Pulsed electric stimulated changes in potatoes during their cooking: DMA and DETA analysis

The internal cortex of potato (two varieties: Dali and Agria) was tested using dynamic mechanical analysis (DMA) combined with dielectric thermal analysis (DETA), in air of 90% humidity in temperature scans between 30 and 90 °C. Temperature plots of storage (SM i.e. elastic) and loss (LM i.e. inelastic) moduli were obtained. DETA, based on alternated current of frequency 20 kHz, continually determined both components of impedance. The PEF (pulse electric field) was applied prior to the DMA/DETA test; the experiments were arranged into four sets: b (no PEF), c (one 10 ms long pulse of alternated field 500 V/cm), d (two same pulses with a 0.1 s interval between them), e (the same pulses with a 1 s interval). The impedance was recalculated giving parameters of a single model represented by parallel connection of a resistor Re and a capacitor with capacitance ωC. The temperature range was divided into three stages: A (30-60 °C), B (60-80 °C), and C (80-90 °C). For set b and stage A parameter Re decreased, whereas capacitance was nearly constant. Both parameters were constant in stage C. In part B, between 70 and 80 °C, Re sharply decreased and the capacitance showed a sharp peak, both indicating either collapse of the cellular membranes or starch gelatinization. Application of PEF led to reduction of the peak but the process was more effective when application of PEF was repeated (sets d and e) and mainly if longer time interval between the pulses was used (e). PEF causes disintegration of cellular membranes and water release from vacuoles so that free vacuole's water makes possible that starch gelatinization appears at lower temperatures. The role of PEF as a disintegration source was tested and it was found that its efficiency is strongly enlarged when it is combined with heating.