If you maintain the product at frozen condition it shouldn't have any effect on biosurfactants. But frequent freezing and thawing will affect the integrity of the molecule. Another way would be, you can extract the biosurfactants from the water medium using solvents like ethyl acetate and remove the solvents by rota-evaporation/vacuum drying and store the crude product. This will avoid the unnecessary reaction of water molecule with biosurfactant during cold storage. 

thank you for the explanation. where can i find related reference?

Multiple freeze-thawing should not have any effect on biosurfactant itself, however, cells will be deteriorated. Cell debris likely will ruin your product quality.

dear Mr. Titus, when I put my biosurfactant on freezer, there wasn't any cells anymore because it has been sentrifuged before. so, could it be concluded that my products of biosurfactants were not affected by that?

Thank you,

the answer should be the same as for products like phospholipids. For these storage in a freezer is recommended to avoid chemical and microbiological change.

i used to think that way too, actually.