128 Produce Degradation: Reaction Pathways and their Prevention
limiting the range of a steady-state atmosphere. The system requires ac-
curate knowledge of the respiratory parameters of the commodity to be
stored. The area of the diffusion membrane must be recalculated as a
function of temperature, respiration rate, and weight of the product
[41]. The facility is cheap and does not consume any gas or chemicals
but is difficult to master.- Flushing method
The flushing method is usable only for laboratory facilities. The CA cham-
ber is flushed with the storage atmosphere from an atmosphere genera-
tor connected to N 2 , O 2 , and CO 2 gas bottles. The flow rate to achieve a
determined atmosphere is dependent on the CO 2 production and O 2 de-
pletion rates in the CA room. The system is easy to run but is gas-con-
suming. The flushing method can be improved using a system described
by Varoquaux et al. [13]. In this CA facility designed for research pur-
poses, plant tissues are placed in airtight cabinets in which initial atmo-
spheres are created with a system allowing the injection of gases. The
atmospheres in the cabinets are regularly analyzed and then adjusted to
preset values. The atmosphere in each cell is successively sampled. If
the concentrations of the different gases are ± 0.1 kPa from the preset
values, the computer orders the opening of the valves of mass flow con-
trollers to input the exact amount of N 2 , O 2 , and CO 2 necessary to re-
store the preset composition. The algorithms are based on the dilution
law since the system operates at about 100 kPa. Since the three mass
flow controllers can be opened at the same time, the system saves time
and gas. This equipment allows the storage of fruits and vegetables in a
controlled atmosphere, which can then be analyzed at intervals for their
sensory, microbiological, and physiological qualities. - Scrubbing method
This is the principle of most current commercial CA facilities. The atmo-
sphere is regulated by a CO 2 scrubber of different types (diethanol-
amine, potassium carbonate, adsorption on activated charcoal or
zeolites). Oxygen concentration is equilibrated by injection of air. The
universal respirometer previously detailed is based on this method, but
O 2 depletion is compensated by injection of pure O 2.
5.2.2.2 Effect of Gas on Ethylene Biosynthesis and Action
Maturation mechanisms of climacteric and nonclimacteric [42] fruits and vegetables
have been reported in Chapter 4 of this book. Ethylene (C 2 H 4 ) is produced by all
plant tissues, either intact or stressed [43]. Harvesting may also be considered as an
injury stress [44]. Exposure of climacteric plant organs to exogenic ethylene triggers
maturation and senescence. A reduction in ethylene production and sensitivity asso-
ciated with the composition of storage atmosphere can extend the shelf life of
climacteric commodities. Ethylene production is reduced by either low O 2 or high
CO 2 , or both, and the effects are additive (Figure 5.8). Oxygen is a substrate of
1-aminocyclopropane-1-carboxylate (ACC) oxidase. Marcellin [45] claimed that a