Kinetics of release from kafirin films

Two separate studies were performed. The first studied release of two different preservatives (lactic acid and calcium propionate) from a biopolymer film to a model food with a water activity of aw=0.95. The second investigated the release of four sugars (fructose, maltose, raffinose and stachyose) with different molecular weights from a biopolymer film to a model food with water activities ranging from 0.85 to 0.95. In both studies, the biopolymer films containing the release components were made from kafirin and placed on the model food, consisting of a gel made of gelatine, sucrose and water with well defined water activity. The amount of preservatives and sugars released from the film to the gel were monitored over time using HPLC at different depths in the model food.The release of preservatives was rapid in both cases, and a gradient of released substances had already formed after 2 h in the model food. The gradients levelled out with time. Calcium propionate had a somewhat faster release than lactic acid.The water activity of the model food had a large impact on the release of sugars. A slow release was observed in the model food with aw=0.85, while a much more rapid release occurred in the model food with aw=0.95. There was a limited diffusion depth in the model food with aw=0.85, probably owing to a limited trial time, while the sugars in the model food with aw=0.95 diffused deeper into the model food until the film was depleted of available sugars.