*Article* **Ultrasonic Assisted Extraction of Quinoa (***Chenopodium quinoa* **Willd.) Protein and Effect of Heat Treatment on Its In Vitro Digestion Characteristics**

**Xingfen He, Bin Wang, Baotang Zhao and Fumin Yang \***

College of Food Science and Engineering, Gansu Agricultural University, Lanzhou 730070, China; hexingfen163@163.com (X.H.); wangbin\_1519@163.com (B.W.); zhaobaotang@126.com (B.Z.) **\*** Correspondence: yfumin@gsau.edu.cn; Tel.: +86-13893337478

**Abstract:** To extract and utilise the protein in quinoa efficiently, we investigated the effect of rate of quinoa protein isolate (QPI) extraction by ultrasound-assisted alkaline extraction and traditional alkaline extraction methods using single-factor experiments and Box-Behnken design. The effect of different heat treatment temperature and time on QPI functional properties and in vitro digestion characteristics were also investigated. The results showed that the optimal conditions of ultrasoundassisted alkaline extraction process were: ultrasonic time 99 min, solid-liquid ratio 1:20 *w:v*, ultrasonic temperature 47 ◦C, and pH 10, and its extraction rate and purity were 74.67 ± 1.08% and 87.17 ± 0.58%, respectively. It was 10.18% and 5.49% higher than that of the alkali-soluble acid precipitation method, respectively. The isoelectric point (pI) of QPI obtained by this method was 4.5. The flexibility and turbidity of QPI had maximum values at 90 ◦C, 30 min, and 121 ◦C, 30 min, which were 0.42 and 0.94, respectively. In addition, heat treatment changed the 1.77–2.79 ppm protein characteristic region in QPI's nuclear magnetic resonance hydrogen spectroscopy (1H NMR). After heating at 90 ◦C and 121 ◦C for 30 min, the hydrolysis degree and total amino acid content at the end of digestion (121 ◦C, 30 min) were significantly lower than those of untreated QPI by 20.64% and 27.85%. Our study provides basic data for the efficient extraction and utilisation of QPI.

**Keywords:** quinoa protein; ultrasounic-assisted extraction; in vitro digestion; amino acid content; nuclear magnetic resonance hydrogen spectroscopy
