Microalgae are promising protein sources due to their overall high protein content. The low aqueous solubility of microalgae proteins, however, limits their application in food, pharmaceutical or personal care systems, unless solubility is enhanced by e.g. hydrolysis. In this study, we examined the interfacial rheological properties at the oil-water interface of insoluble microalgae protein-rich fraction from Chlorella protothecoides and their hydrolysates prepared by hydrolysis in hydrochloric acid at 65 degrees C (Hydrolysates 65) and 85 degrees C (Hydrolysates 85). Results showed increased interfacial activity of the insoluble microalgae protein-rich fraction after hydrolysis: Hydrolysates 65 and Hydrolysates 85 had higher interfacial storage G(i)' and loss moduli G(i)'' compared to the untreated insoluble microalgae protein-rich fraction. Increasing amounts of soluble protein fragments mixed with insoluble protein particles in hydrolysates stabilized interfacial layers. The influence of pH on the interfacial behavior of samples was also determined and revealed that G(i)' and G(i)'' values of treated and untreated protein fractions decreased as pH increased beyond their isoelectric points due to increasing electrostatic repulsions between adsorbed protein fragments and aggregates. The high viscoelasticity of the acid-hydrolyzed insoluble microalgae protein-rich fraction at the oil-water interface indicates a high potential for them to be useful in stabilizing emulsion-based products. (C) 2019 Elsevier Inc. All rights reserved.