Agriculture Reference
In-Depth Information
storage need to be stored initially under condi-
tions that promote final maturation of the native
periderm and the formation of wound periderm.
Favorable conditions for wound healing
and periderm maturation include an environ-
ment with oxygen and CO
2
concentration similar
to those in fresh air, RH above 80%, and tem-
perature of
10-
13°C (Wigginton, 1974). This
temperature range is low enough to slow the
replication of soft rot bacterial and other patho-
genic organisms, but warm enough for wounds
to heal and native periderm to mature within ap-
proximately
2
weeks (Lulai and Orr, 1995; Lulai
and Corsini, 1998; Schreiber
et al
., 2005). Wound
periderm may never achieve the same level of
water permeability as native periderm (Sabba
and Lulai, 2002; Schreiber
et al
., 2005). Consist-
ent with this observation are data showing that
the amount of weight lost by tubers in storage
increases with the severity of harvest injury
(Misener, 1994).
Potato tubers contain lenticels, specialized
pores that are used to promote gas exchange.
Lenticels are particularly apparent when tubers
are removed from wet soils, as they swell open
under these conditions. Although lenticels are a
pathway for relatively unrestricted diffusion of
water vapor from the tuber, water loss from len-
ticels is a small fraction of the total water loss.
This reflects the large difference in area between
the lenticels and the remainder of the tuber sur-
face. Most of the water lost from tubers diffuses
through the periderm.
remove moisture from tubers. Tubers with high
rates of respiration produce heat at higher rates
than tubers with lower rates of respiration. The
increased amount of cooling required to main-
tain a constant temperature will lead to greater
water loss from rapidly respiring tubers relative
to tubers with low respiration rates. Poorly insu-
lated and poorly sealed storage buildings have a
greater requirement for cooling air when ambi-
ent temperatures are high. As a result, tubers in
these storages will have greater weight loss com-
pared to those in well-insulated storages.
Humidification
To minimize water loss from potatoes in storage,
the storage air may be humidified. Various kinds
of humidification systems exist, including wet
fiber pads, centrifugal humidifiers, high-pressure
water spray nozzles, and air-assisted spray noz-
zles (Brook
et al
., 1995). Evaporation from tubers
adds moisture to the storage atmosphere at the
expense of tuber weight loss (Hunter, 1986). To
minimize tuber dehydration, humidity sensors
used to control a humidification system need to
be positioned in the ventilation air upstream of
the tubers.
Minimizing tuber water loss in storage
through humidity management necessitates the
ability to measure storage temperature and hu-
midity. Temperature is measured easily with
temperature probes accurate to <0.5°C. Relative
humidity is more difficult to measure, especially
above 90% RH. The accuracy of humidity probes
is usually no better than ±
2-
3% RH in the range
of
90-
100% RH. This lack of accuracy for RH
measurements can have a significant effect on
tuber weight loss when sensors consistently
report values
1-
3% higher or lower than the
actual RH.
Rate of water loss
The rate of water loss from tubers depends on the
resistance to water vapor diffusion through the peri-
derm, the surface area of the tuber, and the water
VPD between the tuber and the atmosphere. The
resistance to water vapor diffusion and the sur-
face area of the tuber are under genetic control
and are influenced strongly by field management
practices. Since the RH within the tuber is always
close to 100%, the magnitude of the VPD depends
on the management of temperature and RH in
the potato storage atmosphere.
The water-holding capacity of cool air is
less than that of warm air. The significance of
this is that cooling air, which is always at a lower
temperature than the tubers, will inevitably
15.7
Sprout Control
Sprouted tubers are undesirable, because sprouts
promote rapid dehydration, but more import-
antly, sprouted potatoes are unacceptable to
consumers and cannot be sold to processors.
Sprouts on tubers are primary sites for water
loss. Sprout area is directly proportional to rate
