Ultrasonic extraction of soy protein isolate: Characterization and comparison with microwave and enzymatic extraction methods

Abstract Soy meal as an agro‐industrial by‐product produced by the soybean oil processing industry is rich in protein. To valorize soy meal, the present study was aimed at the optimization of soy protein isolate (SPI) extraction by ultrasound treatment, its characterization, and comparison with microwave, enzymatic, and conventionally extracted SPI. Maximum yield (24.17% ± 0.79%) and protein purity (91.6% ± 1.08%) of SPI were obtained at the optimized ultrasound extraction conditions of 15.38:1 (liquid–solid ratio), 51.85% (amplitude), 21.70°C (temperature), 3.49 s (pulse), and 11.01 min (time). The SPI extracted with ultrasound treatment showed a smaller particle size (27.24 ± 0.33 µm) as compared to that extracted with microwave, enzymatically, or conventional treatment. Functional characteristics, namely, water and oil binding capacity, emulsion properties, and foaming properties of ultrasonically extracted SPI were increased by 40%–50% as compared to SPI extracted with microwave treatment, enzymatically, or conventionally. Structural and thermal properties studied by Fourier‐transform infrared spectroscopy, X‐ray diffraction, and differential scanning colorimeter showed amorphous, secondary structural change, and high thermal resistance of ultrasonically extracted SPI. Increased functionality of ultrasonically obtained SPI can enhance its application in the development of various new food products. Practical Application Soybean meal is one of the richest sources of protein and has huge potential to lessen protein‐based malnutrition. Most of the studies on soy protein extraction were found to be based on the conventional methods that yield less quantity of protein. Hence, ultrasound treatment which is one of the novel nonthermal techniques has been selected for the present work and optimized for soy protein extraction. The ultrasound treatment showed significant improvement in extraction yield, proximate composition, amino acids profile, and improvement of functional properties of SPI as compared to the conventional, microwave, and enzymatic methods which proved the novelty of the work. Hence, the ultrasound technique could be used to increase the applications of SPI for developing a wide range of food products.