Processing and Properties of Expanded Starch Materials

Doctoral thesis, 2008

The need to strive towards a more sustainable development of the society is becoming increasingly urgent. As the population of the world is growing so is society´s impact on the environment. This is causing environmental problems with growing amounts of waste. Biodegradable materials from renewable resources would decrease this impact on the environment. Starch is a promising candidate for such materials, fulfilling both the requirement on renewability and that on biodegradability. In order to use starch as a material it needs to have thermoplastic properties and therefore its native semicrystalline structure needs to be disrupted. Disruption of the structure is called gelatinization and is achieved by heating starch in the presence of water. In this study two methods have been used to achieve gelatinization, microwave treatment and extrusion. This work deals with parameters influencing the porosity of starch foams. The molecular composition of starch and the moisture content were of significant importance as they govern both the glass transition temperature and the rheological behaviour of a material, leading to foams with varying porosities. Porous structures of starch, foams, can be used for packaging purposes. It is possible to prepare porous structures of starch polymers both by using microwave treatment and extrusion with water as a foaming agent. Processing of starch requires carefully chosen processing parameters, such as temperature, moisture content, rotational speed of the extruder screw and type of die. Different types of granular potato starches with different amylose/amylopectin ratios were used, which has an influence on many of the properties linked to the foaming capability of the material. The properties of the starch melt such as moisture content, viscosity, melt strength, elongation at break and glass transition temperature were of crucial importance for forming a stable porous structure. The porous structures have been investigated by density measurements, expansion ratio, Environmental Scanning Electron Microscopy imaging and absorption capacity. The potato starch consisting of 100% amylopectin was found to be a better raw material for preparing foams than starches rich of amylose, both when foamed by microwave treatment and by extrusion. The reason suggested is the better water holding capacity of amylopectin which was confirmed by rheological measurements. The moisture content was of large significance throughout the foaming process, at the gelatinization, as a foaming agent and by influencing the glass transition and the rheological behaviour of the starch melts. Gluten improved the processability of starch and led to an increased number of small cells in the foam. The porosity had an influence on the capability of the starch foams to absorb liquid.