Modification of the functional properties of zein and kafirin protein films by oxidation using hydrogen peroxide

Abstract

Maize and sorghum flours do not form viscoelastic doughs similar to wheat flour. However, isolated zein (maize prolamin) can form a viscoelastic dough when first cast into a film and then hydrated in water above its glass transition temperature (Tg). Wheat gluten functionality is strongly influenced by disulphide and dityrosine bonding. Furthermore, kafirin (sorghum prolamin) films are stabilised by disulphide bonds formed through cysteine oxidation. Thus, this study determined the effects of oxidising commercial zein (mainly ?-zein), total zein (?-, ?-, ?- and ?-zein) and total kafirin (?-, ?-, ?- and ?-kafirin) with hydrogen peroxide on film functionality and dough formation. Films were prepared by casting where the prolamins were denatured in 70% aqueous ethanol at 700C, allowing the solution to cool to ambient temperature and then adding hydrogen peroxide (up to 150 mg/g protein) with and without copper catalyst and then evaporating the solvent off. Increasing hydrogen peroxide concentration increased zein and kafirin film aqueous buffer uptake, but not with hydrogen peroxide plus copper catalyst. SDS-PAGE revealed disulphide bond polymerisation of total zein and total kafirin films prepared with hydrogen peroxide, but not with films from commercial zein. DSC showed an increase in Tg and heat capacity change of commercial zein, total zein and total kafirin films prepared with hydrogen peroxide. FTIR indicated an increase in ?-helical conformation of commercial zein and total zein and an increase in ?-sheet conformation of total kafirin with increase in hydrogen peroxide concentration, but not with commercial zein films. The change in total kafirin structure could have been due to kafirin containing more cysteine residues than total zein. The dried films were manipulated with distilled water at 50oC into doughs. Commercial zein films prepared with hydrogen peroxide formed doughs but not the films prepared without hydrogen peroxide. All total zein film preparations formed doughs, whereas all total kafirin film preparations did not. Addition of hydrogen peroxide to commercial zein and total zein increased dough cohesiveness, extensibility and water holding capacity. The zein doughs were extensible when left below Tg for 1min. Inclusion of hydrogen peroxide in commercial zein and total zein promoted fibril formation as was seen under SEM and stereomicroscopy. However, no fibrils were formed with total kafirin. FTIR of commercial zein and total zein doughs indicated a slight increase in ?-helical and an increase in ?-sheet conformation with total kafirin aggregates. Commercial zein, total zein and total kafirin films react differently with hydrogen peroxide treatment. Commercial zein contains few cysteine residues, hence the effects of hydrogen peroxide could be due to hydroxylation of amino acid side chains which would promote hydrogen bond formation. As hydrogen peroxide promotes disulphide bond crosslinking of total zein and kafirin, extensive crosslinking prevents dough formation from kafirin films. Therefore oxidation of zein and kafirin with hydrogen peroxide modifies the film and dough functional properties. However, further research is required on how to form doughs from total kafirin.