How cold can it get before apples are harmed on the tree?
If frost occurs during harvest do not touch or move that fruit until the frost has left the apple. If fruit has been touched (picked), or rubbed by limbs, or other fruit while frozen, the areas of contact will be damaged under the skin.
If frost is imminent, harvest as much fruit as possible and ensure that all harvested fruit is placed under heated cover. Apples that remain on the tree recover from freezing more completely than harvested fruit. Fruit on the tree may undergo several freezings with little injury. Harvested apples seldom recover from even one freeze.
The freezing point of apples is approximately 29º. However, the fruit will not freeze until it is at, or below, the freezing point for some time. The greater the number of freezings, or the longer the freeze period, the more likely the fruit is to be injured and to breakdown in storage. Freezing is likely to increase the number of fruit with symptoms of senescence or old age, because freezing increases the rate of ripening.
Should the fruit freeze, it will have a hard, glossy appearance, wrinkled skin, and a purplish discoloration of the red side of the apple. Frozen fruit can have a fermented off-flavour and also be considerably softer. Severely frozen areas of the fruit will be brownish, and probably mushy if a large area is affected. If a grower is not sure whether the fruit is frozen, do not begin picking until air temperatures have increased and all threat of persistent frost is gone.
Fruit subjected to frost is best packed and sold, not stored. Frozen fruit ripens more quickly and the storage life is shorter and the fruit may be subject to a higher than normal incidence of breakdown and decay. If the fruit is suspected of having been exposed to frost, at harvest, be sure to inspect the fruit periodically, by removing a few apples from storage and leaving them to age at room temperature for a week. If the fruit shows signs of breakdown when cut open, it should be marketed immediately. Never risk storing frozen apples in controlled atmosphere or low oxygen storage.