92 Produce Degradation: Reaction Pathways and their Prevention
apples have lower reflectance than those of undamaged apples in both the near-
infrared and visible spectra, but the injuries are detected more easily in the near-
infrared spectrum. Based on this, Damerow^108 concluded that the reflectance index
at 800 nm could be used to distinguish the grade of injury. Recently, Varith et al.^109
detected bruising in apples using thermal imaging of dropped apples stored at 26°C
for 48 h. The temperature differences between bruised and sound tissues were
possibly due to the differences in thermal diffusivity. Under steady-state temperature,
thermal imaging did not detect bruises, indicating that the temperature differences
were not due to emissivity differences.^109
Electrical conductivity has been investigated in relation to percentage of pared
fruit or damage indexes prepared by manually paring strawberries and stimulating
bruising with a three-dimensional vibrator. The conductivity was associated with the
percentage of pared fruit (0 to 40%) or damage indexes (1 to 5) of strawberries,
with correlation coefficients of 0.938 and 0.917 at P ≤ 0.05, respectively. Further-
more, the bruising caused by the three-dimensional vibrator resulted in increases in
electrical conductivity in response to vibrating time and vibrating force. The decrease
in electrical conductivity of strawberries treated with the vibrator during 2 days of
storage at 25°C and 80% RH may be due to wound healing of the fruit. Electrical
conductivity exhibited potential for quantitatively evaluating damage of strawberries
during transportation and marketing.^110
Analysis based on determining scar tissue, suberin, and resistance to fungi may
be used for evaluating the storage quality of potato tuber lots. In a study of tuber
lots of the variety Nevskii harvested manually and mechanically, tubers without
mechanical damage and visible disease symptoms formed scar tissue more quickly
and had thicker scar tissue and a thicker suberin layer than the rest.^111 Both a muriatic
acid dip test and X-ray computerized tomography have been found to provide some
indication of damage of sweet onions under certain conditions. Muriatic acid has
been found to cause the flesh of a freshly harvested sweet onion to appear yellow
and become a sticky gel in the vicinity of damage, thus enhancing the visibility of
damage. X-rays also aid viewing of the internal structure of a sweet onion and thus
damage that may have occurred.^112
4.8 METABOLIC CHANGES
4.8.1 RESPIRATION
An increase in respiration rate is one of the first responses to mechanical injury. In
general, respiration rates are inversely related to the shelf life of produce.113,114 Fresh-
cut fruits and vegetables best illustrate the effect of wounding on respiration rate.
The increase in respiration rate following fresh cutting normally results in decreased
product shelf life relative to that of the whole fruit.114,115 Fresh-cut products from a
number of fruits show increased respiration rates relative to the whole fruit.115-118
The respiratory rate response to wounding, however, may depend on the commodity.
The rise in respiration after cutting or wounding may be related to α-oxidation of
long-chain fatty acids that results in CO 2 release.114,116,120 Cutting does not appear to
influence the respiration rate of oranges during refrigerated storage,^121 while cut