Microwave-assisted moulding using expandable extruded pellets from wheat flours and starch

Extruded pellets made from wheat flour and purified wheat starch were expanded by applying microwave heating within moulds, a method known as microwave-assisted moulding (MAM). Selection of adequate mould material, pre-treatment of pellets and control of the initial loading of pellets in the mould cavity were found to be the key issues to achieve a uniformly foamed block with good integrity. Polytetrafluoroethylene (PTFE) was found to be an appropriate mould material for the MAM process. The bonding between foamed pellets in a block can be significantly enhanced by soaking the pellets in a NaCl solution before microwave foaming. There exists an optimum initial loading of pellets in the mould for a given pellet formulation, which allows sufficient expansion to achieve an acceptable extent of interfacial bonding and mould filling. The work demonstrated the feasibility of moulding starch block foams and potential applications of the foams in packaging and lightweight composites.