Protein Fractions with Techno-Functional and Antioxidant Properties from Nannochloropsis gaditana Microalgal Biomass

Microalgae are a promising source of high-value products ranging from simple biomass production for food and feed to valuable biorefinery products (biogas, bioethanol, pigments, proteins, fibers, etc.). Thus, protein fractions such as protein extract and hydrolysate from the Nannochloropsis gaditana microalgal biomass represent an unexploited opportunity. The protein extract (PE) from defatted microalgal biomass was obtained after solubilization at alkaline pH and precipitation at the isoelectric point, and the protein hydrolysate (PH) was generated by enzymatic hydrolysis using papain. These fractions were characterized according to their protein content and amino acid composition in relation to the microalgal biomass. Their techno-functional properties (emulsifying and foaming capacity) and antioxidant activity (ORAC method) were also determined. PE showed a high total protein content of 65.9% w/w and a content of essential amino acids comparable to the FAO/WHO/UN reference profile. The emulsifying capacity for PE was greater than 50% and the emulsion was stable after 24 h. Using papain as the enzyme for hydrolysis of PE, a degree of hydrolysis (DH) of 25.4% and the generation of peptides (<10 kDa) were obtained, thus increasing its solubility rate compared to the native protein. However, PH did not show any capacity to form emulsions. The antioxidant activity of PH reached an EC50 value of 0.026 mg TE/mL using the ORAC method, being 28 times greater than that of PE. This study demonstrated that microalgal protein from N. gaditana not only contains important nutritional value but PE could also be considered a biopolymer with good emulsifying properties and their hydrolysate with an increased antioxidant activity. They can be exploited in industrial applications in the food and/or pharmaceutical industries.