Effect of Thermosonic Pretreatment and Microwave Vacuum Drying on the Water State and Glass Transition Temperature in Agaricus bisporus Slices

This study aimed to understand the micromechanism of thermosonic pretreatment and microwave vacuum drying on Agaricus bisporus. The water state and glass transition temperature (Tg) of fresh and thermosonically treated Agaricus bisporus slices during microwave vacuum drying were studied using differential scanning calorimetry (DSC), low-field nuclear magnetic resonance (LF-NMR), and magnetic resonance imaging (MRI). Results showed that four population groups were contained in the initial distribution of transverse relaxation time (T2) data of fresh A. bisporus slices: T21 (0.38–7.05 ms), T22 (9.33–32.75 ms), T231 (37.65–265.61 ms), and T232 (305.39–811.13 ms). Thermosonic pretreatment significantly decreased the initial free water content of A. bisporus sample but was accompanied by a sharp increase in its immobilized water. “Semi-bound water transfer” appeared during microwave vacuum drying (MVD) at moisture contents (Xw) of 0.70 and 0.60 g/g (wet basis (w.b.)) for untreated and thermosonically treated samples, respectively. MVD caused dramatic changes in the water state and enhanced the Tg by decreasing the content and mobility of immobilized water in A. bisporus tissues. The mobility of semi-bound water for thermosonically and MVD-treated samples was higher than for MVD-untreated samples, resulting in Tg values decreasing by approximately 2–11.5 °C, but the uniformity of water distribution in thermosonic-treated and MVD-treated samples was better at Xw ≤ 0.52 g/g (w.b.).