User:Hudaa009/sandbox

PREAMBLE
The increased demand by consumers for fresh-like, safe, nutritious, convenient and flavorful packaged foods have paved the way for the continuous emergence of novel food processing technologies. A number of innovative thermal methods including sous-vide, microwave, radiofrequency and infrared heating and non-thermal methods including high hydrostatic pressure, irradiation and pulsed light technology for processing of packaged foods, have become the subject of active research and development.

SOUS-VIDE PROCESSING
Sous-vide is a French term literally meaning “under vacuum”. This technology allows food to be thermally processed using vaccum-packaging in heat-stable, high barrier or air-impermeable multi-laminate plastics. This form of processing is especially amenable for food consisting of partially cooked ingredients alone or combined with raw foods, requiring low temperature storage until the packaged food is thoroughly heated immediately prior to serving (Ghazala, 2004). In short, sous-vide is an “Assemble-Package-Pasteurize-Cool-Store” process. Figure 1 provides a simplified flow diagram that outlines the basic steps in sous-vide processing.

MICROWAVE HEATING (MW)
Microwaves are a form of electromagnetic radiation characterized by the wavelength and frequency of the waves used in food processing (915-2450 Hz). Microwave heating is generated by the conversion of electromagnetic energy to thermal energy. The technology of microwave heating of foods has garnered scientific and consumer interests due to its volumetric origin, rapid increase in temperature and relative ease of cleaning (Ahmed and Ramaswamy, 2007). Unlike more traditional forms of thermal processing, such as pasteurization and retorting which are characterized by a slow thermal diffusion process, the volumetric nature of heat generated by microwaves can substantially reduce the total heating time, thereby minimizing the overall severity of the process and leading to a greater retention of the desirable quality attributes of the product (Sumnu and Sahin, 2005). According to Tewari (2007), the time required to come to target process temperature is attained within one-quarter of the time typically reached by conventional heating processes. Microwave technology is also amenable to batch processing and therefore can be used to pasteurize pre-packaged products as well as offer the flexibility of being easily turned on or off.

INFRARED (IR) AND RADIOFREQUENCY (RF) HEATING
Another processing method involving dielectric heating is RF heating which has the potential for the rapid heating of solid and semi-solid foods. RF heating refers to the heating of dielectric materials with electromagnetic energy at frequencies between 1 to 300 MHz (Orfeuil, 1987). Research and application of RF sterilization of packaged foods has been fairly limited. Previously, researchers at Washington State University, in conjunction with laboratories at Strayfield UK, the U.S. Army Natick Soldier Center and the U.S. Army Combat Ration Network have designed and created a prototype 27-MHz RF sterilization system to process foods pre-packaged in a polymeric tray or a multi-tray system to mimic large-scale industrial production systems (Ramaswamy and Tang, 2008). The research group demonstrated the efficacy of RF energy to inactivate heat-resistant spores to produce shelf-stable pre-packed foods such as RF-processed eggs, pasta and meats. In addition, research work undertaken by the group also involved the development of computer models to predict RF energy generation in packaged foods; as well as, improvement in the design of RF sterilization systems (Ramaswamy and Tang, 2008). Figure 3 is a schematic diagram of a generic RF dielectric heating unit adapted from Zhao and others (2000). IR uses electromagnetic radiation generated from a hot source (quartz lamp, quartz tube or metal rod) resulting from the vibrational and rotational energy of molecules. Thermal energy is thus generated following the absorption of radiating energy. Although IR is an emerging technology for the food industry, research on its application to process food has been conducted on an experimental or pilot-scale. IR provides instant heating, thereby cutting down on the need for heat build-up. In addition, compared to conventional heating equipment, the operating and maintenance costs are lower and it is a safer and cleaner process.

HIGH HYDROSTATIC PRESSURE (HHP)
Among the array of non-thermal processing technologies, HHP has garnered the most attention since the early 1990s. For the last 20 years, HHP has been explored quite intensively by food research institutions as well as the food industry with the goal to enhance the safety, quality, nutritional and functional properties of a wide variety of packaged foods with minimal deleterious effects on their nutritional and organoleptic characteristics (Welti-Chanes et al., 2005). Figure 4 visually compares untreated and pressure-treated (350 MPa/ 2min/ 4°C) strawberries, blueberries and raspberries. HHP-treated blueberries and strawberries did not undergo extensive quality deterioration although raspberries became noticeably softer.

During HHP, pre-packaged foods are exposed to pressures of the order of 200–600 MPa for a few minutes. Process temperatures during treatment can vary from subzero temperatures to above boiling point of water (100°C) (Caner et al., 2004a). Because HHP is performed on packaged foods, cost-intensive aseptic package sterilization can be avoided (Rastogi et al., 2007).

IRRADIATION
Food irradiation is a process involving the exposure of food to ionizing radiations, such as gamma rays emitted from the radioisotopes cobalt–60 and cesium–137, and high-energy electrons and X-rays produced by machine sources (Ohlsson, 2002). Ionizing radiation can be applied to decontaminate pre-packaged food. Since food and packaging materials are irradiated concurrently, the radiation stability of the packaging material is of paramount importance if the technology is to be implemented successfully. PULSED LIGHT TECHNOLOGY (PL) PL technology is an innovative method for the decontamination and sterilization of foods using very high power and very short duration pulses of light emitted by inert gas flashlamps (Palmieri and Cacacea, 2005). The high power pulses of radiation can be in the spectra of UV, visible (VL) and IR light. PL, in addition to being used for the sterilization of packaging materials and online sterilization of transparent fluids, has also been successfully used for the surface decontamination of foods in plastic packaging. This process has shorter treatment times with concomitantly higher throughput, rendering the process very efficient.

ACTIVE PACKAGING
Active packaging is more dynamic and is able to sense changes in the environments in the interior and exterior of a package by adjusting properties of the package (Han, 2007). Active packaging serves as an effective intervention or hurdle to reduce or prevent microbial growth during food storage. This is achieved by: (i) actively or constantly changing permeation properties or the gas composition in the package headspace during storage, or (ii) actively releasing minute concentrations of antimicrobial agents, incorporated or impregnated into the packaging materials, overtime (Hurme et al., 2003). Figure 5, adapted from Han (2003), depicts the structure of a generic antimicrobial package, consisting of a base such as a plastic film coated with a carrier such as methylcellulose into which antimicrobials such as nisin, lyzozyme etc. can be incorporated.

CONCLUSION
Recalls, largely caused by the transference of pathogens into food products between the final processing step and packaging of perishable foods, have spurred interest in post-packaging decontamination. Improvements to existing designs and development of new technologies for thermal post-packaging applications have included sous-vide processing, microwaving, and RF heating. Non-thermal post-packaging decontamination systems can include high hydrostatic pressure, irradiation, pulsed light technology and active packaging. One can expect continuing research of these technologies in tandem with advances in food packaging materials, given increasing consumer awareness of product safety linked to human health and the potential of economic losses associated with contaminated products. Since non-thermal methods do not incorporate additional exposure of packaged products to elevated temperatures which can further degrade sensory quality and nutrient content, one can easily envision post-packaging decontamination using a non-thermal method, such as HHP, becoming more commonplace in the food industry in order to assure enhanced food safety at a moderate cost.