Biomedical Engineering Reference
In-Depth Information
lettuce, disorders like rib discoloration and pink rib have been reported.
72
Rib
discoloration occurs on the midrib of the outer head leaves usually where the rib
curves. It was postulated that high temperature favors rib discoloration.
145
Pink rib
is a diffuse pink coloration near the base of the outer leaves of the head. High transit
temperature and low O
2
atmospheres were the causes.
145
Temperature and length of
storage are two interacting external factors that affect the storage life of asparagus.
Lipton
146
reported that temperature of 5°C and above accelerated the lignification
of tracheal tubes and increased the alcohol insoluble solids content, two of the criteria
used to quantify texture. Likewise, temperature of 5°C and above was reported to
promote rapid visual deterioration of asparagus.
146
E
FFECT OF
T
EMPERATURE
S
TRESS ON
Q
UALITY AND
S
HELF
L
IFE
S
TABILITY OF THE
P
RODUCE
Scientists have tried to find a universal mechanism to explain chilling injury from
the varying responses of plants exposed to low temperature. Parkin and co-workers
147
reviewed the following possible mechanisms explaining chilling injury: (a) bulk
membrane lipid phase transitions at low temperature resulting in the formation of
gel phase lipids leading to a loss of membrane integrity and physiological dysfunc-
tion advanced by Lyons;
134
(b) domains of lipids undergoing continual transitions
at critical temperature from liquid crystal to gel phase resulting in membrane damage
as shown in a model by Quinn;
148
(c) low temperature-induced changes in the levels,
kinetic properties, and cold lability (loss of activity) of enzymes;
149
and (d) cellular
redistribution of calcium as a primary transducer of chilling injury.
150
Parkin and
co-workers
147
speculated that lipid peroxidation may have a role in the development
of irreversible injury during low temperature stress, where its effect would be similar
to senescent processes of free radical damage to tissue and progressive membrane
hardening. Membrane properties are altered at low temperature and Inaba and
Crandall
151
reported a correlation between low temperature injury and increased
electrolyte leakage.
The major biochemical event that leads to chilling injury is thought to be the
solidification of membrane lipid microenvironments (lateral phase separations). Sub-
sequently, this affects ion transport regulation and alters the catalytic properties of
membrane-bound enzymes.
152,153
The disruption of membrane function affects cel-
lular metabolism such as diminished regulatory control of cellular energy generation,
membrane semipermeability, and metabolite and ion compartmentation.
147
Plant
tissues subjected to shear during wounding and freezing respond by oxidizing their
lipids.
154
Ethane, a product of lipid oxidation, appears before losses in phospholipids
are noted. Lurie and Klein,
130
however, claimed that lipid peroxidation was not
involved in chilling injury of tomatoes because of the absence of ethane in fruits
that were damaged by chilling injury. One symptom of chilling injury in avocado
fruit is the gray or dark brown discoloration in the mesocarp.
155
According to
Lieberman and co-workers,
156
this discoloration is due to the accumulation of oxi-
dized phenolic compounds. This symptom was observed to be more severe at the
stylar end compared to the pedicel end.
157
Flaccidity and a dull, dark gray-green
aspect of the tips and sometimes a portion below the elongation zone are the
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