3. Results and discussion
3.1. Effect of single factors on extraction of oligosaccharides assisted by ultrasound
3.1.1. Solvent type
The selection of solvent is a first crucial step for parameters optimization, and critical for the complex food samples as it will determine the amount and type of oligosaccharides being extracted. In this study scallion was chosen as a test sample for optimization of the parameters because of its high content of total oligosaccharides [27]. Fig. 1A showed that aqueous acetone was slightly better than aqueous methanol and aqueous ethanol in ultrasound extraction of oligosaccharides from scallion under same extraction conditions (50% v/v, 50 C, and 10 min). However, the differences among the extracted concentrations of oligosaccharides
by all solvents applied were not significant (p > 0.05).Johansen et al. [14] showed that the aqueous ethanol (50% v/v) was as effective as 50% (v/v) methanol for conventional extraction of raffinose family oligosaccharides. Therefore, ethanol, which is categorized as GRAS (Generally Recognized as Safe) and is preferred for application in food systems, was chosen as the extraction solvent for the next experiments.