Improved tensile strength of zein films using glyoxal as a crosslinking reagent

The effect of crosslinking reagents glyoxal, methylglyoxal and formaldehyde on physical properties of zein films was studied. Zein was solubilized in 90% (v/v) aqueous ethanol and the pH was adjusted with either hydrochloric acid or sodium hydroxide. Crosslinking reagents were added to give 0.3, 1, 3 and 6% (w/w by zein weight), with the reactions carried out at 60 degrees C. Triethylene glycol was added as a plasticizer. Films were cast and dried at room temperature, with cut bars stored at 50% and 70% relative humidity before testing. Films crosslinked with glyoxal and formaldehyde showed a significant increase in tensile strength under certain pH conditions. Films of glyoxal reactions conducted at basic pH gave the highest overall tensile strength, with a 52% increase compared to the control film. Formaldehyde films had improved tensile strength when reacted at acidic or neutral pH. Methylglyoxal had no effect on the tensile strength of zein films. Zein films crosslinked with glyoxal or formaldehyde were found to swell, rather than degrade, when placed in three compatible solvents. Films crosslinked with glyoxal were resistant to boiling water. Denaturing gel electrophoresis of glyoxal and formaldehyde reactions showed the presence of high molecular weight moieties when compared to control reactions.