The pasting properties of sonicated waxy rice starch suspensions

Abstract

The effect of sonication on the pasting properties of waxy rice starch solutions (5 wt%) was investigated. It has been found that the functionality of starch granules was significantly influenced by the length of sonication and the solution temperature. A comparison of the pasting behaviour showed that the peak and final viscosities of the starch dispersions sonicated at temperatures near the onset temperature of gelatinisation were lower than those of the non-sonicated dispersions. The particle size measurements showed that the size of the heated and sonicated granules were smaller than that of the heated non-sonicated starch granules. Scanning electron microscopy (SEM) observations showed that the starch granule surface was not affected by sonication, and the size exclusion chromatography did not show any reduction in the size of the starch molecules. Based on these observations, the change in the pasting behaviour is explained in terms of the solubilisation of the swollen starch granules and starch aggregates induced by sonication.

Introduction

The use of sonication in the food industry has seen a rejuvenation as it is an effective process, requiring short processing times, with high throughput and low energy consumption [1], [2]. Of particular interest to the present study, sonication of starch can offer an alternative to chemical modification methods, such as the use of surfactants (e.g., sodium dodecyl sulphate, SDS) and alkali in starch isolation [3]. However, sonication treatment can also affect the physico-chemical properties of starch suspensions; amylose and amylopectin, which in turn can affect the pasting properties of the starch suspensions.

In general, sonication treatment can be performed either on non-pasted dispersions of starch granules or on starch pastes. In the first case, which deals with non-pasted starch suspensions, Gallant and co-workers [4], [5] in their pioneering work sonicated potato starch suspensions at ambient temperatures. They reported that under conditions that would promote cavitation, the starch granules exhibited deep conical pits. The extent of this surface granule damage increased with time of sonication and decreased with the concentration of starch in the suspension. These observations allowed them to propose the radial structure of submicroscopic units in the starch grain, which at the time was not yet well-established. Recently Huang [6] sonicated hydroxypropylated cornstarch granules and showed, using scanning electron microscopy (SEM), that the granules presented pores or channels, although X-ray diffraction did not show any change in the crystalline structure of the granules. The pasting temperature of hydroxypropylated sonicated starch increased, but exhibited a lower maximum viscosity, when compared to non-sonicated hydroxypropylated starch paste.

The majority of studies on the effect of sonication on starch have been performed on fully pasted starch dispersions. These starch dispersions are heated above their onset temperature of gelatinisation, where starch granules dissolve into their molecular components, amylose and amylopectin. As early as 1933, it was reported that sound radiation of starch solutions resulted in a decrease in the viscosity of the starch solution [7]. Azhar and Hamdy [8] sonicated boiled sweet potato starch for 5 min in sodium acetate buffer, and reported that 30 min sonication achieved the maximum decrease in viscosity, and that this decrease in viscosity was caused by the damage to starch granules rather than starch molecules. Their hypothesis was drawn from the fact that the rate of the enzymatic action of β-amylase was the same for sonicated and non-sonicated starch slurries. Jackson, Chotoowen, Waniska and Rooney [9] used ultrasound to solubilise starch dispersion, as an alternative method to alkaline or dimethyl sulfoxide (DMSO) solubilisation, prior to molecular weight characterisation by size exclusion chromatography. They reported that mild sonication increased the solubility of the starch molecules, but extensive sonication resulted in the depolymerisation of amylopectin. Seguchi, Higasa and Mori [10] sonicated starch boiled for 3 min in DMSO solutions, and showed using SEM and gel-filtration chromatography that sonication promoted the disintegration of starch molecule aggregates. Chung, Moon, Kim and Chun [11] studied the effect of sonication on the physico-chemical properties of mung bean, potato, and rice starch solutions (5 wt%) heated at 95 °C for 5 min. They reported that the clarity of the solution increased and the viscosity decreased, indicating that sonication-induced disruption of the swollen starch granules rather than breakage of the starch molecules. Czechowska-Biskup, Rokita, Lotfy, Ulanski and Rosiak [12] sonicated gas (Ar, He, O₂) saturated corn starch solution for up to 90 min. They reported a decrease in the molecular weight of the amylopectin and attributed the scission of the macromolecules to the attack by hydroxyl (OH) radicals and the mechanochemical effects of sonication. Finally, Iida, Tuziuti, Yasui, Towata, and Kozuka [13] showed that the viscosity of gelatinised 5–10% starch solutions of different botanical origin could be reduced by two orders of magnitude when sonicated for 30 min. NMR investigation of the sonicated starch solutions showed that there was no alteration in the chemical structure of the starch molecules. They suggested that the decrease in molecular weight observed by high performance gel permeation chromatography was due to the increase in the number of the free mobile macromolecules.

In summary, the published literature shows that sonication can definitely affect the physico-chemical properties of starch solutions in different ways. These effects include: starch granule pitting; starch molecule degradation through molecular scission and chemical attack by hydroxyl radicals; and the solubilisation of starch molecular aggregates and swollen granules. It is likely that these differences are due to the sonication conditions, such as sonication time, temperature, power, frequency, and to the differences in the botanical origin and the concentration of the starch solutions. The aim of the present work was to investigate the pasting behaviour of 5% waxy rice starch suspensions sonicated at different temperatures. Particle size measurement and SEM observation were also used to investigate the changes in the morphology of the starch granules.