GELATINIZATION PARAMETERS OF STARCH AND SOME CEREAL PRODUCTS, AS DETERMINED THERMOMECHANICALLY FROM TORQUE MEASUREMENTS

Marek Wianecki, Edward Kołakowski
Department of Food Technology, Agricultural University of Szczecin, Poland

ABSTRACT

Gelatinization parameters of selected starch materials such as flour grits, native starches, and modified starch were determined thermomechanically, by measuring the respective torque values. The highest torque [Nm] was found to occur during gelatinization of rice grits, potato starch, and distarch phosphate. Pure starches required less time to gelatinize, compared to flour grits. Grits of larger grain sizes required higher initial temperatures (t_P) to gelatinize. The highest gelatinization rates were observed in potato starch and distarch phosphate.

Key words: starch, grits, gelatinization, torque.

INTRODUCTION

Starch functionalities are very often decisive for rheological properties of foods. As a thickener, starch modifies food viscosity and affects texture and structure of foods. The modifications are seen as gelatinization, thickening, and elevated resistance to heating, shock, and ageing [16]. This is particularly important for the quality of sauces, dressings, puddings, jams, jellies [8], and many other products. Processing technologies and quality of products from cereal and potato materials containing starch as a reserve substance are directly related to properties of the polysaccharide in question [3,15].

From the practical point of view, both viscosity of starch gels and their stability are important. Gelatinization rate, which determines starch susceptibility to gelatinization, is an important parameter as well.

Starch gelatinization occurs by destruction of the ordered structures of starch granules. Initially, the granules become swollen as a result of their absorbing about 30-40% of water. As the temperature increases (60-85°C), amylose is released first, followed by the simultaneous occurrence of the two starch fractions. A further temperature increase results in disintegration of the granules to smaller fragments, and in the destruction of amylopectin. Full gelatinization ensues when starch loses its crystalline structure [13,18]. Starch gelatinization is characterised by the initial and final temperatures [6]. Starch gel viscosity is measured to assess functional properties of native and modified starches and/or dextrins [19,14]. Starch gel viscosity depends on starch type, starch concentration, and measurement method. In addition to methodological factors, the actual viscosity value depends on other circumstances, such as starch granule size, amount of phosphoric acid bound to starch, and the presence of electrolytes [2,11,12].

Gelatinization parameters are usually measured using the Brabender amylograph; however, a similar role may be played by any viscometer capable of simultaneous measurement of viscosity and temperature. Viscosity measurements are taken under turbulent conditions and depend on two factors: temperature and shear stress [4].

The present work was aimed at studying the dynamics of gelatinization of some cereal products and selected native starches from changes in their torque ensuing when aqueous starch suspensions, continuously heated to 96°C, were mixed.

MATERIAL AND METHODS

The study made use of commercially available grits, flours, and starches of quality compatible with the following Polish standards:

Rice and pop-corn grits were obtained by grinding the corns in a cereal grinder (Feuma, Germany), separating the husks, if present, and sieving to the grain size required. Grits of the following three grain sizes (g) were used:

g < 0.43 mm       0.43 < g < 0.75 mm       0.75 < g < 1.00 mm Assays were performed using a Brabender (Germany) measurement system containing a power unit (Do-corder), a planetary blender with an in-built thermocouple, a thermostat, and a recorder for simultaneous registration of changes in torque and temperature on mixing (Fig. 1). Gelatinization tests were carried out on samples equivalent to 300 g 15% moisture content standard flour [5], mixed with 1 l distilled water, and heated as described below.

The samples were mixed at a speed of 65 rotations min^-1, and were heated from the initial temperature (20°C) to 95°C at an average rate of ~1°C·min^-1. During the test, the torque [Nm] and the temperature [°C] were recorded. The graphs obtained were used to determine gelatinization parameters, viz. the initial and final gelatinization temperatures, gelatinization time, and the so-called gelatinization rate. The gelatinization rate expresses the increment in torque during 1 min of 1°C temperature increase (Fig. 2).

RESULTS AND DISCUSSION

Torque
The torque recorded during gelatinization is a derivative of viscosity of the mixture being processed. The actual value of torque was found to depend on the type and amount of the starch carrier and on its grain size. The isolated starches usually showed high torque increments (3.85–6.70 Nm), except for the high-amylopectin tapioca starch (1.15 Nm) and high-amylose Hylon 7 which did not gelatinize. The highest torque was recorded during gelatinization of potato starch and distarch phosphate (6.60 and 6.70 Nm, respectively) (Table 1).

Potato starch amylopectin contains one phosphate group per 317 glucosyl residues. The organic phosphorus content in potato starch may amount to 700 ppm [7]. This is a factor responsible for the high viscosity and substantial clarity of starch gels [4].

Gel viscosity depends also on the size of the swollen starch granules. According to Hermansson and Svegmark [3], potato starch granules increase their volume on gelatinization about 100 times, a 30-fold increase only being found in cereal starch granules.

High torque values (5.85-6.80 Nm) were observed to occur also during gelatinization of rice grits. Those torques could have resulted from the rice containing high amounts of amylose and/or longer-chain amylopectin producing, e.g., increased hardness of boiled rice [10].

Wheat starch showed a much higher increase in torque during gelatinization (4.6 Nm), compared to wheat grits (1.7-2.4 Nm) and flours (1.1-1.4 Nm). This is a result of both a lower starch content in wheat grits and flours, and of the effect of the extent of grinding on torque.

Larger grain sizes were accompanied by higher torque values. The trend was visible in rice and wheat grits, but not in corn grits. The corn grit gels prepared from both corn types showed the highest torque to occur at intermediate grain sizes (0.43-0.75 mm).

Regular corn grits showed higher torque increment on gelatinization, compared to the grits produced from pop-corn. As demonstrated by Almeida-Dominguez et al. [1], viscosity of pop-corn gels was by about 1.5-1.7 times lower than that of gels prepared from regular corn.

Sandhu and Singh [14], too, reported a high variability of rheological properties of starch gels prepared from various corn types. At extreme cases, gel viscosities differed by 50%.

Gelatinization time
Gelatinization time, too, was found to be dependent on the type of starch carrier being gelatinized. Pure starches gelatinized faster than cereals. The shortest gelatinization time (9.7 min) was typical of potato starch, the longest (27.7 min) being recorded in 0.43–0.75 mm grain size pop-corn grits. Pure corn starch gelatinized during 17.6 min, while corn grits took 23.0–27.7 min to gelatinize. This can be taken as evidence that the presence of components other than starch in a carrier, e.g., protein, fibre, and minerals, restrict water absorption, hence the elongation of gelatinization time. Some of the energy of the system is used also for swelling and protein denaturation as well as for interactions between the various components.

Almeida-Dominguez et al. [1] reported that larger and more compact corn particles required additional time and energy for hydration preceding swelling and gelatinization. Therefore they recommended that hard corn varieties be ground. The pattern typical of corn grits was only partly repeated in wheat materials. Although the wheat grits needed a longer time (20.1–21.3 min) to gelatinize, compared to pure wheat starch (16.5 min), gelatinization time of the two wheat flours was identical (16.4 min) and very similar to that of native starch. Thus, the extent of diminution of the material plays an important part in controlling gelatinization parameters. Intensive grinding of grains very often results in mechanical damage to starch granules, which increases their hydrophilic properties and facilitates gelatinization, thus affecting the gelatinization rate.

Gelatinization temperature
Ranges of gelatinization temperatures were similar within a cereal type. However, a pattern was observed whereby the initial gelatinization temperature was decreasing as the grain size was reduced. It was only in the case of pop-corn that the pattern did not occur. Reduction in the initial gelatinization temperature is an effect of, i.a., the amount of short-chain amylopectin branches with the degree of polymerisation (DP) of 6-9 [17]. Final gelatinization temperatures decreased as the wheat grit grain size increased, and increased with increasing grain size of corn grits

Gelatinization rate
Gelatinization rate is a parameter unifying all the above mentioned factors active during the process, and may be useful in classifying starch materials and native starches. The highest rates were observed during gelatinization of potato starch [85.1·10^-3 Nm (°C·min)^-1] and distarch phosphate [59.6·10^-3 Nm (°C·min)^-1]. High rates [11.7–27.1·10^-3 Nm (°C·min)^-1] were recorded also in rice grits, pure corn starch [19.9·10^-3 Nm (°C·min)^-1], and wheat starch [17.4·10^-3 Nm (°C·min)^-1]. Corn and wheat grits showed lower gelatinization rates, compared to the corresponding pure native starches. The grain size was another factor affecting the gelatinization rate. Larger grain sizes of rice and wheat grits corresponded with higher gelatinization rates. This trend, however, was not maintained by corn grits. Among the starches analysed, the lowest gelatinization rate was found in the high-amylopectin tapioca starch.

The Brabender (Germany) measurement system ensured high-precision measurements. The rheograms obtained made it possible to determine torque, temperature, and time to 0.05 Nm, 1°C, and 0.1 min, respectively. The use of a large sample (1.3 kg suspension) eliminated accidental errors which often accompany measurements taken at a micro scale.

CONCLUSIONS

1. Among the food materials analysed, the highest torque increment (M_k-M_p) was shown by gels prepared from rice grits, potato starch, and distarch phosphate.

2. Isolated starches usually needed less time (Δτ) and higher torques to form gels, compared with the corresponding cereal grits and flours.

3. In most cases, higher grain sizes of grits were associated with higher initial temperatures of gelatinization (t_p).

4. The highest gelatinization rates (V_p) were typical of potato starch and distarch phosphate, the lowest rates being observed in pop-corn.

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Accepted for print: 26.09.2007

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