Understanding the role of water-ionic liquid mixtures on the gelatinization and dissolution of maize starches

Abstract

The catastrophic impact of petroleum based plastics on our environment has resulted in the development of a new class of biodegradable polymers. Starch is an attractive raw material for biodegradable plastic applications due to its low cost, sustainability and its availability in large quantities. Besides, it can be transformed into thermoplastic material using conventional plastic processing techniques. Despite its attractive potential as a source of biopolymer materials, the use of starch-based plastic products has been restricted to niche markets that do not require high performance applications due to poor mechanical properties and moisture sensitivity of the materials. Starch cannot be thermally processed without water but thermoplastic starch materials developed from water are brittle due to retrogradation. Much research has been devoted to the use of plasticizers other than water. Non-volatile plasticizers such as glycerol, glycol, sorbitol, urea and ammonium derived amines have mainly been used in the plasticization of starch, but due to their high water sensitivity, recent researches have diverted to novel plasticizers, particularly the ionic liquids. This dissertation involves an in-depth understanding of the effect of the ionic liquid 1-ethyl-3-methylimidazolium acetate, [Emim][OAc] on the plasticization or gelatinization of starch. Starch gelatinization is a key component of starch processing and the understanding of the molecular and structural changes in the starch granules during this process is important for the effective control of functional behaviour during development of starch-based materials. In this research, the impact of [Emim][OAc] on starch gelatinization is initially evaluated through information gathered from the thermal, pasting and microscopic properties of thermally treated starch-water-[Emim][OAc] mixtures at various water-[Emim][OAc] mole ratios. DSC, being the most convenient way of determining thermal properties, revealed starch gelatinization to have occurred at a reduced temperature by a certain mole ratio of water-[Emim][OAc] mixture. A further decrease to the water content reduced the phase transition temperature of starch even farther. This result was supported by RVA and microscopy. It is proposed that the phase transition of starch at the reduced temperature is controlled by two processes: gelatinization and dissolution believed to be occurring in a synergistic manner. At lower [Emim][OAc] concentration (water/[Emim][OAc] mole ratio ≥ 25.0/1 mol/mol), gelatinization mainly occurs; while at higher [Emim][OAc] concentration (water/[Emim][OAc] mole ratio ≤ 2.8/1 mol/mol), dissolution occurs. Since [Emim][OAc] was able to gelatinize starch at a reduced temperature close to room temperature, further investigations of the effect of this water/[Emim][OAc] mole ratio (7.2/1 mol/mol) on starch molecular and structural changes was examined under room temperature. Characterization techniques involved in this investigation were photography, microscopy, DSC, XRD, NMR, FTIR and FT-Raman. The results of the different characterization techniques substantiated one another in confirming that the 7.2/1 mol/mol water/[Emim][OAc] effectively disrupted the crystallinity of starch at room temperature. The basis of gelatinization at room temperature is examined through the effect of changing the ionic liquid [Emim][OAc] with 1-ethyl-3-methylimidazolium ethyl sulphate ([Emim][EtSO4]) , acetic acid (CH3COOH), sodium hydroxide(NaOH) and urea (CO(NH)2). Possible explanation for this behaviour is suggested.