Viscosity and hydrodynamic radius relationship of high-power ultrasound depolymerised starch pastes with different amylose content

Highlights

• High-amylose corn starch pastes are degraded less by high-power ultrasound (HPUS).

• HPUS reduces markedly the viscosity and extent of pseudoplasticity of the pastes.

• A relationship between viscosity and hydrodynamic radius of the starch molecules.

• This relationship is explained by the theories developed for synthetic polymers.

Abstract

Corn starch pastes (5 or 10 w%) with different amylose content (∼2.7%, ∼5.1%, ∼29.6%, or ∼52.7%) were treated using high-power ultrasound (frequency 20 kHz, power 13.5 or 29.9 W) for up to 20 min. Changes in the physical properties were determined using viscosity measurements and dynamic light scattering. Results show that both the viscosity (η) and hydrodynamic radius (R_H) decreased markedly with the increase in ultrasound treatment time. FT-IR showed that the molecular scission occurred at the C–O–C bond of α-1,6 glycosidic linkage, and that the extent of breakage was inversely correlated with amylose content. Further, high-amylose starch pastes were found to be more resistant to ultrasound treatment due to their aggregation. A master curve for the behaviour of η and R_H is proposed, which is also confirmed by a similar study carried out on the ultrasound treatment of rice starch pastes with different amylose contents. The findings of this work can be used to tailor starch solutions with different viscosities using high-power ultrasound.

Introduction

The degradation or depolymerisation of macromolecules by high power ultrasound (HPUS) is known since a long time (Schmid & Rommel, 1939). However, in recent years the use of HPUS to reduce the size of large biomacromolecules has been revisited since this method offers an alternative to the conventional enzymatic and chemical methods. Another advantage of the HPUS is that it tends to breakup macromolecules in the middle of the chain, generating monodispersed fragments (Price, West, & Smith, 1994; Van der hoff & Gall, 1977). Of particular interest is the use of HPUS for starch based systems, which range from starch granules with different size and shape depending of their botanical origin to their individual polysaccharide components (Jane et al., 1999, Pérez et al., 2009). When starch granules in aqueous solutions are pasted, that is when heated under shear in the presence of excess water, the starch granules dissociate into their main polysaccharide components, amylose and amylopectin. The ratio of amylose to amylopectin vary for different starches, with waxy starches having a low amylose content (typically less than 5%) and normal or native starches having higher amylose content (>10%).

Amylose is essentially a linear macromolecule consisting of a α- D-glucan chain which is linked through α–D–(1–4) linkage (Bergthaller, 2005, Liu, 2005) with a degree of polymerization (DP) between 1000 and 10,000. A small portion (0.1%) of amylose molecules are branched via (1–6)-α linkages (Bergthaller and Hollmann, 2007, Takeda et al., 1978) and approximately 500–6000 glucose units are distributed among 1–20 chains. The length of these branch chains varies from 4 to 100 DP (Hizukuri, 1996, Takeda et al., 1978). The molecular weight (M_w) of amylose is dependent on the botanical source, and is normally around 0.15–0.4 × 10⁶ g × mol⁻¹ (Hizukuri et al., 2006, Hizukuri, 1996). Because of the presence of hydroxyl groups and its linear structure, amylose molecules have a tendency to approach each other and to bond together. This may cause the reduction of affinity of amylose molecules for water, which affects its solubility (Jane and Chen, 1992, Liu, 2005). Compared to amylose, amylopectin has much larger M_w, ranging from 10⁶ to 10⁹ g × mol⁻¹, depending on the botanical origin of the starch (Hermansson and Svegmark, 1996, Liu, 2005). Amylopectin is a highly branched molecule which is composed mainly of α-(1–4) linked d-glucopyranose (as in amylose) but also has a greater proportion of non-random α-(1 → 6)-linkages at the branching (Jane and Chen, 1992, Liu, 2005, Nikuni, 1978).

In recent years, many studies on starch degradation by HPUS have been published, although the first study reporting the effect of power ultrasound on starch molecules has been published by Szent-györgyi, in 1933 (Szent-györgyi, 1933). The degradation of high molecular weight polysaccharides is associated with the marked reduction in viscosity and molecular weight. Huang, Li, and Fu (2007) observed a decrease in the viscosity of corn starch pastes after sonication (ultrasound power at 500 W for 3–15min) which was related to a breakage of glycosidic linkages and weakening of the biopolymer network. The effects of 360 kHz ultrasound (output power at 100 W for 22 min) on an aqueous solution of chitosan and corn starch have been studied by Czechowska-Biskup, Rokita, Lotfy, Ulanski, and Rosiak (2005). They also observed a reduction in viscosity of starch dispersion and in the molecular weight of amylopectin, and suggested that hydroxyl radicals and the mechanical effects due to ultrasound cavitation are responsible for the depolymerisation of the polysaccharide solutions. Similar observation was also reported by Luo et al. (2008) where they sonicated different corn starch samples (various in amylose content) at 100 W power for 30 min. They suggested that hydroxyl radicals produced from acoustic cavitation could react with linear amylose and the side chains amylopectin. Iida, Tuziuti, Yasui, Towata, and Kozuka (2008) reported the reduction of viscosity of sonicated (power of 120 W, up to 30 min) waxy maize, tapioca, potato and sweet potato starch pastes which decreased dramatically at initial period and then tended slowly to a limiting value.

The main objectives of the current study are, firstly to determine the importance of the amylose content on the extent of starch degradation by HPUS. Secondly, to develop a relationship between the viscosity of the starch pastes and the hydrodynamic radius of the starch molecules. For these reasons, pastes obtained from corn starches having different amylose content were subjected to HPUS under different power and sonication time conditions. The viscosity of the HPUS treated starches were measured, and the size of the starch molecules was determined through their hydrodynamic radius, measured by dynamic light scattering. To validate the relationship between the viscosity and particle size, additional data obtained on waxy and normal rice starches are also used. These experimental data are also obtained using dynamic light scattering and rotational viscometry.