Agriculture Reference
In-Depth Information
operating height above the bulk hopper as it has been known
for trailers to have too high a tipping height to be able to use
a bulk hopper, although this may be overcome with ramps.
The choice of hopper must fit the range of supply
systems and should also avoid having a narrow discharge
point as this can cause churning of the tubers and hence
damage.
The supply from the bulk hopper must be readily adjust-
able so as to give optimum supply to the sorting, sizing
and packing operation. The normal method of adjusting
supply is by a manual operated, variable speed motor but
sometimes proximity sensors or electronic eyes are used.
The objectives of a box unloading system are similar to
those for bulk hoppers but the supply is in discrete one or
half tonne units. The unloading system varies from equip-
ment which will simply empty the box as fast as possible to
systems which will gradually tip the box at the rate required
to feed the line. Systems that will tip gradually can be
controlled manually but more commonly are controlled by
an electronic eye or proximity sensor placed before the
first piece of operation on the line.
Some box unloading systems have the facility to weigh
each box before unloading, count the number of boxes or
have a bar code reader and it is anticipated that these
facilities will become more common in the future.
Box tipplers or rotating buckets can be fixed to fork lift
trucks or tractors and produce a rapid method of moving
potatoes. There will always be damage and cut tubers
around the bucket edge which may not be high in percentage
terms but will need to be graded out at a later stage.
of fingers. This action can break up small clods and also
in rubbing the surface of the tuber remove lightly attached
soil. The use of brushes also can remove lightly attached soil
and the quantity of surface abrasion can be changed by the
stiffness of the brush used. Brushes are sometimes used
with wet cleaning systems often after a soak tank to assist
in reducing soil adhesion to the tuber surface. These
systems can work well in removing small quantities of soil
but can change into mud cylinders if there are large
quantities of soil about.
Spools and discs can be used as a method of soil
extraction using principles similar to those described above
but in this case the forward motion comes from the tubers
behind pushing the crop forward.
Cleaners and washers
Choosing the best time to wash the crop is major element
in quality control. Washed potatoes are more attractive to
consumers and attract price increases. However, washing
also makes them more susceptible to deterioration and
loss. Fingers and Fontes (1999) claimed that the average
storage life of 30 to 40 days in perfect conditions reduced
to 7 to 15 days once potatoes had been washed. Washing is
therefore usually dictated by the imminence of conveying
them to market.
Particularly for the prepack market, it is a requirement
that tubers are washed, and this is normally done using a
barrel washer in which they are tumbled around in water
inside the barrel. The amount of cleaning depends on the
speed of rotation, the inside surface and the time in the
washer which is normally altered by changing the angle of
the washer to the horizontal. The barrel is typically around
15% under water so that the potatoes keep falling back into
the water, which provides agitation and cleaning. It is
possible to have a dry barrel cleaner with sides made of
bars. There can be a high level of damage to the tubers if
they are “thrown” against the sides of the drum but the
insertion of a “brake sail” made of a thick plastic material
can slow down the movement and reduce the variability of
the mechanical load (Geyer & Oberbarnscheidt 1998).
For the disposal of the water from a barrel washer it is
normal to use recycled water from both the cost and
environmental standpoint. Various methods are used for
removing mud, sand and fine organic particles such as
sieves, sedimentation and hydro-cyclonic systems (Geyer
1996). There is a variety of machines available on the
market suitable for washing and drying potatoes.
Stone seperation or removal is important not just because
of the fact that stones are a reject product but because of the
potential damage to potatoes and equipment. Most modern
Cleaners
Except in cases where the potatoes have been harvested
soil free or previously cleaned, some form of cleaning is
required. There are five main forms of tuber cleaning. In all
cases a compromise has to reached between the level of
cleaning and the possibility of damage.
The web-type system of soil extraction consists of a
series of interlocking rods or rods fixed to a side belt. The
webs are normally rubber covered (Huijsmans
et al
. 1990)
or hollow (McRae
et al
. 1990) to reduce damage. The
cleaning system relies on the web gap and agitation to
remove the soil and is similar to that found on a potato
harvester. This system will not remove stones or break up
clods but can be useful as an intial cleaning system or
precleaner.
A star cleaner consists of a series of interlocking rubber
stars which revolve on shafts at right angles to the direction
of product flow. The soft rubber fingers are backward
curving and raise the tubers as they pass onto the next bank
