Nonthermal Processes Yielding Microbial Inactivation

Thermal preservation processes such as retorting canned foods and pasteurizing milk and eggs have the disadvantage of degrading product quality as perceived by the consumer as well as nutritional quality. In recent years, much research has been conducted with success to develop nonthermal food processes that destroy microorganisms. These processes include irradiation, ultrahigh hydrostatic pressurization, pulsed electric fields, oscillating magnetic fields, high-intensity visible and UV light, and ultrasonics (manothermosonication). In all of these processes, microbial inactivation increases with the duration and intensity of the treatment, analogous to thermal processes.

Food irradiation has gained prominence worldwide as a nonthermal method. Irradiation refers to ionizing electromagnetic radiation that inactivates microorganisms by producing high-energy electrons within the food product. Irradiation has been approved for use in the United States on a number of food products (Table 6). Specifically, the

FDA has approved irradiation from sealed units of radioactive nuclei (131Cs or 60Co), electron beams generated from machine sources at energies not to exceed 10 MeV, or X rays generated from machine sources not to exceed 5 MeV.

The legal basis for regulation of food irradiation by the FDA is that irradiation is included in the definition of "food additive" in the 1958 Amendment to the FDCA. The primary concern is that the radiation, in the process of destroying microorganisms and enzymes, will produce toxic substances. However, the Joint FAO/IAEA/WHO Expert Committee on the Wholesomeness of Irradiated Food (JECFI) concluded in 1980 after review of extensive research that irradiation of any commodity up to an overall average dose of 10 kGy presents no toxicological hazard. Despite its safety and potential benefits for a safe food supply, food irradiation still remains a controversial food process with many food processors unwilling to install irradiation systems.

The other nonthermal preservation methods have not found widespread usage and are still under development. Ultrahigh hydrostatic pressure (UHP) has been shown to be effective in inactivating microorganisms (vegetative bacteria, parasites, yeasts, and molds) at pressures between 300 and 600 MPa (43,000 to 87,000 psi). Processes can be made continuous for liquid foods, and processing times range from one minute to one hour at room temperature. The UHP process can also be enhanced by increasing temperatures up to 90°C, to the point of inactivating enzymes and spores. The other nonthermal processes involve exposing the food to high-intensity field strengths for short durations, sometimes including up to 100 rapid pulses. The degree of microbial inactivation depends on the strength of the field, its duration, and the number of pulses. Increasing temperature during these processes also enhances inactivation.

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