A plasticiser. Otherwise the film would be brittle, fragile and glassy, which makes it difficult to prepare a defect-free film, but also difficult to handle the product. Addition of a plasticiser, changes the properties from glassy to brittle, to pliable and elastic, often called rubbery.

It leads to loss of mechanical strength and ultimately loss of coherence of the layer.

Plasticizers for biopolymers are hydrophilic by nature, which means that their addition to the film will lead to an increase in moisture permeability, thereby compromising on the quality of the barrier function.
Further, it should be realised that plasticisers are usually relatively small molecules that may have good interaction with the product substrate. Thus, the plasticiser may slowly migrate from the film into the product, leading to a slow loss of elasticity of the film, and subsequent failure of the film, and possibly deterioration of the product itself.