Starch structures and their usefulness in the production of packaging materials

Environmental concerns about increasing industrial use of plastics and the associated waste are raising the demand for renewable sources to replace petroleum-based products, especially in the packaging sector. This thesis examined the relationship between molecular structures of starch and their material properties in coating materials for food packaging. Chemical modification of starch with citric acid and tailored starches from genetically modified potatoes with higher amylose content was used to improve film formation, reduce water sensitivity and decrease gas permeation. Citric acid was used as a cross-linking agent and as a plasticiser to improve the barrier properties of starch-based barrier coatings. Methods to detect the cross-linking of starch by quantifying the di-ester content of citric acid and measuring molecular weight changes were developed. Starch films containing highest citric acid content showed the highest cross-linking density and lowest water solubility, but also showed the highest degree of starch degradation due to acid hydrolysis. Adjustment of pH in the starch formulation was used to control starch degradation and adapt it to industrial needs. At pH 4, hydrolysis of starch nearly stopped but cross-linking reaction still occurred, leading to minimum gas permeation. Cross-linking occurred already at temperatures as low as 70°C. Hence no curing step was needed to initiate cross-linking reaction. This is suitable for industrial paper coating applications since high temperature is not required. The coating process had a great impact on the molecular structure of starch. Laboratory-scale coatings showed a lower degree of cross-linking and no significant hydrolysis of starch compared with solution-cast films, a difference which could be attributable to shorter drying time in laboratory-scale coatings. The high evaporation rate in industrial pilot-scale coatings promoted cross-linking, but also increased coating surface unevenness, resulting in pinholes, which diminished barrier properties. Studies of tailored starches from genetically modified potatoes with expected increased amylose content showed irregular granules but no weakening of birefringence. It was found that altered amylopectin structure rather than high amylose content resulted in better barrier properties. The amylose-like structure formed in the cast films of starches from genetically modified potatoes decreased oxygen permeability and improved film strength.