Produce Degradation Pathways and Prevention

Packaging and Produce Degradation 123

content (2, 5, and 21%) is very important, but under 12% CO 2 the effect of oxygen
is no longer noticeable. Respiration is considered to be a succession of enzymatic
reactions of Michaelian type and is characterized by an apparent Km (appKm),
which is the substrate concentration that gives half the maximal rate of the reaction
[27]. The use of the Michaelis equation to model the respiration rate of plant tissue
is not rigorous because plant respiration is a multienzyme, multisubstrate system;
moreover, the determination of appKm also takes into account the diffusion of
oxygen through the epidermis and inner tissues, which does not obey the same
equation. In addition, the substrate concentration to consider should be the one in
the reaction medium and not its concentration in the surrounding atmosphere. The
results reported in Figure 5.3 do not permit even a rough estimate of appKm. Values
of appKm are usually measured by fitting the Michaelis-Menten model to sets of
data describing gas exchange of plant tissues as a function of O 2 concentration.
Considering all possible cumulative errors due to intra- and intervariabilities of
batches of fruits and vegetables, appKm cannot be accurately assessed using this
approach [10]. For this reason the respirometer described previously was slightly
modified in order to permit the direct assessment of O 2 consumption rate as a function
of the residual O 2 partial pressure in the head space, which allows the determination
of the apparent Km. First, the CO 2 was continuously trapped; second, the O 2 injected
to balance the pressure between the two vessels was replaced with pure nitrogen.
Under these conditions, the respiration of the plant tissues causes a progressive
decrease in O 2 content in the vessel under a constant partial pressure of CO 2. The
instrument permits the recording of the O 2 depletion as a function of time (Figure
5.4). This curve is fitted to a polynomial equation, the derivative of which is the

FIGURE 5.3Changes in respiration rate (RRO 2 ) in millimoles per kilogram per hour of
broccoli florets cv. Emperor as a function of percentage of O 2 and CO 2 in the atmosphere
after 7 d of storage at 0°C. (From Zagory, D. and Kader, A.A., Food Technol., 42, 70, 1988.
With permission.)

12

5

0

2

5

21

0.2

0.3

0.4

0.5

0.6

Respiration rate(mmole/kg.h)

Carbon dioxide (%)

Oxygen (%)