Mechanical Injury of Fresh Produce 83
4.3 TISSUE DAMAGE
4.3.1 WOUND SEVERITY
The extent of product damage varies according to a number of conditions. Variety,
temperature, water content, specific gravity, and degree of maturity are some of
these. Mechanical parameters of fruits and vegetables depend on cell dimension and
cell wall structure. The initial effect of an impact sufficient to cause damage is cell
rupture and/or loss of tissue integrity extending from the organ surface at the impact
site to several cell layers below. External bruises are caused by failure of the outer
pericarp due to cell rupture or fracture between cells when the tissue receives an
impact above the bio-yield point. Normally, the tissue affected by the impact is softer
than surrounding tissues. Internal bruising is, however, caused by impacts lower than
the bio-yield point if the impact energy is transmitted to the underlying locular tissue
with sufficient force so as to impair ripening. In tomatoes, this is indicated by
subsequent abnormal color development and increased gel viscosity.^30 Sargent et al.,31,32
in a study of four tomato cultivars, determined that internal injury could be cumu-
lative. Tomatoes handled at the breaker stage were significantly more susceptible to
internal bruising than those handled mature-green. Internal bruising can be easily
detected by the unusual increase in ethylene (C 2 H 4 ) production (wound C 2 H 4 ) several
hours later. The CO 2 respiration rate may also be used to evaluate internal fruit
injury.^33
Internal bruising significantly affects chemical composition and physical prop-
erties of pericarp and locule tissues, but not placental tissue. In bruised tomato locule
tissue, carotenoids, vitamin C, and titratable acidity were 37, 15, and 15% lower,
respectively, than in unbruised fruit. For bruised pericarp tissue, vitamin C content
was 16% lower than for unbruised tissue, whereas bruising increased electrolyte
leakage and extractable polygalacturonase activity by 25 and 33%, respectively.
Evidence of abnormal ripening following impact bruising was confined to locule
and pericarp tissues and may be related to the disruption of cell structure and altered
enzyme activity.^34 Firmness plays a critical role in resistance to mechanical damage
resistance of apples. Apples tested using destructive (static and impact penetrome-
ters), nondestructive (sphere probe penetrometer), and subjective techniques indi-
cated that during harvesting the highest coefficient of variation was 16.6% for
bruising resistance.^35 During storage the highest coefficients of variation were 19%
for rupture force and 18% for penetration depth. Puchalski and Bieluga^36 also
demonstrated that cultivars with the firmest flesh consisting of smaller cells and
intercellular spaces are more resistant to mechanical damage. During storage, the
resistance factor fell as a result of ripening processes that weakened the intercellular
bonds and decreased the firmness of the fruit. The damage resistance of apples,
however, cannot be solely assessed from firmness measurements. Absorbed energy
at impact often correlates linearly to bruise volume of apple fruit. The elasticity
theory postulated by Roudot et al.^37 permits calculation of the limit strength of apple
fruit flesh on the basis of the diameter of the impact surface and the depth of tissue
damage. Pressure marks on apples appear to have no effect on parasitic spoilage,
although some physiological defects that may be variety-dependent could occur.