Agriculture Reference
In-Depth Information
The most basic store is a traditional woven basket,
covered in straw, which may be within the home. The
structure of a house that is intended to provide a stabilised
environment for the residents and sometimes animals,
against the extremes of weather is also of benefit in the
storage of potato tubers. The weave of the basket allows for
some natural ventilation and thus reduces the likelihood of
hot spots or localized deterioration.
The tropical low cost store should, if possible, use any
beneficial aspects of the climate and so in addition to some
form of insulation and shading to minimise daytime
heating there should be a form of ventilation with night air
when the temperature is at its coolest and the relative
humidity at its highest. (Hunt
et al
. 1982). This store is
ventilated by the differences in the density of the air between
the warm tubers and the ambient air. The warm air between
the tubers will rise bringing in new, cooler ambient air.
The materials used will depend on what is locally available,
but it is important that the configuration of the roof ensures
that the whole store is in shade during the day. It is also
important that the walls are lined so that all the air moves
in a vertical direction. The simplest system lifts the pota-
toes off the ground and ensures that air can pass through
the tubers all at the time. The night ventilated version has
flaps that can be opened to allow ventilation only at night,
which can be particularly useful where there is a large diur-
nal variation such as at altitude.
A further development of the naturally ventilated store
with night ventilation is described by Bishop and Stenning
(1997) where a solar collector is used, so called solar
cooling. Construction of this solar cooling makes use of
rocks or similar material that are heated up by the sun
during the day and then the flaps are opened at night and
the heat of the rocks induces additional nocturnal ventila-
tion. Initial trials carried out in Kenya at sea level near
Mombassa gave some temperatures slightly lower than
with night ventilation alone but additional development
work is required.
Evaporative cooling in naturally ventilated stores with
or without solar cooling did not show a significant benefit
as the additional resistance to air flow of any damp pad had
a negative effect which balanced the cooling potential
(Bishop & Stenning 1997; Hunt 1990).
Evaporative cooling can work on forced ventilation
stores, in particular in hot, dry conditions, but there have
been many examples where this system has not worked as
well as it theoretically should because of dry patches in the
moist pad, an uneven pad, holes in the pad, or saline
deposits clogging up the pad. Although the capital cost of
evaporative cooling can be low it requires a high level of
management and maintenance.
POST-HARVEST HANDLING OF POTATOES
Grading
The objective of the packhouse is to maximise the profit
from the incoming product. This may involve virtually no
treatment or extensive grading, cleaning and packaging,
depending on requirements of the end user.
There are normally a number of different operations
involved in a packhouse (often referred to as a grading or
packing line) which can simplified as crop cleaning, sorting
and inspection (grading), sizing and packing. As well as
these operations there are the considerations of supply and
removal of the product (and reject/trash) with the ever
present concern of damage limitation.
In all countries there are various laws and regulations in
place which are relevant to packhouse operation although
the range and detail may be different. The legislation may
cover aspects of hygiene, operator safety from machines,
noise, dust and excessive working hours. Other legislation
may involve pollution, disposal of waste and effect on the
surrounding area. As well as various regulations and
legislation the clients of the packhouse may also have their
own additional requirements and it is important to ensure
that all these requirements are considered.
Evaporative cooling
When air carrying less than the total quantity of water pos-
sible at that temperature, less than 100% relative humidity,
passes through a wetted pad or fine mist it will pick up
water thus increasing its relative humidity. At the same
time the energy required to vaporise the liquid water (the
latent heat of vapourisation) comes from the air and so the
air cools. This should mean that evaporative cooling has
considerable use in potato storage in warmer and also drier
climates as it can reduce temperatures and increase the
relative humidity of the air.
Reception facilities
The objectives of any bulk hopper system is to enable the
bulker lorry, trailer, bulk bucket or box tippler to unload
easily and quickly without damage to the crop and to provide
a continuous supply of potatoes to the packhouse line at an
appropriate rate. The size and dimensions of the hopper are
therefore dictated by its being wide enough and the correct
height for unloading and with a volume which can provide a
reserve of tubers so that the line can still work even with
intermittent supply. Typical capacities are between 3 and 15
tonnes. Another dimension that can be important is the
