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

season. The main environmental factors affecting root quality are soil moisture,
nutrition, temperature and pests. These will be discussed separately below.
4.4.1. Freezing injury
Sweet potatoes are generally considered to be very susceptible to low temperature
injury, however, their average freezing temperature of –1.7 °C is lower than that
of many hardier vegetables. Sweet potatoes can be exposed to freezing temperatures
in the field at harvest if night temperatures fall below 0 °C and dug roots remain
in the field overnight. While this is uncommon in the tropics, such temperatures
can be encountered during harvest. Sweet potatoes that have been only slightly frozen
are characterized by yellowish-brown discoloration of the vascular ring and internal
vascular elements and by a yellowish-green water-soaked appearance of surrounding
tissues. When exposure has been long enough for extensive ice formation to occur
within the tissues, they collapse immediately upon thawing and the root becomes
soft and flaccid as water is liberated. Such roots may dry and become brown, discolored
mummies, although they usually decay due to infection by blue mold fungus
(Clark and Moyer, 1988). Sweet potatoes must be harvested before freezing weather
occurs.
4.4.2. Chilling injury
Potatoes exposed to moderate chilling may be dug, cured and sold but they should
not be placed in long-term storage (Pierce, 1987). Lower temperatures significantly
reduce root quality and storage life of sweet potatoes. Low temperatures
may cause internal breakdown in storage roots, internal discolouration, an increase
in the frequency of roots with decay and sometimes, hard core. Hardcore is a disorder
in which roots remain hard after cooking. The cold is thought to modify pectic
substances in the middle lamella so that the tissue remains hard during cooking.
Hardcore develops in roots exposed to 1.5 °C for as little as one day or 10 °C for
at least three days. Chilled roots do not exude latex when cut (Clark and Moyer,
1988).
4.4.3. High temperature and sunscald
Effect of Preharvest Factors 29
The sweet potato plants are relatively resistant to the combined effects of heat and
sunlight. In contrast to the growing plants, storage roots left exposed to the sun
after they are dug are commonly damaged by sunscald. Many harvesting systems
involve removing storage roots from the soil and leaving them on the soil surface.
This facilitates drying and removal of soil from the roots, which are picked in a
second step of the harvest operation. When roots are left in bright sun for as little
as 30 minutes, serious sunscald can result. High temperature (greater than 32 °C)
can also damage harvested sweet potatoes causing sunscald and eventually breakdown
and decay (Pierce, 1987). They become soft near the surface, and within a
few days their exposed surfaces may turn purplish brown. The incidence of storage
rots, especially Rhizopus soft rot, surface rot, Fusarium root rot and charcoal rot,