It is common knowledge in the industry that the freeze/thaw cycle is harmful to proteins in serum. During the freezing portion of the cycle, the formation of ice can cause stresses on proteins which can cause aggregation and possible denaturation. Turbidity in serum after the thawing cycle is normally due to the formation of cryoprecipitates. Turbidity in serum will increase due to repeated freeze/thaw cycles. One of the items that causes turbidity after freeze/thaw cycles, is the protein fibrinogen. This protein is quite soluble and may not completely clot during the processing of whole blood. After freeze/thaw cycles, the remaining fibrinogen in the serum can aggregate and precipitate even though the serum has been sterile filtered. This cloudiness cause customers to question if the serum is contaminated. It has been our experience that this visible cloudiness does not affect the quality of the serum.
The best rule is to purchase serum in volumes that will be completely used at one time. Another way to reduce the effects of freeze/thaw cycles is to aseptically aliquot the serum into the sterile container size of choice.