Ultrasound, an emerging non-thermal technique, has potential to modify the functionality of bio-molecules like protein. In the present study, the impact of ultrasound on whey protein (WP) was assessed with functional, rheological, heat coagulation and transition temperature, SDS-PAGE, FTIR spectra, scanning electron microscopy and transmission electron microscopy. The results of this study showed that the raw WP had broad bimodal particle size distribution while after ultrasonication, modified WP exhibited narrower distribution along with smaller particle size (0.683 ± 0.225 μm) compared to untreated WP (2.453 ± 0.717 μm). The solubility of WP also increased after ultrasonication (72.22 ± 0.68% to 79.21 ± 1.08%). During the rheological evaluation, both the samples exhibited Newtonian behaviour but, the modified WP exhibited dramatically higher storage modulus (G') throughout the temperature profile compared to raw WP mainly due to enhanced proteins aggregation during heating which revealed more elastic and stronger gel. The modified WP exhibited significantly higher (about 6-times) heat stability compared to raw WP which signified that after ultrasonication the WP can withstand higher temperature during processing for longer time. The results were also confirmed by higher transition temperature (T) of modified WP (93.32°) compared to untreated WP (81.93 °C). The SDS-PAGE profile of raw and modified WP showed that the ultrasound significantly decreased the bands density of low molecular weight molecules (β-lactoglobulin and α-lactalbumin). FTIR spectra also showed noticeable difference between the secondary structure component of raw and modified WP. Finally, the structural micrographs of raw and modified WP from SEM and TEM analysis also confirmed the adequacy of modification of WP employing non-thermal techniques. The modified WP revealed smaller, regular and more homogenous and ordered structures compared to untreated sample.

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Response surface methodology was used to optimize processing variable for ultrasound-assisted modification of whey protein. The process was optimized employing Box-Behnken Design with three independent variables i.e. amplitude (20-40%), time (10-20 min) and concentration (10-15%). A second order model was employed to generate response surfaces. Experimental results revealed that analyzed model solutions exhibited the significant influence on various responses signified that the applied statistical model fitted well. The optimized independent variables were found to be 19.77 min time, 20.02% amplitude and 12.78% concentration of feed. The modified whey protein had the solubility, 78.52%; heat stability, 1076.19 s; water solubility index, 92.30%; water holding capacity, 0.469; oil absorption capacity, 1.709; foaming capacity 92.27; foam stability, 27.71 and firmness, 1692.09 g. Analytical response revealed that solubility of modified whey protein exhibited significant positive correlation with water solubility index, emulsion stability index and firmness.