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Influence of type and power of ultrasonic treatment on the drying time of pumpkin (Cucurbita moschata)

High-intensity ultrasonics as novel food processing technology have increased use in the food industry. The cavitation effect of ultrasonic processing can influence a wide array of food processing operations such as mixing, emulsification, inactivation of microorganisms, and drying. Drying is one of the most energy-intensive operations in the industry, and as such could strongly benefit from using novel technologies to help before or during drying. High-intensity ultrasonics have a thermal and mechanical impact on processed food, due to the high energy and pressures obtained in an implosion of cavitation bubbles during the propagation of the ultrasonic wave in high liquidcontent materials. Such impact in the vicinity of the food surface can have an impact on the cellular structure, specifically modifying the matrix and enlarging pores inside processed food. As drying is a mass transfer process, it is heavily influenced by the pore sizes in the food. In this way, ultrasonic pre-treatment can significantly increase mass transfer and consequently shorten drying times. This can potentially lead to a decrease in energy consumption and ensure a more sustainable process. This study investigates the effect of the ultrasonic treatment of pumpkin on the drying time. For the experiment, square pumpkin pieces were processed using two different types of ultrasonic treatment. Ultrasound with the probe was used at 30, 60, and 90 % of maximal amplitude during 3, 6, and 9 min. For the second type of treatment, an ultrasonic bath was used at amplitudes of 30, 60, and 90 % during 30, 45, and 60 min. Drying was performed using a conventional convective tray dryer at 60 oC using an airflow of 0.5 ms-1. Statistical analysis is performed using ANOVA and posthoc tests in Statistica software. MANOVA analysis of obtained results shows a significant change in drying times which correlate to both types of ultrasonic pre-treatment. The drying time of control samples was 305 min, which was a baseline for further analysis. Samples treated using an ultrasonic bath show a statistically significant decrease in the drying time (down to the shortest drying time of 248 min, processed at 90 % of maximal amplitude) which correlates to the increase of the amplitude. This same property was confirmed using the ultrasonic probe, as drying time was shortened to 245 min for samples treated with 30 % of maximal amplitude for 9 min. Further increase of ultrasonic power seems to have no further influence on the drying time. The ultrasonic probe was proven to be more effective in the sonication of pumpkin samples, with much shorter processing times and lower amplitudes needed for the same effect on the drying time. In conclusion, while pre-treatment using an ultrasonic have a significant influence on the decreasing of drying time, such a process would be much more beneficial using high-powered ultrasound with an immersion probe, as it can process the surface of the material with the same amount of mechanical energy in a much shorter amount of time.

pumpkin, ultrasound, waste, drying

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