World Grain - June 2020 - 68

GRAIN OPERATIONS

humidity of corn even if exported
at 14% to 14.5% moisture content.
RELATIVE HUMIDITY AND MOLD
DEVELOPMENT
Fungal spores are activated more by the relative humidity of air surrounding corn kernels than temperature. A relative humidity
above 65% to 70% is the general activation

threshold. This implies that moisture contents in equilibrium with a relative humidity of 65% to 70% at any temperature determines whether fungi will sporulate or corn
will continue to store safely. For example,
corn at 15°C and 15.1% moisture content
will be in equilibrium with air at a relative
humidity of 75% (Table 1, page 70).
Unfortunately, at a relative humid-

ity of 75%, fungal spores will begin to
activate, which often is observed in the
form of surface crusting in corn when
headspace temperatures in silos and
warehouses increase in warm weather.
Ideally, US corn exported to the tropics
should be stored closer to 13% to 13.5%
to assure relative humidity surrounding grain kernels remains below 65% to
70% at temperatures above 20°C.
Condensation associated with handling
and storage of imported US corn in warm
climates is another challenge. It often arrives in tropical ports at a temperature at
least 10°C colder than the ambient air.
Water condenses on the external surfaces
of grain handling equipment when cold
corn is unloaded from vessels and transferred into storage silos or warehouses.
Condensed water on external metal surIDFHVGRHVQRWD൵HFWWKHFRUQDVORQJDVLW
is not allowed to drip onto the corn. Ideally, corn is transferred in enclosed conveyors rather than open belt conveyors that allow warm, humid air to come into contact
with the cold, dry corn. Contact between
this cold grain and humid air causes water
to condense on the exposed corn, which
when placed in storage can result in a
higher equilibrium relative humidity of
the air. This in turn results in activation of
fungal spores followed naturally by onset
of spoilage and self-heating.
A key process technology for reducing
moisture content to safe storage levels is
DUWL¿FLDO GU\LQJ 8QIRUWXQDWHO\ KLJK FDSDFLW\ FRQWLQXRXVÀRZ GU\HUV DUH JHQHUally not found at US export terminals and
overseas import terminals.
The equipment is expensive to purchase,
own and operate. Thus, drying with heated air is typically not an option for managing moisture content in imported corn.
AMBIENT AND CHILLED AERATION
Aeration of grain involves forcing ambiHQWRUFKLOOHGDLUDWORZÀRZUDWHVWKURXJK
a stored grain mass to lower or maintain
its temperature. It requires fans and an
air distribution system to push or pull air
through the grain. Most silos and warehouses at overseas import terminals are
not equipped for aeration. However, if

68

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World Grain - June 2020

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http://digital.world-grain.com/sosland/wg/2009_02_01
http://digital.world-grain.com/sosland/wg/2009_01_01
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