Disclosed herein are compositions comprising aggregates of insoluble alphaglucan with alpha-1, 3-glycosidic linkages. These aggregates have an average hydrodynamic radius of about 50 nm to about 300 nm, and a fractal dimension of about 1.6 to about 2.4. Insoluble alpha-glucan aggregates in some disclosed aspects are arborescent. Further disclosed are methods of preparing these compositions.