Agriculture Reference
In-Depth Information
Table 16.8 Factors Affecting the Severity of
External Damage.
Likelihood of splitting
One damage assessment method was developed by the
Scottish Centre for Agricultural Engineering (Robertson
1970). Different levels of damage were categorised as follows:
Less
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References
1. undamaged (0 points)
2. slightly bruised: the tuber may be only scuffed, where a
bruise may be detectable after a stroke of a hand peeler,
but not after two strokes (to a depth of 1.5 mm) (1 point)
3. moderately bruised: where bruises necessitate peeling
to a depth of 4 mm to remove (3 points)
4. severe bruising: where the bruise can only be removed
by peeling to a depth of more than 4 mm (7 points)
Decreasing drop
height
Increasing drop
height
McRae et al . 1975
Impact flat
surface
Impact web rod
surface
McRae et al . 1975
Shimada 1980
Decreasing
tuber weight
Increasing tuber
weight
Parke 1963
Higher
temperature
Lower
temperature
Johnsonson and
Wilson 1969
The method is used for both harvesting and handling
operations and the total score for handling operations
should be less than 50 per 100 tubers. The two main
drawbacks with this system is that it is stepped and does
not include bruising damage. There are other methods
which give a linear rather than stepped index and include
bruising. Evans and McRae (1996) described a system
which involved the number of peeler strokes which were
needed to remove all damage and bruised areas to give the
damage index.
One difficulty with bruise assessment is that bruises take
time to develop and the standard method used by many
growers and packhouses involves placing the tubers in a
high humidity atmosphere for 24 h at around 37°C before
bruise assessment. It has been shown that holding the
potatoes at a similar temperature but in pressurised oxygen
(1.5 bar) will develop latent bruising (Duncan 1973);
results could be obtained in seven hours. The system was
further developed to give results in five hours with humidi-
fied oxygen (Melrose & McRae 1987). However, owing to
safety and cost considerations work was then carried out
using compressed air at 3 bar which gave similar periods of
time to oxygen (McRae & Melrose 1990).
To assess the susceptability of a tuber to impact damage
it is necessary to measure both mechanical and chemical
properties (Baritelle et al . 1999b).
After impact the flesh of the tuber changes colour and
Dean et al . (1993) suggested that the grey/black colour was
caused by both enzymatic and non-enzymatic oxidation of
phenolic substances by the enzyme polyphenoloxidase
(PPO). This oxidation results in the formation of melanin
pigments. Reeve (1968) suggested that PPO mixed with
phenolic substances when the cell membranes were
disrupted, meaning that damage to the cell wall may not be
required for membrane failiure to occur. Baritelle et  al .
(1999b) have developed a method for producing fast results
for measuring the chemical properties of the tuber in
Flaccid tubers
Turgid tubers
Hesen and
Kroesbergen
1960
Finney and
Findlen 1967
Mature tubers
Immature
tubers
Finney et al .
1964
The importance of falling distance, conveyor belt speed
and materials used has been known for many years and has
been well documented since the 1950s (Vollbracht &
Kuhnke 1956). In a large packhouse the tubers can pass
over up to 100 transfer points during movement from store
to retail bag and this can greatly increase the risk of damage
(Peters 1996).
The factors that influence the extent of external damage
were summarised by McRae (1985) and are given in
Table 16.8.
Instrumented spheres have been developed in a number
of countries, to measure the level of impact that the tuber
experiences. Two examples of the results obtained are
given in Figures 16.7a and b (Maunder et  al . 1990) of
grading into store and from a grading and packing
line.  Instrumented spheres are useful for the assessment
of  individual grading lines and comparison between
packhouses.
Assessment of damage and susceptibility
Assessment of damage is the quantification of damage that
has already ocurred and the assessment of susceptability is
the potential for damage of a specific variety or the poten-
tial for damage in a specific situation. In both cases any
system should be quick to use, repeatable and not operator
dependent.
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