Tannin binding of kafirin and its effects on karirin films

Abstract

Kafirin, the prolamin protein of sorghum grain, could be extracted from the by-products of the sorghum processing industry and used to make films and coatings for food packaging, in particular to extend the shelf-life of fruits and nuts. Protein-based films can be an environment-friendly alternative to synthetic plastic packaging systems. However, the properties of protein-based films are generally inferior to those of synthetic plastics. Modification can alter the properties of protein-based films. In this project, the interaction between phenolic compounds and kafirin was investigated in relation to their potential to modify kafirin films. A range of phenolic compounds was tested in terms of their ability to bind and complex with kafirin in an in vitro binding assay. The protein-phenolic compound interaction was quantified by haze formation and colorimetric determination of total polyphenol bound. Ferulic acid, catechin and extracted flavonoids from condensed tannin-free sorghum did not complex with kafirin. Tannic acid (TA) and sorghum condensed tannins (SCT) complexed kafirin and formed haze. Thus, T A and SCT were selected as potential modifying agents for kafirin films. TA and SCT were added at up to 20% (w/w tannin to protein basis) during kafirin film casting. Both TA and SCT bound to kafirin in the film. Scanning electron microscopy showed that TA modified films were less porous; and the SCT modified films appeared more globular in structure than unmodified film. Modification with both tannins increased the tensile stress and Young's modulus and decreased the tensile strain of the kafirin films. Oxygen permeability of the modified films was decreased, but no change in the apparent water vapour permeabilty. The T9 of the films increased with increased modification level. SOS-PAGE, FT-IR and Raman spectroscopy were used to study TA and SCT interaction with kafirin. SOS-PAGE revealed a high Mr band for kafirin-SCT complexes which did not enter the separating gel. FT-IR of kafirin complexed tannins and tannin modified films showed a decrease in the absorbance at the frequency of about 1620 cm-1, suggesting a decrease in â-sheet structures. FT-IR results also suggested that the â-sheets of kafirin in dry form were probably changed into random coils during kafirin dissolution to make films. Raman spectra showed a shift in the TA peak at about 1710 cm-1 to about 1728 cm-1 in the kafirin- TA complexes, suggesting participation of the carbonyl groups of TA in TA-kafirin interaction. It is proposed that hydroxyl groups of tannin can form hydrogen bonds with carbonyl groups of random coils of kafirin during film casting. Thus, the carbonyl groups are probably not available to be reorganized into â-sheets. The other possible mode of interaction can be hydrophobic interaction between the aromatic rings of tannins and the pyrrolidine rings of proline. Because tannins have numerous aromatic rings with hydroxyl groups, it is also proposed that they can bind with more than one polypeptide chain at the same time to cross-link kafirin. This cross-linking probably produces a high Mr kafirin-tannin complex that leads to haze. The cross-linking would also lead to lower molecular mobility of modified kafirin films. This could decrease oxygen permeability, probably as a result of decreased free volume. Cross-linking could also be responsible for the increased tensile stress and decreased tensile strain of modified kafirin films. The higher tensile stress of modified• kafirin films suggests that they can have the potential to form stronger coatings around fruit such as litchi fruit to possibly reduce pericarp microcracking as an example, and thus may reduce the pericarp browning of litchi. The lower oxygen permeability of the modified films and the potential antioxidant activity of the tannins suggest that these films can be a good coating to prevent rancidity of nuts.