Agriculture Reference
In-Depth Information
through permeable tops and bottoms. In these
regions, seed tubers may be kept in small storag-
es that allow diffuse light to reach the tubers,
promoting the growth of strong, short sprouts
that are less susceptible to damage during plant-
ing than sprouts that have developed in the dark.
In regions where subsurface soils routinely
freeze during the winter, ground storage or
aboveground clamps and cribs may be impracti-
cal. In this case, tubers must be stored in a
well-insulated facility that prevents freezing. The
simplest of these are belowground cellars that
maintain temperatures above 0°C via walls insu-
lated with soil, straw, or synthetic material. The
RH in a belowground cellar depends on the rate
of evaporation from the potatoes and on how
well the walls, floor, and roof are sealed to prevent
air and water vapor movement. Belowground
potato storage is implemented most easily for
small volumes of tubers, but some designs have
capacities of
25
t (Pringle
et al
., 2009).
Large volumes of potatoes can be stored
successfully in bags, boxes, or bulk piles. Each
method has advantages, some of which are re-
lated to climatic factors, storage facility, and the
intended market. When tubers are stored in bags
made of burlap, the bags are typically stacked to
a depth of
2
m or less in buildings where air tem-
perature and RH are controlled to suit the needs
of the crop.
Large-scale storage of potatoes in pallet
boxes is common in the UK (Cunnington, 2008).
Ventilation air, with or without supplemental
humidification, can be supplied through floor
vents or from side walls. In sophisticated storage
systems, boxes are designed to form air ducts
when stacked in place, and this facilitates effi-
cient air movement throughout the entire stor-
age. Box storage allows tracking of small lots of
tubers from individual plots or fields, since each
box can be labeled and followed independently.
Large-scale storage of potatoes in bulk piles
inside a climate-controlled, insulated building is
typical in the USA (Brook
et al
., 1995) and the
Netherlands, and for processing potatoes in
Great Britain (Cunnington, 2008). Tubers are
typically piled 4.5-5.5 m high into rooms or
large bins on either side of a central plenum.
Fans at one end of the plenum force tempera-
ture-controlled air through moist fiber pads and
past supplemental humidification systems. Lat-
eral air ducts located on the bottom of the potato
pile distribute the humidified air throughout the
storage. These ducts can be made of corrugated
steel, with perforations to distribute the air, or
can be in-floor ducts covered with wooden slats.
Air that passes through the pile is drawn back
toward the fans to be recirculated. Fresh air, as
needed to prevent excessive CO
2
build-up and
regulate temperature, is pulled through adjust-
able louvers in the outside wall and blended with
the return air before passing through the hu-
midification system. In many cases, supplemen-
tal refrigeration augments the cooling capacity
available from introducing cool outside air and
the evaporating of water in the humidification
system.
15.3 Physiological Defects
Occurring During Storage
Potatoes in storage may develop quality defects
that decrease the value of the stored crop. Sev-
eral of these are physiological defects that are
independent of pathogen activity.
The most universal storage-induced defect
is pressure bruising of tubers. Pressure bruise
results when tubers within a pile lose their abil-
ity to resist the downward force applied by the
weight of the tubers above. In its mildest mani-
festation, this results in pressure flattening,
which is apparent as flat spots or indentations on
tubers coming out of storage. When the force of
overhead pressure exceeds the ability of the
tubers to withstand compression forces, pres-
sure bruise may be observed (Castleberry and
Jayanty, 2012). Bruising results when the cells
under pressure-flattened areas sustain physical
damage that results in the formation of melanin
pigments (Lulai
et al
., 1996). Pressure bruising
is thought to be promoted by tuber water loss before
harvest or in storage that results in the reduc-
tion of cellular turgor pressure. Susceptibility to
pressure bruise varies with cultivar (Castleberry
and Jayanty, 2012).
Several tuber quality defects in processing
potatoes arise from the accumulation of the re-
ducing sugars, glucose and fructose, in storage.
During high-temperature frying, the carbonyl group
on a reducing sugar reacts with the alpha- amino
group of amino acids in a non-enzymatic, Mail-
lard reaction (Shallenberger
et al
., 1959). Prod-
ucts of the Maillard reaction include dark-colored
