Quality of wheat starch films with amylose-lipid nanomaterials

Abstract

Starch is used in the food industry for many applications, for example, as a thickener and stabiliser. Novel uses of starch include making biodegradable plastic materials. These are environmentally friendly alternatives as they have a lower carbon footprint than synthetic plastics. However, starch plastics have poor mechanical and water vapour barrier properties, and as such have limited applications. Several researchers have noted that modification of starches can result in plastics with increased performance. Some modifications include the additions of filler material, which is thought to improve starch plastic’s performance. Filler materials can include materials such as inorganic clays (kaolin, montmorillonite and sepiolite). An alternative material can be naturally occurring amylose-lipid complexes. These complexes can be made by modifying starch, such as wheat starch, with lipid material, such as stearic acid. This project investigated the effect of making plastic materials from wheat starch modified with stearic acid after different pasting times to produce modified starch with amylose-lipid complexes. The plastic's mechanical, thermal and barrier properties are determined. The work also investigated the effect of adding amylose-lipid nanomaterial to wheat starch films and determined the plastic's mechanical, thermal and barrier properties. Wheat starch was modified, with 0.5% and 1.5% stearic acid, and films were then made from the modified wheat starch after pasting with a RVA (Rapid Visco-Analyser). After 30 minutes pasting time, the wheat starch films modified with 0.5% stearic acid resulted in significantly increased tensile properties compared to the other films, including maximum stress and strain as well as stress at break. These films also showed lower oxygen and water vapour permeability. However, when starch films were made with modified wheat starch during pasting for 120 minutes, a lower tensile stress and higher water vapour and oxygen permeability was obtained. The differences can be attributed to amorphous amylose-lipid complexes formed in situ during short (30 minutes) pasting compared to semi-crystalline amylose-lipid complexes formed during extended (120 minutes). The semi-crystalline amylose-lipid complexes can act as weak point and pin holes for crack initiation and propagation. The presence of amorphous amylose-lipid complexes in the films was suggested to be the main factor that increased the films’ tensile properties. In addition, when amylose takes part in complexation with stearic acid, it is not available to form junction zones for network development. Consequently, weaker films are formed. Amylose-lipid material was isolated from wheat starch that had been modified with stearic acid. This was then characterised to ensure that the isolated material was in fact amylose-lipid material. The amylose-lipid material was found to exist at nano-scale. These nanomaterials were then added to wheat starch paste and films were made. Wheat starch films made with 5% amylose-lipid nanomaterial addition resulted in the highest increases in the films’ tensile properties in terms of the tensile stress and modulus, increased barrier properties in terms of water vapour and oxygen, and most stable thermal properties in terms of glass transition and dynamic mechanical thermal analysis. It was suggested that there is an interaction between the amylose-lipids materials and wheat starch polymers which improved the mechanical properties. The improved mechanical property is thought to arise from the amylose-lipid nanomaterial having acted as a nano filler. By acting as a filler, the nanomaterial provided a tortuous pathway which increased the oxygen and water vapour barrier properties. Amylose-lipid complexes formed in situ during wheat starch pasting with stearic acid or through exogenous addition of amylose-lipid complexes have the potential to improve biodegradable wheat starch films.