Pulsed Electric Field-Assisted Extraction of Juice from Food Plants

Abstract

Solid–liquid extraction from food plants can be effected by pressing (solid–liquid expression) or by solvent (diffusion, leaching). It has various industrial applications: Extraction of fruit juices and vegetable oils, production of sugar and wine, dewatering of fibrous materials (fodder plants), dehydration of biological wastes, etc. (Brennan et al., 1990; Schwartzberg, 1997). The application of electric fields to enhance the extraction from food plants is not new. Flaumenbaum (1949) reported that application of alternating electric fields using 220 V and of industrial frequency of 50 Hz increases the juice yield from prunes, apples, and grapes. Zagorulko (1957) reported that the application of d.c. and alternating electric fields enhanced the extraction of juice from sugar beets. In these first studies, the electric fields with gradients of 0.5–2 kV/cm were obtained by small spacing between electrodes of 1–3 mm. The increase in extraction yield was explained as a result of electrical breakage of cellular membranes and was named electroplasmolysis. But earlier attempts to industrialize this process in the 1950s and 1960s were not successful because of technical difficulties originated by small distance between electrodes (sparking effects, limiting of throughput, etc.) and excess heating of sliced particles. Then, in the 1970s, the studies were mostly devoted to the application of low-gradient electric fields of 10–100 V/cm accompanied by ohmic heating (Rogov and Gorbatov, 1974). The reported enhancement of extraction with low gradient electric fields was attributed to both thermal and electrical destruction of cellular tissue. The interest of application of low-gradient alternating electric fields for enhancing extraction from food plants was renewed in Europe andNorthAmerica in the 1990s. Halden