Effects of ultrasonic pretreatments on thermodynamic properties, water state, color kinetics, and free amino acid composition in microwave vacuum dried lotus seeds

Ultrasonic pretreatments were applied to lotus seeds at acoustic energy densities of 0.29, 0.40, and 0.51 W mL−1 for 10 min. After pretreatments, lotus seeds were subjected to microwave vacuum drying (MVD). Parameters of glass transition temperature (Tg), gelatinization temperature (Tp), water state, color kinetics, and free amino acid content of microwave vacuum dried lotus seeds were determined. With increasing acoustic energy density, MVD elevated the Tg values appreciably by decreasing the content of cytoplasmic bulk water in lotus seeds tissues. The Tp had a positive relationship with the relaxation times of cytoplasmic bulk water (T22), while Tg had a negative relationship with T22. Color kinetics were analyzed by the divisional method during MVD due to different browning reactions, which failed to appear with ultrasonic pretreatment. Free amino acid content ranged from 517.65 to 666.13 mg/100 g dry weight at 0.51 W mL−1.