Microwave sterilization of sliced beef in gravy in 7-oz trays
Introduction
Microwave (MW) heating is a result of interaction between alternating electromagnetic field and dielectric material (Orfeuil, 1987). Compared with conventional heating using water or steam as the heating media for package foods, MW energy has the potential to provide more uniform and rapid volumetric heating.
As a unique thermal processing technology, MW heating has been successfully used in food industry, including tempering or thawing of bulk frozen foods (meat, fish, and others), cooking of bacon and sausage, and drying of pasta and vegetables (Bengtsson and Ohlsson, 1974, Hulls and Shute, 1981, Hulls, 1982, Jones, 1992, Schiffmann, 1992). Research on MW treatment of foods has also been reported for disinfesting of insects in agricultural commodities (Wang et al., 2003a), blanching of vegetables, inactivating of enzyme, pasteurization of breads, cured hams and sausage emulsions, sterilization of food products (Decareau, 1985, Venkatesh and Raghavan, 2004).
Two frequencies, 2450 ± 50 and 915 ± 13 MHz, are allocated by the US Federal Communications Commission for MW heating applications (Decareau, 1985, Metaxas and Meredith, 1983). Two thousand four hundred fifty megahertz is widely used in domestic MW ovens and some industrial applications. Two thousand four hundred fifty megahertz systems have the limitations of small penetration depth (∼1 cm) and multi-mode cavities, causing non-uniform and unpredictable heating patterns in food packages. In general, 915-MHz microwaves can penetrate much deeper (∼3 cm) in foods, and therefore may provide more uniform heating (Mudgett, 1989). Nine hundred fifteen megahertz systems could be established with a single-mode cavity, which could provide predictable electromagnetic field, resulting in predictable and reproducible heating patterns in foods.
As public concerns over food safety continue to grow and the demand for high quality packaged convenience foods increases, MW heating is drawning much attention of researchers in developing novel pasteurization and sterilization processes for packaged foods. A 915-MHz, single-mode, MW sterilization system for processing packaged foods was developed at Washington State University (WSU) (Tang et al., 2006). It was intended for proving the concept and demonstrating the potential of MW technology in pasteurization and sterilization applications, for developing theories and methodologies in support of technology development, for studying various operation parameters, and for collecting engineering data for industrial scale-up. The system has been used for studying the influence of MW sterilization on quality of various foods, including asparagus (Sun et al., 2007), macaroni and cheese (Guan et al., 2002, Guan et al., 2003), salmon, chicken, rice, scrambled eggs, and mashed potatoes. Most of the MW processed products had superior quality, attractive appearance, and high consumer acceptance. This is attributed to the fact that microwave sterilization processes sharply reduced processing time compared with conventional retorting (Guan et al., 2002). During the studies, some necessary processing parameter, such as MW power, flow rate and temperature of circulating water, were established for optimum system operation. Meanwhile, a chemical-maker-assisted computer vision technique for determining heating patterns and cold spots inside MW treated food packages was developed by the WSU MW research group (Pandit et al., 2007a, Pandit et al., 2007b).
For scaling-up the MW system for industrial applications, it is necessary to study the MW sterilization and collect processing data for various foods. The objectives of this research were to investigate the technical feasibility of MW sterilization of a highly inhomogeneous food, sliced beef in gravy prepackaged in polymeric trays, and to establish appropriate procedures for developing and validating a MW sterilization process for the inhomogeneous food with the 915 MHz MW technology. Four major steps were taken to achieve the objectives: (1) choosing a model food to emulate the real food for determination of heating patterns, (2) determining cold spots in the food trays, (3) developing schedules for MW sterilization process of the sliced beef in gravy in 7-oz trays, and (4) verifying microbial safety of the MW processed foods by inoculated pack studies.
Section snippets
MW sterilization system setup and operation
The pilot MW sterilization system at WSU (Fig. 1) was used in this project. The system consisted of two 5-kW 915-MHz MW generators, waveguides, two MW heating cavities, loading and unloading cavities, a sample tray conveyor system, a water circulation system, and a control and data acquisition system. MW power from the generators was transmitted to the MW heating cavities through waveguides during operation. The loading cavity was used for loading food sample trays and also served as a
Shrinkage of sliced beef during heat treatment
Fig. 5 shows the characteristic of beef shrinkage in boiling water with 0.5% salt. Most of the shrinkage (about 70%) took place in the first four minutes; the sample weight and area reduced slowly between 4 and 20 min, and did not change after 20 min. The 4-min heating in the boiling water was considered adequate to set the geometry of beef samples for MW processing schedule development. Similar method was used in the food industry to reduce shrinkage and control solid content in thermally
Conclusions
A microwave (MW) sterilization process for sliced beef in gravy prepackaged in 7-oz polymeric trays was developed in this study. For processing schedule development, sliced beef was pre-treated for 4 min in boiling water with 0.5% salt before processing to limit possible movement of samples inside trays and make it possible to monitor temperatures at cold spots. Whey protein gel (35% Whey Alacen 878, 1% d-ribose, 0% salt, and 64% water) was chosen as a model food to emulate the pre-treated
Acknowledgments
The research work was financially supported by Washington State University Agricultural Research Center and USDA National Integrated Food Safety Initiative, Grant No. 2003-51110-02093, titled “Safety of foods processed using four alternative processing technologies”. The authors would like to thank Yu Wang for her help in doing experimental work.
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