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Structure and Functional Properties ofAcid Thinned Sorghum StarchHarinder Singh a , Navdeep Singh Sodhi a & Narpinder Singh aa Department of Food Science and Technology , Guru Nanak DevUniversity , Amritsar-IndiaPublished online: 21 Aug 2009.

To cite this article: Harinder Singh , Navdeep Singh Sodhi & Narpinder Singh (2009) Structure andFunctional Properties of Acid Thinned Sorghum Starch, International Journal of Food Properties, 12:4,713-725, DOI: 10.1080/10942910801995614

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International Journal of Food Properties, 12: 713–725, 2009Copyright © Taylor & Francis Group, LLCISSN: 1094-2912 print / 1532-2386 onlineDOI: 10.1080/10942910801995614

713

STRUCTURE AND FUNCTIONAL PROPERTIES OF ACID THINNED SORGHUM STARCH

Harinder Singh, Navdeep Singh Sodhi, and Narpinder SinghDepartment of Food Science and Technology, Guru Nanak Dev University, Amritsar-India

The structural and physicochemical properties of acid thinned sorghum starch prepared bytreating starch with 2.2 M HCl at 30°C for different durations (0–72 h) were studied.Amylose content and swelling power decreased and crystallinity increased with increase inacid hydrolysis. The scanning electron microscopy observations demonstrated that the acidthinning did not cause any disruption of the granular crystalline structure. However, thethermal properties observed by DSC showed a decrease in onset temperature (To) andenthalpy of gelatinization (DHgel) and an increase in conclusion temperature (Tc) upon acidhydrolysis. A significant reduction in starch pasting viscosities and gel hardness was alsoobserved with acid thinning.

Keywords: Sorghum starch, Acid thinning, Physicochemical, Morphology, Thermal,Pasting, Gel strength, Crystallinity.

INTRODUCTION

Acid modified starches are extensively used in food, textile, paper, and other indus-tries.[1] These are typically prepared by treating starch slurry at a temperature lower thanthe gelatinization temperature of starch with mineral acid, for one or many hours ofreaction time. Hydrochloric acid and sulphuric acid are commonly used for the manufac-turing of acid hydrolyzed starches.[2,3] Studies have shown that hydrolysis of cereal,[4–6]

legume,[6] and potato[7] starches proceed in two stages. Initially, fast hydrolysis of amor-phous region followed by slow hydrolysis of crystalline regions. Kainuma and French[8]

suggested that cleavage of starch chains in the amorphous regions allows extensivereordering of the chain segments to give a more crystalline structure with a sharper X-raypattern. Acid hydrolysis is reported to cause a decrease in amylose content for corn,[9,10]

arrowroot,[11] potato, rice,[12] and tapioca[13] starches indicating a preferential hydrolysisof amorphous regions. An increase in gelatinization enthalpy for cereal,[14,15] potato,[16,17]

and pea[16] starches is observed. Kang et al.[18] reported an increase in rate of retrograda-tion with the progress of acid hydrolysis for rice starch. A decrease in viscosity of acidtreated corn starches[10] and decrease in rigidity of acid hydrolysed oat starches[19] ascompared to native gels is quoted in the literature.

Received 22 September 2007; accepted 19 February 2008.Address correspondence to Navdeep Singh Sodhi, Department of Food Science and Technology, Guru

Nanak Dev University, Amritsar 143005, India. E-mail: navdeep_fst@yahoo.co.in

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714 SINGH, SODHI, AND SINGH

Sorghum (Sorghum bicolor (L) Moench), also known as jowar in India, is one of themajor cereal grains grown worldwide with an estimated production of 64.6 × 106 metrictons. India is the third largest producer of sorghum with an annual production of 7.40 × 106

metric tons.[20] It is an important crop in semiarid regions because of its droughtresistance[21] and requirement of fewer inputs.[22] It sustains the lives of the poorest ruralpeople and often referred to as “coarse grain” or “poor people crop.”[23] However, withincreasing world population and decreasing water supplies, it is foreseen as an importantfuture crop.[22] Despite its importance, our knowledge about sorghum processing is lim-ited as compared to other cereals. Although, recently, there has been increased interest insorghum because its potential for industrial starch production being comparable tocorn.[24] Moreover, the availability of corn to Indian starch industry is decreasing, day byday, because of its increased demand by industries involved in the production of breakfastcereals and snacks. So, sorghum being cheaper than corn, as it can be grown with mini-mum inputs, is exploited as an alternative starch source for diverse industrial applications.Thus, the present study was undertaken to produce acid modified sorghum starch with anobjective to compare its physicochemical, morphological, thermal, pasting and gel textureproperties with the native sorghum starch.

MATERIALS AND METHODS

Materials

Sorghum cultivar (CSV-16) was procured from National Research Centre forSorghum (NRCS), Hyderabad, India.

Starch Isolation

Starch was extracted by using the method of Beta et al.[25] Sorghum grain (100 g)was steeped in 200 ml of NaOH (0.25%, w/v) at 5°C for 24 h. The steeped grains werewashed and ground with an equal volume of water using a blender for 3 min. The slurrywas filtered through a 200-mesh screen. The material remaining on the sieve was rinsedwith water. Grinding and filtering were repeated on this material. After rinsing, thematerial remaining on the sieve was discarded. The filtrate was allowed to stand for 1 h.The filtrate was centrifuged at 760 × g for 10 min. The grey colored, top protein-rich layerwas removed using a spatula. Excess water was added to resuspend the sample, andcentrifugation was done for 5 min. Washing and centrifugation was repeated several timesuntil the top starch layer was white. The starch was dried for 24 h at 40°C.

Acid Treatment

Starch was acid modified with the method reported by Raja.[26] Starch (5 g) wastransferred into glass stopper conical flasks of 250-ml capacity. Two-hundred ml offreshly prepared 2.2M HCl acid was carefully added into the flask to accomplish auniform dispersion of acid throughout the starch. The flasks were then immersed in waterbath maintained at 30°C. The flasks were periodically swirled to ensure uniform contactbetween acid and starch granules. For preparation of the zero hour samples, the starch wasprepared by filtration under vacuum, allowing as little a contact time as possible with acid.The remaining samples were prepared by steeping in acid and then separating the starch

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STRUCTURAL PROPERTIES OF ACID THINNED SORGHUM STARCH 715

after completing the respective time periods of 8, 16, 24, 48, and 72 h. The wet starch thusobtained was thoroughly washed with distilled water. Starch was then dried in oven for24 h at 40°C.

Amylose Content, Swelling Power, and Solubility

Amylose content of the native and acid modified starches was determined by usingthe method of Williams et al.[27] Swelling power and solubility were determined using 2%aqueous suspension of the starch by the method of Leach et al.[28]

Morphological Properties

Scanning electron micrographs of native and acid modified starches were obtainedwith a scanning microscope (Jeol JSM-6100, Jeol Ltd., Tokyo, Japan). Starch sampleswere suspended in ethanol to obtain 1% suspension. One drop of the starch-ethanol solu-tion was applied on an aluminum stub, and the starch was coated with gold-palladium(60:40). An acceleration potential of 10 kV was used during micrography.

Thermal Properties

Thermal properties of native and acid modified starches were analyzed using aDSC-821e (Mettler Toledo, Switzerland) equipped with a thermal analysis data station.Starch (3.5 mg, dwb) was weighed in 40 μl capacity aluminum pan (Mettler, ME-27331)and distilled water was added with the help of a Hamilton micro syringe to achieve astarch-water suspension containing 70% water. Samples were hermetically sealed andallowed to stand for 1 hour at room temperature before heating in DSC. The DSC analyzerwas calibrated using indium and an empty aluminum pan was used as reference. Samplepans were heated at a rate of 10°C/min from 40 to 100°C. Onset temperature (To), peaktemperature (Tp), conclusion temperature (Tc) and enthalpy of gelatinization (DHgel) werecalculated automatically. Because the peaks were symmetrical the gelatinization range(R) was computed as (Tc-To) as described by Vasanthan and Bhatty.[4] Enthalpies werecalculated on starch dry basis.

Pasting Properties

The pasting properties of starches were evaluated with a Rapid Visco Analyser(RVA-4, Newport Scientific, Warriewood, Australia) using RVATM Crosslinked andSubstituted Method, No. 9, Version 4.[29] Starch (12 g, 14%, mb) was weighed directly inthe aluminum RVA sample canister, and 25 ml distilled water was added. A programmedheating and cooling cycle was used where the samples were held at 50°C for 1 min, heatedto 95°C in 3.7 min (heating and cooling rate 12°C/min), held at 95°C for 2.5 min beforecooling to 50°C in 3.8 min, and holding at 50°C for 2 min. Parameters recorded werepasting temperature (PT); peak viscosity (PV); hot paste viscosity (HPV) (minimumviscosity at 95°C); cool paste viscosity (CPV) (final viscosity at 50°C); breakdown (BD)(=PV – HPV); and set back (SB) (=CPV – HPV). All measurements were replicated thrice.For native and 0 h sample, the viscosity values exceeded the Rapid Visco Analyser(RVA-4, Newport Scientific, Warriewood, Australia) limits, so 3 g (14%, mb) sample wastaken instead of 12 g.

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716 SINGH, SODHI, AND SINGH

Gel Hardness

Hardness of RVA starch gels was evaluated as described by Bhattacharya et al.[30]

The starch slurry formed in the canister after RVA testing was covered and kept at 4°Covernight and allowed to gel. The gel formed in the can (37-mm diameter, 20-mm height)was used directly for texture analysis. The textural analysis was performed using TA/XT2Texture Analyzer (Stable Micro Systems, Surrey, England). The gels were punched to adistance of 10 mm using a flat cylindrical probe (5-mm dia.) at a cross-head speed of1 mm/s. The peak force obtained during this punching was recorded as gel hardness.[31]

Twelve repeated measurements were performed for each sample.

X-Ray Diffraction

X-ray diffractograms of fully moistured starch granules (exposed to 100% relativehumidity for 3 days) were recorded by a copper anode X-ray tube using an AnalyticalDiffractometer (Pan Analytical, Phillips, Holland).[32] The diffractometer was operated at30mA and 40kV with a scanning speed of goniometer of 4°/min.

Statistical Analysis

The data reported in all the tables are an average of triplicate observations. The datawere subjected to statistical analysis using Minitab Statistical Software (Minitab Inc., USA).

RESULTS AND DISCUSSION

Physicochemical Properties

The amylose content of the starch decreased with increase in duration of acid treat-ment (Table 1). Native sorghum starch showed amylose content of 17.4%, whichdecreased to 14.7% and 8%, respectively, after 24 and 72 h of acid treatment. Thissuggests that the action of acid was more targeted towards the amorphous region bypreferential hydrolyzing the amylose.[33,34] The presence of which otherwise could havefacilitated complex formation with iodine, as in the native starch. Robin et al,[35] Manignatand Juliano,[36] and Raja[26] in their studies on linternized potato, rice, cassava and maizestarches, respectively, also showed a decrease in amylose content with acid treatment.

Table 1 Amylose, swelling power, and solubility index of native and acid thinnedsorghum starch.

Acid treatment Amylose (%) Swelling power (g/g) Solubility index (%)

Native 17.4e 14.0b 7.8c

0 h 17.0e 15.1b 10c

8 h 16.8e 1.35a 77b

16 h 15.9d 0.97a 81ab

24 h 14.7c 0.88a 82a

48 h 9.6b 0.82a 83a

72 h 8.0a 0.78a 85a

Values with similar superscripts in column do not differ significantly (p < 0.05) (n = 3).

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STRUCTURAL PROPERTIES OF ACID THINNED SORGHUM STARCH 717

Swelling power and solubility provide evidence of the magnitude of the interactionbetween starch chains within both the amorphous and crystalline domains. The extent ofinteraction is influenced by the ratio and characteristics of amylose and amylopectin interms of molecular weight distribution, degree of branching, length of branches and con-formation of molecules.[37] Swelling power decreased and solubility increased with theincrease in duration of acid treatment (Table 1). Swelling power of native starch was 14 g/g,which decreased to 0.78 g/g after 72 h acid treatment, however it improved with 0 h acidtreatment. Jayakody and Hoover[15] also observed an increase in swelling factor for cerealstarches at the first stage of acid hydrolysis and suggested this change to the interactionbetween hydrolyzed amylose and water. The decrease in swelling power with increase inacid hydrolysis duration corroborated with the increase in crystallinity indicated in X-raydiffractograms (Fig. 1). Kainuma and French[8] associated the decrease in swelling powerwith acid treatment to the stiffness of entangled amylopectin network in the crystallineregion of the starch. The effect of disruption of hydrogen bonding between adjacent starchpolymers and erosion of the amorphous regions during limited and prolonged acid treat-ment, respectively, on swelling power cannot be ruled out.[15]

The solubility index increased from 7.8% for native to 85% after 72 h acidtreatment. The increase in solubility was greater during the initial 8 h of acid treatment.

Figure 1 X-ray diffractograms of native, 24 h and 72 h acid treated sorghum starches.

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718 SINGH, SODHI, AND SINGH

The increase in solubility of starch with increase in acid treatment may be attributed to anincrease in low molecular weight linear fraction with hydroxyl groups that facilitatedsolubilization in warm water.[38] Similar results, are also reported for acid treated corn[9]

and arrowroot[11] starches.

X-Ray Diffraction

X-ray diffractograms of sorghum starch showed a typical A-type pattern (Fig. 1).Acid treatment did not cause any change in crystalline pattern. However, a change fromcrystalline C- to A-type with acid treatment of Rhizoma Dioscorea and Radix Puerariaestarches has been reported.[39] Jane et al.[40] postulated that the branch points of B-typeamylopectin like in potato starch are mainly located in the amorphous region making thesame susceptible to acid hydrolysis, but branch points of A-type starch amylopectin likethose of cereals are scattered in both amorphous and crystalline regions, and thus, are lesssusceptible to acid hydrolysis. This might be the reason that the pattern of sorghum wasnot changed with acid treatment. A progressive increase in peak intensities with increasein acid treatment indicates increase in crystallinity. The reordering of the chain segmentsresulted into more crystalline structure following cleavage of starch chains in the amor-phous regions during acid treatment has been reported to be responsible for sharperpeaks.[8] Waigh et al.[41] also explained increase in crystallinity of starch following acidtreatment using side chain liquid crystalline model.

Morphological Properties

Scanning electron micrographs of native starch showed presence of irregular shapedgranules consisted of elongated and spherical shaped granules (Fig. 2). The microscopydid not show any significant destruction in the granular structure of starch during acidtreatment. Similar observations have been made earlier by Shi and Seib.[42] Jiping et al.[39]

reported that B-type Fritillaria starch granule started to crack at the hydrolysis period of4 days while C-type Rhizoma Dioscorea and Radix Puerariae starch granule did not crackuntil the hydrolysis progressed up to 16 days.

Thermal Properties

Gelatinization patterns of native and acid treated starches were found to differ withrespect to gelatinisation temperatures (To, Tp, and Tc), range of gelatinisation (R) andgelatinisation enthalpies (DHgel) (Table 2). Native starch showed To, Tp, and Tc of 66.1,70.5, and 75.4°C, respectively. Upon acid hydrolysis the onset gelatinisation temperature(To) has been observed to decrease with duration of acid hydrolysis. Atichokudomchaiet al.[13] proposed that gelatinization temperature seemed to correlate with the partlyhydrolyzed amylose content left after hydrolysis. However, conclusion gelatinizationtemperature (Tc) increased with duration of acid hydrolysis. This increase could be due tolonger amylopectin double helices resulting from acid hydrolysis than in unhydrolyzedamylopectin molecule, because branch points in unhydrolyzed amylopectin may reducethe length of helix-forming side chains.[43]

DHgel was observed to decrease with acid-thinning of starches, which is in agree-ment with the studies of Sandhu et al.[9] on waxy and normal maize starches, Biliaderiset al.[44] on pea starch and Muhr et al.[17] on potato and wheat starches. This decrease in

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STRUCTURAL PROPERTIES OF ACID THINNED SORGHUM STARCH 719

DHgel can be attributed to the loss of some degree of order of amorphous regions prior togelatinization.[17] Blanshard[45] also reported a decrease in DHgel due to the preferentialhydrolysis of amorphous regions as these regions play an important role in the thermody-namics of gelatinization. Contrarily, Jayakody, and Hoover[15] reported increase in DHgelresulting from acid hydrolysis suggesting the formation of more double helical starch dueto the interaction between amylose-amylose and amylose-amylopectin chains during acidhydrolysis.

Transition temperature range (R) progressively increased with the increase in dura-tion of acid treatment. R of native starch was 9.3, which increased to 10.2, 17.5, and 29.9,respectively, with 8, 16, and 72 h of acid treatment. Earlier studies carried out by Muhret al.[17], Shi and Sieb,[42] and John et al.[11] reported similar observations. The broadening

Figure 2 SEM (×1500) of native and acid treated sorghum starches. (A-Native; B-0h; C-8h; D-16h; E-24h;F-48h; G-72h).

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720 SINGH, SODHI, AND SINGH

of gelatinization endotherm of acid treated starches may be attributed to the easier pene-tration of water in the acid treated starches due to preferential hydrolysis of amyloseregions and delay in penetration of water molecules in the crystalline regions, which hadincreased during acid treatment. The broadening of R may also be attributed to changes indouble helices length, reported to increase with decrease in proportion of the double heli-ces. The gelatinization endotherm of starch containing shorter helices has been reported tobe broader than that of starch with longer helices.[41,46]

Pasting Properties

Pasting properties are dependent on granule swelling, friction between swollen gran-ules, amylose leaching, starch crystallinity and chain length of starch components.[47,48]

Various pasting properties (Peak viscosity-PV; Hot paste viscosity-HPV; cold paste viscosity-CPV; breakdown-BD; set back-SB; pasting temperature-PT) for acid hydrolysed starcheswere measured using RVA. The results revealed a significant influence of acid hydrolysisduration on sorghum starch (Table 3, Fig. 3). All the pasting parameters showed a decreas-ing trend upon increase in duration of acid hydrolysis. Native sorghum starch showed PT

Table 2 Thermal properties of acid thinned sorghum starch.

Acid treatment To, °C Tp, °C Tc, °C DH, J/g R

Native 66.1b 70.5c 75.4a 9.37b 9.3a

0 h 65.8b 69.9b 74.9a 11.0c 9.1a

8 h 68.0c 72.6d 78.2b 10.8bc 10.2a

16 h 66.1b 73.7d 83.6c 9.9b 17.5b

24 h 65.5b 73.8d 84.1cd 9.0b 18.6b

48 h 55.8a 64.5a 85.1de 5.09a 29.3c

72 h 56.3a 68.6b 86.2e 4.98a 29.9c

To = onset temperature; Tp = peak temperature; Tc = conclusion temperature;R = gelatinization range (Tc - To); DHgel = Enthalpy of gelatinization (dwb,based on starch weight). Values with similar superscripts in column do notdiffer significantly (p < 0.05) (n = 3).

Table 3 Pasting properties of acid thinned sorghum starch.

Acid treatment PV (cP) HPV (cP) CPV (cP) BD (cP) SB (cP) PT (°C)

Native 4698e 2053c 3238d 2645e 1185b 75.15c

0 h 4214d 1969c 3392de 2245d 1423c 75.00c

8 h 2389c 418b 3687e 1971c 3269d 75.05c

16 h 406b 94a 1149c 312b 1055b 74.20c

24 h 322a 55a 1065c 267ab 1010b 72.60b

48 h 135a 28a 236b 107a 208a 70.90b

72 h 107a 9a 46a 98a 37a 66.05a

Ptemp : pasting temperature; PV: peak viscosity; HPV: hot paste viscosity (minimum viscosity at 95°C); CPV:cool paste viscosity (final viscosity at 50°C at the end of 13 min. cycle); BD: break down (=PV − HPV); SB: set-back (CPV-HPV). Starch sample of 3 g (14%, mb) was used in native and 0 h where as 12 g (14%, mb) starchsample was used for 8, 16, 24, 48, and 72 h acid treated sample. Values with similar superscripts in column donot differ significantly (p < 0.05) (n = 3).

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STRUCTURAL PROPERTIES OF ACID THINNED SORGHUM STARCH 721

of 75.15°C, which decreased to 66.05°C for 72 h acid treated starch. PV of 4698cP wasobserved for native starch, which decreased to 4214cP for 0 h acid treated starch. Uponfurther treatment, the PV values decreased from 2389cP for 8 h to 107cP for 72 h acidtreated starch. The reduction in PV by acid treatment may be attributed to increase inhydrolysis of amorphous regions and production of low molecular weight dextrins. HPV isinfluenced by the rate of amylose exudation, amylose-lipid complex formation, granuleswelling, and competition between exudated amylose and remaining granules for freewater, while CPV is largely determined by the retrogradation tendency of the solubleamylose upon cooling.[49] HPV ranged from 2053cP to 9cP, while CPV varied from3238cP to 46cP for native and 72 h acid hydrolysis starch, respectively. BD, which is themeasure of susceptibility of cooked starch granule to disintegration and SB, which ismeasure of recrystallization of gelatinized starch during cooling,[50] also decreased withacid treatment. BD and SB drastically reduced from 2645cP to 98cP and 1185cP to 37cPfor native and 72 h acid treated starch, respectively. The low setback of acid treated starchmay be due to Newtonian behaviour of its starch gel and due to the insufficient time forthe starch molecules to align themselves during this measurement.[51] Similar decrease inpasting viscosities upon acid treatment of corn starch has been reported earlier by Sandhuet al.[9] and Wang et al.[10]

Gel Hardness

Hardness of RVA starch gels determined using textural profile analysis on TextureAnalyser are shown in Table 4. Gel hardness of native and 0 h acid treated starches were

Figure 3 Pasting profiles of native and acid treated sorghum starches. (A-Native; B-0h; C-8h; D-16h; E-24h;F-48h; G-72h).

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722 SINGH, SODHI, AND SINGH

0.31 and 0.38 N, respectively. For other acid hydrolyzed samples, a maximum value of5.20 N was observed for 8 h hydrolysis followed by 3.77, 2.81, 0.63, and 0.39 N, respec-tively. The mechanical properties of starch gels depend on various factors, including therheological characteristics of the amylose matrix, the volume fraction and rigidity ofgelatinized starch granules, as well as the interactions between dispersed and continuousphases of the gel.[52] These factors, in turn, are dependent on the amylose content and thestructure of amylopectin.[53] Acid treatment caused a significant decrease in gel hardness.Wurzburg[2] and Fleche[3] also reported reduction in rigidity of starch gel following acidtreatment. Amylose was reported to be the main factor in the short-term development ofgel structure, while amylopectin was correlated with long term development of gelstructure.[54,55] Starches that exhibited harder gels tended to have higher amylose contentand longer amylopectin chains. [56]

CONCLUSION

The sorghum starch treated by 2.2 M HCl for different durations, lasting up to 72 h,showed significant changes in its functional properties. Acid thinning caused decreasein amylose content, swelling, gel hardness, and increase in solubility of starch. Increasein crystallinity and decrease in starch pasting viscosities was observed upon acidhydrolysis, as well. The information thus generated can be used in producing acidmodified sorghum starches with desired properties after critically selecting duration ofhydrolysis.

ACKNOWLEDGMENTS

The authors wish to thank Dr. N. Seetharama, Director, National Research Centre for Sorghum(NRCS), Hyderabad for providing sorghum sample for this research work. The research facilitiesprovided by Regional Sophisticated Instrumentation Centre (RSIC), Panjab University, Chandigarhfor Scanning Electron Microscopy; and Indian Institute of Technology (IIT), Delhi for X-ray diffrac-tograms are also acknowledged.

Table 4 Hardness of acid thinned sorghumstarch gels.

Acid treatment Hardness N

Native 0.31a

0 h 0.38a

8 h 5.20d

16 h 3.77c

24 h 2.81b

48 h 0.63a

72 h 0.39a

Starch sample of 3 g (14%, mb) wasused in native and 0 h where as 12 g (14%,mb) starch sample was used for 8, 16, 24,48, and 72 h acid treated sample. Valueswith similar superscripts in column do notdiffer significantly (p < 0.05) (n = 12).

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