Plasticization of kafirin films

Abstract

Despite the potential of kafirin (sorghum prolamin protein) films, up until now there has been no in-depth investigation on the plasticization of kafirin films, similar to that done on zein films. Since protein films tend to be very brittle, plasticization is a very important aspect. Cast films were produced from kafirin and plasticized with different combinations of plasticizers (glycerol (G), polyethylene glycol 400 (PEG) and lactic acid (LA)) according to a rotatable central composite statistical design. The effects of the different plasticizer combinations on the film properties (tensile-, Tg-, moisture and oxygen barrier properties), were investigated through a series of tests performed on the films. Plasticization of kafirin films was investigated further by determining the effect of an emulsifier, diacetyl tartaric ester of monoglyceride (DATEM) and an acidulant, glucono-_-lactone (GDL) on the films. To investigate the distribution and migration of the plasticizers in kafirin films, the films were studied by light microscopy. It was clear that G, PEG and LA together were necessary to plasticize kafirin films. G and PEG were found to be effective plasticizers, leading to a decrease in film strength and an increase in strain as the plasticizer amount increased. LA was, however, found to act rather as a solvent for kafirin during film casting, instead of acting as a plasticizer. An increase in plasticizer content also brought about a lowering in the Tg of the films, as well as an increase in film permeability to water vapour and oxygen. G and PEG were found to attract water from the atmosphere, which proved to be very influential on the properties of the films; the more plasticizer present in the film, the more moisture attracted, the greater the effect on the film properties. DATEM was not found to be a plasticizer for kafirin films. However, GDL did bring about changes in film properties, similar to G and PEG. It caused film strength to decrease and film strain to increase, but with less detrimental effects on the moisture barrier properties of kafirin films. Microscopy showed that the plasticizer migrated over time, apparently leading to plasticizer molecule coalescence, and the formation of plasticizer pools. The plasticizer combination of G, PEG and LA improved the qualities of kafirin films, reducing film brittleness, but it is not an ideal plasticizer combination due to the fact that it attracts water to the film and it is not stable over time. Moisture is another plasticizer to be taken into account, since it will be absorbed by the plasticizers in high relative humidity areas and will have an additional plasticizing effect on films properties. GDL proved to have potential as a kafirin film plasticizer as it affected the barrier properties of the films less. Further research is recommended into the plasticization mechanism of GDL.