Mechanical Injury of Fresh Produce 85
treatments such as dipping in calcium hypochlorite, ethyl alcohol, benomyl, or
Dichloran do not yield consistent results.^46 It is postulated that the wound healing
response, effective in many other storage tissues, is insufficient in cassava to prevent
further damage.^45 It is suggested that genetic manipulation of cassava might be
beneficial in regulating wound response genes. Potato tuber cells with high turgor
pressure (nearly 0.4 MPa) when hit by a pendulum striker created shatter bruises.
With a decline in turgor to < 0.2 MPa, little or no shatter bruising occurred. Effects
of cultivar on severity of tissue damage was demonstrated by Laerke et al.,^47 who
found that tubers of the potato cultivar Maris Piper (MP) were 10 times more
blackspot-susceptible than those of the cultivar Colmo (CM). The membranes of
MP leaked more K+, Mg2+, and Ca2+ than those of CM. Both cultivars showed a
very fast increase in K+ leakage immediately after impact. Thereafter, CM seemed
to reconstitute the membranes, while a high efflux of K+ from MP continued for up
to 32 h of incubation. It has also been reported that potato tubers from 0 and 10%
potassium treatments had a significantly lower Young’s modulus (a measure of
stiffness) than those grown at full-strength Hoagland’s solution. Significantly lower
failure stress and shock waves of impact were also propagated through both of
them.^48
4.3.2 MECHANICAL HANDLING
Several reports have shown that bruising is linearly related to impact energy,^49 and
mechanically stressed fruit exhibit visible degeneration of mesocarp and endocarp
during storage. Other factors that determine differences in bruising susceptibility of
fruit include methods of packaging, transportation, and harvesting; storage condi-
tions; and fruit characteristics. Mechanical stress itself can also modify fruit metab-
olism, with important consequences on quality of the product. Mechanical handling
on grading lines is one of the most hazardous operations as far as mechanical damage
is concerned. Fruit bruising occurs as an overall effect of two combined factors:
machine roughness and intrinsic fruit susceptibility.^50 While electronic fruits similar
to the fruit in size and shape may be used in grading lines to assess machine
roughness,^51 relationships between bruise susceptibility and physical and rheological
properties of fruit are used to assess intrinsic fruit susceptibility.^52 Thus, bruise
susceptibility increases for lower curvature radius of surfaces in contact, higher tissue
turgidity, higher viscoelastic behavior, and lower fruit firmness.^53 Beilza et al.^53
recently described a method to estimate bruise probability by means of logistic
regression that can be used for prediction and simulation models to describe the
flow of fruits along the grading line and to evaluate impact at each transfer point.
The model was also used to estimate the percentage of damaged fruit at the end of
the line as a function of its intrinsic susceptibility.
The steps at which onions are most likely to suffer damage are generally the
drops between conveyor belts at various stages of harvest and postharvest handling.
Holik^54 determined the degree of damage for onions in which 35- to 50-g onions,
dried in the field, were dropped from heights of 0 to 1,000 mm. Whereas onions
seemed to tolerate falls of up to 400 mm, falls of around 1,000 mm caused consid-
erable mechanical damage. Machinery and storage containers that minimize the