Agriculture Reference
In-Depth Information
treated onions was less than 5 mm, whereas non-dipped onions had grown by
an unacceptable 10-20 mm. The heat treatment increased respiration rate
above control levels by about 40% but did not reduce visual quality or shelf-life.
To reduce the microbial population, minimally-processed products like
green onions are washed in chlorinated water, e.g. 50 N mg/l NaOCl (sodium
hypochlorite). Hot water dips also reduced the microbial population and, by
adding sodium hypochlorite to the hot water at rates of up to 200 mg/l, the
plate count of aerobic microorganisms was reduced 200-fold compared with
an unheated wash in hypochlorite (Cantwell
et al.
, 2001).
Microbial contamination can also be reduced by gamma radiation. Green
onions trimmed into 1 cm leaf rings, washed in 100 mg/l sodium hypochlorite
and subjected to gamma radiation at 1 kGy showed a 300-fold reduction in
microbial count immediately after treatment compared with non-irradiated
material and a sixfold reduction after 14 days at 3°C, along with improved
visual quality. Doses of 2 and 3 kGy caused membrane damage and resulted in
loss of aroma and visual quality, although the microbial count was below the
detectable limit throughout 14 days (Fan
et al.
, 2003).
Green
A. fistulosum
washed, trimmed and cut into 10 cm lengths showed a
response of respiration rate to oxygen concentration that could be described by
the Michaelis-Menten equation from enzyme kinetics (Hong and Kim, 2001):
R = V
m
[O
2
]/(K
M
+ [O
2
])
(Eqn 7.1)
where
R = respiration rate
[O
2
] = oxygen concentration of atmosphere
V
m
= maximum respiration rate
K
M
= [O
2
] at half maximal respiration rate (0.5V
m
), the Michaelis constant
K
M
had an average value of 1.67% oxygen.
Maximum respiration rate, V
m
, increased with temperature, approxi-
mately threefold for every 10°C between zero and 20°C. At 10°C it had a mean
value of 30 ml/kg/h of oxygen uptake or carbon dioxide output. Other reports
have given a V
m
of 43 ml/kg/h and a K
M
of 1.24 oxygen for green
A. fistulosum
and a V
m
at 10°C of 49 ml/kg/h and K
M
of 0.46% oxygen for green
Allium
wakegi
(Hong and Kim, 2001).
The relationship between maximum respiration rate and temperature for
green onions followed the Arrhenius equation over the range 0-20°C (Hong
and Kim, 2001). At 0, 10 and 20°C the respiratory quotient (carbon dioxide
output:oxygen uptake) remained unity until oxygen concentration decreased
below 1%, after which it began to increase sharply, indicating a shift from
aerobic to anaerobic respiration. This leads to the accumulation of ethanol and
acetaldehyde, which are toxic to cells and cause off-flavours in fresh produce.
These respiration parameters should be useful for modelling expected
respiration and designing appropriate modified-atmosphere packaging for
fresh-cut green onions (Hong and Kim, 2001).
