**4. Conclusions**

In this study, the effects of ultrasonic-assisted and traditional alkali-dissolving and acid-precipitation methods on the extraction of QPI were compared. The effect of heat treatment on the functional properties and in vitro digestion properties of QPI were studied. The results indicate that the factors influencing the extraction rate of QPI by ultrasoundassisted alkaline extraction method were solid-liquid ratio > ultrasonic time > ultrasonic temperature, in order of their influence. The optimal conditions for extraction were as follows: ultrasonic time 99 min, solid-liquid ratio 1:20 *w:v*, ultrasonic temperature 47 ◦C, and pH 10. Under these conditions, the extraction rate reached 74.67 ± 1.08% and the purity of QPI obtained was 87.17 ± 0.58%. In comparison with the traditional alkaline dissolution and acid precipitation method, the extraction rate and purity of QPI extracted by ultrasound-assisted method were increased by 10.18% and 5.49%, respectively. The pI of QPI is 4.5. Heat treatment had a significant effect on the 1H NMR spectrum, turbidity and flexibility of QPI. Heat treatment changed the turbidity of QPI by affecting the 1.77 to 2.79 ppm region in the 1H NMR spectrum of QPI. After heat treatment, the degree of hydrolysis and amino acid content of QPI in vitro digestion decreased. The results of this study provide a basis for processing and utilization of quinoa protein isolate.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/foods11050771/s1, Figure S1: The effects of time (A), solid-liquid ratio (B), temperature (C), pH (D) and the response surface methodology and contour plots for the effects of various factors (E, F, G) on the extraction rate of QPI. Different letters represent significant differences (*p* < 0.05). Figure S2: Effect of different heat treatment temperatures on 1H NMR integrated spectra of QPI.;Table S1: Codes and levels of factors for response surface methodology experiments. (Ultrasonic); Table S2: Central composite arrangement for independent variables A (Ultrasonic time, min), B (Solid-liquid ratio, g·mL<sup>−</sup>1), C (Ultrasonic temperature, ◦C) and their response (QPI extraction rate, %); Table S3: Regression equation analysis of variance. (Ultrasonic); Table S4: Codes and levels of factors for response surface methodology experiments. (Alkali-solution and acid-isolation); Table S5: Central composite arrangement for independent variables A (Time, min), B (Solid-liquid ratio, <sup>g</sup>·mL−1), C (Temperature, ◦C) and their response (QPI extraction rate, %); Table S6: Regression equation analysis of variance. (Alkali-solution and acid-isolation).

**Author Contributions:** Conceptualization, X.H.; methodology, X.H.; software, X.H.; validation, X.H.; formal analysis, B.W.; investigation, B.W.; resources, B.W.; data curation, X.H.; writing—original draft preparation, X.H.; writing—review and editing, B.W.; visualization, B.Z.; supervision, B.Z.; project administration, F.Y.; funding acquisition, F.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Key Research and Development Project of Gansu Provincial Science and Technology Department grant number 18YF1NA076.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

