Anoxic and other treatments

One of the solutions used to decrease post-harvest losses is controlled atmosphere storage; certain crops are stored under different conditions (low oxygen concentration, low temperature, high CO2 concentration, etc.) to slow down metabolic processes like ripening. With this in view, it is important to monitor the

Respiration tca,o2

C6H12O6 C3H4O3 \

acetaldehyde ethanol

Ethanolic fermentation Fig. 12.9 Respiration and ethanolic fermentation processes.

C2H5OH

metabolic responses of harvested crops under these conditions in order to develop new storage systems.

One of the processes under study is the balance between respiration and alcoholic fermentation, which is dependent on oxygen concentration condi-tions.71,141,142 Under normal aerobic conditions (21% O2), plants produce energy through respiration by oxidation of pyruvate, through the tricarboxylic acid cycle (TCA) and oxidative phosphorylation in presence of the atmospheric O2, to CO2 and water. Under anoxic conditions, the plant has to produce ATP (adenosine triphosphate) with no consumption of O2 by different fermentation processes; this phenomenon is very common in nature, for example under flooding or ice-encasement conditions, and plants have developed different fermentation pathways that play an important role in survival during long periods of anoxia. One of the most common fermentation pathways is ethanolic fermentation, in which pyruvate is first decarboxylated to acetaldehyde by pyruvate decarboxylase (PDC) and this is then quickly reduced to ethanol by the action of alcohol dehydrogenase (ADH). Both processes are schematically shown in Fig. 12.9.

As the balance between the two processes depends on the oxygen concentration, study of the parameters determining the respiration to fermentation ratio is very important in order to optimise crop storage at low O2 concentration. For this reason, the emission and effects of ethanol and acetaldehyde under anoxic and/or hypoxic conditions have been of outstanding interest.143145

Acetaldehyde is, in general, toxic to plant cells owing to its high reactivity. Several investigations have been conducted on its effects on the ripening processes by the direct exogenous application of acetaldehyde and/or ethanol.146 In tomato, low acetaldehyde concentration has been shown to inhibit ripening but the results are dependent on the initial fruit maturity, the applied concentration and the duration of exposure; in contrast, acetaldehyde accelerated senescence in pears and blueberries.

One important factor to have in mind when storing crops in low oxygen controlled atmosphere is the restoration rate of the aerobic conditions. After a period of oxygen deprivation, re-exposure to air can cause important damage to plant tissues, in some cases being more detrimental than the lack of oxygen itself.147 The causal agent of this post-anoxic injury in plant tissues is acetaldehyde. During anoxia, the plant obtains its energy through alcoholic fermentation and consequently ethanol is being accumulated in the tissues. When re-exposed to oxygen, the ethanol is oxidised to acetaldehyde, which is believed to be responsible for this post-anoxia injury. Acetaldehyde emission takes place a few minutes after the recuperation of normoxic conditions, as has been demonstrated for red peppers,19 a clear indication that the ethanol oxidation is due to rapidly formed active oxygen species (AOS) like hydrogen peroxide.148,149 It has been shown that gradually restoring normoxic conditions could reduce the adverse effects of re-aeration. This has been proved, for example, in red bell pepper by measuring the post-anoxic upsurge in acetaldehyde emission as a function of the restoration rate of the O2 concentration,60 as can be seen in Fig. 12.10.

Figure 12.10(a) shows the acetaldehyde emission of a red bell pepper under anoxic conditions; the onset of the fermentation is clearly noticeable by the plateau about 3.5 hour after the insertion of the fruit into the anoxic environment. As indicated above, switching directly to a normoxic atmosphere leads to a sudden rise in the acetaldehyde concentration; in this example an attempt to suppress this acetaldehyde upsurge was made by a post-anoxic addition of only 0.4% O2, but even this low O2 concentration yielded to a high release of acetaldehyde for about 20min, as is clearly shown in Fig. 12.10(a). In subsequent experiments, (Fig. 12.10b) lower O2 concentrations were used in the re-aeration of the red bell pepper sample. After about 9 hour under anoxic conditions, 0.05% O2 was introduced into the cuvete, leading to only 20% increase in the acetaldehyde emission in 1 hour; from this point gradually increasing the O2 content produced a smooth decrease in the acetaldehyde production.

This finding opens the way to subsequent investigations into optimising the conditions under which normoxic atmosphere is restored in low oxygen CA storage facilities, aiming to suppress the acetaldehyde upsurge and, consequently, pos-anoxic injury in fruits and vegetables. In later investigations by the same group, the acetaldehyde upsurge could not be prevented in avocado fruits by slowly restoring normoxic conditions after anoxia.150 Obviously, further studies are still necessary to devise general methods to suppress post-anoxic injury in plants.

Anoxic treatments have been also investigated in the authors' laboratory aiming to elicit phytoalexin trans-resveratrol in post-harvested grapes. The results obtained showed an increase in the resveratrol content with treatments up to 24 hour, but the time course of evolution shows that high resveratrol content is better maintained after short anoxic treatments (i.e. 6 hour).

Finally, a development in the effects of inducing stress in fruits (namely grapes) uses the UV irradiation on the fruit.151-155 A good example of this method is that developed by Cantos et al.156 in which an 11-fold enhancement of the resveratrol content was achieved 3 days after a very short (30 s) irradiation of the grapes at l = 534 nm, leaving the main sensory characteristics of the fruits unchanged. On the other hand, it has been shown that UV irradiation can reduce

Time (hour)

Fig. 12.10 (a) Acetaldehyde release in a red bell pepper under different oxygen conditions. The post-anoxic acetaldehyde upsurge caused the introduction of only 0.4% O2 is clearly noticeable. (b) Suppression of the post-anoxic acetaldehyde upsurge in a red bell pepper by the gradual restoration of the oxygen concentration (adapted from

Oomens et al.60).

Time (hour)

Fig. 12.10 (a) Acetaldehyde release in a red bell pepper under different oxygen conditions. The post-anoxic acetaldehyde upsurge caused the introduction of only 0.4% O2 is clearly noticeable. (b) Suppression of the post-anoxic acetaldehyde upsurge in a red bell pepper by the gradual restoration of the oxygen concentration (adapted from

Oomens et al.60).

post-harvest decay of table grapes.157,158 The latter studies only investigated the time evolution of the damaged grapes and no chemical analysis was performed. However, the correlation between the enhancement of the natural resistance of the grapes observed and the elicitation of the resveratrol content is clearly demon-

strated in references 151-156. An example of this correlation is shown in the next section.

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