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 CaCl 2 stress was explored in soaking. • Both CaCl 2 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. In order to fortify γ-aminobutyric acid (GABA) of brown glutinous rice (BGR), pre-germination strategy was employed, and effects of low-frequency (28 kHz) ultrasound treatment combined with CaCl 2 stress on the sprout length, germination rate, morphology, color, water, total polyphenol content (TPC), starch, protein, GABA contents and relevant metabolites were investigated. The germination rate would be inhibited under CaCl 2 concentration ≥ 2.0 % during 24 h soaking without ultrasound treatment, and no significant difference was also observed combined with 9 h ultrasound treatment. Ultrasound treatment was beneficial to water absorption, TPC enrichment, energy metabolism, lipid metabolism and protein hydrolysis. Higher contents of GABA (3.29 folds), pyruvic acid (7.63 folds), glycerol (4.88 folds), glutamate (2.02 folds) and glucose (1.32 folds) were obtained due to the antagonistic effect between the 30 w ultrasound treatment and 2.0 % CaCl 2 stress at the 9 h pre-germination, and energy, lipid and protein metabolomic pathways were all involved in the GABA accumulation. [ABSTRACT FROM AUTHOR]