does not
directly affect the disease loading of the bulbs, they tend to
remain healthy for longer than usual: when bulbs age nor-
mally, the outer scales rapidly become senescent when
sprouts start to grow and hence become susceptible to
pathogens. Kubilius and Bushway (1998) recently devel-
oped a new, sensitive method for detecting MH in onion
tissues capable of detecting levels as low as 2 μl l
−1
.
at 18°C delayed 50% initial sprouting by 1 or 2 weeks
respectively although the mechanism by which ethylene
inhibits sprout growth in onion is still unknown (Bufler
2008). Benefits of ethylene treatment include the elimina-
tion of detectable residues caused by MH, the use of
low concentrations (10 μL L
−1
) which pose no hazards to
workers and the potential integration of ethylene treatment
with controlled atmosphere, low- or high-temperature stor-
age (Johnson 2006).
The ethylene binding inhibitor 1-methylcyclopropene
(1-MCP) is approved for use on a range of produce in
several countries (Watkins 2006). Sprout growth inhibition
has previously been shown in onions cv. SS1 treated after
curing with 1 μl l
−1
1-MCP as a single 24 h treatment as
compared to the control when stored at 4°C and 12°C
(Chope
et al
. 2007c). However, when stored at 20°C,
Chope
et al
. (2007c) found treated onions cv. SS1 had
longer sprouts than the control although this was not
significant. This agrees with Bufler (2008) who found
onions cv. Copra sprouted earlier when treated with 0.25 μl
l
−1
1-MCP for 5 h (20°C) then stored at 18°C suggesting
sprout suppression using 1-MCP may only be effective
prior to low-temperature storage.
Irradiation
Another means of slowing bulb degeneration is to use
controlled gamma irradiation prior to storage. The World
Health Organization permits gamma radiation up to a
strength of 0.15 kGy to preserve onions and garlic
(Kobayashi
et al
. 1994). This technique has repeatedly been
shown to reduce bulb storage losses and is being adopted
in preference to MH in some countries, such as Algeria
(Benkeblia 2000). Toxicological tests have concluded that
irradiation of onion bulbs is not detrimental to eating qual-
ity or harmful to humans (Brewster 1994). Irradiation can
be used in conjunction with other methods of maintaining
quality such as controlled temperature or atmosphere
conditions. Irradiation is expensive and is therefore often
only used for processes such as sterilisation when there are
no alternative methods (Andrews
et al
. 1998). Irradiated
food is also not acceptable to consumers in some countries;
however, there seem to be few reasons for refusing food
treated in this way. Certainly it has been shown to be effec-
tive in assisting onion storage over long periods, and may
have the advantage of killing off certain pathogens and pests
(mites) which can attack the stored crop (Ignatowicz 1998).
Irradiation is also very effective in extending the life of
stored garlic (reviewed by Iglesias & Fraga 2000).
Scientists in Argentina have developed methods to eluci-
date whether garlic has been irradiated or treated with MH,
using the appearance of the bulbs at 120 days after harvest
in order to distinguish which of the treatments was applied
(Pellegrini
et al
. 2000). Onions and garlic from Chile,
imported into Cuba, were irradiated before transport and
this allowed them to retain quality substantially better than
the nontreated bulbs, even after six months of storage at the
destination (Iglesias
et al
. 2001).
Controlled atmosphere
For a long time, studies of changed atmospheric concentra-
tions of oxygen and carbon dioxide on onions were only of
theoretical interest (Chawan & Pflug 1968; Adamicki &
Kepka 1974). Studies on the effects of changing the gas
concentrations of O
2
and CO
2
during storage began to be
translated into practice in the past 15 years, first to extend
the marketing season of very sweet onions in the United
States (Smittle 1988) and more recently to try to find
methods of extending long-storage of onions (Adamicki &
Saltveit 1997; O'Connor 2002). However, recent work by
Chope
et al
. (2007a) suggested that removal from CA
storage (5 kPa CO
2
, 3 kPa O
2
) caused a stress-related
increase in respiration rate which in some short-storing,
mild cultivars such as cv. SS1 could be sufficient to initiate
sprouting, and thus reduce shelf-life. The usual recommen-
dation for onions is to reduce oxygen to 3% and allow CO
2
to rise to 5% (higher levels may be damaging) and to store
at near zero (1-2°C) temperatures (Smittle 1988). Stores
are fitted with sensors to control temperature, RH and gas
levels, and the gas mixture is circulated through the onions.
In some seasons, losses may be experienced in store
because the onions in the fields were already infected by
bacterial or fungal diseases which can continue to grow at
low temperatures. Table 17.7 summarises studies on the
effect of CA on storage and shelf life.
Ethylene
Recently, systems have been introduced which produce a
continuous supply of ethylene to suppress sprouting of
stored onions and potatoes (10 μL L
−1
and 4 μL L
−1
respec-
tively) reviewed by Chope and Terry (2008). Continuous
ethylene treatment (10.6 μl L
−1
) for 4 or 9 months storage
