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P-STARCH-12

The effect of alkali treatment on the granular morphology and pasting properties of rice flour

Kunyarat Reepholkul and Sanguansri Charoenrein∗

Department of Food Science and Technology, Faculty of Agro-Industry,

Kasetsart University, Bangkok 10900, Thailand

∗ Corresponding author E-mail: fagisscr@ku.ac.th

Abstract Alkali agents are important for the preparation of many traditional starch-based food products e.g. Chinese noodles and some Thai desserts. They were used to improve and develop some product characteristics such as color and texture; however, the reason of their effect is not clear. The main objective of the present study was to investigate the effect of alkali on the granular morphology and pasting properties of rice flour. Rice flour from cultivar Lueng 11 was treated with a 0.025 N alkali solution (potassium hydroxide, calcium hydroxide and sodium tetraborate) for 1 hour at 25+2 °C. Changes in the morphology of the starch granules after heating at 50 and 60 °C were examined using a light microscope. The alkali treatment induced the rupture of starch granules during heating. A Rapid Viscosity Analyzer (RVA) was then used to study the pasting properties. The pasting properties of alkali treated flour showed increases in the peak viscosity, breakdown and final viscosity whereas the setback was decreased. Potassium hydroxide showed stronger effects on the granular morphology and pasting properties of the rice flour than calcium hydroxide and sodium tetraborate. This study shows that alkali play an important role in the pasting properties of starches and granular morphology. The information gained from this study provides further understanding of the behavior of alkali treatments on starch-based food. Keywords: Alkali treated flour; rice flour; granular morphology; pasting properties 1. Introduction The main storage energy source for many higher plants is starchwhich is an α-D-glucan polymer produced in granules of seeds, roots, and tubers. The use of starch in food products as a food ingredient is usually not based on nutritional value but on functional value (Zobel and Stephen, 2006).Food starches can impart desirable properties, including texture or mouth feel, thickening, gelling, binding and stability (Hermansson and Svegmark, 1996). The selection of starches used for a particular food system is determined by the processing requirements and the characteristics required in the finished product. Alkali agents are important in the preparation of many traditional starch-based food products such as in the production of masa (whole corn kernels in excess water containing lime or calcium hydroxide; Ca(OH)2(Gomez et.al., 1992; Bryant and Hamaker, 2004), lod-chong (rice flour with calcium hydroxide; Ca(OH)2), pretzels (wheat flour with sodium hydroxide; NaOH), waxy rice dumplings (waxy rice flour with sodium hydroxide; NaOH) (Lai et.al., 2002), yellow alkali noodles and pasta (wheat flour with sodium hydroxide; NaOH and sodium carbonate; Na2(CO3)). Inclusion of an alkali can change such characteristics as color aroma and flavor, as well as provide a firm and elastic texture to the resulting product (Lai

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et.al., 2004; Karim et.al., 2008, Nadiha et.al., 2010). Several studies have been carried out on changes in alkali treated starch properties. Treatment with an alkali can modify some of the microstructure and physico-chemical properties of starches including amylose content, swelling properties and pasting behavior (Lai et.al., 2002; Han and Tyler, 2003; Lai et.al., 2004; Karim et.al., 2008, Nadiha et.al., 2010). These changes are varied and depend on the type of alkali, concentration used and type of starch all of which can affect the quality of the modified product. We are particularly interested in studying the effects of alkali on rice (Oryza sativa L.) flour since this method has been used extensively in traditional starchy foods such as snacks and dessert. (Lai et.al., 2004). Information on the susceptibility of rice flour to alkali hydrolysis is interesting and the reasons for the desired effects are not clear. Therefore, the main objectives of the present study were to investigate the effect of alkali, mainly potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2) and Sodium tetraborate (Na2B4O7) on granular morphology and pasting properties using light microscopy and RVA respectively. The information gained from this study further enhances our understanding of the behavior of alkali in starch-based foods. 2. Materials and Methods 2.1 Materials The rice flour (‘Lueng 11’ cultivar) was provided from Kalasin, Thailand. KOH was obtained from Merck, Germany. Ca(OH)2 was obtained from Fulka, Germany, and Na2B4O7 was obtained from Ajax Finechem Pty Ltd, Australia. All chemicals were analytical grade. 2.2 Methods Preparation of alkali treated with flour Rice flour suspension (40%w/v) was prepared with the alkali solutions (KOH, Ca(OH)2 and Na2B4O7) and left to stand at room temperature (25+2°C) for 1 hour. Before and after the alkali treatment, the pH of the flour suspension was measured to determine any changes in pH due to steeping. The samples were then washed five times with distilled water prior to centrifuging at 5000 rpm for 15 min. The flour was then dried in an air-oven at 40°C for 16 hours, then ground and sieved though a 100-mesh sifter before analysis. Control flour was prepared by adding distilled water instead of the alkali solution. Granular Morphology Rice flour suspensions (8%w/w) were heated in a water bath shaker at 50 and 60 °C for 30 min and were then observed promptly using a light Microscope (Leica) with 100x magnification. Pasting properties The pasting properties of the rice flour suspension were then measured with an AACC Approved Method using a Rapid Visco Analyzer (Newport Scientific, Australia). All measurements were done in triplicate. Statistical analysis The experiments were ordered as a completely randomized design and the differences in means were determined by the Duncan New Multiple Range Test. The data were analyzed using SPSS 16.0 for windows.

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3. Results and Discussion The results showed that the alkali treatment significantly influenced rice flour characteristics, particularly, the morphology and pasting properties. 3.1 Changes in granular morphology of alkali treated flour The light microscope images of the rice flour granules heated at 50 and 60°C for 30 min are shown in Figures 1 and 2. At 50°C, the microstructure of the alkali treated flour showed deformed granules whereas the alkali treatment induced the rupture of starch granules after heating at 60°C. KOH treated starch granules seemed to be more affected than in other treatments.

Figure 1 Microstructure of alkali treated starch granules after heating at 50°C for 30 min. (a) Control (b) KOH treated flour (c) Ca(OH)2 treated flour and (b) Na2B4O7 treated flour (100x, Bar = 5 µm)

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Figure 2 Microstructure of alkali treated starch granules after heating at 60°C for 30 min. (a) Control (b) KOH treated flour (c) Ca(OH)2 treated flour and (d) Na2B4O7 treated flour (100x, Bar = 5 µm) 3.2 Pasting properties of alkali treated flour Figure 3 also shows the pasting characteristics of native and KOH, Ca(OH)2 and Na2B4O7 treated flour. All treatments showed increases in the peak viscosity, breakdown and final viscosity while only the setback value was found to decrease. These results indicate that the hydroxyl group of alkali can interact with flour and induce consistent changes to the behavior of flour (Nadiha et.al., 2010). Since the peak temperature decreased, that indicates there was fastergranule swelling and therefore less thermal energy was required to cookthe rice. These changes in peak temperature and swelling correlate well with themorphology change of the starch granules when treated with alkali.Moreover, KOH showed a stronger effect on the granular morphology and pasting properties of the rice flour than Ca(OH)2 and Na2B4O7. These results were due to the ability of each alkali which was dependent on the molecular size of alkali system, as well as the particular reactions which may be involved in the viscosity changes (Han and Tyler, 2003).

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Figure 3 Pasting properties of native and alkali treated rice flour at 1 hour. (a) Control (b) KOH treated flour (c) Ca(OH)2 treated flour and (b) Na2B4O7 treated flour. 4. Conclusion This study found that the granular morphology and pasting properties of rice flour could be adjusted by the use of an alkali treatment. The alkali treatment induced the rupture of starch granules during heating. The pasting properties of the alkali treated flour samples were found to exhibit an increase in the peak viscosity, breakdown and final viscosity whereas the setback and pasting temperature were found to decrease. Nevertheless, alkali-induced properties of flour were found to be dependent on the type of alkali. KOH showed stronger effects on the granular morphology and pasting properties of rice flour than Ca(OH)2 and Na2B4O7. Acknowledgements This work was financially supported by the Graduate School, Kasetsart University, Thailand. References AACC. 1995. Approved methods of the AACC. Method 61-02. Determination of the pasting

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Hamaker, B.R. and B.A. Larkins. 2002. Maize Food and Feed: A Current Perspective and Consideration of Future Possibilities, pp.1-18. In Y.H. Hui, A. McHughen, R. Scorza, W.K. Nip and G. G. Khachatourians eds. Transgenic Plants and Crops.Marcel Dekker, Inc., Arizona.

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