A comparative study of infrared and microwave heating for microbial decontamination of paprika powder

Abstract

There is currently a need in developing new decontamination technologies for spices due to limitations of existing technologies, mainly regarding their effects on spices' sensory quality. In the search of new decontamination solutions, it is of interest to compare different technologies, to provide the industry with knowledge for taking decisions concerning appropriate decontamination technologies for spices. The present study compares infrared (IR) and microwave decontamination of naturally contaminated paprika powder after adjustment of water activity to 0.88. IR respectively microwave heating was applied to quickly heat up paprika powder to 98°C, after which the paprika sample was transferred to a conventional oven set at 98°C to keep the temperature constant during a holding time up to 20 min. In the present experimental set-up microwave treatment at 98°C for 20 min resulted in a reduction of 4.8 log units of the total number of mesophilic bacteria, while the IR treatment showed a 1 log unit lower reduction for the corresponding temperature and treatment time. Microwave and IR heating created different temperature profiles and moisture distribution within the paprika sample during the heating up part of the process, which is likely to have influenced the decontamination efficiency. The results of this study are used to discuss the difficulties in comparing two thermal technologies on equal conditions due to differences in their heating mechanisms.

Keywords: infrared heating; microbial decontamination; microwave heating; paprika powder; quality.

FIGURE 1

Schematic picture of the IR heating set-up.

FIGURE 2

Temperature profile to reach 98°C in the paprika powder, with an initial water activity of 0.88, treated by (A) infrared heating; step (1) 22.6 kW/m², step (2) 11 kW/m², and step (3) 0 kW/m², respectively (B) microwave heating; step (1) 650 W and step (2) 0 W. The Petri dish was moved to the conventional oven during step 3 and step 2 for the infrared and microwave treatment respectively.

FIGURE 3

Thermal images of the paprika powder after IR heating for (A) 20 s, (B) 40 s, and corresponding images of microwave heating after (C) 20 s and (D) 40 s. Upper row of images are taken from above, and lower row are images taken of the cross-section of the paprika sample. In the first case the petri dish cover was removed after heating; in the latter case the petri dish was split with bottom and cover still in place.

FIGURE 4

Reduction of (A) aerobic mesophilic bacteria and (B) bacterial spores in paprika powder, after 10 and 20 min holding times at 98°C. The paprika powder was conditioned to a water activity of 0.88 prior to the IR and microwave treatments.

FIGURE 5

Water activity of the top, center and bottom layer of the paprika bed after the heating up part of the process (0 min holding time), 10 min holding time and 20 min holding time for the (A) infrared and (B) microwave treated samples. The overall water activity of the paprika sample was 0.88 before the treatment.