Protein isolates were prepared from the defatted mustard meal (DMM) by ultrasound-assisted alkaline extraction and precipitation at the isoelectric point (pH 4.5). Response surface methodology was used to determine optimum conditions for the extraction of protein from the DMM. Box–Behnken design includes independent variables such as pH (8, 9.5, and 11), particle size (150, 375, and 600 μm), ultrasonic amplitude (50%, 70%, and 90%), and ultrasound treatment time (8, 10, and 12 min), which were used to get the optimal conditions for highest protein yield (dependent variable). Maximum yield (44.87%) was obtained when pH, particle size, amplitude, and time were kept at 11, 375 μm, 90%, and 10 min, respectively. Functional properties were studied for the protein isolate obtained at optimal conditions with the highest yield. Results showed that ultrasound pretreatment increased the protein yield as compared to conventional methods. Protein isolate obtained by ultrasound assessment had comparable functional properties to those produced by traditional methods. SDS-PAGE profile of proteins does not show any change in the molecular weight. XRD analysis showed that the difference between the particle distance of protein obtained with and without ultrasound-assisted extraction was very small. The protein content (dry basis), water holding capacity, oil holding capacity, emulsifying activity, foaming capacity, and foam stability were 93.33 ± 0.57%, 0.68 ± 0.01 g/g, 1.86 ± 0.05 g/g, 63.16 ± 0.25%, 58 ± 0.47%, and 63 ± 0.84%, respectively.

Practical applications

Mustard seed is used for edible oil production and the meal is its by-product. The meal contains a good amount of protein, which can be utilized in two ways: for nutrition and as a functional ingredient in various food products. The use of ultrasound-assisted extraction increases the yield as well as modifies the functional properties. This process can be used practically to add the value of the mustard meal.