Food Chemistry 121 (2010) 1053–1059

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Food Chemistry

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Comparative susceptibilities of sago, potato and corn starches to alkali treatment

M.Z. Nor Nadiha a, A. Fazilah b, Rajeev Bhat b, Alias A. Karim b,*

a Halal Products Research Institute, University Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysiab Food Biopolymer Research Group, School of Industrial Technology, Universiti Sains Malaysia, 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 17 September 2009Received in revised form 25 November 2009Accepted 25 January 2010

Keywords:Alkali treatmentSagoPotatoCornStarches

0308-8146/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.foodchem.2010.01.048

* Corresponding author. Tel.: +60 4 6532268; fax: +E-mail address: akarim@usm.my (A.A. Karim).

The effects of steeping starch (sago, corn, and potato), in 0.025 M of sodium hydroxide for 0, 15, and30 days at 30 �C, on its granular structure and other physicochemical properties were investigated.Changes in the morphology of starch granules indicated that the alkaline solution affected the granularstructure of the starch. Pasting studies showed that the peak viscosity, breakdown, and setback of sagoand potato starch decreased significantly, whereas that of corn starch increased significantly, whensteeping time was prolonged. Swelling power increased significantly for treated potato and corn starches,but it decreased for sago starch. The amylose content of all alkali-treated starches also decreased signif-icantly after treatment. Onset and peak temperatures of gelatinization (as analyzed with a differentialscanning calorimeter) increased significantly, but the enthalpy decreased, for both gelatinization and ret-rogradation. The results showed that the physicochemical properties of starch of various botanical originswere affected to variable degrees when it was treated with alkaline solution.

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1. Introduction

Food starches impart certain desirable properties when addedto foods, e.g. texture or mouth feel, thickening, gelling, binding,and solubility (Lai, Karim, Norziah, & Seow, 2004). In cold water,starch granules undergo limited reversible swelling (Roberts &Cameron, 2002), and gelatinization in foods is usually achievedby thermal treatment. However, low-temperature swelling andgelatinization of starches can be induced by the addition of aque-ous alkali (Wooton & Ho, 1989). Alkalizing agents, such as sodiumhydroxide, are widely used in the production of many traditionalfood products, e.g. tortillas (Campus-Baypoll, Rosas-Burgos,Torres-Chavez, Ramirez-Wong, & Serna-Saldivar, 1999), waxy ricedumplings and yellow alkaline noodles (Lai et al., 2004). Alkalizingagents are believed to enhance desirable product quality character-istics such as colour (Bhattacharya & Corke, 1996; Maher, 1983;Miskelly & Moss, 1985; Moss, Miskelly, & Moss, 1986), aroma,and flavour and they confer a firm and elastic texture to the prod-uct (Lai et al., 2004).

During alkaline steeping, starch can undergo changes inmicrostructure and physicochemical properties (Gomez, Lee,McDonough, Waniska, & Rooney, 1992; Gomez, Waniska, & Roo-ney, 1990). These changes are varied and depend on the type ofalkalizing agent, concentration used, duration of steeping, and typeof starch. Compared to other starch modifications, such as enzymeand acid treatments, very few studies of alkali hydrolysis have

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been conducted. Recent studies of alkali hydrolysis have mainly fo-cused on corn starch (Bryant & Hamaker, 1997; Mistry & Eckhoff,1992; Mondragón, Bello-Pérez, Agama-Acevedo, Betancur-Ancona,& Pena, 2004) because it is the most commonly used starch in thefood industry. Others have reported changes caused by the alkalitreatment, such as the leaching out of protein from the granule sur-face during gelatinization of rice starch extracted using the alkalinemethod (Han & Hamaker, 2002), production of annealed starchmolecules during nixtamalization due to the increase of gelatiniza-tion temperature and amylose content (Méndez-Montealvo,Trejo-Espino, Paredes-Lópoz, & Bello-Pérez, 2007), and also higherpeak viscosity in the pasting profile after treatment with alkali (Laiet al., 2004).

Changes in starch properties also occur after acid hydrolysis.Differences in the rate and extent of acid hydrolysis between cer-eal, tuber, and legume starches exist and have been attributed todifferences in granule size (Jane, Wong, & Mcpherson, 1997); thisscenario might also apply to alkali treatment. For example, Robertsand Cameron (2002) reported that the ratio of starch/sodiumhydroxide (NaOH) influences the swelling and gelatinization ofstarch.

The composition and structure of starch granules vary consider-ably between different plants, thus affecting the properties andfunctions of starches from different crops. Corn (cereal) and potato(tuber) starches are widely used commercially for various applica-tions. Sago starch is perhaps the only example of a commercialstarch derived from the stem of palm (sago palm). Sago palm(Metroxylon spp.) contains a large amount of starch in its trunk.Sago starch granules are generally bigger than those of rice

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(3–10 lm), corn (5–20 lm), wheat (22–36 lm), or cassava (5–25 lm), but smaller than those of potato (15–85 lm).

To date, the effects of alkali on starch characteristics have notbeen studied as extensively as have other types of starch modifica-tion, such as acid and enzyme hydrolysis. Therefore, this study wasdesigned to determine the comparative susceptibilities of starchesof different botanical origins to alkali treatment. The results can beusefully applied to the development of novel products with im-proved textural attributes and extended shelf life.

2. Materials and methods

2.1. Materials

Native sago, corn, and potato starches were supplied by SIMCompany Sdn. Bhd., Penang, Malaysia. Analytical-grade NaOHand iodine (I2) were obtained from R&M Marketing, Essex, UK.Potassium iodide (KI) was purchased from Fisher Scientific, Lough-borough Leics, UK, and sodium azide (NaN3) was obtained fromSigma–Aldrich, Steinheim, Germany. Dimethylsulfoxide (DMSO)was purchased from Fisher Scientific, UK. Commercial starchesand chemicals were used directly without further purification.

2.2. Sample preparation

Aqueous NaOH solution 0.1% (w/v), containing 0.1% sodiumazide as a chemical preservative, was prepared. For each type ofstarch, a slurry (in duplicate) was prepared in alkali solution(15%, w/v) and left at room temperature (30 ± 2 �C) for 0, 15, and30 days, with shaking, by hand, once a day. The pH of each starchdispersion was measured, before and after the treatment, to deter-mine any changes in pH after steeping. The samples were thenwashed with distilled water until the pH of the starch supernatantwas nearly neutral. The starch sediment was collected and dried inan oven at 40 �C for 48 h before further analysis. Starch weight wasrecorded before and after treatment with alkali to determine thestarch yield. Control starch was prepared by adding distilled waterinstead of alkaline solution for each type of starch and this under-went same process as did alkaline-treated starch.

2.3. Moisture content

Moisture contents of native and alkali-treated samples weredetermined by drying triplicates of 5 g samples to a constantweight in an air oven at 105 �C.

2.4. Scanning electron microscopy

Native and alkali-treated starch samples were examined inpowder form. About 2 mg of each sample were applied to an adhe-sive metal tape attached to a specimen stub. The starch granuleswere then evenly distributed on the surface of the tape and coatedwith gold–palladium (60:40). Scanning electron micrographs(SEM) were obtained with a scanning electron microscope (FieldEmission Leo Supra 50vp, Zeiss, Oberkochen, Germany).

2.5. Apparent amylose determination

Native and alkali-treated starches (10 mg, dry basis) wereadded to 2 ml of DMSO and heated at 85 �C for 15 min. Deionizedwater was added to the dissolved starch to bring the volume upto 25 ml. Starch solution (1 ml) was pipetted into a 50 ml volu-metric flask and make up to volume. Iodine (5 ml) was added.Absorbance was read at 600 nm (Mcgrance, Cornell, & Rix,1998), using a UV–visible spectrophotometer.

2.6. Pasting properties

A Rapid Visco Analyzer Model 3-D (Newport Scientific Pty. Ltd.,Narrabeen, Australia) was used to study the pasting behaviour ofthe starch samples. Briefly, a starch sample (10% w/w, dry weightbasis) was subjected to the following heating and cooling pro-gramme: equilibrated at 50 �C for 1 min, heated to 95 �C in7.5 min, held at 95 �C for 5 min, cooled to 50 �C in 8.5 min, and heldat 50 �C for 3 min (Karim et al., 2007). All measurements were per-formed in triplicate.

2.7. Swelling power and solubility

To determine the swelling factor, a starch sample (0.4 g, dry ba-sis) was weighed accurately in a centrifuge tube before 40 ml ofdistilled water were added. The slurry was heated at 80 �C in awater bath for 30 min. Subsequently, after samples were cooledto room temperature, the solution was centrifuged at 3000 g for15 min. The supernatant obtained was carefully removed, andthe swollen starch sediment was weighed. The aliquot of superna-tant was evaporated overnight (110 �C). Analyses were performedin triplicate. Swelling power and solubility were calculated asfollows:

Swelling powerðg=gÞ ¼Weight of the wet sedimentðgÞWeight of the dry starchðgÞ ð1Þ

Solubilityð%Þ ¼Weight of dried supernatantðgÞWeight of the dry starchðgÞ � 100 ð2Þ

2.8. Thermal analysis

The thermal properties of starch were studied using a differen-tial scanning calorimeter (DSC-Q100, TA Instruments, New Castle,DE, USA). Starch slurries were prepared at 1:3 dry starch/ratios,hermetically sealed using a DuPont encapsulation press (DuPontCo., Delaware, USA), and reweighed. Samples then were heatedfrom 20 to 120 �C at 10 �C/min. After the DSC run, samples werestored at 4 �C for 7 days for retrogradation studies. Onset (To), peak(Tp), and completion (Tc) temperatures, as well as enthalpy, werecalculated. Each sample was run in triplicate.

2.9. Statistical analysis

The data were statistically analyzed using the SPSS programme(Statistical Package for Social Science) version 11.0 (SPSS Inc., Illi-nois, USA). Analysis of variance (ANOVA) was performed and themeans separation was conducted by LSD (P 6 0.05).

3. Results and discussion

3.1. Yields

Starch yield, after the treatment and washing process, relativeto untreated starch, decreased. After 30 days of alkali treatment,the yields of starch were 93.6%, 93.4%, and 86.5% for corn, sago,and potato, respectively. The washing process for the starch slurrychanged the pH of the starch from �12.0 to pH �7.0. The markedloss of starch yield after the washing process, especially for potatostarch, suggested that the starches had been solubilized by alkaliduring the treatment and that potato starch was hydrolyzed morerapidly than were the other starches tested.

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3.2. Changes in granular structure of alkali-treated starch

Figs. 1–3 show SEM micrographs for native and alkali-treatedsago, corn and potato starches. Scanning electron micrographsillustrate the presence of pores and cavities only on the surfaceof native corn starch (Fig. 2D). However, a previous study (Fannon,Hauber, & BeMiller, 1992) suggested that all starch granules mightcontain pores and channels that are unobservable with SEM butthat are large enough for water, reagents or enzymes to passthrough.

In the first 15 days (Fig. 1–3D) of the experiment, the surfaces ofthe starch granules (especially for potato starch) began to erodeand exhibited holes, indicating that the alkali solution might havediffused into the starch granules. We observed more severe dam-age on the surface of starch granules after 30 days of alkali treat-ment (Fig. 1–3F). We postulate that the alkali continued overtime to act on the granules and gradually disrupted the surface,hence damaging the granule. However, the extent of damage dif-fered among the starches, suggesting that starch granules of differ-ent botanical origins have varied susceptibilities to alkalitreatment.

Compared to corn and potato starch, the surface of sago starchgranules exhibited few holes and little major damage (Fig. 1D andF). The lesser susceptibility of sago starch to enzyme treatment wasreported by Srichuwong, Sunarti, Mishima, Isono, and Hisamatsu(2005), who showed that sago starch is degraded to a smaller de-gree than are other starches when hydrolyzed with porcine pan-creatic a-amylase. Alkali-treated corn starch granules seemed tobe most affected, possibly due to the presence of natural poresand cavities that facilitate the penetration of alkaline solution intothe granules.

3.3. Apparent amylose content of alkali-treated starch

Table 1 shows the apparent amylose contents for native and al-kali-treated starches at 0, 15, and 30 days. The amylose contentsfor native sago, corn, and potato starches were 31.2%, 27.2%, and27.8%, respectively. Amylose content of all alkali-treated starchesshowed a marked decreased after 30 days of treatment. Apparent

Fig. 1. Scanning electron micrographs of native and alkali-treated sago starch: (a) 500�;and (f) 30 d treated, 3000�.

amylose content of sago and potato starches did not change signif-icantly during the first 15 days of alkali treatment, but it decreasedwhen the treatment was prolonged to 30 days. In contrast, appar-ent amylose content of alkali-treated corn starch decreased signif-icantly in the first 15 days of treatment and remained constantthereafter.

The reduction of apparent amylose content of alkali-treatedstarches (Table 1) could be attributed to the disruption of theamorphous region that contains amylose chains (Karim et al.,2007). In addition, the alkali probably affects the amylose ratherthan the amylopectin molecules and/or regions of the granules(Lai et al., 2004). Lai et al. (2004) suggested that the ions in alkalisolution diffuse into the amylose-rich amorphous regions of thegranules, break intermolecular bonds, and cause the granules toswell to a higher degree, with a concomitantly higher exudationof amylose.

The insignificant changes in apparent amylose content for sagoand potato starch during the first 15 days of the experiment sug-gest that the alkalizing agent did not intensively affect the amor-phous region of either starch during this period. A similar trendwas found when potato starch was subjected to acid treatment(Wang & Wang, 2001). The amorphous regions in different starchesmight differ in terms of dimension and molecular arrangement(Wang & Wang, 2001). The large degree of polymerization (DP)of potato amylose (3320–21,800) (Hoover, 2001) might slow downthe hydrolyzing action of the alkali solution. Wang and Wang(2001) suggested that a larger DP requires more time for thehydrolyzing agent to degrade the chain length in starch molecules.This scenario is consistent with our finding that the amylose con-tent of potato starch only decreased significantly after 30 days oftreatment. Thus, the alkali solution needed a longer time to hydro-lyze potato starch, which has a larger DP than has corn starch.

After 30 days of alkali treatment, corn starch had the highestpercentage reduction of amylose content (10.5%), followed by po-tato (4.5%) and sago (2.5%) starch. However, these data might notprovide the exact amount of amylose content decrease caused bythe alkali steeping process. According to the yield data, the starchyield collected after the washing process decreased, especially forpotato starch. The washing process might remove the solubilized

(b) 3000�; (c) 15 d treated, 1000�; (d) 15 d treated, 3000�; (e) 30 d treated, 1000�;

Fig. 2. Scanning electron micrographs of native and alkali-treated corn starch: (a) 500�; (b) 300�; (c) 15 d treated, 500�; (d) 15 d treated, 3000�; (e) 30 d treated, 500�; and(f) 30 d treated, 8000�.

Fig. 3. Scanning electron micrographs of native and alkali-treated potato starch: (a) 500�; (b) 3000�; (c) 15 d treated, 500�; (d) 15 d treated, 3000�; (e) 30 d treated, 1000�;and (f) 30 d treated, 3000�.

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starch, together with the supernatant, and the removed solubilizedstarch might contain solubilized amylose.

The greatest decrease, occurring in alkali-treated corn, might bedue to the existence of pores and natural cavities on the surface ofcorn starch granules (Fannon et al., 1992), which would allowgreater access of alkali into the granules’ internal structures. Largemolecules, including enzymes or ions, could easily penetrate intothe granule interior through these pores (Fannon et al., 1992).Baldwin, Adler, Davies, and Melia (1994) reported the presenceof pores in potato starch, but they are not as abundant as are thosein corn starch. Thus, the susceptibility to the alkali might be great-er for corn, which would allow the OH� ions to disrupt the amor-phous region that contains amylose chains. There are no reports of

the existence of natural pores and cavities on the surface of sagostarch, which could explain the minimum decrease of amylosecontent of this starch compared to the others.

3.4. Pasting properties of alkali-treated starch

Table 1 also shows the pasting characteristics of native and al-kali-treated sago, corn, and potato starches. Overall, similar trendswere recorded for alkali-treated sago and potato starches, whereasalkali-treated corn starch exhibited the opposite trend.

There was a decreasing trend of peak viscosity, breakdown, andsetback for alkali-treated sago and potato starch. This result agreeswith that of Karim et al. (2007), who reported a similar decrease in

Table 1Pasting properties and amylose content for native and alkali-treated sago, corn, and potato starches at 15 and 30 daysa.

Starch Pasting parameters Amylose content (%) Swelling (g/g) Swelling (g/g)

Treatment Peak viscosity (cP) Breakdown (cP) Setback (cP) Past tempearture (�C)

Sago Native 1916.4de ± 1.2 1158.0e ± 6.0 523.2e ± 1.2 75.6f ± 0.2 31.2h ± 0.6 10.9ef ± 0.8 1.7de ± 0.1Control (15 days) 2144.4f ± 39.6 7422.0i ± 7.2 523.2e ± 12.0 76.3g ± 0.4 31.1h ± 0.6 8.9bc ± 0.6 1.4c ± 0.1Control (30 days) 1886.4de ± 9.6 6248.4h ± 1.2 546.0e ± 12.0 75.6fg ± 0.2 32.2i ± 0.1 9.5cd ± 0.4 1.5cd ± 0.4NaOH (15 days) 964.8b ± 2.4 507.6bc ± 10.8 290.4b ± 2.4 76.3g ± 0.2 31.4h ± 0.5 8.2b ± 0.1 2.4g ± 0.0NaOH (30 days) 739.2a ± 3.6 482.4bc ± 2.4 200.4a ± 3.6 76.3g ± 0.2 30.4 ± 0.1 6.6a ± 0.2 1.3c ± 0.1

Corn Native 1816.8d ± 4.8 445.2ab ± 26.4 432.0d ± 54.0 80.4i ± 1.1 27.3de ± 0.5 11.3fg ± 0.8 1.7de ± 0.1Control (15 days) 1876.8de ± 28.8 440.4ab ± 68.4 626.4f ± 52.8 77.3b ± 0.3 23.6b ± 0.6 11.8fg ± 0.7 2.5g0.2Control (30 days) 1690.8c ± 14.4 345.6a ± 170.4 529.2e ± 52.8 5.4a ± 0.1 22.4a ± 0.8 10.0cde ± 0.1 2.4g ± 0.1NaOH (15 days) 1872.0de ± 10.8 637.2d ± 13.2 753.6g ± 30.0 75.1f ± 0.4 24.5c ± 0.3 10.5def ± 0.2 2.6g ± 0.1NaOH (30 days) 1933.2e ± 6.0 598.8cd ± 7.2 754.8g ± 28.8 76.1g ± 0.4 24.4c ± 0.0 12.2g ± 0.3 2.1f ± 0.1

Potato Native 9650.4d ± 54.0 7827.6j ± 178.8 1366.8h ± 22.8 65.4b ± 0.4 27.9ef ± 0.0 13.5h ± 1.1 1.9ef ± 0.2Control (15 days) 10615.2k ± 56.4 7422.0i ± 10.8 290.4b ± 30.0 67.3c ± 0.3 27.5e ± 0.4 13.4h ± 1.3 0.3a ± 0.2Control (30 days) 9044.4h ± 94.8 6248.4h ± 12.0 426.0d ± 31.2 67.6c ± 0.3 26.7d ± 0.3 16.8i ± 0.3 0.7b ± 0.1NaOH (15 days) 9331.2i ± 66.0 4686.0g ± 46.8 367.2c ± 3.6 69.6d ± 0.4 28.3f ± 0.0 21.5j ± 1.4 0.4a ± 0.0NaOH (30 days) 6994.8g ± 24.0 2422.8f ± 14.4 523.2e ± 9.6 70.8e ± 0.2 26.6d ± 0.4 24.6k ± 0.1 1.3c ± 0.1

a For a particular type of starch, means with different letters in same column differ significantly (p < 0.05), n = 3.

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pasting profile of alkali-treated sago starch. The higher peak vis-cosity exhibited by native starches in this instance was due tohigher granule rigidity and integrity, contributed by the presenceof amylose (Karim et al., 2007). However, for alkali-treated starch,the amorphous region (likely containing amylase) was largely dis-rupted by treatment, thus weakening the granular structure. Con-sequently, when shear was applied to the starches during pasting,the granules could not attain their maximum swelling capacity,resulting in reduction of the peak viscosity.

The increase in peak viscosity observed in corn starch could berelated to the presence/absence of surface proteins (Han & Ha-maker, 2002). The protein on the surface of the starch granulewould be partially removed when treated with alkali (Han & Ha-maker, 2002). This premise is supported by our protein determina-tion, which showed that the protein content for 30 day alkali-treated corn starch (0.08%) was decreased significantly comparedto that of native corn starch (0.3%). The removal of surface proteinwill allow the granule to swell more rapidly. In addition, otherstudies (Liukkonen, Kaukovirta-Norja, & Laakso, 1992; Seidel, Oro-zouich, & Medcalf, 1984) have reported that alkali extraction canalso remove the lipid content from the surface of the starch gran-ule. The removal of lipid and protein from the starch surface mightincrease the swelling of starch granules during pasting (Tester &Morrison, 1992), which may in turn increase the viscosity. A nega-tive correlation (0.483) between peak viscosity and amylose con-tent suggests that there was no relationship between starchviscosity and percent amylose content in the starch granule.

Breakdown is a measure of the response of starch pastes toshear-thinning during the holding period at 95 �C (Lai et al.,2004). Thus, breakdown viscosity indicates the tendency of starchto resist shear force during heating (Karim et al., 2007). A decreasein breakdown viscosity with increased steeping time, for sago andpotato treated starch, suggests that the granule structure of alkali-treated starch is more stable toward shear, and hence less prone tolose viscosity upon holding and shearing. The stability of the gran-ule structure might be due to the presence of the OH� ions in thealkali solution, which would strengthen the bonding force withinthe granules and thereby prevent the physical breakdown of starchgranules even after holding for a certain period of time at 95 �C(Mistry & Eckhoff, 1992). This might explain the decrease in break-down viscosity for alkali-treated sago and potato starches. On theother hand, the greater breakdown of alkali-treated corn starchindicates weaker granular structure and a greater tendency to loseviscosity upon holding and shearing. This result is consistent with

the significant reductions in amylose and protein contents of thealkali-treated corn starch, which render the granules more proneto rupture.

Total setback is the difference between final viscosity andtrough (hot viscosity) in the pasting curve (Lee, Hettiarachchy,McNew, & Gnanasambandam, 1995). In other words, it is a mea-sure of aggregations of gelatinized starch during cooling, and it ishighly correlated with amylose content and DP (Sandhya Rani &Bhattacharya, 1995). This might not be true for our situation. Table1 shows that the amylose content for alkali-treated corn starch de-creased significantly after 30 days, but the setback value increased.The increase of the total setback value for alkali-treated corn starchmight be due to the presence of ions that restrict the tendency ofthe starch molecules to realign after cooling, thus facilitating lowersetback. Because of the natural features of the corn starch granule,the effect of the OH� ions might be more pronounced compared tosago and potato starches.

Pasting temperature was recorded when the rate of change ofviscosity above the set point was first achieved (Jin-song, 2008),which indicates the onset gelatinization temperature of the starch.According to Table 1, the significant loss of amylose content in thealkali-treated starch might have facilitated the swelling of thestarch, thus increasing the viscosity in the system.

3.5. Swelling and solubility of alkali-treated starch

The swelling and solubility data illustrate the integrity andrigidity of the starch granules. Swelling is primarily a property ofamylopectin and amylose, whereas lipids can inhibit swelling (Tes-ter & Morrison, 1992). Table 2 shows the swelling power and sol-ubility of native and alkali-treated starches. There was aprogressive increase in swelling power for alkali-treated potatostarch compared to that for native starch. A gradual increase ofswelling for corn starch was observed, whereas sago starch exhib-ited a significant decrease of swelling after the treatment.

Therefore, the increases in swelling power of alkali-treated cornand potato starches can be attributed to the disruption of theamorphous region in the granule, which presumably reduced therestraining effect of amylose, thus allowing the granule to swellmore freely (Karim et al., 2007). The swelling power of sago starchwas decreased after alkali treatment. It is possible that, under thetest conditions used in this study (heating at 80 �C for 30 min),sago starch granules were prone to rupture, thus yielding a low

Table 2Swelling and solubility for native and 15 and 30 days alkali-treated sago, corn andpotato starchesa.

Sample Swelling (g/g) Solubility (%)

Sago Native 10.9a ± 0.8 1.7de ± 0.1Control (15 days) 8.9bc ± 0.6 1.4c ± 0.1Control (30 days) 9.5cd ± 0.4 1.5cd ± 0.2NaOH (15 days) 8.2b ± 0.1 2.4g ± 0.0NaOH (30 days) 6.6a ± 0.2 1.3c ± 0.1

Corn Native 11.3fg ± 0.8 1.7de ± 0.1Control (15 days) 11.8fg± 2.5g ± 0.2Control (30 days) 10.0cde ± 0.1 2.4g ± 0.1NaOH (15 days) 10.5def ± 0.2 2.6g ± 0.1NaOH (30 days) 12.2g ± 0.3 2.1f ± 0.1

Potato Native 13.5h ± 1.1 1.9ef ± 0.2Control (15 days) 13.4h ± 1.3 0.3a ± 0.2Control (30 days) 16.8i ± 0.3 0.7b ± 0.1NaOH (15 days) 21.5j ± 1.4 0.4a ± 0.0NaOH (30 days) 24.6k ± 0.1 1.3c±0.1

a For a particular type of starch, means with different letters in same columndiffer significantly (p < 0.05), n = 3.

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value of the swelling power. This is consistent with the results ofpeak viscosity (Table 1).

The solubility of alkali-treated starches shows inconsistent re-sults after 15 and 30 days of treatment. This inconsistency mightbe due to the removal of some soluble material during the filtrationprocess after the alkali treatment. From the percentage yield ofstarch obtained after the treatment (average of 85–95%), we as-sume that some of the soluble components of the treated starchwere removed during the washing process. Hence, the solubilityresults were not completely accurate.

Table 3 shows the thermal profiles of alkali-treated starches.The higher gelatinization and onset temperatures for treatedstarches are in agreement with results from other studies (Karimet al., 2007; Méndez-Montealvo, Sánchez-Rivera, Paredes-López,& Bello-Pérez, 2006; Méndez-Montealvo et al., 2007). The highergelatinization temperature of alkali-treated starch might be dueto the rearrangement of polymer chains into more stable configu-rations when the steeped starch is held below the gelatinizationtemperature (Mondragón, Bello-Pérez, Agama-Acevedo, Betancur-Ancona, & Pena, 2004). In addition, the Na+ ions likely diffuse intostarch granules through small pores on the surfaces that enhancethe stability of the granule through electrostatic interaction

Table 3Transition temperatures and enthalpy associated with gelatinization and retrogradation o

Gelatinization (�C)

Starch Treatment To Tp TC D

Sago Native 67.5 g ± 0.7 73.7h ± 0.3 82.1f ± 0.9 1Control (15 days) 67.4h ± 1.0 73.4hi ± 0.2 81.0ef ± 0.5 1Control (30 days) 68.5h ± 0.3 73.6i ± 0.0 80.0e ± 0.8 1NaOH (15 days) 68.9g ± 0.1 74.0j ± 0.1 80.6ef ± 1.3 1NaOH (30 days) 68.8h ± 0.2 74.3k ± 0.2 82.1f ± 0.3 1

Corn Native 64.6d ± 0.1 70.4f ± 0.1 77.6d ± 0.1 1Control (15 days) 64.8de ± 0.1 70.5f ± 0.1 77.4d ± 0.0 1Control (30 days) 65.2de ± 0.1 70.6f ± 0.1 77.2d ± 1.5 1NaOH (15 days) 65.4e ± 0.2 70.7f ± 0.1 78.1d ± 0.3 1NaOH (30 days) 66.1f ± 0.3 71.3g ± 0.2 78.0d ± 1.2 1

Potato Native 59.8a ± 0.2 64.0a ± 0.3 72.5a ± 1.1 1Control (15 days) 61.9b ± 0.1 65.4b ± 0.1 73.8ab ± 0.1 1Control (30 days) 62.6c ± 0.4 66.0c ± 0.5 74.3bc ± 1.7 1NaOH (15 days) 64.9de ± 0.0 67.6d ± 0.1 74.1abc ± 0.6 1NaOH (30 days) 65.3e ± 0.1 68.6e ± 0.0 75.6c ± 0.7 1

a For a particular type of starch, means with different letters in same column differ s

between the Na+ ions and the hydroxyl groups of starch (Bryant& Hamaker, 1997; Méndez-Montealvo et al., 2007; Karim et al.,2007). In Karim et al. (2007) study, the ions that diffused throughthe small pores into the granules could not be removed, eventhough the samples were washed thoroughly with distilled waterand reached neutrality (Karim et al., 2007). They found that starch,after 30 days of alkali treatment, contained approximately 4–6times more ions than did the control starch.

In our study, alkali-treated potato starch showed a significantincrease of gelatinization temperature compared to the other al-kali-treated starches. This is an interesting finding because weassumed that the natural features of corn starch granules wouldallow more OH� ions to diffuse into the granule, thus providingstability and increasing the gelatinization temperature. Severedisruption of the amorphous region of alkali-treated corn starch(based on apparent amylase content results) might have re-duced the ability of the OH� ions to interact well with the hy-droxyl groups, resulting in a lower gelatinization temperaturecompared to potato starch. Lower enthalpy values were ob-served in the gelatinization and retrogradation study for alltreated starches. The reduction of enthalpy in alkali-treatedstarch compared to native starch (in the gelatinization and ret-rogradation samples) illustrates that the energy transformationthat occurs during melting and uncoiling of the double helicesof crystallite amylopectin was decreased. The effect of depoly-merization of alkali-treated starch granules might be the reasonfor this reduction.

4. Conclusions

Our results have shown that the physicochemical properties ofstarches of various origins were affected to variable degrees whenthey were treated with alkaline solution. Amylose content of all al-kali-treated starches showed a marked decrease after 30 days oftreatment. The reduction in amylose content resulted in variableextents of granule swelling and starch solubility. Pasting propertiesof alkali-treated starch were significantly altered with prolongedsteeping time. In general, alkali-treated starch also showed a sig-nificant increase in onset gelatinization temperature (as deter-mined by DSC), with potato starch showing the highest increase(compared to sago and corn starches). The different degrees of sus-ceptibility of starch to alkaline treatment reflect the complexity of

f native and alkali-treated sago, corn and potato starches for 15 and 30 daysa.

Retrogradation (�C)

H (J/g) To Tp TC DH (J/g)

6.2e ± 0.3 46.7cd ± 0.8 56.5b ± 0.1 67.0 d ± 0.6 6.0bc ± 0.15.8de ± 0.0 46.5bcd ± 0.8 56.4b ± 0.3 65.1bcd ± 1.1 6.4 c ± 3.10.9a ± 0.9 47.6de ± 0.1 57.0b ± 0.8 65.2bcd ± 1.7 3.6 a ± 0.62.1ab ± 1.1 46.4bcd ± 0.7 56.1 b ± 0.6 65.6cd ± 1.3 5.0abc ± 0.35.3cde ± 0.3 46.3bcd ± 0.2 56.0 b ± 0.5 64.8bc ± 1.0 4.4abc ± 0.3

1.0a ± 1.4 44.5a ± 0.2 54.7 a ± 0.3 64.5bc ± 0.5 5.4abc ± 0.22.3abc ± 1.1 45.1ab ± 0.4 54.6 a ± 0.4 64.5bc ± 0.2 4.0ab ± 0.11.0a ± 1.6 45.2ab ± 1.4 54.0 a ± 1.8 62.4a ± 0.4 3.6a ± 1.02.0ab ± 0.3 45.6abc ± 1.0 54.4 a ± 0.4 63.2ab ± 0.8 4.3ab ± 0.31.8a ± 0.8 48.0bcd ± 1.5 59.2 a ± 3.2 64.1abc ± 1.2 4.9abc ± 0.2

6.3e ± 4.5 49.1ef ± 1.0 61.0 c ± 0.4 72.0ef ± 1.0 4.91abc ± 0.56.2e ± 1.0 47.6 de ± 1.2 60.8 c ± 0.5 71.2ef ± 0.6 5.0abc ± 1.02.9abcd ± 3.0 50.0 f ± 1.0 61.3 c ± 0.9 70.4e ± 1.8 3.4a ± 1.43.7abcde ± 0.7 48.1 e ± 1.0 61.6 c ± 1.5 73.1f ± 2.9 5.8bc ± 0.94.9bcde ± 0.8 48.9ef ± 0.4 61.2 c ± 0.4 72.5f ± 0.7 4.7abc ± 0.4

ignificantly (p < 0.05), n = 3.

M.Z. Nor Nadiha et al. / Food Chemistry 121 (2010) 1053–1059 1059

the granular structure and molecular architecture of differenttypes of starch.

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