Agriculture Reference
In-Depth Information
With time, green leaves senesce and yellow, cut ends of tissues brown, microbial
populations on the tissue increase and nutritional quality - in terms of sugars,
vitamins and flavour - declines. Such changes are common to all fresh, leafy
vegetables. Problems specific to cut alliums include: (i) extension of the roots on
those root-trimmed to leave the stem base intact; (ii) geotropic curvature if the
plants are stacked horizontally; and (iii) if the stem base is trimmed off leaving just
pseudostem tissue, the leaf sheaths may extend after trimming, the inner ones
growing more than the outer to give an unsightly effect known as 'telescoping'
(Cantwell et al. , 2001). Products thus trimmed, or processed to a greater extent -
for example, chopped into leaf 'rings' 1 cm long (Fan et al. , 2003) or shredded
and mixed with other leaves in ready-to-eat mixed salads (Vieira et al. , 2003),
both for retail or sale to food service companies - represent an expanding market
for green onion products.
The time for which such produce can be maintained acceptably fresh can
be extended by storage at low temperatures, high humidity (> 95%) and in an
atmosphere of low oxygen and high carbon dioxide concentration. A range of
packaging films exist which vary in their permeability to oxygen and carbon
dioxide. Within a package that is fairly impermeable to these gases, carbon
dioxide concentration initially increases and oxygen decreases as the product
respires. Depending on both the relative permeability of the packaging film to
oxygen and carbon dioxide and the respiration rate of the product, a modified
atmosphere can be established within the package that prolongs storage life
(Fonseca et al. , 2000). Establishment of the desired atmosphere can be aided by
initially filling the package with the desired gas mixture. Currently, green
alliums are packaged in films resulting in < 1% oxygen and 8-10% carbon
dioxide (Hong et al. , 2000).
The benefits of cold temperatures are well known, and green alliums have a
shelf-life of up to 4 weeks at 0°C (Hruschka, 1974). However, the cleaned
produce frequently has to be kept at 4-5°C, and in these temperatures shelf-life
can be extended by modified atmospheres. Hong et al. (2000) reported that
trimmed green hybrid ( A. cepa
A. fistulosum ) onions had a shelf-life of no more
than 7 days at 10°C, regardless of storage atmosphere. At 5°C an atmosphere of
0.1-0.2% oxygen with 7.5-9.0% carbon dioxide slowed leaf discoloration,
prevented geotropic curvature, prolonged high visual quality and, in onions
with an intact base plate, kept root extension to negligible levels over 21 days.
This treatment reduced, but did not stop, inner leaf extension (telescoping) in
trimmed onions. Very similar results were given by 0.2% oxygen alone, but in
2% oxygen responses were little different to those in normal air. With modified
atmospheres good quality was maintained for more than 2 weeks at 5°C. At 0°C
high visual quality and zero curvature was maintained for 3 weeks, but by then
a low-oxygen atmosphere in addition was more preventive of leaf discoloration.
In a subsequent study, extension growth of cut onions was reduced by
dipping 4 cm of the basal end in hot water at 52.5 or 55°C for 4 or 2 min,
respectively (Cantwell et al. , 2001). After 10 days at 5°C the extension of heat-
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