Conjugation of Microalgal Phenolics and Protein for Bioactivity and Bioaccessibility Enhancement

Microalgae are rich in protein and phenolics, thereby having great potential for production of functional foods and nutraceuticals. However, despite featuring high nutritional value, these compounds often suffer from low stability and bioaccessibility. In this study, phenolics and protein extracted from Chlorella vulgaris were conjugated at different ratios (2.5-10%) and the structure and bioactivity of the conjugates were comprehensively characterized. The fluorescence intensity of protein decreased from 340 to 130-98 a.u. after conjugation and the UV-vis absorbance dropped from 1.6 to 0.5 a.u., which confirms the alteration of the chromophore area. The FTIR spectra revealed shifts in the C=O, N-H, and C-N bands, and the ¹H NMR spectra showed the broadening of signals and appearance of new peaks, indicating covalent bond formation through the Schiff base and Michael addition reactions. Conjugation significantly increased the antioxidant activities, in terms of ABTS inhibition by 644%, 257%, and 97%, as well as the ACE inhibitory activity, by 13.5%, 17.5%, and 19.7% for the 2.5%, 5% and 10% conjugates, respectively. The 2.5% conjugate showed the highest bioaccessibility (144%), which was 2.5 times that of free phenolics. Overall, this study proves that conjugation is an effective approach to enhancing the bioactivity and bioaccessibility of microalgae-derived compounds and unravel the structure-activity relationship of conjugates. The findings can promote the valorization of microalgae for product development in the food and nutraceutical industries.