Agriculture Reference
In-Depth Information
there can be pressures to lower the store temperature to
benefit the product that has been in store for some time.
To monitor the progress of wound-healing, it is suggested
that a few tubers are cut and placed on the corner post of
boxes and checked daily to see when they have healed over.
system used, the size, number and type of fan, the control
system, wall design and whether there is refrigeration or a
mixer box (blending air from inside the store with ambient
air which is in itself too cold by itself) is used The maximum
depth of potatoes is 3-5 m depending on the air distribution
system (Pringle
et al
. 2009)
The type of air distribution system will depend upon a
main duct with smaller ducts or laterals off it. These laterals
may be above or below ground. The main duct should be
large enough for the store operator to be able to walk down
easily when it is necesary to open or close laterals. If the
duct is difficult to walk down it does not encourage regular
store management, In a number of recorded cases where
the store management has not been satisfactory the ease of
operation has been cited as an important reason (Holloway
1990). The maximum air speed that should be permitted
inside the main air duct is 10 ms
−1
(0.1 m
2
for each cubic
metre of fan capacity at the appropriate rating). Preferably
the inside surface of the main duct should be free of any
projections that could cause turbulence any the cross
sectional area of the duct should be measured inside any
internal strengthening members.
The smaller side ducts or laterals can be above or below
ground. The advantages of below ground ducting is such
that the store can be more easily filled without the laterals
having to put down as the store is filled or being an
obstruction as the store is being emptied. Also with the
below ground laterals do allow the building to be used for
another use. Above ground ducts are normally of the
“A” type, although plastic drainage piping can be used
(Bishop 1994).
Ambient ventilation
Although there has been a trend to refrigerated storage in
Northern Europe, Storey
et al
. (1994) stated that in a PMB
(Potato Marketing Board) survey that over half the storage
in the UK was in boxes and 46% had refrigeration. The use
of ambient air for maintaining the storage environment is
still popular in tonnage terms. The actual ventilation
strategy and the ventilation rate varies considerably with
geographic areas. In the United States the recommended
rates range from 0.005 m
3
/st
−1
(Sparks 1973; 1980) to
0.0075 m
3
/st
−1
(Sawyer
et al
. 1965). In central and northern
Europe (e.g. the Netherlands and United Kingdom) they
range from 0.019 to 0.0278 m
3
/st
−1
(Rastosky & Van Es 1981)
in southern Europe (e.g. Italy) rates as high as 0.0308 + m
3
/st
−1
are suggested (Bertolini & Guarnieri 1990).
Reasons for these variations depend on local climate,
market requirements and the condition of the tubers going
into store; in northern Europe it may be necessary to dry or
partially dry potatoes going into store. The ventilation rate
required will also vary depending on whether the potatoes
are stored in bulk or in boxes or crates. Also important in
the decision of ventilation rate is the expected or potential
quantity of soil on the tubers at storage as well as any
moisture.
Forbush and Brook (1993) carried out work over three
seasons to investigate the influence of airflow rate, in bulk
stores, on the temperature, moisture and market quality of
process potatoes using the variety Atlantic. The ventilation
rate used were 0.016, 0.032 and 0.010 (for one season) m
3
/
st
−1
. The temperature was maintained in all three cases with
also no difference in sugar, fry colour or quantity of defects.
There was also no statistical difference in weight loss how-
ever “potato loss uniformity was proportional to increased
ventilation rates”.
In some cases the ventilation may be continuous (Hymlo
et al
. 1976; Grahs
et al
. 1977; Hymlo
et al
. 1979) or it may
be variable depending on how much cooling is required.
The variation may be achieved by altering the number of
fans in operation or by having a variable speed fan.
Box storage
The reasons for the popularity of box stores is that they
allow for different varieties of potatoes to be stored
together, but kept distinct. Traceability is much easier with
box storage as each grower/harvest date/field can be seper-
ately labelled. Normally the information on the crop is
written on a card and stapled to the box but bar codes are
becoming more commonly used. Disease transmission is
much slower in a box store and may well be confined to a
specific box. Almost all stores are using wooden boxes of
nominally one tonne (1.2 m × 1.8 m × 0.9 m) although in
practice the capacitty is nearer to 950 kg. Figure 6.4 shows
a typical system used to fill boxes.
Bulk storage systems
There is only one basic design of forced ventilation bulk
store which involves air passing through the crop from
below. There are differences in the type of air distribution
Ventilation systems in box storage
There are four basic methods of ventilating box stores:
nonducted, differential ventilation, letterbox or serpentine
ventilation and postive ventilation system.
