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H. DOGAN¹, Z. Hicsasmaz², S. Katnas³, and D. Kocer². (1) Food Science and Technology Research Institute, TUBITAK, P.O. Box 21 Gebze, Kocaeli, 41470, Turkey, (2) Food Engineering Department, Middle East Technical University, Ankara, 06531, Turkey, (3) Chemical Engineering Department, Gazi University, Faculty of Engineering and Architecture, Maltepe, Ankara, 06570, Turkey

JUSTIFICATION: Starch-lipid, protein-lipid, starch-protein and protein-protein interactions occur during extrusion cooking that cause network formation which affect extrudate microstructure, physical and functional properties.

OBJECTIVES: To assess the effect of extrusion conditions on interaction mechanisms and effects of these interactions on extrudate properties.

METHODS: We used a Brabender counter-rotating twin-screw extruder. The independent variables were the feed composition, barrel temperature and screw speed. Interaction of lipids with feed mixture components was evaluated by simple petroleum ether extraction. We considered lipid available to extraction before extrusion and that becomes unavailable after extrusion to interact with protein, starch, or both. We determined iodine binding capacity of feed mixtures and extruded samples to calculate percentage starch complexation. We studied protein-protein interactions using urea, sodium dodecyl sulfite, 2-mercaptoethanol, and SDS+2-ME aqueous solvents with specific chemical action on proteins.

RESULTS: Increase in protein content increased percentage free lipids after extrusion. Thus, lipids are more likely to form complexes with starch than proteins. At high lipid concentrations percentage bound lipids decreased. The occurrence of excess lipid in the feed caused slippage, thus underprocessing. Percentage lipid complexation increased at high moisture levels corresponding to complete gelatinization. High moisture-high temperature combinations increased percentage starch complexation. Presence of proteins decreased the extent of complexation. At high lipid concentrations complex formation increased. Extrusion of lentil flour decreased solubility of proteins due to noncovalent interactions since more protein is soluble in the SDS-containing buffer than in plain buffer. More solubility in SDS+2-ME buffer than in SDS-buffer indicates formation of new disulfide bonds. Content of soluble proteins in the extrudate at 185°C extrusion temperature was slightly higher than that at 165°C. Protein aggregation was partially disrupted during higher temperature extrusion reducing the molecular weight of proteins.

SIGNIFICANCE: The interrelationship between processing conditions, microstructure, functional and physical properties can be explained by studying component interactions.