122 Produce Degradation: Reaction Pathways and their Prevention
(RRO 2 or RRCO 2 ) when temperature increases by 10°C. The example in Figure 5.2
shows RR (RRO 2 or RRCO 2 ) of litchis measured at four temperatures. The Q 10 of
oxygen consumption is equal to 2.07, which is consistent with previous results for
litchis ranging from 2.04 to 2.8. The Q 10 of most plant tissues ranges from 2 to 3
[11] but may exceed 3, as with blueberries [12]. Benkeblia et al. [10] reported that
the Q 10 of onion respiration rates were 1.67 and 1.84 for RRO2 and RRCO2, respec-
tively, and as a consequence a rise in temperature will increase the RQ.
5.2.1.3 Influence of Gas Composition on Respiration Rate and
Physiological DisordersRespiration rates also depend on the O 2 and CO 2 composition of the atmosphere.
The respiration rate of plant tissues is also reduced with decreasing O 2 and increasing
CO 2 concentrations. In fermentative catabolism (anaerobic pathway), ethanol pro-
duction requires decarboxylation of pyruvate to CO 2 without O 2 uptake [8]. A high
respiratory quotient (above 2) is a good indicator of anaerobic catabolism [12]. It
is established that high CO 2 concentrations inhibit several enzymes of the Krebs
cycle, including succinate dehydrogenase [14]. This would inhibit the aerobic path-
way and result in the accumulation of succinic acid, which is toxic to plant tissue
[15]. CO 2 levels as low as 5% may induce physiological disorders in the common
mushroom [16], and asparagus exhibits surface pitting when stored in atmospheres
above 10% CO 2 [17]. The average CO 2 toxicity threshold ranges from 7 to 30%
depending on plant and storage factors. Crisp head lettuce in storage with elevated
CO 2 is strongly affected by O 2 concentration [18]; however, this is not the case for
romaine lettuce [19]. Cultivation conditions such as irrigation, climate, and fertili-
zation can modify plant tissue susceptibility to CO 2 injury. Krahn [20] found that
the outer leaves of crisp head lettuce are not injured by 2% CO 2 but the inner leaves
and midribs show damage. The effect of CO 2 on cell ultrastructures [21] and mem-
branes [22] could account for its toxicity. It can be postulated that CO 2 dissolution,
which enhances acidity in the cell medium, may participate in the physiological
disorder. Optimum concentrations of CO 2 should minimize the respiration rate
without danger of anaerobic metabolism.
Commodities vary widely in their tolerance of different atmospheres [23]. A
classification of fresh fruits and vegetables according to their tolerance to reduced
O 2 and elevated CO 2 has been presented by Kader et al. [24]. According to Zagory
and Kader [25], numerous vegetables such as tomatoes, bell peppers, artichokes,
and cabbages do not tolerate over 2% of CO 2 in their storage atmosphere. Apples,
peaches, peas, cauliflower, eggplants, carrots, and radishes can be stored with up to
5% CO 2. Leeks, asparagus, beans, onions, cucumbers, garlic, potatoes, strawberries,
and raspberries still survive under 10% CO 2. Lastly, some plant tissues such as
cherry and broad-leafed endive are very resistant to this gas (up to 20%). It should
be noted that most fresh-cut plants are less sensitive to CO 2 than their intact coun-
terparts [26].
Figure 5.3 shows some results for broccoli florets (cv. Emperor). The increase
in CO 2 concentration (0, 5, and 12%) inhibits broccoli’s respiration rate. Without
any carbon dioxide (0%) the increase in respiration rate (RRO 2 ) as a function of O 2