638 Produce Degradation: Reaction Pathways and their Prevention
fumigation with sulfur dioxide.^11 The most common fungal infection of grapes,
Botrytis cinerea, grows across nearly the entire temperature range in which grapes
are handled. Growth of this fungus is slowed as the temperature is lowered until it
is almost stopped at the optimum storage temperature. Therefore, it is important that
fruit temperature is lowered promptly.
The turgidity of grapes increases as the temperature is lowered.^57 This turgidity
can result in splitting of the fruit. With prolonged storage, the fruit loses its turgidity
due to moisture loss and begins to soften. Temperature is a determining factor in
the amount of water lost from harvested grapes.
At least three symptoms result with water loss from grapes.^11 First to appear are
shriveled stems that usually become brittle and break easily when handled. Since
the stems of grapes serve as a handle to move the fruit, breaking the stems essentially
means the fruit is lost for all practical purposes even though the berries still look
and taste good. The rate of stem drying is directly related to temperature and
increases logarithmically.
The second symptom of water loss is browning of the stem, which detracts from
the appearance of the fruit.^11 The rate of stem browning increases more rapidly with
increases in temperature than does the rate of stem drying. From 21 to 38°C there
is a threefold increase in stem drying after 8 h, whereas for stem browning the
increase is more than fourfold.
The third symptom of water loss is shrinkage and subsequent softening of the
berries.^11 At 4 to 5% moisture loss the berries feel softer. This softening is related
to temperature before cooling. Grapes held 8 h at 38°C lost 3% of their initial weight
and 75% of the berries were rated soft. Berries held at 21°C lost only 0.3% of their
weight and only 45% of the berries were rated soft.
Muscadine grapes (Vitis rotundifolia) are indigenous to the southern United
States and have been under cultivation since the colonial days.^58 Muscadine fruit is
produced in tight, compact clusters that do not ripen uniformly. Muscadine grapes
for the wine and juice market are usually harvested mechanically, while those for
table use are hand-harvested.^57
Muscadines have a tendency to “shatter” or break away from the stems during
harvest. The result is a torn stem cap called a “wet scar” that is an ideal entry point
for microorganisms and contributes to shortened storage life. Research has shown
that muscadine grapes decayed twice as fast at 20°C as at 10°C and three times as
fast at 10°C as at 0°C.^59 Grapes with wet stem scars stored for 1 week at 10°C or
3 weeks at 0°C had 6 to 10 times more decay than grapes with dry stem scars.
A comparison of the effects of storing muscadines at 20, 4.5, and 0°C revealed
that decay developed rapidly at the higher temperature, making it impractical to
store the grapes for more than 3 d.^57 At the refrigerated temperatures, decay was
slowed; however, the fruit softened gradually during a 24-d study. In that study, the
percentage of soluble solids, percentage of titratable acidity, individual sugars, and
organic acids did not change significantly during storage regardless of storage
temperature. Total phenol content increased and pectin decreased during storage,
and the higher storage temperature resulted in the greatest changes.