The effect of cooking on proteins from acha ( Digitaria exilis ) and durum wheat

The effect of cooking on proteins from acha and durum wheat was assessed from an analysis of protein extractability, gel electrophoretic profiles, in-vitro protein digestibility (IVPD) and the amino acid compositions of wholemeal flour and residue proteins. Heating wholemeal flour samples at 100-140 degrees C (t = 10-40 min) resulted in 0-30% and 45-55% decreases in acha and durum protein solubility, respectively. In general, high molecular weight (30-70 kDa) protein subunits were more susceptible to heat damage. For both cereals, sodium dodecyl sulphate (SDS; 10 g litre-1) and/or dithiothreitol (DTT; 10mM) increased protein solubility in unheated and heated samples. The IVPD index was 90-91% and was not significantly altered by cooking (100-120 degrees C, t = 40 min). Cooking at extreme temperatures (140 degrees C, t = 40 min) reduced the IVPD by 8% (P = 0.05). Osborne fractionation resulted in a durum or acha residue level of 7.8% or 55.2%. Treatment with solvent containing propanol, SDS and/or DTT at room temperature followed by SDS-polyacrylamide gel electrophoresis of non-solubilised proteins showed that the glutelin fraction of acha, with the exception of a 65 kDa subunit, was insoluble owing to strong inter-subunit hydrophobic and disulphide interactions. Wholemeal acha flour and residue protein showed a significantly greater level of hydrophobic and sulphur amino acids as well as glutamine which is associated with H-bonding. The possibility that cereal protein solubility is also dependent on protein-carbohydrate links is discussed.