304 Produce Degradation: Reaction Pathways and their Prevention
soil fertilized with typhoid excreta; and in 1912 it was demonstrated that lettuce and
radishes grown in soil containing Bacillus typhosa (now Salmonella typhi) contained
the microorganism on their surface for 31 days^ (Beuchat, 1998). Fruits and vegetables
can be contaminated with pathogenic microflora during cultivation, harvesting, trans-
port, distribution, marketing, and manipulation at home. Bacteria such as Clostridium
botulinum, Bacillus cereus, and Listeria monocytogenes are frequent in soil. Salmo-
nella, Escherichia coli, and Campylobacter reside in the intestinal tracts of animals,
including humans, and can contaminate raw fruits and vegetables that come in
contact with feces, sewage, untreated irrigation water, or surface water (Bibek, 1996;
Beuchat, 1998). To minimize safety concerns, the principles of good hygiene practice
and the preventive HACCP concept should be applied to avoid or minimize the
contamination of products and to improve effectiveness of preservative methods.
10.2 METHODS FOR PRESERVING FRESH AND
PROCESSED PRODUCEAfter harvest, deterioration of fruits and vegetables commences, mainly by physio-
logical and enzymatic changes in the tissues. The preservation methods used to
extend the shelf life of fresh produce involve optimizing or slowing down of ripening
and senescence.
10.2.1 PREVENTION OF PHYSIOLOGICAL CHANGES
Undesirable physiological changes of fresh produce can be prevented by optimiza-
tion of environmental conditions. Manipulation of environmental conditions is usu-
ally performed to lower respiration of products and to reduce the growth of decay
organisms without inducing physiological injury. Temperature and relative humidity
are the principle factors. The optimal conditions for manipulation and storage of
fresh produce have been summarized in several sources (Flores Gutiérrez, 2000).
10.2.1.1 Temperature
Temperature is a major factor in the control of decay organisms, respiration, and
transpiration. Quick cooling after harvest is critical, especially in the case of rapidly
respiring tissues such as spinach or broccoli (Shewfelt, 1986). Rapid precooling of
produce can be achieved by room cooling, contact or package icing, hydrocooling,
forced-air cooling, hydraircooling (fine-mist spray combined with forced-air cool-
ing), vacuum cooling, and cryogenic cooling. Recent developments in precooling
techniques and their comparison have been reviewed by Brosnan and Sun (2000).
During handling and storage, chilling or freezing injury should be avoided. If
the shelf life requirements are too short compared to the time needed to develop
low-temperature disorders, such as overnight holding prior to processing, chilling
injury may not be a major concern. The most common prevention of chilling injury
is to store chilling-susceptible produce at or above the temperature at which no
injury occurs or to use various postharvest techniques to alleviate chilling injury.
These methods include, for example, preconditioning, such as holding produce at