Florida Agricultural Experiment Station


 earliest effects of frost to be noted internally are either water-
 soaked areas on the membranes or the formation of white
 specks, composed of hesperidin crystals, on the walls of the seg-
 ments of the pulp of the fruit (Fig. 84). These crystals average
 about 1/50 inch in diameter and are formed as a result of the
 chemical changes in the fruit. They are usually seen within
 from 5 to 10 days after the fruit has been frozen and, while
 common in most citrus fruits, are of rare occurrence in grape-
 fruit. The presence of these crystals will serve to identify frozen
 fruit even before it begins to dry out.
 When frozen fruits are cut across the segments, they often
 show a pale area part way between the center of the fruit and
 the rind. The pulp cells of frozen fruits tear apart more easily
 when cut across than with unfrozen fruit. Other internal effects
 of freezing that develop later are changes in the specific gravity,
 drying out, and thickening of the rind when partially injured
 fruits remain on the trees for further development. Freezing
 to such a degree as to cause considerable quantities of ice to form
 in the interior of the fruit may seriously disrupt the tissues.
 Although the fruit may not be sufficiently injured to show on
 the outside, the rind may be affected in such a way as to permit
 the evaporation of the juice of the pulp, particularly at the stem
 end. This drying out frequently causes separation of the pulp
 segments. When sound and frozen fruit are picked and later
 compared, it is found that the sound fruit increases while the
 frozen fruit decreases in specific gravity. Freezing kills the pro-
 toplasm, or cell sap, changing the semi-permeable membrane to
 a dead porous mass, thus allowing the water of the juice to evap-
 orate through the rind. This evaporates without much change
 in the volume of the fruit and the rind becomes abnormally
thickened in case the frozen fruit is left on the tree. Eventually,
little or nothing is left in the fruit but a chaffy mass of juice-
sacs. In the sound fruit, on the other hand, the rind becomes
thinner while the interior juices remain practically intact. If,
however, the fruit has been severely frosted it falls within a few
days after the injury (Fig. 82).
 The temperature at which citrus fruit will freeze is affected
by the degree of maturity it has reached and the length of time
it is subjected to the low temperature. In Florida the freezing
point of fully ripe oranges is usually placed at 260F. Grapefruit
withstands a low temperature for a longer period because of the
greater thickness of the rind.