Mechanism of the formation of insoluble structures in a protein extract of the microalga Chlorella protothecoides at pH 3

By lowering the pH of a dispersion prepared with a water-soluble protein extract from the microalga Chlorella protothecoides, a minor insoluble protein fraction is formed. The aim of this study was to reveal what kind of insoluble structure is formed at pH 3. For that purpose, the spatial fractionation of proteins and carbohydrates, and the effect of urea addition was studied. Results showed that mainly proteins precipitate from solution at pH 3, whereas no significant change (p > 0.05) was found in carbohydrate concentration during the shift in pH. Moreover, urea dissolved aggregates, showing that mainly hydrophobic interactions were causing the aggregate formation. Taken together, the results imply that the formed insoluble structures at pH 3 are composed of protein aggregates and not electrostatically stabilized protein-polysaccharide complexes.