Structural changes and plasticizer migration of starch-based food packaging material contacting with milk during microwave heating

The effect of microwave treatment on the plasticizer migration and the changes in multi-scale structures of starch ester films for food packaging were evaluated, and the relationship between these structural changes and plasticizer migration was revealed. The plasticizer migration from the starch ester film to the milk system was accelerated during microwave heating compared with during simple immersion at 30 °C without microwave. After microwave heating, the water and/or fat molecules of the milk system rapidly permeated the film interface to enlarge the interchain spaces. The plasticizer molecules distributed near the boundary more readily migrated out than those located in the matrix interior. The changes in the plasticizer/starch ester interaction and film structures weakened the stability of the ether bond both in the plasticizer and starch ester molecules. The crystalline structure of the film was changed with a certain degree of destruction and the relevant interplanar spacing was decreased. The milk permeation into the amorphous region of the film impeded the shrinkage and aggregation of amorphous starch ester chains and even enlarged the inter-chain distances in this region. However, the ordered microaggregations were shrunk with reduced polydisperse size distribution. In consequence, the aggregation structure showed less restriction to the activated plasticizer molecules as a result of microwave treatment. These structural changes, including the expanded amorphous region and shrunk ordered microregions, could be the reason for the greater plasticizer migration during the microwave treatment. This knowledge will help us in further designing the starch-based materials with restrained plasticizer migration by controlling the molecular interaction, crystalline structure and ordered aggregation structure within the film matrix for safe and reasonable application of this type of packaging materials.