Development and Validation of Processes for Continuous Flow Microwave Processing of Foods Containing Sweetpotato Particulates

Continuous flow microwave processing has been successfully utilized in commercial production of homogenous foods, and now there is an opportunity to use this technology to enhance the quality of multiphase foods. However, due to the high temperatures and mechanical stress incurred by continuous processing, it is challenging to maintain food particulate shape, which would be necessary for consumer acceptance in products like soups. Covington, NC 413 and Oriental sweetpotato cubes (Ipomoea batatas), with orange, purple and white flesh, respectively, were subjected to various pretreatments in order to investigate their effects on reducing degradation of texture due to thermal processing. Control and treated samples were loaded into a stainless steel cell, heated in an oil bath to an internal temperature of 125 °C and held for 30 sec. Among the pretreatments examined a 2-step pretreatment that included soaking in 0.3M Na2CO3 at 25 °C for 1 h followed by low temperature blanching at 62 °C in 1% (w/v) CaCl 2 provided the greatest firmness retention for all sweetpotato cultivars. Covington cubes, which are the most sensitive to thermal degradation, were pretreated by the 2-step process and evaluated at temperatures ranging from 115-130 °C for 0-12.22 min. Across all temperatures and times, peak force to fracture of the tested cubes showed no significant difference (p<0.05) illustrating the robustness of the pretreatment. The Hunter L*a*b* color values showed that the pretreatment application and subsequent processing caused significant decreases in all components, but the values stayed within what has been commonly reported for sweetpotatoes.;Since texture measurements indicated sweetpotatoes prepared by the 2-step pretreatment can maintain firmness at a level adequate to survive commercial processes, potential for use in high temperature thermal applications such as microwave processing was investigated. Dielectric property measurements showed that while dielectric constant did not change due to pretreatment, dielectric loss factor increased for pretreated Oriental and NC 413 sweetpotatoes when compared to raw samples and this was attributed to the application of CaCl2 during the pretreatment. Five microwave runs with the target temperatures of 115, 121, and 125 °C were conducted utilizing a pilot scale 100 kW system. Pretreated cubes were inserted into a carrier fluid of orange-fleshed sweetpotato puree and received microwave application at three points. After passing through the hold tube they were cooled, and collected in a pressurized tank outfitted with a sieve. All inserted sweetpotato cubes were recovered, and subjected to firmness and color measurements. For all cultivars, microwave processing caused a significant decrease in firmness as measured by peak compression force (N), however texture was firm enough for all cubes to stay intact after going through the microwave heating process. Microwave processing also caused a significant decrease in color components, but not to a level outside of what has been previously reported.;After creating a food particulate that could withstand continuous flow microwave processing, microbiological validation of a pilot scale 100 kW microwave system was attempted utilizing immobilized spore beads of Geobacillus stearothermophilus. Immobilized beads were placed in prefabricated cube-shaped particles made of polymethylpentene, which has been proven to heat more conservatively than food particles made from various vegetables. Prefabricated particles also contained magnets, which tracked their movement throughout the system and allowed for calculation of residence times. The prefabricated particles were inserted into a stream of orange-fleshed sweetpotato puree utilized as the carrier fluid and subjected to microwave application. At the end of the process they were collected and the immobilized spore beads were enumerated to determine surviving populations. Magnetic tracking showed that each particle was accounted for as it moved throughout the system unobstructed and spent 78+/-1 sec in the hold tube. Hold tube exit temperatures ranged from 96.9-129.9 °C due to variable microwave power. Log inactivation of G. stearothermophilus spores ranged from 0.20--2.05 and was most consistent when temperatures at the hold tube exit were stable. Based on the success of achieving free particle flow and utilization of immobilized spore beads as bioindicators, this study shows promise in achieving microbiological validation for a continuous flow microwave system.