Commercially Available Solutions
Another solution that is available for freezing protein solutions at large scale uses stainless steel vessels (cryovessels)
from Sartorius-Stedim Biotech (Aubagne, France). These cryovessels are available in multiple sizes (125-L, 200-L, and 300-L)
and consist of a jacketed stainless steel tank with an internal radial finned-heat exchanger. This effectively divides the
tank into six (or 10 for 125-L) longitudinal sections and has the effect of reducing the heat-transfer distance and improved
heat transfer across the entire volume. Dendritic ice formation is promoted, thus avoiding the potentially damaging effects
of cryoconcentration.14 The vessels are cooled and heated by an external refrigeration system that circulates heat transfer fluid through the jacket
and fin system. The temperature profile of the heat transfer fluid is programmable and results in reproducible temperature
profiles in the vessel. The vessel is kept stationary through the freezing process below 0 °C, but is gently agitated by rocking
during the thawing process. The lack of agitation during freezing prevents solutes from moving and promotes the formation
of dendritic ice. Agitation during thawing promotes rapid mixing of the thawed material, thereby removing concentration hot
spots and maintaining uniform temperature in the solution with rapid thawing. The lowest working temperature for the equipment
is –60 °C.
A variation on the bulk freezing technology is the FreezeContainer from Zeta Holdings (Styria, Austria). Jacketed vessels
(currently limited to 300-L) are cooled or heated through an internal circulation system (mounted in the lid). Heat exchange
is accomplished by an external refrigeration system by a circulating heat transfer fluid. The temperature profile is programmable.
The entire container is agitated during thawing.
A large-scale bag freezing system called Celsius from Sartorius Stedim Biotech uses upright bags made of Stedim71 film (ethylene
vinyl acetate product contact material) that are filled with the solution to be frozen and held with slight compression between
two plates that serve as heat exchange surfaces. These plates are cooled or heated by circulating heat transfer fluid from
an external programmable refrigeration unit. The slight compression provides improved contact and heat transfer resulting
in a frozen bag in the shape of a pillow. The bag is kept in frames so as not to stress the material during handling and transport.
The sizes of nominal bags are 16.6 L and 8.3 L, with fill volumes ranging between 4.2 L and 16 L, and 2.1 L and 8 L, respectively.
Six bags can be simultaneously processed in the cryo unit.