Packaging and Produce Degradation 135
and Skog [110]. They owed this enhancement to increased cell-to-cell adhesion,
which increased by 60% after CO 2 treatment. The changes in the pH of the apoplast
and consequent precipitation of soluble pectins were thought to be responsible for
the gain in cell-to-cell adhesion. However, whether the pH of the apoplast was
increased or decreased due to CO 2 treatment was not clearly stated. A decrease in
the resistance of the apoplast was observed and was supposed to be due to the
accumulation of H+ and HCO 3 – ions. Holcroft and Kader [111] postulated that
changes in the intracellular pH of produce treated with elevated CO 2 atmospheres
depend on the size and pH of the vacuole and concentration of weak organic acids
that affect the buffering capacity of the cells. They observed an increase in intrac-
ellular pH of strawberries, which have large and acidic vacuoles, with unchanged
firmness. Yong and Soon [112] reported a similar increase in pH for fig fruit accom-
panied by enhancement in firmness. On the other hand, the pH of cut lettuce tissues,
which do not accumulate high concentrations of organic weak acids, decreased after
treatments of elevated CO 2 [113]. Wszelaki and Mitcham [96] and Perez and Sanz
[114] have reported similar beneficial effects of high-oxygen atmospheres on firm-
ness of strawberries. However, the mode of action of high-oxygen atmospheres on
firmness retention of strawberries is not well understood.
Gas composition in the storage atmosphere may have an indirect effect on
firmness. For example, the fungistatic effect of CO 2 on models prevents or delays
the rot of plant tissue. In some cases the indirect effect of gas composition on decay
is more complex.
Pectinolytic Pseudomonas fluorescens and Pseudomonas viridiflava are well
known as soft rot bacteria on stored vegetables [115]. They may also induce spoilage
of shredded endive leaves [116]. Strains of pectinolytic Pseudomonas marginalis
isolated from fresh-cut broad-leafed endives show a strong spoilage capacity on the
commodity though these bacteria are present in both spoiled and apparently sound
packs [80]. CA enriched in CO 2 up to 50% reduces the in vitro growth of
pseudomonads and Erwinia spp. Surprisingly, the same atmosphere does not modify
epiphytic proliferation of Pseudomonas marginalis in green salad leaves, but CA or
MA containing over 15% CO 2 reduces or eliminates soft rot on endive leaves that
were previously inoculated with heavily concentrated Pseudomonas marginalis sus-
pension. The same phenomenon is observed when leaves are inoculated with a sterile
(ultrafiltered) growth medium of this bacterium. The presence, in the ultrafiltrate,
of active pectinolytic enzyme can account for the spoilage. Increasing CO 2 to 20%
reduces the necrosis, and at 40% it prevents any damage. It is remarkable that MA,
with high CO 2 concentrations up to 40%, has no effect on the soft rot induced by
Aspergillus niger growth medium. Because P. marginalis produces pectate lyases
[117] with an optimum pH of 6 to 8, and Aspergillus niger a polygalacturonase active
at pH 4 to 5, it is postulated that the beneficial effects of CO 2 on soft rot induced by
P. marginalis are due to the acidification of cell medium by dissolved CO 2.
5.2.2.5.2 Flavor
At least three mechanisms are responsible for off-flavors in MAP fruits and vege-
tables: (1) enzymatic action, (2) a switch to anaerobic catabolism, and (3) microbial
spoilage.