Bottles and carboys are used for frozen storage and transport of biopharmaceutical formulations under a wide range of conditions. The quality of freezing and thawing in these systems has been questioned due to the formation of heterogeneous ice structures and deformation of containers. This work shows that during freezing of bulk protein solutions, the liquid at the air-liquid interface freezes first, forming an ice crust and enclosing the liquid phase. As the enclosed liquid freezes, internal pressure rises, pushing the liquid phase through the porous ice crust towards the air interface, leading to interfacial stress and protein aggregation. The aggregation of bovine serum albumin was more intense in the foam-like ice mound that was formed at the top, where bubbles were entrapped. This was characterized experimentally with the assistance of magnetic resonance imaging (MRI). An isothermal cover is proposed to prevent the early freezing of the liquid at the air interface, attenuating substantially interfacial stress to proteins and releasing hydrostatic pressure, preserving the shape and integrity of the containers.
Keywords: freezing; ice crust; interfacial stress; isothermal cover; pressure increase.