The influence of ultrasound on the degree of oxidation of hypochlorite-oxidized corn starch

LWT - Food Science and Technology 50 (2013) 439e443

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The influence of ultrasound on the degree of oxidation of hypochlorite-oxidized corn starch W.T. Chong, U. Uthumporn, A.A. Karim, L.H. Cheng* Food Technology Division, School of Industrial Technology, Universiti Sains Malaysia, USM, 11800 Minden, Penang, Malaysia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 December 2011 Received in revised form 27 August 2012 Accepted 30 August 2012

Oxidized corn starch was prepared with and without sonication at different levels of sodium hypochlorite (0e2 g active chlorine/50 g starch) and treatment time (15 and 30 min). Gel permeation chromatography revealed that weight average molecular weight decreased while polydispersity index increased with increase in active chlorine concentration and treatment time. It was found that carbonyl and carboxyl contents, and swelling power of oxidized corn starch generally increased with progressive increase in active chlorine level, treatment time and in the presence of sonication. A reversed trend was observed for the solubility. Peak viscosity was found to decrease with increase in active chlorine concentration to a threshold level of 1 g active chlorine/50 g starch, after which peak viscosity increased as the active chlorine level was increased. Breakdown values and final viscosities were increased and decreased respectively with progressive increase in active chlorine concentration. These phenomena became significant as the starch was subjected to sonication and longer treatment time. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Corn starch Starch oxidation Sodium hypochlorite Ultrasound Gel permeation chromatography Pasting properties

1. Introduction Starch is the main source of energy for humans (Miyazaki, Hung, Maeda, & Morita, 2006). Native starch has been used in various food preparations primarily based on its thickening or binding properties (Jobling, 2004; Rengsutthi & Charoenrein, 2011). The limitation in usage is due to the fact that native starch shows poor tolerance to a broad range of processing conditions and poor functional properties (Thomas & Atwell, 1999). However, these shortcomings can be overcome through chemical or enzyme modification, and physical treatment and this has made starch becomes a useful polymer in industrial applications as modification can enhance starch positive attributes and/or to overcome the inconsistency of starches (Jobling, 2004). One of the common chemical modifications of starch is oxidation. Oxidized starch shows lower viscosity at high solid content, lower retrogradation property, better clarity, film forming and binding properties than the native ones. All these have diversified starch applications in food and non-food industries. In food applications, oxidized starch is used in batters and breading, as fillings in bakery products (Kuakpetoon & Wang, 2001; Thomas & Atwell, 1999). The common oxidants used to produce oxidized starch are permanganate (Takizawa, Silva, Konkel, & Demiate, 2004), hydrogen

* Corresponding author. Tel.: þ60 4 6535209; fax: þ60 4 6573678. E-mail address: [email protected] (L.H. Cheng). 0023-6438/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.lwt.2012.08.024

peroxide, hypochlorite (Sangseethong, Termvejsayanon, & Sriroth, 2010) and oxygen (Ye et al., 2011), Among the abovementioned, sodium hypochlorite is the most popular used oxidizing agent for producing oxidized starches at commercial level (Kuakpetoon & Wang, 2001). According to Wurzburg (1986), there are two major reactions taking place during starch oxidation. The first reaction occurs mainly on the hydroxyl groups at the C-2, C-3 and C-6 positions of D-glucopyranosyl with these hydroxyl groups being oxidized to carbonyl groups and then to carboxyl groups. The second reaction is the depolymerization of starch molecules by primarily hydrolyzing amylose and amylopectin molecules at a-D-(1 / 4) glycosidic linkages. Hence, the carbonyl and carboxyl contents as well as the degree of degradation are generally used to indicate the extent of starch oxidation. The degree of hypochlorite oxidation is affected by many factors such as starch molecular structure, starch origin, packing of crystalline lamellae and the size of amorphous lamellae, pH, temperature, concentration of oxidants and catalyst (Kuakpetoon & Wang, 2001; Tolvanen, Mäki-Arvela, Sorokin, Salmi, & Murzin, 2009; Wang & Wang, 2003; Wurzburg, 1986). The application of ultrasonic irradiation in food processing has been increasing from the past few years because it shortens the processing times required and it also lowers the energy consumption, making it an effective process (Jambrak, Lelas, Mason, Kresi c, & Badanjak, 2009; Mason, Paniwnyk, & Lorimer, 1996). When liquid is irradiated with high intensity ultrasound, microbubbles are formed, with dissolved gas acting as the nucleic. These microbubbles oscillate, grow into slightly larger size during

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the expansion cycle then shrink during the compression cycle. Once reaching a critical size called the resonant size, the microbubbles will eventually implode violently and generates very high pressure and temperature which lead to production of very high shear energy waves and turbulence. This process is called the cavitation (Leighton, 1994; Mason & Lorimer, 2002; Suslick, 1994). The power ultrasound through cavitation can cause drastic changes in chemical, functional and physical properties of solute present in the liquid medium (Jambrak et al., 2009). Zuo, Knoerzer, Mawson, Kentish, and Ashokkumar (2009) studied the pasting behaviour of sonicated waxy rice starch suspensions. They found that the length of sonication time and the solution temperature markedly influenced the functionality of starch granules. Recent study by Jambrak et al. (2010) on the effect of ultrasound on physical properties of corn starch reported that ultrasound irradiation ruptured and mechanically disrupted the starch granules. In short, the results of previous works have showed that sonication does affect the physicochemical properties of starch in different manners such as degradation, solubilization and swelling. The aim of this work was to investigate the effect of sonication on the degree of oxidation of oxidized corn starch produced with different levels of sodium hypochlorite (NaOCl).

phase was an aqueous solution of 0.1 mol equivalent/L NaNO3 at a flow rate of 1.0 mL/min. Starch sample (0.1 g) was prepared in dimethyl sulphoxide (DMSO), shoke for 1 h in a 95  C water bath, and was then stirred at room temperature for 24 h. An aliquot (1.5 mL) was taken and added with 6 mL of absolute alcohol. The precipitated starch was recovered by centrifugation at 2000g for 30 min and washed twice with ethanol by centrifugation for 15 min to remove dimethyl sulphoxide (DMSO). Then, the precipitated starch was redissolved in 5 mL deionized water by stirring for 30 min in a heating water bath. The mixture was then centrifuged at 2000g for 15 min to remove insoluble residue prior to injection. 2.4. Carbonyl and carboxyl contents determination Carbonyl and carboxyl contents of native and modified corn starch samples were determined according to the titrimetric method of Kuakpetoon and Wang (2006). 2.5. Swelling power and solubility determination Swelling power and solubility of starch samples were determined in quadruplicates as described by Chan, Bhat, and Karim (2010).

2. Materials and methods 2.6. Pasting properties 2.1. Materials Commercial corn starch containing 26.47 g amylose/100 g was purchased from Sim Company Sdn. Bhd, Penang, Malaysia. Sodium hypochlorite (NaOCl) containing 10 g active chlorine/100 g was used in this experiment. All other chemicals used were of analytical grade.

The pasting properties of starch were evaluated in quadruplicates according to the method of Chan et al. (2010) using a Rapid Visco-Analyzer (RVA) (Model RVA-4; Newport Scientific Pvt. Ltd., Warriewood, Australia) and software program Thermocline for Windows (TCW). The viscosity was measured in Centipoise (Cp).

2.2. Sample preparation

2.7. Statistical analysis

Corn starch slurry was prepared by dispersing 50 g of starch in 450 g of deionized water. The starch slurry was maintained at 35  C in a water bath and pH was adjusted to 9e10 with 2 mol equivalent/ L NaOH with consistent stirring using an overhead stirrer. Active chlorine content was adjusted to 0.06e2 g/50 g starch in 30 min while maintaining the pH at 9e10 with 0.5 mol equivalent/L H2SO4. Sonication was carried out in an ultrasonic bath (Transsonic TI-H10, Elma, Germany) with power ultrasound of 200 W with 100% amplitude at a frequency of 25 kHz and the intensity was 13 W cm2. Sonication was continued for 15 min or 30 min under continuous stirring to prevent the starch granules from settle to the bottom. After which, the starch slurry was adjusted to pH 7.0 with 0.5 mol equivalent/L H2SO4; vacuum filtered, washed with deionized water, washed with absolute ethanol and dried in a vacuum oven at 30  C overnight. Parallel control of these five level hypochlorite-oxidized corn starches were also prepared by employing the same procedure with only stirring for 15 min and 30 min without sonication. Control samples were also prepared by applying same conditions as oxidized starch without any hypochlorite addition.

The data reported were averages of quadruplicate observations. The data were subjected to statistical analysis using SPSS 14.0 (SPSS Inc., Chicago, USA). One-way analysis of variance (ANOVA) was performed. Duncan’s multiple range test (p < 0.05) was carried out to evaluate significant differences between means.

2.3. Gel permeation chromatography The weight average molecular weight (Mw) and polydispersity of unmodified and modified starches were determined using a Malvern Viscotek Triple Detection GPCMax (Light Scattering Detector, RI Detector and Viscometer) gel permeation chromatography (GPC) system which consisting of an inline eluent degasser, a syringeloading sample injector equipped with a 100 mL sample loop and dual linear (connected in series) Viscotek A6000M Aqueous GPC/ SEC column packed with hydroxymethacrylate polymer. The mobile

3. Results and discussion 3.1. Gel permeation chromatography The weight average molecular weight (Mw) and polydispersity index of samples prepared are tabulated in Table 1. Results show that corn starch subjected to modification exhibited a marked difference in molecular mass properties compared to the unmodified counterpart. Starch subjected to sonication only shows higher Mw value, whereas the Mw values of oxidized and soni-oxidized starches are lower as compared to the native starch. The Mw values of oxidized samples decreased with active chlorine content and treatment time. The lower Mw could be attributable to depolymerization of starch molecules, i.e. amylopectin and amylose chains during the modification process. This observation is consistent with the study of Wang and Wang (2003), who reported that the degree of starch molecules degradation increased with oxidant concentration. It is noted that at a specific active chlorine level, greater depolymerization was evident with the use of sonication. The increase in Mw for sonicated starch was unexpected and the plausible explanation could be intermolecular crosslinks formed between the functional groups. However, further analysis is needed to substantiate this explanation. Polydispersity index (Mw/Mn) is generally used as an indicator of the broadness of a molecular weight distribution of a polymer.

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Table 1 Molecular weight, polydispersity index, carbonyl and carboxyl contents of starch samples prepared. Starch sample

Active chlorine content (g/50 g starch)

Molecular weighta (106)

Polydispersity indexa (Mw/Mn)

Carbonyl contenta (g/100 g)

Carboxyl contenta (g/100 g)

Native corn starch

e

20.31  1.72J

1.92A

0.0079A

0.0055A

Corn starch with sonication 15 min Sonication 30 min Sonication

e e

28.65  3.29K 35.16  2.75L

2.05A 2.04A

0.0125AB 0.0194CD

0.0076A 0.0101AB

Oxidation without sonication (conventional method) 15 min Treatment 0.06 0.5 1.0 1.5 2.0 30 min Treatment 0.06 0.5 1.0 1.5 2.0 Oxidation with sonication 15 min Sonication

30 min Sonication

a

0.06 0.5 1.0 1.5 2.0 0.06 0.5 1.0 1.5 2.0

8.64 5.91 5.69 3.55 2.42 7.48 5.11 5.02 3.34 2.32

         

0.45I 0.10FG 0.21FG 0.22CDE 0.16ABC 0.30HI 0.28EF 0.15EF 0.15CD 0.18ABC

2.44BC 2.62BCD 2.63BCD 3.42G 4.43I 2.46BC 2.81DE 2.94EF 3.69H 4.67IJ

0.0153BC 0.0403E 0.0790F 0.1113H 0.1417J 0.0221D 0.0425E 0.0797F 0.1123H 0.1521K

0.0080A 0.0547D 0.1285G 0.1867K 0.2449M 0.0189BC 0.0830EF 0.1388H 0.2166L 0.2772N

8.10 5.76 4.56 3.19 1.72 6.92 4.49 4.67 3.08 1.61

         

0.26HI 0.25FG 0.19DEF 0.20BCD 0.05AB 0.33GH 0.23DEF 0.30DEF 0.26ABCD 0.08A

2.40B 2.70CDE 2.69CDE 3.75H 4.78JK 2.52BC 2.87DEF 3.09F 4.48I 4.99K

0.0203CD 0.0447E 0.0842F 0.1281I 0.1648L 0.0256D 0.0463E 0.0976G 0.1403J 0.1745M

0.0145ABC 0.0758E 0.1569I 0.2189L 0.2898 0.0203C 0.0894F 0.1743J 0.2785N 0.3969P

Means  1 SD (n ¼ 4). Means within the same column followed by different superscript letter are significantly different (P < 0.05).

Broader molecular weight distribution will have larger polydispersity index (Chan et al., 2011). Results show that polydispersity index increased with active chlorine concentration and also treatment time. This indicates that the molecular weight distribution of starch molecules is positively correlated with both oxidation level and treatment time. Chromatograms (data not shown) also show that the peak of molecular weight distribution skewed to lower molecular weight region (longer retention time). At a specific active chlorine concentration, starch oxidized with the presence of sonication showed higher polydispersity index. 3.2. Carbonyl and carboxyl contents The carbonyl and carboxyl contents of native and modified corn starches are shown in Table 1. According to Wurzburg (1986), hydroxyl groups of D-glucopyranosyl were first oxidized to carbonyl groups and then to carboxyl groups during starch oxidation. From Table 1, it is noticed that the carbonyl and carboxyl contents of modified corn starches were markedly higher than the native corn starch. Besides, it is observed that both carbonyl and carboxyl contents of oxidized corn starches generally increased with progressive increase in active chlorine concentration and treatment time. Interestingly, the carboxyl content of oxidized starches increased at a much faster rate than the carbonyl content. This could be due to the possibility that as the level of active chlorine and treatment time increases, more hydroxyl groups being converted to carbonyl groups and promptly oxidized to carboxyl groups. This may then result in a higher carboxyl content formed in oxidized starch. From Table 1, it can be noted that at a specific active chlorine concentration, soni-oxidized starches showed higher carbonyl and carboxyl contents than those prepared with conventional method. This shows that sonication was effective in enhancing starch oxidation. A report by Makino, Mossoba, and Riesz (1983) stated that when the sonicated medium is water, H and OH radicals are

generated during cavitation. Thus, it is believed that these highly reactive radicals may help in enhancing starch oxidation. Besides, Kardos and Luche (2001) revealed that many chemical reactions could be initiated by sonication because of the highly reactive conditions created during cavitation. 3.3. Swelling power and solubility Fig. 1(a) shows that swelling power of oxidized starch decreased with active chlorine level and treatment time, and swelling power of corn starch oxidized at low level of active chlorine (0.06 g active chlorine/50 g starch) was slightly higher than the native corn starch. Higher swelling power shown by sample treated with 0.06 g active chlorine could probably due to breakdown of inter and intra hydrogen bonds of starch molecules, allowing larger amount of water molecules forming hydrogen bonds with exposed hydroxyl groups. Nevertheless, longer treatment time and excessive active chlorine led to a greater degree of structural disruption and this may subsequently weaken granule integrity to sustain swelling. This weakening effect became profound as the starch sample was subjected to cavitation forces of sonication. A reversed trend was demonstrated in solubility (Fig. 1(b)). In general, solubility increased as the extent of oxidation was progressed. Hodge and Osman (1996) reported that increase in solubility after oxidation was due to disintegration and structural weakening of the starch granule. Lawal, Adebowale, Ogunsanwo, Barba, and Ilo (2005) also revealed that the increase in solubility was probably influenced by leaching of amorphous region of starch granule. 3.4. Pasting properties Pasting properties of starch samples are shown in Fig. 2. It is observed that there were significant differences (p < 0.05) in the pasting properties among native starch and modified starches. It is

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Fig. 1. Swelling power (a) and solubility (b) of starch samples. Oxidized corn starch prepared with (C) and without (B) sonication for 15 min; with (-) and without (,) sonication for 30 min. [Typical coefficient of variation (C.V.) for quadruplicate measurements did not exceed 10%.]

notable that no significant differences in peak viscosity among slightly oxidized starch in Fig. 2(a). At 1 g active chlorine/50 g starch, treated samples showed lowest peak viscosity value. After which, peak viscosity increased progressively with active chlorine

concentration. At a specific active chlorine level, starch samples subjected to sonication showed higher peak viscosity as compared to starch samples prepared by conventional method without sonication. Highest peak viscosity is observed for starch sample treated at 2 g active chlorine for 30 min. This could be attributed to the higher carbonyl and carboxyl contents formed. Breakdown is a measure of cooked starch resistance to disintegration during holding phase of the heating and cooling cycle at 95  C (Sandhu & Singh, 2007). Higher breakdown viscosity indicates granule disruption or the lower tendency of starch to resist shear force during heating (Karim et al., 2008). The breakdown values of oxidized and soni-oxidized starches were found to be higher than the native sample and samples subjected to sonication (15 min, 30 min) only (Fig. 2(b)). The values increased with active chlorine concentration, treatment time and in the presence of sonication. Highest breakdown value was shown by sample oxidized for 30 min in the presence of sonication. According to Chan et al. (2010), high breakdown viscosity is correlated with high degree of swollen starch granules collapse, thus reduce the resistance of starch pastes to shear force. Fig. 2(c) shows that final viscosity decreased with increasing active chlorine concentration and treatment time. It can be seen that starch samples subjected to sonication showed a greater decrease in final viscosity as compared to those samples prepared with conventional method. Reduction in final viscosity could be attributed to the degradation of starch molecules and greater amount of carbonyl and carboxyl formed upon treatment. Setback is the measure of the reassociation of solubilized starch molecules or the extent of retrogradation of gelatinized starch during cooling phase (Lai, Karim, Norziah, & Seow, 2004; Thomas & Atwell, 1999). Fig. 2(d) shows that corn starch subjected to

Fig. 2. Pasting properties of starch samples. (a) Peak viscosity; (b) breakdown value; (c) final viscosity; (d) setback value. Oxidized corn starch prepared with (C) and without (B) sonication for 15 min; with (-) and without (,) sonication for 30 min. [Typical coefficient of variation (C.V.) for quadruplicate measurements did not exceed 10%.]

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sonication only gave higher setback value as compared to unmodified starch. Generally, oxidized and soni-oxidized starches had higher values of setback as compared to the native sample at 15 min treatment time whereas starch samples subjected to 30 min modification process had lower setback values. This trend became profound in the presence of sonication. Higher setback value could be due to greater amount of solubilized starch chains released which were capable of reassociating during the cooling cycle (Chan et al., 2010). According to Sandhu, Kaur, Singh, and Lim (2008), the retrogradation tendency of a hot starch pastes upon cooling is determined by the affinity of hydroxyl groups to interact or associate with each other. Hence, the substitution of hydroxyl groups with larger size carbonyl and carboxyl groups during oxidation which can hinder the interaction between starch chains could have caused the oxidized starch molecules to show a lower setback value (Tavares, Zanatta, Zavareze, Helbig, & Dias, 2010). 4. Conclusions The degree of starch oxidation and the resultant starch physicochemical properties varied greatly with the amount of active chlorine, treatment time, and also the presence of sonication. Oxidation of starch in the presence of sonication degraded the starch polymer chains significantly. Sonication was found to accelerate the rate of oxidation by giving higher carbonyl and carboxyl contents at a specific active chlorine concentration. In the present study, it was found that structural disruption experienced in slightly oxidized starches resulted in higher peak viscosity. At a higher level of oxidation, structural disintegration occurred in conjunction with the formation of higher carbonyl and carboxyl contents which led to higher peak viscosity, high breakdown value and lower final viscosity. Acknowledgements This work was funded by Universiti Sains Malaysia Research University Grant Scheme (Grant No: 1001/PTEKIND/814145). Chong acknowledges the Fellowship awarded by Universiti Sains Malaysia. References Chan, H. T., Bhat, R., & Karim, A. A. (2010). Effects of sodium dodecyl sulphate and sonication treatment on physicochemical properties of starch. Food Chemistry, 120, 703e709. Chan, H. T., Leh, C. P., Bhat, R., Senan, C., Williams, P. A., & Karim, A. A. (2011). Molecular structure, rheological and thermal characteristics of ozone-oxidized starch. Food Chemistry, 126, 1019e1024. Hodge, J. E., & Osman, E. M. (1996). Carbohydrates. In O. R. Fennema (Ed.), Food chemistry. New York: Marcel Dekker.   Jambrak, A. R., Herceg, Z., Subari c, D., Babi c, J., Brn ci c, M., Brn ci c, S. R., et al. (2010). Ultrasound effect on physical properties of corn starch. Carbohydrate Polymers, 79, 91e100. Jambrak, A. R., Lelas, V., Mason, T. J., Kresi c, G., & Badanjak, M. (2009). Physical properties of ultrasound treated soy proteins. Journal of Food Engineering, 93, 386e393. Jobling, S. (2004). Improving starch for food and industrial applications. Current Opinion in Plant Biology, 7, 210e218.

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