Injuries From Chilling and Freezing

Lacy P. McColloch.

Fresh fruits and vegetables are kept cool on their way to market so as to slow down the processes that cause their over-ripeness and decay. Refrigeration is the foundation of the big industry that makes it possible for us to have fresh produce the year around.

But some commodities that are subjected too long to low temperatures are so injured or weakened that their physiological processes are impaired or slowed to the point of inability to function, and decay becomes far more extensive than normal.

The fruits and vegetables that require high temperatures for growth generally are the ones most subject to injury by chilling or low temperature. Among them are bananas, citrus fruits, cucurbits, eggplant, peppers, potatoes, sweetpotatoes, and tomatoes. In handling them one has to remember that each has its own requirements or limitation as to the temperatures at which it can be held safely.

Chilling injury, which differs from freezing injury, results from holding a commodity at low (32 to 50 F.) but not freezing temperatures for enough time to impair its life processes. The effect is not well understood, but it appears that the tissues, unable to carry on normal metabolism, gradually become weakened. Chilling injury therefore is relatively slow.

Freezing injury results when ice crystals form in the tissues of fruits or vegetables. It usually occurs if they are subjected to temperatures corresponding to their freezing points or lower. Freezing injury may take place in a few hours and in spots or throughout the commodity. Tissues injured by freezing generally look as if they were soaked in water.

Chilling injury often is not apparent at the time fruits and vegetables are removed from low temperatures but becomes evident several days later. The symptoms are more noticeable in some commodities than in others. It is hard to diagnose chilling injury definitely because clear-cut symptoms are absent.

BANANAS are highly sensitive to unfavorably low temperatures. Injury so severe as to make them unsalable may occur if they are held at 45 F. or below in still air for 12 hours. The upper limits of temperatures that may cause chilling injury are not sharply defined, but the lowest temperature at which bananas should be held is 56 .

Ripe fruits are slightly more susceptible than green fruits. Chilling injury is confined mainly to the peel, in which some of the surface cells are killed. Severely chilled green fruits may have dark-green, water-soaked specks or nearly the entire surface may be dark. The latex exudes little or not at all in chilled green bananas when they are broken. It is clear, rather than milky or cloudy. That condition, however, is not a definite indication of chilling it also is characteristic of bananas that are beginning to ripen. If chilled to a lesser degree, green fruits develop a dull, smoky appearance on ripening, rather than a bright-yellow color.

Chilling injury does not become apparent on ripe fruits unless they are held at low temperatures a long time. The characteristic dull appearance soon develops, however, when chilled ripe fruits are removed to higher temperatures.

One has to know a good deal about bananas to recognize chilling injury, especially in green fruits, because factors other than chilling may cause a similar appearance.

CITRUS FRUITS are not chilled to the point of injury during the normal transportation period, but may be injured in storage if unfavorably low temperatures prevail. Storage may be necessary for citrus fruits in order to spread the marketing period. Because the demand for lemons is greatest in warm weather, a large part of that crop is stored for a while.

The storage behavior of citrus fruits is influenced by their degree of maturity and their inherent differences. The storage temperatures recommended, however, are those found most desirable for most of them. Low-temperature injury results in the development of various disorders, some of which are common to all citrus while others are specific for certain kinds.

Grapefruit stored at 32 to 40 F. for 2 or 3 months might suffer pitting, watery breakdown, scald, and browning of the oil glands. The pits in the rind are a type of low-temperature injury that may occur in all citrus fruits. Pitting is especially serious if grapefruits are stored for 2 months or more at 32 to 40 . The pits may occur anywhere on the surface and often are numerous. Occasionally the collapsed tissues appear bleached, but usually they are darker than the healthy rind. Pits developed on fruits stored at 32 are smaller than those on fruits stored at 36 and 40 .

Watery breakdown may develop in any citrus fruit stored at low temperatures for 2 or 3 months. Grapefruit picked late in the season are more susceptible than fruits picked earlier. Affected fruits are soft, spongy, and water-soaked in both peel and flesh and look as if they had been frozen. Affected fruits develop a fermented odor when they are held at room temperature.

Scald involves a superficial and fairly uniform browning of large areas of the rind. Affected areas at first are firm, but in severe cases the surface becomes spongy and soft and resembles an early stage of watery breakdown.

Another type of browning is confined entirely to the oil glands. It usually occurs at 32 and 36 F. Affected glands close together give the appearance of mass discoloration. Actually the discoloration is confined to the oil glands and the surrounding tissues are only slightly discolored.

Lemons also are subject to pitting, watery breakdown, and scald. The handling and storage problems of lemons are quite different, however, from those of grapefruit or oranges. Because lemons can be picked at a more immature stage than grapefruit and oranges, they can be held at a higher storage temperature for the 3 months needed to carry the winter and spring crop to summer.

Lemons stored at 58 F. until ready to ship will escape the disorders I mentioned. Even at that temperature, however, lemons are somewhat subject to a browning of the membranes a disorder known as membranous stain, which cannot be detected until the fruits are cut. It is greatly increased by storing lemons at 40 . If lemons are stored after reaching the market at temperatures of 32 to 40 for 60 to 90 days, low-temperature disorders may be expected to develop. For short-term Storage (2 to 4 weeks) in the market, 32 is perhaps the most desirable temperature for lemons because membranous stain does not occur then and other injuries do not become serious in that period if the fruits have not previously been stored at a low temperature.

Pitting, brown stain, and watery breakdown usually develop if oranges are stored at low temperatures for a long time. Such disorders can be avoided if the storage period is shortened and fruits with longer storage-life expectancy are selected.

Pitting is worse on early and mid-season varieties of Florida oranges than on late-maturing varieties. The variety Pineapple is one of the most susceptible. The variety Valencia is quite resistant to pitting and other low-temperature disorders. Valencia oranges can be stored at 32 to 34 , but the storage period should not exceed 12 weeks.

Brown stain is characterized by a superficial and fairly uniform browning of a large area of the rind. Like watery breakdown of grapefruit, it usually develops in certain varieties of oranges stored at 32 for 2 months, or longer. None of the low-temperature disorders has been serious in Florida-grown Valencias stored at 32 for 8 to 12 weeks.