Drying characteristics of potato slices via electrohydrodynamics

Potato has been one of the most favorite foods with the largest production and consumption in China. However, the potato tuber with high water content can easily germinate difficult to store, leading to a serious waste of resources. Drying can be used to effectively reduce the moisture content of potatoes, and then extend the storage period for the added value. The main drying methods are hot air drying, microwave vacuum drying, freeze drying, and infrared radiation drying at present. There is also some negative impact on the color, shrinkage, and nutrient content of dried products. The existing drying has one or more defects, such as equipment, energy consumption and quality. It is necessary to explore new drying technology. Electrohydrodynamic (EHD) drying without heat can be suitable for heat-sensitive food and biological products, due to the energy saving, low cost of equipment manufacturing, simple operation, rapid control of airflow speed, and sterilization. There was also excellent retention of the color, nutrient composition and shape of the material. However, it is still lacking on systematic studies on the drying and physicochemical characteristics of potatoes using EHD drying. In this study, the potatoes were dried by electrohydrodynamic drying. A systematic measurement was then carried out of the moisture content, drying rate, Rehydration ratio and effective moisture diffusion coefficient of potatoes under different drying voltages. The effects of electrohydrodynamic drying on the molecular structure, chemical composition and water transport of potatoes were also investigated by infrared spectroscopy and low-field nuclear magnetic resonance. Corona discharge was generated by a high-voltage electric field during drying. The voltage and current waveforms were plotted to obtain the outstanding filamentary discharge in half of the cycle. The higher the voltage in a single cycle was, the higher the discharge frequency and current amplitude were. The uneven discharge promoted the discharge, and then inhibited the discharge around, indicating positive feedback. High-voltage electric field discharge produced N⁺, N²⁺, O^2-, and OH^- ions. A large number of neutral particles were mixed to blow away from the needle electrode, and then form the ionic wind. As such, the ionic wind was also attributed to the main mechanism of electrohydrodynamic drying. A falling-rate period was then found when the electrohydrodynamic drying was acted mainly on the surface of the potatoes. The drying rate increased with the increase of voltage, whereas, the increment gradually decreased. Electrohydrodynamic drying significantly increased the effective moisture diffusion coefficient and Rehydration ratio of potatoes. The highest average drying rate, effective moisture diffusion coefficient and rehydration ratio were 0.922 8 g/(g·h), 2.265 3×10^-10 m²/s, and 4.78 at 30 kV voltage. There were generally similar positions of characteristic peaks in the infrared spectra under different drying voltages. The characteristic peak intensity at 22 kV was the largest. Potato protein was dominated by β-sheet structure. The β-sheet structure shared an increasing trend with the increase in voltage. Thus, electrohydrodynamics led to the transformation of the protein's secondary structure from the order to the disorder. The internal structure of the potato was destroyed to reduce the binding force of less mobile water, particularly for the improved fluidity of free water. The migration and removal of three kinds of water were promoted in the potato, resulting in a decrease in water content. Infrared spectroscopy analysis showed that there was no significant change in the functional groups after drying. The finding can also provide the experimental and theoretical reference for electrohydrodynamic drying in potato processing.