Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

394 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
A good skin set is an indication of tuber maturity. A properly matured periderm helps in
reducing bruise and is a major factor that determines harvest timing. Typically, 2–3 weeks
after vine killing is required for tubers to have a mature periderm. A torque meter is often
used to quantify the tuber resistance to skinning, and it is a measure of skin set in tubers (Lulai
and Orr, 1993). Experiments on tuber resistance to skinning indicate that tensile strength of
phellum has a minor role and changes little during the process of maturation (Lulai, 2002).
The major factor in tuber resistance to skinning is the phellogen shear component. As
periderm matures, the phellogen shear component also increases. The immature periderm
is prone to skinning injury, which is due to phellogen single tangential plane of fracture.
During development, these cells undergo changes resulting in physical strengthening known
as skin set. Pavlista (2001) evaluated skin set on different cultivars using a torque meter by
measuring resistance to skinning after vine killing. He concluded that resistance to torque
exerted by the torque meter is an accurate measurement of skin set, and it can be used to
determine optimum harvest conditions. McGee et al. (1985) found that the effect of maturity
on wound healing is a temporary phenomenon and is not seen after storage.
19.2.1 Ideal conditions for wound healing
Potato tubers that are bruised due to harvest and grading operations are more prone to weight
loss. This is due to evaporation of water from the wounded area. These are potential points
of entry for microorganisms during storage. Wound healing usually occurs over a 2–3-week
period after the tubers are placed in storage. The rate of healing is dependent on temperature.
The viability of tubers in long-term storage is dependent on the effectiveness of the curing
process. There is significant variability among different cultivars in terms of storability,
which is largely attributed to effectiveness of the curing process (McGee et al., 1985).
Wound healing is essential because wounds are ideal entry points for pathogens that
cause Fusarium dry rot and Pythium leak, and they further lead to loss of water and solutes
(Spooner and Hammerschmidt, 1992). Wound healing takes place when temperatures
are maintained at 15–20 ◦ C with 95% relative humidity. Temperatures less than 10 ◦ C significantly
slow down the healing process. Following curing, potato tubers moved to lowtemperature
(5–10 ◦ C) chambers with high humidity for long-term storage.
During the curing process, potato tubers form new layers of cells that protect the internal
living tissues from dehydration, injuries, and pathogens. These multiple layers of tissue
called “cork” or “phellum” to protect damaged or injured tissue. Potato periderm or skin
consists of three tissue types: phellum, phellogen, and phelloderm. Cork, or phellem, is the
outermost layer that is formed by the phellogen (cork cambium). Cork tissue is programmed
irreversibly to be dead at maturity. This phenomenon is also seen in secondary (mature)
stems, roots, and tubers. Cork cells are impermeable due to the deposition of suberin and
waxes. Cork also contains triterpenoids and phenylpropanoid derivatives, which may deter
pathogen attack. Cork that is formed as a result of a wound is also called wound periderm.
19.2.2 Chemistry of wound healing
In potato, during the process of curing, new layers of cells that form wound periderm
are deposited with polyphenolic (aromatic ligninlike domain) and polyaliphatic (cutinlike)
compounds by a process described as suberization (Bernards, 2002). The suberized skin of