In the thermal processing of foods, heat inactivates both microorganisms and enzymes to extend the shelf life of the treated foods. With application and incorporation of nonthermal processing methods the same microorganisms and enzymes in the food are usually targeted with the same expectation of inactivation or control; however, the actual mechanisms of microbial inactivation and protein denaturation are usually different when comparing thermal processing to nonthermal processing methods. Different mechanisms of inactivation mean the rates of inactivation are different and the degree of effectiveness is also usually different. Consequently, the usual primary goal in adapting the nonthermal methods for commercial use is to maintain the same high degree of safety enjoyed with thermal processing while minimizing changes to the desirable sensory qualities and nutrition in the product. That can be an immense challenge depending on the food product. Each nonthermal process is somewhat distinct with its own set of limiting factors that can include an inability to denature browning enzymes, limited or nonexistent inactivation of viruses or bacterial endospores, regulatory hurdles, high costs for equipment and maintenance, and a lack of background information usually provided by past industrial experience. Examples of areas of insufficient information for these new technologies include standardization of industrial process procedures, and methods and surrogate identification for process validations. These factors make the commercialization of new food products utilizing nonthermal processes complex and daunting.
With the exploding demand for fresh fruits and vegetables by consumers in North America comes the necessary importation of produce from areas of the world where crops can be grown and harvested all year round. For example, the Produce Marketing Association stated that imports of fresh produce increased from 13.8 billion pounds in 1993 to 20.2 billion pounds in 2000. The quality of fruit in winter is now nearly equivalent to fruit sold in the warmer months due to significant improvements in storage, transportation, and distribution. Unfortunately, the incidence of foodborne illness also coincides with this increased bounty of fresh fruits and vegetables (both domestic and imported). For example, in the marketplace the most dangerous food related to acute illness is not derived from meat, milk, eggs, or seafood, but is a plant product: sprouts . In fact, while meat is the most regulated and monitored food commodity, in comparison fruits and vegetables receive only a fraction of government attention. Federal health surveillance of foodborne diseases from 1993 to 1997 documented 2,751 outbreaks that involved 12,537 individual cases of foodborne illness related to contaminated fruits and vegetables, compared with 6,709 cases involving meat products. Outbreaks linked to fruits and vegetables are often the result of fecal contamination caused by inadequate hygiene during production and harvest. Imported fruits and vegetables now appear to harbor exotic and emerging parasites once unknown in North America, and traditional pathogens, such as salmonellae, hemorrhagic Escherichia coli, and Listeria monocytogenes, can be anticipated in both domestic and imported produce as a consequence of poor agricultural practices involving the composting and distribution of manure. Landmark outbreaks involving cyclospora in Guatemalan raspberries, salmonella-contaminated sprouts, E. coli in fresh cider, and Mexican scallions tainted with hepatitis A have become well-known reference points to American consumers who worry about the safety of the foods they eat.
The aim of this chapter is to furnish an overview of the use of nonthermal processing technologies applied for the preservation of fruits, vegetables, and their byproducts, most notably juices. The nonthermal processing methods that are covered in this chapter include high hydrostatic pressure processing, and applications of ionizing irradiation, high-intensity pulsed electric fields, ultrasonic waves, and electrolyzed oxidizing water.
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