Ohmic heating of Japanese white radish Rhaphanus sativus L.

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Abstract
Summary: Ohmic heating was applied to Japanese white radish, Rhaphanus sativus L. at 50 Hz‐10 kHz and 40 V cm−1 to examine the effects of frequency on the heat generation. Of the frequencies examined, 50 Hz gave the sharpest initial rise of temperature and the shortest time to raise the temperature at the mid‐part of radish to 80°C. The heating rates above 60°C were found to be almost the same and linear for all the frequencies. The pressurization (400 MPa, 25°C, for 10 min) of radish eliminated the sharp initial increase of temperature observed at 50 Hz, and gave an almost linear rise of temperature similar to those above 60°C. When radish was treated with a square wave (10 ms with 10 ms interval) of 45 V cm−1 for 30 s, its impedance decreased and never recovered even after storage at room temperature for 20 h. When radish was treated at 50 Hz and 40 V cm−1 until its mid‐part reached 30°C, or heated to 80°C in hot water and then cooled to 30°C, 1H‐NMR imaging analysis showed more free movement of liquid components than in untreated radish. These results suggest that the initial rapid heating up at low frequency is caused by the electroporation of radish tissue membrane, resulting in the reduction of its impedance.

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