Thermal processing of hermetically packaged low-acid foods using microwave-circulated water combination (MCWC) heating technology

The feasibility of sterilizing packaged foods using a 915 MHz Microwave-Circulated Water Combination (MCWC) heating technology was studied with a pilot-scale microwave heating system. This system combines microwave energy with hot water (123∼125°C) and can heat food products up to 121.1°C or above. The products are held at high temperature to reach desired degrees of sterilization (F0 value). The MCWC heating system shortens the overall processing time and provides microbiologically safe products with acceptable sensory quality.; For microbial challenge studies, thermal resistances of PA 3679 spores in a macaroni and cheese entree were determined using Thermal Death Time (TDT) mini-retorts. Inoculated pack studies by Count-reduction and End-point Methods were conducted. The results indicated that the MCWC heating system can process products with designed degrees of sterilization (F0 value). Sensory qualities of macaroni and cheese entree processed by MCWC heating system were compared with freshly cooked macaroni and cheese entree. The MCWC heat sterilized entrees received higher scores than a popular boxed macaroni and cheese entree.; To better understand the interaction between foods and microwaves, dielectric properties of a mashed potato entree were measured using open-ended coaxial-line probe technique over frequencies between 1 MHz and 1800 MHz and temperatures between 20 and 120°C. Influence of moisture and salt on dielectric properties was considered. Descriptive equations at 27 MHz and 915 MHz were developed; and penetration depths were calculated. The results indicated that the dielectric constant and loss factor of mashed potato decreased with frequency. The dielectric constant increased with temperature at 27 MHz, changed little at 40 MHz and decreased with temperature at 433, 915 and 1800 MHz. Dielectric loss factor increased with temperature. Salt increased the dielectric loss factor, especially at 27 and 40 MHz. Power penetration depth decreased with temperature and frequency.; Heating patterns of the mashed potato entree processed by the newly-developed 915 MHz single mode MCWC heating system were determined using an infrared thermal image method. Computer simulation results using the Finite-Difference Time-domain (FDTD) method were compared with those patterns experimentally obtained. The results indicated that heating patterns of mashed potatoes were repeatable and predictable.; The study demonstrates that 915 MHz MCWC heating system has potential to sterilize food products while maintaining a relatively good sensory quality; the newly-developed 915 MHz single mode MCWC heating system can provide predictable and repeatable heating pattern.