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

26 R. M. Madakadze and J. Kwaramba
short periods of time. Most cultivars are short day plants but others appear to be
day length neutral. For a high acceptable quality of cabbage removal of outer wrapper
leaves should reveal the typical shape and colour of the cultivar (green, or purple
or pale yellow-green), firm, heavy for the size head, and the head must be free of
insect, decay, seed stalk and other defects. Leaves should be crisp and turgid
(Cantwell and Suslow, 2001). Some disorders caused by environmental factors
that impact on quality of cabbages are discussed below.
4.3.1. Bolting
This refers to production of a seed stalk instead of a head. In the tropics this rarely
happens but it can occur at very low temperatures (especially night temperature)
accompanied by extreme variations with day temperatures. This is undesirable as
cabbage is grown mainly for the heads. High-veld areas are generally colder therefore
cabbages can be grown in summer, autumn and spring to avoid bolting as winter
temperature cause bolting even in the tropics. Deterioration in storage can also be
associated with bolting (Cantwell and Suslow, 2001).
4.3.2. Frost injury
Heavy losses of cabbage occur annually as a result of freezing, both in the field
and during storage. Cabbage tissue has one of the highest freezing points
(–0.6 °C) found among vegetables (Pierson et al., 1971) even though it is a cool
season vegetable. There is no way of determining by examination of a frozen head
whether it will exhibit injury when thawed. Immediately after thawing, frozen
areas appear water soaked due to suffusions due to water liberated by the thawing
of ice in intracellular spaces. If cells have not been killed, some of this water will
be reabsorbed and the tissue will only appear slightly wilted or shriveled due to
excessive water loss. The margins of lower leaves become flaccid and turn brown
and may even die. There is often rupturing of the epidermis on the underside of
leaves and cracking of main veins. The veins then become spongy, pithy, and
tough, losing their characteristic flavour. Some heads can withstand freezing several
times before the effects become pronounced; others are injured the first time. If cells
have been killed, water does not re-enter them and is either lost due to evaporation,
with attendant drying out of the tissues, or it remains and the tissues become
a leaking, disorganized mass which soon succumbs to saprophytic fungi and bacteria
attacks. The outer leaves of cabbage appear to be more resistant to frost damage
than the inner leaves and stem. They generally thaw without injury. Following
prolonged exposure to freezing temperatures, the inner tissues, especially the stem
pith, are killed and subsequently become affected with bacterial soft rot. A good
quality cabbage should be crunchy and fresh.
4.3.3. Head cracking
Cabbage heads crack when approaching maturity during hot months. Uneven provision
of soil moisture coupled with high temperature cause cracking. Very dry