Agriculture Reference
In-Depth Information
chlorophyllase activity was found to either increase
(Rodriguez
et al
. 1987) or decrease (Majumdar
et al
.
1991) during the senescence of leaves and ripening of
fruit. This suggests that chlorophyllase may or may not be
responsible for chlorophyll degradation. However, it
seems likely that chlorophyllase is important for chloro-
phyll degradation in prickly pear cactus stems.
Microbial population started to increase after 15-20
days of storage (Figure 13.10, Guevara
et al
. 2003). The
least increase in total aerobic mesophiles (AeM) popula-
tion was in cladodes that were kept in semi-active MAP
with an initial of 20% CO
2
, followed by those maintained
in semi-active MAP with an initial concentration of 40%
CO
2
, and the highest increase was in cladodes maintained
in semi-active MAP with an initial of 80% CO
2
, and in
cladodes that were maintained without packaging. AeM
population reached up to 1.9 × 10
5
CFU g
¯
1
after 35 days in
storage. Total anaerobic mesophiles (AnM) were highest in
cladodes that were held in semi-active MAP with an initial
of 80% CO
2
and least in those held without packaging and
in those held in semi-active MAP with an initial of 20%
CO
2
. The highest AnM count was 2.0 × 10
5
CFU g
¯
1
.
The highest increase in mould and yeast (MY) was found
in cladodes that were held in semi-active MAP with an
initial of 40% CO
2
, followed by those maintained without
packaging, while the least MY population was in cladodes
maintained in semi-active MAP with initial concentrations
of 20% and 80% CO
2
.
The highest MY population reached up to 5 × 10
4
CFU
g
¯
1
. Isolation of single colonies was performed by randomly
selecting the square root of the total number of colonies
counted on each plate. On the basis of the macro- and
microscopic characteristics of AeM bacteria isolated and
the biochemical test done, identified bacteria were from
the genus
Leuconostoc
,
Pseudomonas
,
Micrococcus
and
Bacillus
. The AnM were identified in the same way as the
AeM, and the bacteria found were of the
Ruminicoccus
genus. In the case of MY the fungi identified were
Absidia
,
Cladosporium
and
Penicillium
and the yeast
Pichia
, which
produces necrosis of tissue.
(a)
25
20
15
10
5
0
(b)
8
6
4
2
0
0
14
21
25
35
7
Days sat 5°C
Figure 13.8
Changes in weight loss (%) and overall
quality scores in passive and semi-active
MA-packaged prickly pear cactus stems stored at
5
C for up to 35 days. Nonpackaged ( ), passive
MAP ( ), 20% CO
2
( ), 40% CO
2
( ) and
80% CO
2
( ). Vertical bars represent standard
error of the mean (Guevara
et al
. 2003, with
permission).
°
Chlorophyllase activity was lowest in cladodes main-
tained in MAP (Figure 13.9d; Guevara
et al
. 2003). The
lowest activity was in cladodes packaged in passive MAP
and in semi-active MAP with 20% CO
2
, and the highest
activity was found in cladodes packaged in semi-active
MAP with an initial concentration of 40% and 80% CO
2
,
and in cladodes kept without packaging. The highest
chlorophyllase activity present in treatments with elevated
CO
2
concentrations (semi-active MAP with initial 40%
and 80%) is probably due to the stress caused by CO
2
injury. There was a clear relation between chlorophyll
degradation and chlorophyllase activity. Conditions that
show the highest chlorophyll degradation also had the
highest chlorophyllase activity. This is in disagreement
with Baardseth and Von Elbe (1989) who concluded
that gradual chlorophyll disappearance in spinach did
not coincide with the activation of chlorophyllase. The
Summary of findings on MA packaging
It is possible to extend the shelf life of prickly pear cactus
stems by MA by generating an atmosphere with O
2
levels
of up to 8% and CO
2
levels of up to 7% in passive MAP, or
up to 20% in semi-active MAP. CO
2
concentrations
between 7% and 20% decreased the loss in colour, firm-
ness and fibre content, and reduced chlorophyllase activity,
and microbial flora load on the stems. These benefits are
due to atmospheric modification and not to increased
