Potatoes - Crop Post-Harvest: Science and Technology - page 384

subjects the tuber to a mechanical vibrational input which
then produces a number of characteristic resonance peaks
as a response; the second resonance frequency corresponds
to the principal mode of vibration for the flesh and this
frequency has been observed to fall as the tuber softens.

Conveyors and elevators
Cushioning is important at many points in the handling
system and “any cushioning material must combine high
surface wear resistance with retention of resilience over
a long period” (McRae 1990). Armstrong et al . (1995)
summarised the requirements of any cushioning material as
absorbing at least 60% of the impact energy to minimise
product rebound, the durability and ability to clean must be
good and the uniformity between different production lots
should be low. Bollen and Dela Rue (1995) evaluated a
number of cushioning materials and found that “a closed
cell PVC foam was the best material with polyethylene
foam and neoprene rubber exhibiting adequate characteris-
tics over the energy range of the tests”. For the purposes of
testing, the padding materials were impacted by an instru-
mented sphere (Zapp et al . 1989) at six energy levels
between 0.3 J and 1.8 J using a pendulum impact device.
Frictional damage is an area which has received only
limited interest, the dynamic coefficient has been
investigated by Schaper and Yaeger (1992) in 25 kg lots
and by Bishop (2007) which both show significant differ-
ences in between surfaces and for clean or dirty dry or wet
tubers. The potential for frictional damage or scuffing can
be further increased if tubers are transferred from one con-
veyor to another where the tuber is rotating in the opposite
direction to the second conveyor giving an increased effec-
tive contact velocity (Bishop 1990).
Although all drops during potato handling should be
minimized, the size of drop may vary with the level of
tubers such as in a holding hopper. Work done by O'Brien
et al . (1980) on filling systems for fruit and vegetable crops
found that an elevator with height sensor was the most
effective. In some situations it may not be possible to have
a height or proximity sensor and in these cases a method
using a telescopic “zig-zag” system may be used, in which
the direction of the potatoes is constantly changed and the
downward velocity slowed.
The level of dust produced in a potato packhouse can be
very much influenced by the design of the handling system
and, for instance, large drops of soil from soil extraction
systems can greatly increase dust problems. McGovern
(1991) describes dust extraction systems for use in grading
lines. Increasingly, to satisfy health and safety considera-
tions, air suction points are required at any drops in the

Figure 16.6 An inspection line for potato tubers
with lighting placed immediately above the
conveyor belt.

Zegers and van den Berg (1988) showed that the inspection
should be carried out with an illumination of between 500
and 2000 lux with no significant differences of performance
within this range. Often the illumination level, in practice, is
below this and in work carried out by Hyde (1991) in the
state of Washington the levels varied between 350 and
700 lux.
Zegers and van der Berg (1988) also suggested that the
colour of the light should be at least 85 on the colour
rendering index for effective inspection. The colour
rendering index is effectively a measure of whether there
is a preponderance of a particular colour in the light used;
this could make it more difficult to see certain defects on
the tuber surface. Natural light has a colour rendering
index of 100. The colour rendering index found to be
most common by Hyde (1991) was 62 with a cool-white
flourescent tube.

Vision grading
For a number of years there has been the potential for
potatoes to be graded automatically with systems available
to sort by size, colour, shape and blemish (Muir et al . 1989;
Tao et al . 1990). Rates of 30-40 tubers per second have
been achieved. Although there has been much interest in
these systems and one or two have gone into commercial
locations there has been no large uptake for potatoes. The
initial capital cost and the precise definition of what is
acceptable have proved obstacles to their adoption.
Although not directly connected to vision grading the
use of acoustic resonance sensing to assess firmness is one
of the other areas that could have increased importance in
the future (Terdwongworakul et al . 1995). This method