Temperature Effects on Produce Degradation 633
become mushy upon thawing. Once the tissue is frozen, the interchange of metab-
olites among the various cellular components is seriously hampered.^2
Ice formation may be extracellular or intracellular.^7 The first ice crystals form
in moisture on the surface of the cell wall. The crystals then grow in the intercellular
space as water migrates through the cell wall in response to a vapor pressure gradient.
As the ice crystals grow, the cells are progressively dehydrated and shrink, or in
some cases, the protoplast shrinks away from the cell wall and ice may then form
on the inside of the wall. This ice formation and desiccation are not lethal to the
material until the ice formation involves up to approximately three-fourths of the
water originally present in the cell. This dehydration progressively increases the
concentration of the cell solution, lowering its freezing point and providing protec-
tion against intracellular freezing.
The process of extracellular freezing is possible only in material with large
enough intercellular space to allow ice crystals to form from a considerable propor-
tion of water in the tissue.^7 In tissue with small intercellular spaces, the extent of
extracellular freezing is slight before intracellular freezing becomes prominent.
If there is no membrane damage, material with extracellular freezing can thaw
without showing freezing injury.^7 However, if membrane damage has occurred as a
result of the dehydration associated with extracellular freezing, affected protoplasts
eventually disintegrate and die.
If material in which extracellular ice formation has occurred remains at a tem-
perature below the freezing point of the sap, the ice formation spreads into the
cytoplasm and vacuolar sap.^7 This freezing disrupts the cytoplasm and nucleus and
is always fatal. On thawing, the frozen material collapses and exudes liquid. Intra-
cellular freezing may occur without prior extracellular freezing if ice formation is
initiated in material exposed to extreme supercooling.
20.4.3 HIGH TEMPERATURES
Produce exposure to elevated temperatures may occur for a variety of reasons,
including exposure to direct sunlight, hot air in the field, or heat treatments used for
FIGURE 20.15The water-soaked appearance seen on the lower celery stalks indicates freez-
ing injury. (Photo courtesy of A.A. Kader, University of California, Davis.)