Impact of microwave treatment on the structure of common black bean (Phaseolus vulgaris L.) protein concentrate

The common black bean (Phaseolus vulgaris L.) is a legume with distributed cultivation, rich in proteins (approximately 21% m/m) and has great potential for use as an ingredient in the food industry. Microwave technology, which is based on the application of electromagnetic waves, has proven to be effective to improve the extraction of plant compounds. However, its application for the physical modification of proteins has not been widely explored. This study aims to characterize black bean protein concentrates obtained by alkaline extraction and to evaluate the influence of microwave application on the structure of proteins. Black bean protein concentrates obtained by alkaline extraction (pH 9.5-35ºC) were solubilized in water and treated by microwaves under four different conditions that varied the powers (100 and 300 W) and protein concentrations (10 and 50 mg/mL). The treated concentrates were characterized according to particle size, free sulfhydryl content and secondary protein structure by circular dichroism (CD). All treatments resulted in smaller particle size compared to the native protein. The treatment at the lowest power (100 W) and lowest protein concentration (10 mg/mL) induced the greatest reduction of free sulfhydryl content, as well as the smallest reduction in particle size, indicating that greater formation of disulfide bonds and protein aggregation may have occurred. The condition of higher applied power (350 W) and higher concentration (50 mg/mL) resulted in greater reduction in particle size, greater reduction in α-helix content and a significant increase (p ≤ 0.05) in β-sheet and random coil structures, indicating that after denaturation there was reorganization into less stable structures. Although microwave technology is an emerging industrial technique, the results demonstrated its potential to improve the functional properties of isolates obtained through alkaline extraction, especially in reducing aggregate formation.