Production Practices and Quality Assessment of Food Crops. Vol. 1

12 R. M. Madakadze and J. Kwaramba
Table 4. Relative susceptibility of fresh vegetables to freezing injury.
Most susceptible Moderately susceptible Least susceptible
Asparagus Broccoli Beets
Snap beans Cabbage Brussel sprouts
Cucumbers Carrots Cabbage (savoy)
Eggplant Cauliflower Kale
Lettuce Celery Kohlrabi
Okra Onions (dry) Parsnips
Sweet peppers Parsley Rutabagas
Summer squash Peas Turnips
Sweet potatoes Radishes
Tomatoes Spinach
Winter squash
Adapted from Lutz and Hardenburg (1977).
depends upon how low the temperature and, the rate at which it falls, the duration
of the freezing temperature, and the condition of the plant tissues prior to freezing
(Flurry et al., 1977).
Frozen leafy tissues and storage tissues such as turnips and rutabagas, lose their
natural luster and appear glossy. Immediately upon thawing, they become water
soaked. Water-soaked areas of leafy green tissues also appear dirty or a muddy green
colour. In colourless or fleshy tissues like cauliflower heads, there is no initial
discoloration, later, the more sensitive tissues such as the fibro-vascular bundles may
turn yellowish – brown to black. Fleshy roots, such as turnips, radishes, rutabagas,
and horseradish, often show no discoloration except for the vascular tissues.
Vegetables may arrive at the market bearing symptoms of frost damage incurred
during some earlier stage of their growth. Glove artichoke (Lynara scalymus L.)
is an exceptionally long-season crop that can become frozen in the field when the
temperature drops below –1.7 °C. Severe freezing kills the immature flower heads
and causes them to turn black. Slight freezing results in breaking, cracking, and
blistering of the epidermis on exposed outer bracts. The damaged epidermis becomes
whitish and buds may become brown. This detracts from the market appearance
of the buds (Ramsey et al., 1967).
When freezing injury occurs prior to harvest in Irish potatoes (Solanum
tuberosum) it is often referred to as field frost. This condition can usually be
diagnosed by the presence of bluish-gray blotches beneath the skin. Tissues at the
stem end of tubers are more sensitive than those at the bud end, and the differentiated
vascular cells, such as tracheae, sieve tubes, and tracheids, are more susceptible
than are the starch-filled parenchymatous cells. Any freezing during storage and
transit may manifest as or all the necrotic patterns known as ring necrosis, blotch,
or net necrosis. Generally the symptoms of freezing progresses from the ring to
net to blotch type as the freezing progress and as the freezing interval lengthens.
Often the different types of symptoms will overlap. The various internal symptoms
of freezing injury may not occur unless potato tubers are bumped, jarred, or dropped
during the freezing period. Varieties of potatoes differ in their reaction to low