Packaging and Produce Degradation 125
from 14% for high O 2 concentration (from 10 to 20%) to 3% for oxygen concen-
tration lower than 7%. The temperature dependence of appKm was reported by
Cameron et al. [12] and Ratti et al. [30]. Song et al. [31] measured the respiration
rate of six blueberry cultivars and reported appKm values ranging from 0.1 to 0.78%
at 15°C and from 0.1 to 5.2% at 25°C, confirming the very large variability in the
determination of appKm with traditional techniques. Based on a dynamic model of
gas exchange of respiring produce, Hertog et al. [28] stated that appKm was inde-
pendent of temperature. They postulated that the observed increase in appKm with
temperature was probably due to interfering phenomena such as the onset of fer-
mentation at high temperature or was within the margin of error of the experiment.
Benkeblia et al. [10], using the respirometer described above, found appKm values
of onion respiration that were reproducible and highly dependent on temperature
(1.6% at 4°C and 6.3% at 20°C).
The effect of CO 2 on respiration rate is quantified by its inhibition constant (Ki
as a percentage or in kilopascals). Ki is the CO 2 concentration or partial pressure in
the surrounding atmosphere that reduces the respiration rate twofold compared with
a control sample without inhibitor. Few Ki values are available in the literature;
nevertheless Peppelenbos and Van’t Leven [32] found 7.68% for apples but much
higher concentrations for tomatoes (268% CO 2 ) and endive (chicon) (181%). It is
obvious that CO 2 , at concentrations usable for fruit and vegetable MAP, will not
have any visible effect on the respiration of the two latter commodities. The calcu-
lation of Ki depends on the type of inhibition that CO 2 displays with the enzymes
involved in the respiration chain. This issue was discussed by Peppelenbos and Van’t
Leven [32], but the determination of competitive, noncompetitive, and uncompetitive
Ki is too complex for practical use.
5.2.1.4 Influence of Other Parameters
The most marked effect of bruising or wounding (fresh-cut processing) plant tissues
is the activation of their catabolism, which includes respiration rate and ethylene
production. The response depends on the magnitude of the stress. The O 2 respiration
rate of shredded endive is only 1.2 times that of the intact organ [33]. This ratio
increases to 1.4 for broccoli florets [34] and to 2 for shredded iceberg lettuce [35].
For more drastic stresses such as grating, respiration reaches 3 to 7 times that of
unaltered tissue in carrot [36].
Other interfering phenomena can change the apparent respiration rate of plants,
the most important of which is duration of exposure [37]. As shown in Figure 5.6a,
the apparent respiration rates of bean sprouts measured in air at various temperatures
from 1 to 20°C immediately after harvesting are consistent with the Arrhenius law.
After a lag period depending on storage temperature (from 7 d at 1°C to immediate
increase at 20°C), the apparent respiration rates of bean sprouts increased from 0.1
to 1.9 mmole·kg–1·h–1 at 1°C in 4 d and from 2 to 8 mmole·kg–1·h–1 at 20°C after 1 d.
Thereafter, their apparent respiration rate stabilized and decreased. This particular
behavior strongly suggested that the increase in apparent RR was mainly due to a
substantial and synchronous (Figure 5.6b) growth of microorganisms and that the
subsequent decrease corresponded to a progressive death of plant tissues and a