Abstract
Sweep frequency ultrasound- (SFU-) assisted alkali extraction was conducted to increase the yield and content of heat-sensitive protein of walnut meal under a relatively mild condition. The physicochemical and structural characteristics of the proteins obtained by SFU-assisted alkali extraction and the conventional alkali extraction were compared. It was found that the optimal parameters for the SFU-assisted extraction were the solid-liquid ratio of 1 : 12, pH value of 9, initial temperature of 25°C, ultrasonic frequency of 28 kHz, sweep frequency amplitude of 1.5 kHz, sweep frequency cycle of 100 ms, duty ratio of 77%, and ultrasonic time of 90 min. Under this condition, a vast improvement in the walnut protein yield (34.9%) and the walnut protein content (9.8%) was observed. Such improvement was due to the structural changes of the sonicated protein; e.g., SFU decreased the intermolecular/intramolecular hydrogen bond force of proteins and, therefore, caused more order secondary structures and more loosen microstructures. This helped to improve the thermoplastic and solubility of the heat-sensitivity protein. Thus, SFU treatment could be an effective auxiliary technology in the alkali extraction of heat-sensitivity walnut protein. It might also be a promising technology for the extraction of heat-sensitivity protein from other agricultural by-products.
Highlights
Sweep frequency ultrasound- (SFU-) assisted alkali extraction was conducted to increase the yield and content of heat-sensitive protein of walnut meal under a relatively mild condition. e physicochemical and structural characteristics of the proteins obtained by sweep frequency ultrasound (SFU)-assisted alkali extraction and the conventional alkali extraction were compared
The protein ingredient in a walnut meal has not been well used in food processing because of the limited information on its structural and functional properties [8]. e proteins’ structural and functional properties correlate to their preparation method. ere are many studies on the extraction and structure of plant proteins [9, 10], but the studies of walnut protein mainly focused on the long-time and high-concentration alkalisoluble extraction [11, 12]
SFU-Assisted Alkali Extraction Process of Walnut Protein. e walnut meal was dissolved to a fixed solid-liquid ratio, and the pH was adjusted to a fixed initial pH by adding 4.0 mol/L NaOH. en, the sample solution was poured into an ultrasonic bag, sealed, and submerged into the ultrasonic reactor of the SFU equipment with 9 L added water. e SFU equipment (Figure 1) was manufactured by Jiangsu University with upper and lower ultrasonic transducers. e sample solution in the ultrasonic bag was treated under the ultrasonic power intensity of 133.3 W/L at a fixed initial temperature for a fixed ultrasonic time
Summary
SFU-Assisted Alkali Extraction Process of Walnut Protein. E walnut meal was dissolved to a fixed solid-liquid ratio, and the pH was adjusted to a fixed initial pH by adding 4.0 mol/L NaOH. E other extraction parameters including solid-liquid ratio (1 : 9, 1 : 12, 1 : 15, 1 : 18, 1 : 21, w/v), initial pH value (7, 8, 9, 10, 11), initial temperature (15, 25, 35, 45, and 55°C), and ultrasonic time (5, 10, 18, 30, 60, 90, 120, and 150 min) were considered and optimized. E content and yield of walnut protein were set as indices for the optimization of these parameters. En, the protein solution was diluted 20 times, and the protein concentration was determined by the Folin-phenol method [20]. Where n is the dilution time (20); v is the volume of protein solution (500 mL); m1 is the weight of weighted protein powder (5 g); m is the weight of collected all protein powder (g); M is the weight of input walnut meal (g)
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