Role of carbohydrate conjugation on the emulsification and antioxidant properties of intact and hydrolysed whey protein concentrate

The conjugation of proteins with carbohydrates generates covalent bonds which may improve their techno- and biofunctional properties and therefore expand their applications in the food industry. In the present study, a design of experiments (DOE) approach was used to determine the effect of conjugation of whey protein concentrate (WPC) and WPC hydrolysates with carrageenan (CGN) on its emulsifying and antioxidant properties. The DOE was composed of 3 levels with 3 factors, i.e., WPC:CGN (1:1.0 1:3.5, 1:6.0), time of conjugation (6, 27 and 48 h) and degree of hydrolysis (DH; unhydrolysed, low DH and high DH). The conjugated samples were characterised for their oxygen radical absorbance capacity (ORAC) and emulsion activity (EA). Two samples, C3 (1:1.0, 6 h and unhydrolysed) and C4 (1:1.0, 6 h and low DH) with ORAC and EA values of 601.30 ± 14.71 and 709.32 ± 11.11 μmol Trolox equivalents g−1 freeze-dried powder and 0.51 ± 0.01 and 0.58 ± 0.01 absorbance units, respectively, were selected for further study. Emulsions were generated with WPC, non-conjugated (NC3 and NC4) and conjugated (C3 and C4) samples and their functional properties were compared. The NC3 and NC4 samples had higher viscoelastic moduli (G′ and G″) than C3, C4 and the WPC samples which indicated that more stable emulsions may be formed with non-conjugated samples. However, the NC3, NC4 and WPC samples showed low emulsion stability (ES) after 28 days storage with values ranging from 78.6 ± 3.5–85.6 ± 3.6% whereas the conjugated C3 and C4 samples had improved ES with significantly (p < 0.05) lower coalescence index values (64.2 ± 2.5 and 66.7 ± 3.7%, respectively). However, conjugation of the hydrolysate (C4) had a significant (p < 0.05) negative effect on lipid oxidation (0.62 ± 0.04 equivalents of 1,1,3,3-tetraethoxypropane (TEP)/kg emulsion) compared to the unhydrolysed conjugate C3 (0.42 ± 0.04 eq TEP/kg emulsion).