Modification of lentil protein isolate (Lens culinaris) by ultrasonic and heat treatment for the stability of high internal phase emulsions

Heat (HT) and ultrasound (US) treatment can improve the emulsifying properties of proteins. This study evaluated the effect of HT (80°C/20 min), US (60% amplitude/7 min), and a combination of these treatments (HUT) (60%/7 min and 80°C/20 min) on lentil protein isolate dispersions (LPI) aiming at improving the stability of high internal phase emulsions (HIPE). HIPEs were produced with 25% of LPI dispersion (2%, w/w) in four different conditions (untreated, HT, US, and HUT) and 75% of soybean oil using a rotor-stator at 15500 rpm for 1 min. The LPI dispersions were characterized by particle size (volume-weighted mean diameter (D(4,3)), solubility, and intrinsic fluorescence. HIPEs were evaluated in terms of droplet size (D(4,3)), oil loss after centrifugation, microscopy, and Turbiscan stability index (TSI). The results of LPI treatment indicated that HT, US, and HUT reduced the particle size (42.78±0.10 µm, 12.66±0.06 µm, and 14.56±0.10 µm, respectively, compared to 56.78±0.14 µm from untreated LPI) and increased the solubility (34.91±0.83%, 84.43±0.64%, and 92.96±0.23%, respectively, compared to 15.79±0.37% from untreated LPI). HT, US, and HUT increased the exposure of hydrophobic amino acid residues, such as tryptophan, onto the protein's surface. This is confirmed by the fact that these samples showed high values of intrinsic fluorescence emission compared with untreated LPI. HIPEs have been formulated with these LPIs, HT(HIPEHT), US (HIPEUS), and HUT (HIPEHUT). HIPEHT and HIPEHUS showed smaller droplet sizes than HIPE with native LPI (HIPE), and there were no changes throughout 30 days of storage (25°C). HIPEHT, HIPEUS, and HIPEHUT showed lower oil loss values and TSI than the HIPE with native LPI. Therefore, it is confirmed that these treatments contributed to forming more stable HIPEs compared to the HIPEs produced with native LPI dispersions, as they presented lower values of oil loss and TSI.