The lower desert areas experience freezing temperatures several times during the winter and it is not uncommon winter vegetables injured. There are two types of injury that low temperatures can cause on vegetables. Chilling injury is what happens to some vegetable crops of tropical origin held at the wrong storage or transit temperature, but a temperature above 32°F (0°C). Generally these are temperatures around 41-50°F (12.5°C). Chilling injury occurs at temperatures well above freezing point. The tissue becomes weakened leading to cellular dysfunctions. Symptoms include surface lesions/pitting, internal discoloration, water soaking of the tissue, failure to ripen normally and increased susceptibility to decay organisms such as Alternaria. Maturity at harvest also affects the susceptibility to chilling injury in products such as tomatoes, honeydew melons and peppers.

The second type of injury is frost/freezing injury and it can occur in a field when temperatures drop to or below 32°F (0°C). It can also occur during cold storage if temperatures were below the freezing point of the product. This exposure to freezing temperatures may have a drastic effect upon the entire plant or affect only a small part of the plant tissue, resulting in reduced yields or poor product quality. Ice formation on the plant and in the plant tissue is what causes the damage, especially when the tissue thaws. Postharvest researchers have worked on identifying the freezing temperatures for most crops and have published tables with the recommended optimum storage temperatures. No vegetable or fruit cold storage facility should be without these tables and they are available for download at the UC Davis Postharvest web site (http://postharvestucdavis.edu ). Researchers have categorized fruits and vegetables into three groups based on their sensitivity to freezing (Table 1): most susceptible are those that are likely to be injured by one light freeze, moderately susceptible are those that will recover from one or two light freezes and least susceptible are those that can be lightly frozen several times without serious injury. Table 2 summarizes freezing symptoms for a range of common desert grown vegetables.

Table 1. Susceptibility of fresh fruits and vegetables to freezing injury.

Most susceptible

Moderately susceptible

Least susceptible

Symptoms of freezing, chilling injury

Table 2. Symptoms of freezing and chilling injury on some desert grown vegetables

Artichoke: Freezing injury will be initiated at 29.9°F (-1.2°C). Symptoms of light freezing injury are blistering of the cuticle and a bronzing of the outer bracts. This may occur in the field with winter-harvested buds. More severe freeze injury results in water soaked bracts and the heart becoming brown to black then gelatinous in appearance over time.

Asparagus: Freezing injury (water-soaked appearance leading to extreme softening) will likely result at temperatures of 30.9°F (-0.6°C) or lower.

The tip becomes limp and dark; the rest of the spear is water-soaked. Thawed spears become mushy. Chilling Injury occurs when spears are held more than 10 days at 32°F (0°C) and symptoms of chilling injury include loss of sheen or glossiness and graying of the tips. A limp, wilted appearance may be observed. Severe chilling injury may result in darkening near tips in spots or streaks

Snap Bean: Freezing Injury appears as water-soaked areas that subsequently deteriorate and decay. Freezing injury occurs at temperatures of 30.7°F (-0.7°C) or below. The typical symptom of chilling injury in snap beans stored below 41°F (5°C) but above freezing point for longer than 5-6 days is a general opaque discoloration of the entire bean. A less common symptom is pitting on the surface. The most common symptom of chilling injury is the appearance of discrete rusty brown spots which occur in the temperature range of 41-45°F (5-7.5°C). These lesions are very susceptible to attack by common fungal pathogens. Beans can be held about 2 days at 34°F (1°C), 4 days at 36°F (2.5°C), or 8-10 days at 41°F (5°C) before chilling symptoms appear. No discoloration occurs on beans stored at 50°F (10°C). Different snap bean varieties differ significantly in their susceptibility to chilling injury.

Bell Pepper: Freezing injury symptoms include dead, water-soaked tissue in part or all of the pericarp surface; pitting, shriveling, and decay follow thawing. Symptoms of chilling injury include surface pitting, water-soaked areas, decay (especially Alternaria spp.), and discoloration of the seed cavity.

Broccoli: will freeze if stored at 30.6°F (-0.6°C) to 30°F (-1.0°C). This may also occur if salt is used in the liquid-ice cooling slurry. Frozen and thawed areas on the florets appear very dark and translucent, may discolor after thawing and are very susceptible to bacterial decay. The youngest florets in the center of the curd are most sensitive to freezing injury. They turn brown and give off strong odors upon thawing.

Cabbage: Freeze damage appears as darkened translucent or water-soaked areas that will deteriorate rapidly after thawing. Freeze damage can occur if round cabbages are stored below 30.4°F (-0.9°C) and if Chinese cabbage is stored below 31°F (-0.6°C). Leaves become water-soaked, translucent, and limp upon thawing; the epidermis can also separate from the leaf as it does in lettuce.

Carrot: Freeze damage includes blistered appearance, jagged length-wise cracks. Carrot interior becomes water-soaked and darkened upon thawing. Freeze damage occur at below 29.5°F (-1.4°C) but can vary depending on the sugar content of the carrot.

Cauliflower: Freezing injury will be initiated at 30.6°F (- 0.8°C). Symptoms of freezing injury include a water-soaked and greyish curd and water-soaked or wilted crown leaves. The curd will become brown and gelatinous in appearance following invasion by soft-rot bacteria. These brown curds have a strong off-odor when cooked.

Celery: Freezing injury will be initiated at 31.1°F (- 0.5°C). Symptoms of freezing injury include a water-soaked appearance on thawing and wilted leaves. Mild freezing causes pitting or short streaks in the petiole which develop a brown discoloration with additional storage. Leaves and petioles appear wilted and water-soaked upon thawing. Petioles freeze more readily than leaves.

Eggplant: Freezing injury will be initiated at 30.6°F (- 0.8°C), depending on the soluble solids content. Symptoms of freezing injury include a water-soaked pulp becoming brown and desiccated in appearance over time. Eggplant fruit are chilling sensitive at temperatures below 50°F (10°C). At 41°F (5°C) chilling injury will occur in 6-8 days. Consequences of chilling injury are pitting, surface bronzing, and browning of seeds and pulp tissue. Accelerated decay by Alternaria spp. is common in chilling stressed fruit. Chilling injury is cumulative and may be initiated in the field prior to harvest.

Beet: Freezing injury symptoms include external and internal water-soaking; sometimes blackening of conducting tissue.

Garlic: Freezes at temperatures below 28°F (-2°C) due to its high solids content. Thawed cloves appear grayish-yellow and water-soaked.

Green Onion: Freezing injury will be initiated at 30.6°F (-1°C). Symptoms of freezing injury include a water-soaked appearance of bulb or leaves and wilted or gelatinous leaves, after thawing. The bulb will become soft or gelatinous in texture in outer tissue. Freeze injury is rapidly followed by bacterial soft-rot decay.

Lettuce Romaine and Crisphead: Freeze damage can occur in the field and cause separation of the epidermis from the leaf. This weakens the leaf and leads to bacterial decay during storage. Freeze damage can occur during storage if the lettuce is held at 31.7°F (<-0.2°C). This appears as darkened translucent or water-soaked areas that will turn slimy and deteriorate rapidly after thawing. The blistered dead cells of the separated epidermis on outer leaves become tan and there is increased susceptibility to physical damage and decay.

Okra: Freeze damage occurs at temperatures of 28.7°F (-1.8°C) or below. The typical symptoms of chilling injury in okra are discoloration, pitting, water-soaked lesions and increased decay (especially after removal to warmer temperatures, as during marketing). Different cultivars may differ in their susceptibility to chilling injury. Calcium dips and modified atmospheres have been reported to reduce chilling symptoms.

Onion: Freezing injury symptoms include soft water-soaked scales that rapidly decay from subsequent microbial growth. Thawed bulbs are soft, grayish-yellow, and water-soaked in cross section; often limited to individual scales.

Potato: Freezing injury will be initiated at 30.5°F (-0.8°C). Symptoms of freezing injury include a water-soaked appearance, glassiness, and tissue breakdown on thawing. Mild freezing may also result in chilling injury. Freezing injury may not be externally evident, but shows as gray or bluish-gray patches beneath the skin. Thawed tubers become soft and watery. Chilling injury can occur at storage temperatures near 32°F (0°C) after a few weeks and may result in a mahogany discoloration of internal tissue and eventually complete internal breakdown. Much longer periods (months) of storage are generally required for chilling injury at higher temperatures 36-41°F (2-5°C).

Radish: Radish ideally stored and transported just above the freezing point 30.5°F (-1.0°C), but freeze injury is not uncommon. Shoots become water-soaked, wilted, and turn black. Roots appear water-soaked and glassy, often only at the outer layers if the freezing temperature is not too low. Roots become soft quickly on warming and pigmented roots may "bleed" (lose pigment). Thawed tissues appear translucent; roots soften and shrivel.

Spinach: Freezing injury will be initiated at 31.5°F (-0.3°C). Freezing injury results in water soaking typically followed by rapid decay by soft-rot bacteria.

Tomato: Freezing injury will be initiated at 30°F (-1°C), depending on the soluble solids content. Symptoms of freezing injury include a watersoaked appearance, excessive softening, and desiccated appearance of the locular gel. In partially frozen fruits, the margin between healthy and dead tissue is distinct, especially in green fruits. Tomatoes are chilling sensitive at temperatures below 50°F (10°C) if held for longer than 2 weeks or at 41°F (5°C) for longer than 6-8 days. Consequences of chilling injury are failure to ripen and develop full color and flavor, irregular (blotchy) color development, premature softening, surface pitting, browning of seeds, and increased decay (especially Black mold caused by Alternaria spp.). Chilling injury is cumulative and may be initiated in the field prior to harvest.

Turnip: Freezing injury symptoms include small water-soaked spots or pitting on the surface. Injured tissues appear tan or gray and give off an objectionable odor.

Special thanks to:

Richard Smith, UCCE Monterey whom shared cauliflower freezing symptoms and Marita Cantwell, PhD, Postharvest Specialist, UCD whom shared her insights and experience.

References

1. Commercial Cooling of Fruits, Vegetables and Flowers (2008)