Freeze Injury Of

a. General. Freeze injury is exposure of the product to a temperature below the freezing point which results in formation of ice crystals and tissue injury. The extent of freeze injury will vary with the item, length of exposure, and temperature to which the product is exposed.

b. Loss of Color. A loss of color indicates freeze injury. Loss of color usually results in a glassy or transparent appearance.

c. Texture. Textural changes include softening and a water-soaked appearance. Usually, the outer layers of the product will peel. In some cases, the texture will be dry or parchment-like.

d. Appearance. Freezing injures vegetables and melons because ice crystals form in the cells and then rupture the cell walls. The damaged or dead cells lose their resistance to dehydration and to microbial infections. Further, freeze-damaged tissue loses its normal rigidity and becomes mushy upon thawing, leading to the water-soaked appearance commonly associated with thawed vegetables.

e. Off-Odors. Freezing also may lead to development of strong off-odors upon cooking the damaged vegetable. Broccoli is a serious offender because almost unnoticeable freeze injury to broccoli yields a very objectionable odor.

f. Level of Resistance to Freeze Injury. Figure 5-3 lists some vegetables according to their susceptibility to freeze injury. Unfortunately, the type of vegetable does not necessarily yield a clue to the susceptibility to freeze injury. Among leafy vegetables, lettuce is very readily injured, whereas cabbage is not. Green beans are readily damaged; green peas are not.

VEGETABLE

SUSCEPTIBILITY

Artichoke

High

Asparagus

High

Broccoli

High

Brussels sprouts

Fairly resistant

Cabbage

Moderate

Carrot

Moderate

Cauliflower

Moderate

Celery

High

Corn, sweet

High

Endive

Moderate

Green beans

High

Lettuce

High

Onions

Moderate

Peas

Moderate

Radish

Moderate

Spinach

Moderate

Turnip

Fairly resistant

Figure 5-3. Susceptibility of vegetables to freeze injury. 5-7. CHILL INJURY OF FF&V

a. General. Chill injury is exposure of the product to a temperature above its freezing point, but below the temperature normally recommended for storage of the item. The extent of the injury to the product will depend upon time and temperature.

b. Susceptibility to Chill Injury. Chill injury affects only certain vegetables, mainly those which had their origins in the tropics or in the subtropics. Susceptible plants that originated in temperate zones are dormant during the cold season.

c. Signs of Chill Injury. The signs of chill injury are as follows:

(1) Decay. Chilled vegetables decay readily because the low temperature reduces the resistance of cells to invasion by pathogens. Decay can spread rapidly in severely chilled vegetables because the pathogens are growing on dead or dying tissue.

(2) Discoloration. The discoloration may occur externally as in beans, or internally, as in eggplant. The spots or areas may be tan, brown, or black. They may become evident while the product is at low temperature or show primarily after transfer of the item to a nonchilling temperature. Internal discoloration may be noticeable immediately upon cutting the vegetable or only after the injured tissue has been exposed to air.

(3) Pitting. Pitting of the surface is an early symptom of chill injury. Under relatively dry conditions, the injured cells apparently lose moisture more rapidly than it can be transported to them. Desiccation then results in the collapse of the cells and the formation of pits.

(4) Abnormal ripening. The prevention of normal ripening caused by chill injury is seen in mature-green tomatoes and honeydew melons. In tomatoes, coloration is uneven. Desirable softening is delayed in both, and severe chilling may prevent the fruit from ever reaching an edible stage even if decay is absent. However, sensitivity to chilling decreases progressively in some fruits as they ripen, so that ripe tomatoes can tolerate low temperature more readily than pink fruits. In turn, the pinks can tolerate low temperatures better than mature-green fruits.

(5) Texture changes. The development of hard core in sweet potatoes is a texture change due to chill injury. This defect is characterized by the development of a mass of tissue that does not soften even upon cooking.

d. Level of Resistance to Chill Injury. Figure 5-4 lists the susceptibility of various vegetables and melons to chill injury.

PRODUCT

SUSCEPTIBILITY

Asparagus

Low

Banana

High

Lima bean

Moderate to low

Snap bean

Moderate to low

Cucumber

High

Eggplant

High

Cantaloupe

Low

Honeydew melon

High

Okra

Moderate

Bell pepper

Moderate

Potato

Low

Sweet potato

High

Tomato (ripe)

Low

Watermelon

Moderate

Figure 5-4. Susceptibility of vegetables and melons to chill injury.

Figure 5-4. Susceptibility of vegetables and melons to chill injury.

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