Elucidating protein-lipid interactions in wheat gluten using acetic acid

Lipid, a relatively minor component of gluten, has been found to be associated with proteins in this structure and might partially contribute to the high extensibility and elasticity of gluten. Currently, knowledge of protein-lipid interactions in the gluten complex is limited. Accordingly, this research firstly aims to investigate the protein-lipid interactions on the basis of alterations in lipid and protein distribution following various treatments and fractionation using acetic acid. Secondly, the presence of specific protein-lipid interactions in gluten is further studied through analyses of the protein composition of the gluten fractions. Initially, the free and bound lipid components from flour and gluten were separately extracted using a three-step solvent extraction with petroleum ether, chloroform and ethanol. All lipid classes in these extracts were determined using an HPLC method. A comparison of the composition of free and bound lipid from flour and gluten indicated that most of the non-polar lipids originally found in the flour were retained in the gluten with an increasing level of the bound form. A small proportion of the glycolipids and phospholipids from flour were recovered in the gluten but all were present in the bound form. This demonstrates that non-polar lipids could be entrapped within the gluten structure or, alternatively, may become closely associated with at least some of the protein components during gluten preparation. In the investigation of protein-lipid interactions using acetic acid, three distinct approaches have been carried out. The first was to treat gluten with two different concentrations of acetic acid (0.01 and 0.1 M). Following these treatments, non-polar lipid became more readily extractable with solvents of low polarity whilst the level of bound glycolipids and phospholipids were not affected. In the second approach, gluten protein was fractionated into supernatant and pellet using various concentrations of acetic acid (0.01, 0.05 and 0.1 M). Gliadins were found to be the majority of the proteins solubilised at the lower acetic acid concentration (0.01 M) whilst glutenins were more soluble at the high concentrations of acetic acid (0.05 and 0.1 M). Phospholipids were preferentially extracted with the gliadins, indicating the interaction of phospholipids with these proteins. The third approach utilised sequential fractionation with acetic acid at a low concentration (0.01 M) followed by a higher concentration (0.1 M). The fraction soluble in the more dilute acetic acid was predominantly gliadins, while that soluble at the high higher concentration contained both gliadins and glutenin. On the basis of the pattern of proteins, in conjunction with the distribution of free and bound lipid in the gluten fractions it is concluded that the entrapment of lipid in gluten occurs only for the non-polar lipids, whilst polar lipids interact with proteins through specific bonding mechanisms. The data also indicates that glycolipid is probably associated with glutenin through hydrogen bonds and hydrophobic interactions while the phospholipids preferentially interact with gliadins. These findings have been used as the basis for the development of a new model of the interactions between proteins and lipids in the gluten matrix