Enhancement of γ-aminobutyric acid and relevant metabolites in brown glutinous rice (Oryza sativa L.) through salt stress and low-frequency ultrasound treatments at pre-germination stage

•Pre-germination was used to improve the nutritional value of brown glutinous rice.•Antagonism between ultrasound treatment and CaCl2 stress was explored in soaking.•Both CaCl2 and ultrasonic treatments improved GABA production during germination.•Energy metabolism, lipid metabolism and protein hydrolysis were enhanced.•Low-frequency ultrasound with salt stress would short the germinated time.