Gaulin homogenization: a mechanistic study.

Free radical-based oxidation has been detected in the normal operating regime of the Gaulin homogenizer, demonstrating that cavitation occurs in this important industrial bioprocessing equipment. Free radical generation is suppressed by imposition of back pressure, proving that such cavitation occurs in the impingement section. The calculated value of the cavitation number is consistent with submerged jet cavitation, wherein a high-speed jet exiting from the valve gap accelerates fluid in the impingement region, creating the vacuum conditions for cavitation. Using polysaccharides as a model shear-sensitive compound, their breakage pattern in the homogenizer was characterized by molecular size and polydispersity and compared to those of fluid shear flows in capillary tubes and cavitating flow from a sonic horn. The results indicate that breakage occurs primarily by fluid shear, although a contribution by cavitation is also apparent when back pressure is applied. Because biological molecules can readily react with free radicals and the alterations caused thereby are subtle in nature, a thorough evaluation of the impact of free radicals in upstream homogenization is warranted.