Finish Drying and Microbial Inactivation of Paprika by Combined Microwave-Infrared Radiation

Abstract

Paprika, a major spice of India becomes contaminated with bacteria, molds, and mycotoxins during different post-harvest operations. Hence, it is obligatory to diminish the microbial load for ensuring safety towards consumption. The present work was undertaken for the finish drying and inactivation of Salmonella Typhimurium and Aspergillus flavus in paprika by combined microwave-infrared assisted heating. Before heating of paprika, the moisture dependent physicochemical, thermal and sorption characteristics of paprika were studied. The microwave-assisted heating of paprika was done in the range of 5 to 25 W g-1 power density. The drying and quality characteristics were studied to identify the appropriate microwave power density by taking the charring of paprika into account. It was observed that during heating whenever the temperature of paprika was exceeded its corresponding glass transition temperature, it resulted in the accelerated moisture diffusion and quality degradation. The microwave-infrared heating experiments were carried out using the central composite design with the microwave power density, infrared heater temperature, the distance between the infrared heater and paprika layer, and heating time in the range of 4-12 W g-1, 75-375°C, 6-14 cm and 15-75 s, respectively. The responses collected were the average temperature of paprika, water activity, color, ascorbic acid, antioxidant capacity, peroxidase activity, and capsaicinoids contents. The response surface methodology was used to find the optimum condition for MW-IR assisted heating of paprika. The optimum or good set of conditions that meets all the goals was found at 10 W g-1 microwave power density, 300°C infrared heater temperature, 8 cm distance between the infrared heater and paprika layer, and 60 s heating time. The drying and quality degradation kinetics were modeled by conducting additional experiments at an optimum power density of 10 W g-1 and at 8 cm distance between the infrared heater and paprika layer, and by varying three infrared emitter temperatures at 150°, 300°, and 390°C with heating time up to 120 s. The inactivation of Salmonella Typhimurium and Aspergillus flavus in the absence of natural microbial flora in paprika was investigated during MW-IR assisted heating. The average surviving population of the cocktail culture of Salmonella Typhimurium (NCIM-2501 and MTCC-3224) was 2.450 × 107 CFU g-1 and log reduction in the cocktail culture of Salmonella Typhimurium was in the range of 2.308 to 3.909 during 10 s of MW-IR heating followed by 30 s incubation in MW-IR oven cavity. The time required to reach the detection limit for the cocktail culture of Salmonella Typhimurium was found to be 20 s of heating

followed by 30 s incubation period in the MW-IR oven cavity. The log reduction in the survival population of Aspergillus flavus spores was in the range of 2.178 to 6.182 during 20 s of MW-IR heating followed by 30 s incubation in MW-IR oven cavity. Besides the inactivation of Salmonella Typhimurium and Aspergillus flavus in the presence of natural microbial flora, paprika showed the 8.849 log reduction in total plate count during 60 s of MW-IR heating and 7.372 log reduction in yeast and mold count during 50 s of MW-IR heating. The temperature profile of paprika was predicted using a one-dimensional unsteady state heat conduction equation and process lethality was estimated. These results propose that MW-IR assisted heating could be adopted to inactivate the foodborne pathogens in paprika with minimum quality degradation. The shelf life of MW-IR treated paprika (4.40% db or 4.21% wb) under the domestic condition was found to be 101 and 31 d when stored in HDPE and LDPE packages, respectively. In the case of industrial storage condition (5°C and 70% RH), the predicted shelf life of the same product was 5.47 years in HDPE and 1.68 years in LDPE packages. The shelf life of paprika can be extended by storing it at moisture content where physicochemical reactions are highly restricted (monolayer moisture) and refrigerated condition.