Prediction of the effect of sucrose on equilibrium swelling of starch suspensions

Highlights

• Effect of sucrose on equilibrium swelling of maize and rice starch characterized from granule size measurments.

• Flory Huggins model predictions of starch swelling agreed with data.

• Effects of gelatinization and elastic resistance influenced swelling.

Abstract

Equilibrium swelling of 8 wt % suspension of waxy (WMS) and normal (NMS) maize, waxy(WRS) and normal(NRS) rice when heated to 80 °C at different sucrose concentrations (0–30 wt %) were characterized. The swelling power reached maximum values of 10.5 and 14.5 at sucrose concentration of 5 wt % and 10 wt % for NMS and WRS respectively, increased with sucrose concentration for WMS from 11 to 15.5 and decreased for NRS from 11 to 8.2. The onset temperature of gelatinization increased with sucrose concentration for all starches whereas enthalpy of gelatinization increased for normal maize starches and did not vary significantly for waxy starches. The interaction parameters for three component system of starch, water and sucrose were inferred from static light scattering. The predicted equilibrium swelling for all starches at different sucrose concentrations using Flory-Huggins theory agreed well with experimental data.

Introduction

Starch is an essential food ingredient that is used in many formulations as a nutrient as well as thickening and stabilizing agent. It consists of mainly amylose, a linear polymer with α (1–4) glucosidic bonds and amylopectin, a branched ploymer with α (1,6) glucosidic bonds, with their composition being dependent on the starch type (Dongowski, 1997). Starch is present as discrete granules which swell upon heating in excess water above the gelatinization temperature (Evans and Haisman, 1980). Swelling occurs due to diffusion of water into the starch granules as a result of chemical potential gradient. The rate and the extent of swelling depends on the composition and type of starch. Waxy starches with higher amylopectin content exhibit more swelling than regular starches (Blazek and Copeland, 2008). Swelling is found to be influenced by the moisture content of the suspension and increase both with temperature as well as time (Desam et al., 2018) (Zhang et al., 2017). A rapid phase of swelling follows an initial phase of negligible swelling eventually reaching a final phase of maximum swelling (Tester and Morrison, 1990) (Lagarrigue et al., 2008). The swollen granules eventually reaches equilibrium with the surrounding medium because of equal chemical potential of water inside and outside the granule. A significant increase in volume fraction of granules due to swelling leads to the formation of closely packed deformable granules forming a paste. In addition to starch, food formulations consist of several other ingredients such as sugar, salt, polysaccharides etc. The interaction of these ingredients with water and starch granules influence swelling and hence pasting. Understanding the effect of composition of these ingredients on their ability to alter swelling and pasting behavior of starch is therefore important. This manuscript focuses on the effect of sugar on starch swelling.

Extensive investigation of the effect of sugar on water activity and other colligative properties in sugar solutions have been made. A compilation of experimental data of vapor-liquid equilibrium, boiling point elevation and freezing point depression of sucrose solutions of different concentration at different temperatures is given by Starzak and Mathlouthi (2006). Water activity is found to decrease with sucrose concentration and temperature. Water activity coefficient decreases with sucrose concentration with this decrease being pronounced above a concentration of 40%. Group interaction parameters for sugar molecules have been determined by fitting different colligative properties, such as elevation of boiling point (BPE), lowering of vapor pressure, freezing point depression (FPD), osmotic pressure and heats of solubility and dilution, for solutions of various carbohydrates in water and other solvents (Abed et al., 1992) (Ferreira et al., 2003). A four factor empirical Margueles equation was proposed and the parameters fitted for glucose-water system at different temperatures (Starzak and Mathlouthi, 2006) for the prediction of water activity.

Sugar influences the swelling of starch granules thereby affecting the rheological properties of suspension. For wheat starch, swelling increases in the range of 0–20% (w/w) sucrose concentration in aqueous phase with maximum increase occurring at 10%. Beyond 20% sucrose concentration there is a decrease in swelling (Cheer and Lelievre, 1983). For most systems, starch swelling increased at low sugar concentration and decreased above approximately 50 wt% for sucrose and maltose (Bean and Yamazaki, 1978).

The effect of sucrose on swelling can be attributed to its effect on gelatinization temperature and enthalpy of gelatinization. The addition of sugar and sugar alcohols have been found to increase the gelatinization temperature and enthalpy of gelatinization, which has been demonstrated for wheat starch (Wootton and B amunuarachchi, 1980; Ghi-asi, Hoseney and Varrianomarston, 1982; Sopade et al., 2004), corn starch (Chinachoti et al., 1990), amaranth starch (Paredes-Lopez and HernÃąndez-LÃűpez, 1991) (Paredes-LÃşpez and HernÃąndez-LÃşpez, 1991), rice flour and rice starch (Chungcharoen and Lund, 1987), sago starch (Maaurf et al., 2001), mung bean starch (Ahmed, 2012), pressurized tapioca and potato starches (Rumpold and Knorr, 2005), sweet potato starch (Kohyama and Nishinari, 1991a), and oat starch (Hoover and Senanayake, 1996). This increase in gelatinization temperature and enthalpy of gelatinization is believed to be the result of modification of the amorphous regions on starch granules possibly due to starch-sugar and sugar-water interactions (Chiotelli et al., 2000). Trisaccharides and disaccharides influence the gelatinization temperature more than monosaccharides (Kim and Walker, 1992) since they have higher number of hydroxyl groups in their structures which result in stronger interaction with starch; their influence on gelatinization depends marginally on the type of monomers (glucose, xylose, fructose) and their structure. Sugar alcohols result in higher gelatinization temperatures of starch compared to their corresponding sugars. However, unlike sugars, the corresponding sugar alcohols are found to result in a lower gelatinization enthalpies which may be attributed to complex arrangement between sugar alcohols and starch. Among monosaccharides, Xylose provides greatest increase in enthalpy of gelatinization, while isomaltose and trehalose provides the greatest enthalpy increase among disaccharides (Baek et al., 2004).

In general, for starch- sugar systems, the gelatinization temperature increases in the following order: sucrose $>$ 10 DE maltodextrin $>$ maltotriose $>$ lactose $>$ maltose $>$ glucose $>$ mannose $>$ fructose $>$ ribose $>$ water alone (control) with an increase in sugar concentration until a plateau is reached, (Slade and Levine, 1987; Perry and Donald, 2002).

The rheological behavior of starch pastes can be altered by adding other components, the most common component being sugar (Bean et al., 1978). The nature of decrease in peak viscosity with sugar concentrations was found to be similar to that for gelatinization temperature, but the effect of sugar on setback viscosity varied depending on the starch and sugar (Deffenbaugh and Walker, 1989).

The present study develops a mechanistic mathematical model for the prediction of equilibrium swelling of waxy maize, normal maize, waxy rice and normal rice starch in the presence of sucrose in order to quantify the effect of sucrose concentration on starch swelling. Flory-Huggins polymer solution theory for the three component system consisting of starch, sucrose, and water is employed for the prediction of swelling. The model accounts for gelatinization temperature, enthalpy of gelatinization, sucrose-starch and starch-water interactions. The model predictions are compared with experimental data for the four starch systems.