Biomedical Engineering Reference
In-Depth Information
can be obtained by replacing the atmosphere of the storage space with the desired
gas mixture. In the first case, the reduction of O
2
to levels of 2 to 3% may require
up to 25 days
1,20
and depends on the characteristics of the commodity and the degree
to which the storage room is airtight or the gas permeability characteristics of the
packagingfilm.
2,21
When a non-biological or active modification system is used, O
2
can be reduced to levels of 2 to 3% within hours. It has been shown, for some
commodities, that a shorter O
2
reduction or pull-down period results in better post-
storage quality.
1,20,22
Rapid O
2
pull-down or rapid CA requires the shortest possible
time for crop harvest, room loading, product cooling, room closure, and O
2
reduction.
Rapid O
2
reduction is obtained by either N
2
flushing or catalytic combustion. In
addition, O
2
absorbers may be used, especially in MA storage. There are a wide
variety of O
2
absorbers, e.g., dithionite, ascorbic acid, alkali containing glucose, H
2
gas in the presence of palladium catalyst, and iron-based products.
23
Most commer-
cially available O
2
absorbers utilize iron oxide (FeO), which becomes iron oxides
(Fe
2
O
3
and Fe
3
O
4
) and hydroxides (Fe(OH)
2
and Fe(OH)
3
) after absorption of O
2
.
9,23
From the reaction mechanism of the O
2
absorber(s) and volume of the storage space,
it is possible to determine the amount of FeO needed to lower the concentration of
O
2
to approximate desired pre-chosen values.
Modified atmosphere packages are dynamic systems where respiration and per-
meation occur simultaneously. In order to achieve and maintain a satisfactory atmo-
sphere within a package, the rates of CO
2
production and O
2
consumption by the
food product must be equal to diffusion rates of the respective gases through
the package at a given temperature.
8,24
Factors affecting respiration of produce in the
package include mass and type of food product, temperature, O
2
and CO
2
partial
pressures, ethylene levels, and light. Factors affecting gas permeability through the
package include type, thickness, and surface area of the packaging film, as well as
temperature, relative humidity, and gradient of CO
2
and O
2
in the sealed package.
2,7,8,25
In intact produce this is a function of genotype, anatomical part, and physiological
state at harvest. Even within a species, the same anatomical tissue can have different
respiration rates depending on the cultivar. Cutting of produce can also influence
TABLE 5.1
Respiration Rates of Selected Commodities
Respiration Rate at 5°C
(mL CO
2
kg
-1
h
-1
)
Commodities
Dried fruits and vegetables, nuts
<5
Apple, grape, potato (mature), onion, garlic
5-10
Apricot, carrot, cabbage, cherry, lettuce, peach, plum, pepper, tomato
10-20
Blackberry, cauliflower, lima bean, raspberry, strawberry
20-40
Brussels sprouts, green onion, snap bean
40-60
Asparagus, broccoli, mushroom, pea, spinach, sweet corn
>60
Adapted from Rizvi.
26
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