This work was conducted to understand the covalent interactions of whey protein isolate (WPI) with some phytochemical compounds including chlorogenic acid (CQA), rosmarinic acid (RA) and quercetin (Q) (0.3 mmol/g protein). The influences of such interactions on the chemical, structural and functional properties of the WPI were studied. The changes in protein–phenolic conjugates properties were characterized by the change in free amino and thiol groups, tryptophan content, UV–Vis scan and particle size. Moreover, the electrospray ionization-quadrupole-time of flight (ESI-Q-TOF) mass spectrometry, RP-HPLC, TEAC and DPPH assays were also used to evaluate these changes. In addition, the change in isoelectric points of modified proteins was examined using the change in the zeta potential at different pH values. The results showed that these interactions caused a decrease in both free amino and thiol groups and tryptophan content, where the addition of CQA, RA and Q led to decrease in the thiol groups content by about 47.6, 96.1 and 65.8%, respectively. While, the content of tryptophan residue decreased from 29.9 ± 0.3 (nmol/mg protein, for WPI control) to 19.3 ± 0.1, 7.9 ± 0.1 and 17.2 ± 1.0 after interacting with CQA, RA and Q, respectively. The structure of WPI- phenolic conjugates altered compared to the structure of WPI control. The modified proteins displayed very slight shifts in the isoelectric points compared to the unmodified ones. Moreover, these interactions increased the antioxidant capacity of proteins about 2.7 to 3.4 folds compared to WPI control. Therefore, it is strongly recommended to use the modified WPI for preparing functional food products with a high antioxidative power.
Whey protein isolate ESI-Q-TOF–MS Rosmarinic acid Covalent interactions Zeta potential
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The author would like to thank Egyptian Government for Financial Support and staff of the division of Food Technology, Institute of Human Nutrition and Food Science, University of Kiel, Germany for his technical support analysis and providing him with whey protein isolate.
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Conflict of interest
The author declares that he has no conflict of interest.