*3.3. Comparison of Ultrasound-Assisted Extraction and Traditional Alkaline Extraction of Quinoa Protein*

In an early phase of this study, single-factor and response surface test optimisation was carried out for the traditional alkaline extraction method using solid-liquid ratio, temperature, pH, and time as process parameters (Figure S1). The optimal conditions were as follows: time 139 min, solid-liquid ratio 1:29 *w:v*, temperature 56 ◦C, and pH 10; under these conditions, the protein extraction rate was 67.77 ± 1.50% and the purity of QPI was 82.63 ± 2.85% (Tables S4–S6). It was observed that the ultrasound-assisted extraction method significantly improved the extraction rate of QPI, i.e., by 10.18%, and increased the purity by 5.49% compared to the traditional alkaline extraction method. Moreover, using the ultrasound-assisted method, the extraction time and temperature were reduced by 40 min and 9 ◦C, respectively, thereby greatly improving the efficiency of quinoa protein extraction. Similar results were obtained by Li et al. [39] in the study of ultrasound-assisted extraction of capsicum seed protein. Thus, ultrasound-assisted quinoa protein extraction has a significant energy saving effect. In the pH range 3.0–6.5, the extraction rate of quinoa protein first increased, reached the maximum value at pH 4.5, and then decreased (Figure 2). Thus, the pI of QPI was 4.5, which is in agreement with the results obtained by Mir et al. [40] and Elsohaimy et al. [36].

**Figure 2.** Isoelectric point of QPI. Different letters (a–f) represent significant differences (*p* < 0.05).
