Ultrasound-assisted vacuum impregnation and convective drying of potato (Solanum tuberosum cv. Colomba)
Dominik Mierzwa1*, Justyna Szadzińska1, Elżbieta Radziejewska-Kubzdela2, Roża Biegańska-Marecik2
1Poznań University of Technology, Institute of Technology and Chemical Engineering,
ul. Berdychowo 4., 60-965 Poznań (Poland)
2 Poznan University of Life Sciences, Institute of Food Technology of Plant Origin,
ul. Wojska Polskiego 31, 60-624 Poznań, Poland
The vacuum impregnation (VI) process of plant material allows modifying the structure of the tissue without affecting its integrity . However, the wider use of this method is limited due to the high rigidity and low effective porosity of the plant tissue. To overcome this issue, the intensification of mass exchange processes is necessary. Ultrasound (U) has been recognized as a very effective factor intensifying both heat and mass transfer  and has been successfully implemented in a wide variety of unit operations (extraction, drying, emulsification, mixing, etc.). However, its application during vacuum impregnation is an innovative approach.
This research aimed to analyze the influence of the ultrasound on the effectiveness of vacuum impregnation and kinetics of the further convective drying. To complete the analysis, the quality of products of individual unit operations was also assessed.
Potato (Solanum tuberosum cv. Colomba) was used as experimental material. Cubes with a 15 mm side were impregnated in 4 different processes in a water solution of ascorbic acid (5% w/w). Particular processes differentiate from each other with the period of ultrasound application. During VI the impregnation was conducted without ultrasound enhancement, whereas in the UVI1, UVI2, and UVI3 processes the ultrasound of frequency 25 kHz was continuously applied during the vacuum period, relaxation period, and the whole process respectively. In vacuum period, a pressure of 50 mbar was applied for 30 min.Then, the atmospheric pressure was restored and sample was kept immersed for 30 min (relaxation period). Next, impregnated samples were dried at 70° C (airflow 2 m/s). Drying kinetics, the energy consumption of processes and product quality (ascorbic acid content, color, water activity) were analyzed.
It was found that ultrasound may effectively enhance the vacuum impregnation process, however, the effects depend on the period of application. For UVI1 and UVI2, the ascorbic acid content was visibly higher, but in the case of UVI3, it was comparable to the VI process. The highest value of ascorbic acid content was found in the samples impregnated in the UVI2 process (19.1±0.95 mg/100 g of dry matter), Analysis of the drying kinetics allows to state that the application of ultrasound during pretreatment improved tested drying parameters. A reduction of the drying time and energy consumption was observed for samples pretreated with ultrasound enhancement (UVI1-UVI3). The average gain on the drying rate equals 14%, which resulted in a reduction of the specific energy consumption by about 15%. It was also found that ultrasound did not affect the quality of the obtained products which were characterized by similar water activity and color change.
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The research was supported by National Science Centre (Poland) in the framework of SONATA project no. 2018/31/D/ST8/00627.