Thermal and dynamic rheology of insoluble starch from basmati rice

Abstract

Insoluble rice starch obtained as a by-product of the sequential solvent extraction of proteins from commercial Basmati rice flour was evaluated for thermal and rheological properties. Differential scanning calorimetry (DSC) was employed to gather data on gelatinization temperature and heat of gelatinization of rice starch at a constant heating rate. A small amplitude oscillation shear rheometer was used to study gel rigidity and other rheological characteristics. Elasticity (G′) of thermally induced starch gel increased as function of starch concentration and decreased with heating rate. Swelling power (SP), close packing concentration and pasting properties of rice starch were also investigated. Isothermal heating of starch in the temperature range of 70–85 °C exhibited a systematic decrease in elasticity (G′) with heating time where as a reverse trend was observed above 90 °C. Non-isothermal heating of starch at a linear rate of temperature increase with time exhibited peak elastic modulus (G′) values at specific temperatures which corresponded to starch gelatinization temperatures as observed by DSC. During non-isothermal starch gelatinization kinetics of rice starch, G′−t data up to peak value were considered and a rate equation was evaluated. A first-order reaction kinetics adequately described the rice starch gelatinization and the process activation energy obtained ranged between 32.3 and 42.2 kJ/mol.

Introduction

Starch is a complex food hydrocolloid (polymer of α-d-glucose and partially crystalline polymer) where one would expect two-phase transitions during heating in presence of excess water (Maurice, Slade, Sirett, & Page, 1985). Starch granules absorb water resulting in swelling up to several times their original size and lose their crystallinity. The complete process is known as gelatinization. Gelatinization of starch paste involves changes in amylose and amylopectin (Kim, Lee, & Yoo, 2006). Kinetics of starch gelatinization can be studied either by heating it in water or steam in vitro using pure starch or in situ using whole grain. In case of the in vitro gelatinization, there is no physical barrier between starch granules and water molecules, and starch is readily accessible (Turhan & Gunasekaran, 2002).

Gelatinization kinetics of starch has been studied extensively by different techniques. Karapantsios, Sakonidou, and Raphaelides (2002) studied starch gelatinization kinetics by changes in electrical conductance with time. Several investigations on gelatinization kinetics of starches have been reported by iodine blue value technique (Birch & Priestley, 1973; Njintang & Mbofung, 2003). However, most of the researchers (Baik, Kim, Cheon, & Ha, 1997; Riva, Schiraldi, & Piazza, 1994; Spigno & De Faveri, 2004) have used differential scanning calorimetry (DSC) for the study where the degree of gelatinization is directly measured from gelatinization enthalpy. Starch gelatinization has been reported to follow a first-order reaction kinetics (Njintang & Mbofung, 2003; Ojeda, Tolaba, & Suarez, 2000; Riva et al., 1994; Turhan & Gunasekaran, 2002) irrespective of the measuring technique. It is believed that during starch gelatinization, crystallites melt and both molecular and crystalline structures get disrupted (Cooke & Gidley, 1992). However, gelatinization occurs in a non-equilibrium state and, therefore, knowledge of reaction kinetics is essential to predict reaction mechanism more precisely.

Small amplitude oscillatory shear (SAOS) measurements is a particularly useful method to study the gelation/gelatinization phenomenon, in monitoring the kinetics of network development, provided that the measurements are within viscoelastic limit (Biliaderis, 1992) and the strain is restricted to less than 5%. The techniques afford the measurement of dynamic rheological functions, without altering food texture and are far more reliable than steady shear measurement (Bistani & Kokini, 1983). Compared with DSC-based starch gelatinization technique, much less is known about the rheological approach; nevertheless, some studies on such kinetic approaches have been published (Ahmed, Ramaswamy, & Alli, 2006; Kubota, Hosokawa, Suzuki, & Hosaka, 1979; Yoon, Gunasekaran, & Park, 2004). Recently, Yamamoto, Makita, Oki, and Otani (2006) studied alkali-induced rice starch gelatinization kinetics using conventional steady shear measurements. The results were determined by power-law model to elucidate the normality dependence of a flow behavior index and a consistency coefficient.

The reaction kinetics in food systems are commonly studied under isothermal heating conditions. The process usually is simpler to conduct and evaluate the kinetic parameters than under non-isothermal condition (Dolan, 2003). However, the isothermal process has some practical limitations especially when dealing with samples which are difficult to heat instantaneously to testing temperatures. The rate of heating of food materials depends on the state (liquid vs. solid or semi-solid), size, thermal conductivity, viscosity/density and moisture content. In some instances, especially at higher temperatures, it might take even longer to achieve the target temperature than to hold it at that temperature to complete the reaction. In most situations, thermal lag corrections are applied to compensate for the non-isothermal contribution to the total process. The thermal behavior of starches is complex compared to other destruction kinetics because of the several physicochemical changes that occur during heating may involve gelatinization, melting, glass transition, crystallization, change of crystal structure, volume expansion, molecular degradation and motion of water (Yu & Christie, 2001). Kinetic data gathering under non-isothermal conditions has been recently practiced which allows parameter estimation from a single experiment where temperature is varied over the range of interest, and samples are taken at various intervals (Dolan, 2003; Yoon et al., 2004). Parameters are also estimated from a dynamic environment closer to real processing conditions, and thermal lag problems are overcome (Cunha & Oliveira, 2000).

Several studies have been carried out on non-isothermal kinetics for food systems (Ahmed et al., 2006; Claeys, Ludikhuyze, van Loey, & Hendrickx, 2001; Dolan, 2003; Rhim, Nunes, Jones, & Swartsel, 1989; Yoon et al., 2004). It would be interesting to study the influence of non-isothermal heating on starch rheology. Such studies with SAOS measurement techniques in the linear visco-elastic range would provide a broader insight to the gelatinization kinetics. The order of reaction and the necessary energy requirement to achieve critical gel rigidity (activation energy) can be calculated from such thermorheological data. These studies could provide a better insight into gelling mechanisms as well as useful data for potential substitution of one starch for another in food product development procedures. Although Basmati rice has enormous market throughout the globe among Asian people for its characteristics popcorn like flavor, there has been no systematic study on gelatinization kinetics and dynamic viscoelastic characteristics which is the objective of this study. An implicit objective of the work was to evaluate non-isothermal heating kinetics of starch gelatinization by rheological approach.