Properties of Oxidized Cassava Starch as Influenced by Oxidant Concentration and Reaction Time

Transcription

1 Properties of Oxidized Cassava Starch as Influenced by Oxidant Concentration and Reaction Time P-STARCH-26 Kunruedee Sangseethong 1 and Klanarong Sriroth 2,3 1 Cassava and Starch Technology Research Unit, National Center for Genetic Engineering and Biotechnology, Bangkok 10900, Thailand 2 Department of Biotechnology, Faculty of Agro-Industry, Kasetsart University, Bangkok 10900, Thailand 3 Kasetsart Agricultural and Agro-Industrial Product Improvement Institute, Kasetsart University, Bangkok 10900, Thailand Abstract The effects of oxidant concentration (active chlorine 1,000 30,000 ppm) and reaction time ( min) on the physicochemical properties of hypochlorite oxidized cassava starch were investigated. The amount of carboxyl groups formed in the modified starch was highly related to the oxidant concentration used in the reaction whereas the amount of carbonyl groups did not depend on the concentration of oxidant. The pasting characteristics of modified starch were significantly influenced by the amount of oxidant. At lower levels of hypochlorite (1,000 and 2,500 ppm of active chlorine), the modified starches showed high peak viscosity, reduced breakdown and high setback. On the other hand, at higher level of hypochlorite (active chlorine > 5,000 ppm), the resulting modified starches exhibited low peak viscosity, little breakdown and low setback. In the study of reaction time effect, starch was modified with a fixed level of hypochlorite at 30,000 ppm of active chlorine. Results showed that the amount of carboxyl groups increased with increasing reaction time while the paste viscosity decreased. The paste viscosity of modified starch prepared with lower reaction time was not stable during storage at 50 C. The viscosity stability was improved with increasing reaction time. The paste of oxidized starch with reaction time of at least 90 min had stable viscosity over 8 hr of storage at 50 C. The viscosity stability of starch paste was found to relate well with the amount of carboxyl groups formed in the starch molecules. Introduction Oxidized starch is widely used in both food and nonfood industries where film formation and adhesion properties are desired. The major application of oxidized starch is as a surface sizing agent and a coating binder in paper industry. Oxidized starch is produced by reaction of starch with oxidizing agent under controlled temperature and ph. Several oxidizing agents have been used; however, hypochlorite is the most common chemical used for the production of oxidized starch in an industrial scale. During the reaction, hydroxyl groups on starch molecules are oxidized to carbonyl and carboxyl groups respectively. At the same time, depolymerization of starch molecules occurs resulting in viscosity reduction of starch paste (Wurzburg, 1987). Low viscosity of oxidized starch allows preparation of starch paste at relatively high solid content, which is desirable for application in paper industry where starch content as high as 30% may be needed (Maher and Cremer, 1987). Reaction conditions such as amount of oxidant, ph, temperature and time are known to be main factors in controlling the course of all reactions occurred during starch oxidation (Forssell et al., 1995; Parovuori et al., 1995; Wang and Wang, 2003). The combination of all changes in starch molecules i.e. the number of carbonyl and carboxyl groups and the extent of starch degradation dictates the properties of the resulting modified starch. Although several studies have reported on the effects of reaction conditions on the structural changes of starch molecules, relatively few studies reveals such effects on the physicochemical properties of the modified starch. The aim of the present work was to study the physicochemical properties of oxidized starch as influenced by oxidant concentration and reaction time. 337

2 Starch Update2005 Materials and Methods Preparation of oxidized starch A cassava starch slurry containing 40% dry solid was prepared and the ph was adjusted to 10.5 with 3% sodium hydroxide solution. The temperature of the slurry was brought to 30 C and sodium hypochlorite was added slowly to the stirred slurry to reach different final concentrations of active chlorine (1,000 30,000 ppm). During the reaction, the ph of the slurry was maintained between with 3% sodium hydroxide solution. After 3 hours, the ph of slurry was adjusted to 6.5 with hydrochloric acid, filtered, washed three times with distilled water and oven dried at 50 C. For the reaction with 30,000 ppm of active chlorine, the sample was collected at different reaction times from minutes. Determination of carboxyl content The carboxyl content of hypochlorite-modified cassava starch was determined by the ISO method (ISO-11214, 1996). Starch sample (5 g) was stirred in 0.1 M HCl for 30 min. The slurry was then filtered and washed with distilled water until free of chloride ions. The filtered cake was transferred to 300 ml water and the starch slurry was heated in a boiling water bath with continuous stirring until gelatinized and continue stirring at that temperature for another 15 min. The hot sample was titrated with 0.1 M NaOH using phenolphthalein as an indicator. Determination of carbonyl content The carbonyl content was determined as described by Kuakpetoon and Wang (2001). Starch sample (4 g) was slurried in 100 ml of distilled water. The slurry was gelatinized in a boiling water bath for 20 min, cooled to 40 C, adjusted to ph 3.2 with 0.1 M HCl, and 15 ml of hydroxylamine reagent was added. The flask was stoppered and placed in a water bath at 40 C. After 4 hour, the sample was rapidly titrated to ph 3.2 with 0.1 M HCl. The hydroxylamine reagent was prepared by dissolving 25 g hydroxylamine hydrochloride in 100 ml of 0.5 N NaOH. The final volume was then adjusted to 500mL with distilled water. Pasting profile of starch The pasting properties of starch were determined with a Rapid Visco Analyser (RVA-4, Newport Scientific, Australia) using standard program Number 1. The starch sample was 3.00 g (on 14% moisture basis). The starch suspension was held at 50 C for 1 min and subsequently heated to 95 C at 12.2 C/min. Holding time at 95 C was 2.5 min. The sample was then cooled to 50 C at 12.2 C/min, and kept at that temperature for 2.1 min. A rotation speed of the paddle was at 160 rpm. Viscosity stability A suspension of starch (15%, w/w) in distilled water was heated in a water bath at 95 C for 15 minutes with continuous stirring in the first 5 minutes. The starch paste was then maintained at 50 C in a temperature-controlled water bath. When a temperature of starch paste reach 50 C, the initial viscosity of the sample was measured with a rotational rheometer (Brookfield Engineering Laboratories, MA, USA; model DV-III + ) equipped with coaxial cylinder cell. Depending on the range of viscosity determined, spindle No. SC4-27 and SC4-31 were used. The sample was loaded into the sample cup, where the temperature was kept at 50 C. The measurements were carried out after thermal equilibrium was achieved. The viscosity measurements were taken at a fixed rotational speed of 60 rpm. The viscosity stability of starch paste was determined by measuring the viscosity of the sample after being maintained at 50 C for 1, 2, 3, 4, 6 and 8 hours. Results and Discussion Carboxyl and carbonyl contents of cassava starch oxidized with various amounts of hypochlorite are shown in Table 1. increased as the concentration of active chlorine increased whereas the amount of carbonyl groups did not show any correlation with the oxidant levels. The amounts of carboxyl groups formed in starch molecules were substantially higher than the amount of carbonyl groups, especially at higher oxidant levels. This is in agreement with other studies which reported that alkaline conditions favors the formation of carboxyl groups (Wurzburg, 1987). 338

3 Table 1 Carboxyl and carbonyl contents of cassava starch oxidized with different levels of active chlorine NaOCl level (ppm) Carbonyl content 1,000 2,500 5,000 10,000 20,000 30, Pasting characteristics of modified starch were greatly influenced by the concentration of active chlorine used in the reaction (Figure 1). Starch oxidized with low level of active chlorine (1,000 ppm) exhibited higher peak viscosity, lower breakdown and higher setback than native starch which are characteristics of slightly crosslinked starch. Wang and Wang (2003) also observed similar results for the oxidation of common corn and waxy corn starches but with the higher concentration of active chlorine (2,500-10,000 ppm and 2,500-7,500 ppm for common and waxy corn starches respectively). They suggested that carboxyl groups played an important role in forming the crosslinks between starch molecules. When using low level of oxidant, the crosslinks formed thus stabilized the swollen granules and overrode the effects of minor depolymerization. At higher level of active chlorine, depolymerization of starch molecules became dominant, resulting in starch paste with lower peak viscosity and setback. The low viscosity of starch oxidized with 30,000 ppm of active chlorine is typical for commercial oxidized starch and desirable for application in paper industry. Figure 1 Pasting profiles of cassava starch oxidized with different levels of active chlorine 339

4 Starch Update2005 The properties of cassava starch oxidized with 30,000 ppm of active chlorine at various reaction times were determined. Results showed that the amount of carboxyl groups increased with longer reaction time, especially in the initial stage of reaction (Table 2). After 2 hr of reaction, carboxyl content seemed to be stable. When compared to carboxyl groups, the amount of carbonyl groups formed under the conditions used in this study was very low and did not depend on the reaction time. It was quite stable throughout the period of reaction time between 30 and 240 min. Table 2 Carboxyl and carbonyl contents of cassava starch oxidized with 30,000 ppm of active chlorine at various reaction times Reaction time (min) Carbonyl content Figure 2 shows viscosity of cassava starch, oxidized with 30,000 ppm of active chlorine at different reaction times from min, during storage at 50 C for 8 hr. The viscosity of native starch was not determined because it was too high and exceeded the limitation of the measurement equipment (the maximum limitation was at 4,167 mpa.s). The results from the initial viscosity of all samples demonstrated that hypochlorite-modification caused the substantial reduction in viscosity of starch especially in the first 30 min of reaction. This signified that depolymerization of starch molecules mainly occurred in the initial stage of reaction. Although the viscosity reduction was significant in the initial stage, viscosity of the starch paste prepared with this short reaction time was not stable with storage time. Sample with 30 min reaction time set gel shortly after storage at 50 C while, after 8 hr storage, viscosity of the sample with 60 min reaction time increased approximately 2.5 times of the viscosity of fresh sample. The increase in viscosity of starch paste during storage has been considered to be responsible by the reassociation of starch molecules which is known as retrogradation. Higher amount of carboxyl groups introduced to starch molecules at longer reaction time (at least 90 min) effectively minimized retrogradation process leading to considerable improvement in viscosity stability. The stability in viscosity of starch paste was found to relate well with the amount of carboxyl groups formed in the starch molecules (Figure 3). Conclusion Properties of oxidized starch were significantly influenced by the amount of hypochlorite and the reaction time employed during starch modification process. Under alkaline conditions used in this work, the amount of carboxyl groups introduced to starch molecules increased with amount of oxidant and reaction time while the amount of carbonyl groups did not depend on either of these two factors. Starch oxidized with different levels of hypochlorite exhibited different pasting characteristics. With 30,000 ppm of active chlorine, viscosity reduction mostly occurred in the initial stage of reaction; however, longer reaction time was required to produced oxidized starch with high viscosity stability. 340

5 Figure 2 Viscosity of oxidized starch paste (15% solid content) during storage at 50 C. (Only viscosity of fresh sample shown for sample with 30 min reaction time because the stored samples had viscosity exceeding the limitation of the measurement equipment.) Change in viscosity Reaction time (min) 0.2 Viscosity change Figure 3 Change in viscosity of starch paste after 8 hr storage at 50 C and the carboxyl content of oxidized starch prepared with different reaction times. (Change in viscosity for sample with 30 min reaction time not determined because sample set gel after 8 hr storage) Note: Change in viscosity = (Viscosity at 8 hr storage Viscosity of fresh sample) x 100 Viscosity of fresh sample References Forssell, P., Hamunen, A., Autio, K., Suortti, T. and Poutanen, K Hypochlorite oxidation of barley and potato starch. Starch/Starke 47: Kuakpetoon, D. and Wang, Y. J Characterization of different starches oxidized by hypochlorite. Starch/Starke 53: Maher, S. L. and Cremer, C. W Use of modified starches: paper industry. In: O.B. Wurzburg (Ed.). Modified Starches: Properties and Uses. CRC Press, Inc. Boca Raton, Florida. Parovuori, P., Hamunen, A., Forssell, P., Autio, K. and Poutanen, K Oxidation of potato starch by hydrogen peroxide. Starch/Starke 47: The International Organization for Standardization (ISO) Modified starch determination of carboxyl group content of oxidized starch (ISO-11214). The International Organization for Standardization, Switzerland. Wang, Y. J. and Wang, L Physicochemical properties of common and waxy corn starches oxidized by different levels of sodium hypochlorite. Carbohydrate Polymers 52: Wurzburg, O. B Converted starches. In: O.B. Wurzburg (Ed.). Modified Starches: Properties and Uses. CRC Press, Inc. Boca Raton, Florida. 341