138 Produce Degradation: Reaction Pathways and their Prevention
LAB counts did not markedly differ from one pack to another, and Carlin et al. [3]
demonstrated that the growth in both LAB and yeast was faster when the CO 2 content
within the packs increased above 20% and O 2 decreased below 1%. These authors
[3] claimed that microorganisms were not the primary cause of spoilage because
the growth of L. mesenteroides on a sterile medium was not affected by either O 2
or CO 2 concentration in the range from 1 to 10% O 2 and from 1 to 40% CO 2.
Storage of shredded carrots in CO 2 -enriched and O 2 -deprived atmosphere
resulted in high K+ leakage [3], and Barry-Ryan et al. [90] demonstrated that the
main factor in the switch to anaerobic catabolism of MAP shredded carrots is O 2
depletion. It may be postulated that both CO 2 accumulation and O 2 depletion par-
ticipate synergistically in physiological disorder. Other electrolytes and nutrients,
including sugars, were also expelled from the carrot tissues. This exudate provided
microorganisms with a good growth substrate. Modified atmosphere containing less
than 20% CO 2 and more than 2% O 2 prevented physiological disorder and, as a
consequence, microbial spoilage. Conversely, very permeable films favored high
respiration and induced a fast consumption of sugars, which caused a noticeable
loss in palatability of the shredded carrots. It should be noted that the MA passively
generated in packs is highly dependent on the storage temperature because the Q 10
of plant tissue respiration rate is higher than that of the gas diffusion rate through
a polymeric film. At low temperature (from 1 to 3°C) physiological activity and
bacterial growth are sufficiently reduced to delay the spoilage for over a week even
with the least permeable film (OPP, oriented polypropylene, from 30 to 40 μm in
thickness) used to pack fresh-cut carrots in Europe.
5.2.2.5.3 Color
Color changes during the storage and distribution of fresh fruits and vegetables result
from many mechanisms: yellowing, or conversely greening, of green tissues, enzy-
matic browning, oxidation of carotenoids and flavonoids, etc.
5.2.2.5.3.1 Chlorophylls
The chlorophyll molecule consists of a magnesium-chelated tetrapyrrole with a fat-
soluble “tail” (phytol). The change from bright green to a yellowish color is due to
the replacement of the magnesium atom by hydrogen to form pheophytin [127].
Chlorophyll in plant tissue is protected from acidic cytoplasm by its linkage with
the protein. This reaction (pheophytinization) is responsible for the degreening of
broccoli during storage [34]. Piagentini et al. [128] found that the type of packaging
film (OPP and LDPE) did not affect chlorophyll retention in fresh-cut spinach.
Destruction of chlorophyll by exposure to ethylene has been reported and was
correlated with an increase in chlorophyllase activity [129]. The chlorophyll changes
may result from the loss of membrane integrity that occurs with senescence hastened
by ethylene [130]. Other degradative pathways may participate in chlorophyll break-
down, such as chlorophyll oxidase, lipase, and lipoxidase. The results reported by
Watada et al. [120] suggest that the chlorophyll degradation mechanism probably
differs among plant species (more information is given in Chapter 8). Degreening
of plant tissues can be reduced by lowering ethylene production and keeping the
cell membranes in good physiological condition. Wang et al. [131] found that the
storage of green asparagus at 1°C in CA with 5% CO 2 and high relative humidity