Imidazole-based deep eutectic solvents for starch dissolution and plasticization

Highlights

• We prepared imidazole-based deep eutectic solvents with dissolving activity for starch.

• Investigated DESs are suitable for dissolving high amylose starch.

• Influence of water content in starch and DES on starch modification was analyzed.

• Our thermoplastic starch films do not exhibit retrogradation after 1 month storage.

Abstract

Potato starch and high-amylose starch were treated with imidazole-based deep eutectic solvents (DESs) as dissolution and plasticization media. Beside imidazole (IM) for two-component DESs preparation choline chloride (CC), glycerol (G) or carboxylic acids (citric or malic) were used. An influence of water content in starch (as well as an extra water in the starch/DES system) on polymer dissolution and plasticization processes was investigated. Dissolution and gelatinization of starch in DESs were followed via DSC and laser scanning microscopy. A rheometric characteristics revealed an influence of starch/DES system storage time on the plasticization process. The tendency to recrystallization of compression-molded-starch films was evaluated using XRD technique. High dissolution and plasticization effectiveness of CC/IM and G/IM and a low tendency to film retrogradation of thermoplasticized starch were noted.

Introduction

Starch is one of the most abundant biopolymers and is widely used in food and pharmaceutical sectors. Despite its non-toxic and biodegradable character, the application of starch in non-food or non-pharmaceutical industries is rather limited because of its hydrophilic feature as well as weak mechanical and barrier properties.

Because of the strong hydrogen bond that occur between polysaccharide chains starch is insoluble in water and conventional organic solvents, with the exception of dimethyl sulfoxide (DMSO) or its combinations with some additives (Morrison and Laignelet, 1983, Radosta et al., 2001), solutions of dimethylacetamide/LiCl (Heinze, Talaba, & Heinze, 2000) or some amines and their derivatives (Hung and Yu, 2006, Koganti et al., 2011). A semicrystalline structure of starch also hinders its processing via physical or physicochemical methods. Thermoplasticization is known as an industrially important methodology of native starch modification. In the presence of relevant polar plasticizer, high temperature (also shear stress action) starch granules become amorphous and such material is more elastic, forming transparent and homogenous thermoplastic starch (TPS). Water and glycerol (G) are the most often used starch plasticizers, but other polyalcohols (e.g. sorbitol, xylitol, hexylene glycol), amines (e.g. ethanolamine), amides (e.g. urea, formamide), and polycarboxylic acids (e.g. citric acid) are also applied for starch plasticization (Mekonnen et al., 2013, Nafchi et al., 2013, Niazi et al., 2015). However, the above mentioned plasticizers have some drawbacks and allow for starch recrystallization which results in material stiffness and brittleness.

Ionic liquids (ILs) belong to novel starch plasticizers. ILs are the salts liquid below 100 °C, often at room temperature. Some papers related to apply of molecular ILs (especially imidazolium type) for starch dissolution or plasticization were reported (Lappalainen et al., 2013, Mateyawa et al., 2013, Ning et al., 2009a, Ning et al., 2009b, Sankri et al., 2010, Wilpiszewska and Spychaj, 2011). However, despite of many advantages of ILs their green character is disputable (Gericke, Fordim, & Heinze, 2011) and from the economic point of view their high cost is the primary disadvantage. Deep eutectic solvents (DESs) are emerging as new class of ionic liquids that are the mixtures of two or more compounds: one being hydrogen acceptor (e.g. choline chloride) and the other hydrogen donor (e.g. imidazole, glycerol, citric acid, malonic acid) (Abbott et al., 2003, Francisco et al., 2012, Francisco et al., 2013). The melting temperature of DES is much lower than the relevant values of individual mixture components. In comparison to common ILs, DESs are easily to prepare by mixing of components and heating at elevated temperature (usually up to 100 °C) without the generation of any side products and necessity of purification. Furthermore, DESs are often biodegradable, less or nontoxic in comparison to ILs or other organic solvents (Petkovic et al., 2010, Radošević et al., 2015); moreover, they are also less expensive.

There have been few reports on using DESs for starch dissolution (Biswas et al., 2006, Dai et al., 2013, Francisco et al., 2013, Zdanowicz and Spychaj, 2011) or its plasticization (Abbott et al., 2014, Abbott et al., 2012b, Leroy et al., 2012, Ramesh et al., 2012a, Ramesh et al., 2012b, Zdanowicz and Spychaj, 2011). Biswas et al. (2006) used four DESs based on choline chloride (CC) and urea (U), citric acid (CA), zinc chloride or oxalic acid, respectively, for starch dissolution preceding its acetylation. The best effects (i.e. clear, viscous solutions) were found for CC/U (solubility: 9.1 wt%), and CC/CA (solubility: 8.3 wt%) after heating at 100 °C, whereas the use of molecular IL (1-butyl-3-methylimidazolium chloride) allowed to obtain a starch solution up to 15 wt% at 80 °C. Similar results were described when potato starch (dried or humid with 3 and 14 wt% of water, respectively) was dissolved in CC/U, CC/CA and CC/succinic acid at 118–130 °C/60 min, to obtain 5 wt% solutions (Zdanowicz & Spychaj, 2011). Moreover, compression molded transparent and colorless films were formed (140 °C/20 MPa/10 min). In other work DESs based on green natural components were tested as the starch solvents (Francisco et al., 2012). Among 26 tested DESs the highest dissolution efficiency (after heating up to 100 °C for 24 h) exhibited these ones based on malic acid (MA) with CC (solubility: 7.1 wt%), glycine (solubility: 7.7 wt%) and proline (solubility: 5.9 wt%). Ramesh et al. recently tested the efficiency of CC/U (in the presence of organic lithium salt) for corn starch plasticization. An increase of the amorphous phase content in the plasticized starch (Ramesh et al., 2012a) as well as ionic conductivity improvement were reported (Ramesh et al., 2012b). Abbott et al., 2012b, Abbott et al., 2014 and Leroy et al. (2012) investigated the properties of CC/U and CC/G as corn starch plasticizers after processing by compression molding or extrusion. The CC/U plasticized starch exhibited the mechanical properties similar to low density polyethylene (Abbott et al., 2012b, Abbott et al., 2014). It was also stated that the water content in the starch/DES system was a key factor determining starch dissolution or plasticization behavior (Abbott et al., 2012b, Abbott et al., 2014, Zdanowicz and Spychaj, 2011).

Deep eutectic solvent CC/IM with eutectic point 56 °C was obtained when the components were mixed at 3:7 molar ratio (Hou et al., 2008). It exhibited the complete miscibility with some polar solvents (e.g. water) and high glucose solubility (ca. 43.4 wt%). Up to the present imidazole based DES has not been used for starch dissolution or plasticization.

Francisco et al. (2012) and Francisco et al. (2013) used a general term “low transition temperature mixtures – LTTMs” instead of deep eutectic solvents for systems that do not have the melting temperature but exhibit only glass transition peaks. Although some of the here tested eutectic mixtures have the glass transition peaks besides or instead of melting temperature, for simplicity we call them DESs. Some of the above references are cited here even if report dissolution of lignocellulosic materials in ionic liquids and knowing that cellulose dissolution is “different world” than starch dissolution. Reasons for that are: (i) general consideration of ILs green character and some objections about it, (ii) in context of other (than cellulose) polysaccharide types dissolution, (iii) in connection to new term LTTM instead of DES.

DESs based on G or polycarboxylic acids: CA and MA and IM with the same molar basic ratio of the components 3:7 (or other molar ratios for comparison) were tested as starch solvents and plasticizers. An influence of water presence in the starch/DES systems on the dissolution and plasticization behavior was also evaluated. Taking into account a good starch solubility in molten imidazole (Jordan, Schmidt, Liebert, & Heinze, 2014) the aim of this study was to check imidazole-based DES efficiency for starch dissolution and plasticization in comparison to DES previously used for this purpose, i.e. based on CC and U.