Quantification of 21 free amino acids in traditional and nontraditional soybean pastes

The amino acid concentration of soybean products is important for nutritional and health benefits along with desirable sensory characteristics, but these properties could be affected by the origin of raw ingredients, processing method, and storage. Here, we aimed to evaluate the compositional characteristics of free amino acids in soybean paste and identify potential differences between those generated by traditional and nontraditional manufacturing processes. We developed and validated a high-performance liquid chromatography-diode array detection (HPLC-DAD) method involving sample derivatization to analyze 21 free amino acids in soybean pastes. All amino acid compounds were separated within 45 min using a C18 column (250 mm × 4.6 mm, 3 μm). Validation of the optimized 21 free amino acid analysis method showed good linearity (R2 > 0.993), accuracy (≥ 80%), intra- (≤10.63%) and inter-day (≤ 11.58%) repeatability, and limit of quantification (0.025–1.91 µg/g). The levels of 21 free amino acids in nontraditional and traditional soybean pastes detected based upon the optimized method were total amino acid detection levels at 55.3–150.9 (average 92.7) and 49.6–143.4 (average 86.8) mg/kg, respectively. Moreover, we confirmed that the composition ratios of glutamic acid, histidine, and sacrosine were higher in nontraditional soybean paste manufacturing methods (p <  0.1 and p < 0.05 ), which could serve as a quality indicators during manufacture and storage of soybean paste.