In all crystallization processes the final crystal product size is a main characteristic for product quality and is controlled by crystal generation, growth and destruction. Secondary nucleation is crystal generation from other crystals in solution and is the major source of new crystals in a mixed suspension crystallizer. For brittle materials crystal collisions result in attrition or breakage and subsequent growth of fragments produce secondary nuclei. During crystal impact the fragmentation rate is dependant on the impact velocity, crystal size and crystal mechanical properties.

Enzymes or protein food additives can be separated from their natural sources or from fermentation broths by bulk crystallization. Crystallization offers cheaper separation than other commonly used separation techniques with high selectivity and a good yield of a concentrated, stable and active product.

Due to the large size of protein molecules, interstitial spaces between molecules in a protein crystal lattice permit moisture and dissolved compounds sufficiently smaller than the protein to move in and out of the crystal. The hardness of dry crystals without interstitial liquid is different from that of wet crystals. The mechanical properties (elastic modulus, hardness and fracture toughness) of dry and wet crystalline protein and some commonly crystallized small molecules are measured by indentation.

Through these measurements we assess the relative brittleness of crystalline protein and some commonly crystallized small molecule salts to determine the importance of attrition-based secondary nucleation in industrial protein crystallization.