Physicochemical properties and textile utilization of low- and moderate-substituted carboxymethyl rice starches with various amylose content

Abstract

The effects of low and moderate levels of degree of substitution (LDS ∼0.2 and MDS ∼0.6) on physicochemical and textile printing properties of carboxymethyl rice starches (CMRS) containing different amylose (AM) content were investigated. With a similar level of AM, MDS CMRS displayed higher cold water solubility (CWS), yield stresses, and apparent viscosities than those of LDS CMRS. Furthermore, they were superior to LDS CMRS when used as textile print paste. AM also influenced the properties of CMRS with similar DS. When AM increased, CWS of LDS CMRS increased. However, yield stresses and apparent viscosities of CMRS decreased with increased AM. MDS waxy CMRS print paste provided the least color strength among other MDS CMRS. Nonetheless, DS was shown to have a far greater effect than AM on determination of CMRS properties.

Introduction

Carboxymethylation is an etherification reaction which has been commonly used for chemical modification of starch. The goal of the modification is to substitute carboxymethyl groups (CH₂COO⁻), which are negatively charged, for hydroxyl groups (–OH) in starch molecules (Volkert, Loth, Lazik, & Engelhardt, 2004). As a result of the substitution reaction, many functional properties of carboxymethyl starch (CMS) such as cold water solubility, paste and film clarity, and paste and gel storage stability are significantly improved (Kittipongpattana et al., 2007, Kwon et al., 1998, Sangseethong et al., 2005). This has resulted in various applications of carboxymethyl starch: in pharmaceutical excipients, textile printing pastes, adhesives, waste absorbents, paper, and medical poultices (Tijsen, Voncken, & Beenackers, 2001). To determine levels of starch modification, a degree of substitution (DS) – the average number of substituents, which in this case is the number of carboxymethyl groups per anhydroglucose unit (AGU) of the starch polymer – is used as an indication. DS values range from 0 to 3 (Jie et al., 2004, Sangseethong et al., 2005, Volkert et al., 2004).

Several previous studies have focused on factors affecting values of DS of CMS, and optimization of CMS production with specified DS (Hebeish and Khalil, 1988, Kittipongpatanaa et al., 2006, Kwon et al., 1998, Ragheb et al., 1997, Sangseethong et al., 2005, Stojanovic et al., 2000, Tijsen et al., 2001, Volkert et al., 2004). However, studies of the effects of DS on the physicochemical and functional properties of CMS have been quite limited. Bhatacharyya, Singhal, and Kulkarni (1995) reported that when DS of CM corn and amaranth starches increased from 0.10 to 0.20, both peak viscosity and freeze–thaw stability of CMS paste samples decreased. Sangseethong et al. (2005) found that the viscosity of CMS samples increased when their DS increased up to 0.40. However, the study mentioned that all cassava CMS samples with 0.03–0.70 DS were completely soluble in cold water, regardless of their DS. It is interesting to further explore whether more properties of CMS besides those properties are affected by their DS level.

The amylose content (AM) of starch also influences many physicochemical and functional properties of starch products. The hardness of rice starch gels, the textural properties of cooked rice (Yu, Ma, & Sun, 2009), and the tensile strength of starch film (Lloyd & Kirst, 1963) are examples of the properties affected by the ratio of AM. It is also worthwhile to investigate whether AM plays an important role on the properties of CMS with similar DS level.

To the authors’ knowledge, to date no study has been done on the physicochemical and functional properties of CMS with different DS and AM. To gain this knowledge, it is potentially beneficial to industrial end-users in order to facilitating their decision making on selection of CMS with the DS value, and possibly providing the AM criteria for starch material selection to suit their needs. Therefore, the objective of the study was to investigate the effects of two different levels of DS, low and moderate DS (DS 0.2 and 0.6), and the influence of AM content (waxy, low, and high AM) on the physicochemical and textile printing properties of carboxymethyl rice starches. To eliminate the possible effects of different botanical sources of starch on those properties of CMS, all starches in this study are from rice.