Young apple fruits can be injured when freezing temperatures occur at blossom time or soon thereafter. The injuries persist throughout the development of the fruits and appear as blemishes when the fruits are mature. The most typical condition is a band of russeted tissues partly or completely encircling affected fruits about midway between the ends (top photo). In addition, frost russet may occur in irregular spots, in the calyx end, or as diffuse irregular areas characteristic of spray injury and rain russet.
Another type of frost injury seen only rarely on apples is the persistence of green color at the calyx end on mature fruits that are otherwise normally colored. The amount of the surface affected and the intensity of the persistent greening varies. Affected areas are occasionally so flattened and so dark green that they resemble quince rust spots, but any discoloration of internal tissues would not extend as deeply into the core tissues as that of quince rust. If spore-bearing pustules are present on affected areas, they are a definite indication of quince rust. Badly damaged specimens are nearly always distorted at the blossom end, they may have only a few poorly developed seeds or none at all, and the flesh underlying the green areas is somewhat streaked or blotched with brown.
Mature apples may be injured when temperatures fall below the freezing point of the fruits in the orchard, in cold storage, or during transit. Variations in symptoms caused by apple variety, temperature level, duration of freezing, and conditions under which the fruits were thawed make it impossible to determine from the symptoms on a single fruit the time when and the conditions under which the fruit was frozen.
Late-maturing varieties such as Winesap and Rome Beauty may be frozen by unseasonably cold weather while still on the trees. Deep conical bruises are likely to result from picking and handling such fruits while they are frozen.
Freezing may occur in cold storage on loose fruits on the tops of stacks of boxes or bins that are in the air blast coming from the refrigeration coils. This can be a troublesome problem in poorly designed cold-storage plants in which the desired storage temperatures can be maintained only by operating the refrigeration coils at temperatures several degrees below the freezing point of the fruits.
Much of the freezing injury on apples from the Pacific Northwest occurs in the winter months when fruits in shipments to the East must travel long distances at high elevations and across the northern tier of states where low temperatures may persist for days. The development and widespread use of trucks and railroad cars with forced-air heating systems has greatly reduced freezing in transit since the last revision of this publication. Although there are increasing numbers of railroad cars with heating systems operated by diesel units, most of the cars have air circulation systems that operate only when the cars are moving. With the latter type of equipment, freezing of the fruits can occur when cars are stalled by mechanical failures or by weather conditions. Serious losses occurred to fruits in transit when the cars were stalled by deep snow that blocked the rails for several days in early 1968.
Freezing in transit first occurs at the doorway, along the floor and side walls, and near the bunkers; upon prolonged exposure of the fruits to low temperature, injury may extend deeper into the load. If freezing injury is absent in the outer parts of the load, there should be no transit freezing elsewhere in the car. If the injury is uniformly scattered through the packages and through the load, the injury probably occurred before the fruit was loaded.
Apples that have been only slightly frozen may show no visible symptoms, but will be a little softer and will have a shorter storage life than apples that have not been frozen.
Fruits that have been subjected to somewhat lower temperatures may exhibit no external symptoms, although they may be injured internally. A crosswise cut through the middle of an apple reveals 10 small dots, the main vascular bundles, located about midway between the center and the outside of the fruit. These, along with the numerous, less conspicuous, thread-like strands that ramify throughout the flesh, constitute the main food-and-water-conducting system of the fruit, which had been connected with a similar system in the twigs and branches when the fruit was on the tree. In a normal apple the vascular strands are green or yellowish-green, but may be tinged with red in Winesap and some other varieties. In thawed apple, injured by freezing, the main strands and possibly some of the smaller ones are brown (second photo).
While severely frozen, apples become temporarily shriveled and exhibit a network of wrinkles rather than the parallel lines of shrinkage that are produced by normal evaporation. In this condition the volume of the fruit may be reduced by as much as 10 percent. These fruits can be severely damaged by rattling around in the boxes. Upon thawing, however, the fruits will regain their original volume, unless freezing was prolonged and there was serious moisture loss because of low relative humidity.
Both external and internal symptoms occur in severely frozen apples after they thaw. External symptoms consist of irregular-shaped areas with a water-soaked appearance. Some fruits may show brown discolored areas that resemble ordinary scald. Later the discoloration intensifies and may become almost black, depending on the apple variety, temperature at which the fruits thawed, and the length of time after they were thawed. Apples thawed at 40 °F or above discolor more than those thawed at 32°.
Apples that are so severely frozen that they cannot recover when thawed have an internal appearance strikingly different from any other apple condition. The flesh is somewhat spongy, and if a cut fruit is squeezed, juice oozes from the tissues. The vascular tissues are brown even if the flesh is not discolored. The flesh varies from a golden brown to a dark brown or almost black, depending on the variety of apple and the circumstances of freezing and thawing.
Apples that have been bruised while frozen in transit frequently show flattened areas, 1-1/2 to 2 inches in diameter. They are somewhat sunken and soft toward the center and have a dull-brown or slate color over most or all of the center.
Freezing injury may occur when temperatures drop below the freezing point of the fruits. The freezing points for all major apple varieties have been determined experimentally. They range from 27.3° to 29.4 °F, and average 28.4°. When an apple freezes, ice crystals form between the cells. Small crystals form at temperatures slightly below the freezing point of the fruit. A further decrease in temperature will result in the formation of larger crystals in the spaces between the cells as more water is withdrawn from the cells. Injury occurs from the physical forces exerted on cells by the ice crystals, and from chemical reactions within cells from which excessive amounts of water have been removed. The injuries result in the death of the cells. Generally the cells in the vascular tissues are somewhat more sensitive to injury than those in other tissues, and the first symptoms are most likely to appear in the vascular tissues.
Little can be done to prevent the freezing that occurs on late maturing varieties in the orchard, but damage can be minimized by waiting until the fruits have thawed before picking or handling them.
Freezing in storage can be reduced by careful planning of the refrigeration equipment in new installations and the updating of equipment in older storages to eliminate the necessity for operating refrigeration coils at excessively low temperatures. The construction of baffle boards to shield fruits from the air blast directly from the refrigeration coils is helpful.
The use of heater protective service in shipments of fruits that are likely to travel through areas of sub-freezing temperatures will greatly reduce the hazard of freezing in transit.
Freezing injury after thawing
Spring frost, Braeburn
Spring frost, Golden Delicious
Spring frost, Red Delicious
Spring frost, Red Delicious
Chilling injury or low-temperature breakdown