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Starch/Stärke 52 (2000) 283–289 283

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1 Introduction

Highly crystalline tapioca starch can be prepared by mildacid hydrolysis, normally by using hydrochloric or sulfuricacid. When the reaction time of hydrolysis is increased,the crystallinity of the starch increases while its amylosecontent decreases (Tab. 1) [1]. It is known that the acidpreferentially attacks the amorphous regions and then thehigh crystallinity regions [2, 3] . Highly crystalline starch is used in many industries, including food and textile man-ufacture. It has also been used often for structural studiesof starch granules [4, 5].

Scanning electron microscopy (SEM) is often used tostudy starch grain morphology, both in native and modi-fied starches [6]. In the past 10 years, research has beenfocused particularly on the behavior of the starch gran-ules subjected to enzyme hydrolysis [7], and less on thebehavior of granules subjected to acid hydrolysis. There-fore, the objective of this study was to investigate the mor-phology of highly crystalline tapioca starch granules andthe mode of their acid hydrolysis. It was hoped that the re-sults of the study would provide information on both na-tive and highly crystalline tapioca starch structure andthat the information could be used to describe the proper-ties of tapioca starch and further enhance its utilization.

2 Materials and Methods

Tapioca starch was the product of Choheng Co., Ltd.(Thailand). Sodium hydroxide and hydrochloric acid

were obtained from Merck (Germany) and were reagentgrade.

2.1 Preparation of highly crystalline tapiocastarch by acid hydrolysis

Samples of 400 g (dry basis) of tapioca starch were hy-drolyzed by suspension in 600 mL of 6% (w/v) HCl solu-tion at room temperature for various periods of time (i.e.,12, 24, 48, 96, 192, 384 and 768 h). At the end of the timeperiods the reaction was stopped by neutralization with10% (w/v) NaOH solution. The suspensions were thenwashed three times with distilled water by centrifugationat 1000 min–1 for 2 min and decanting. The wet acid-mod-ified starch was either air-dried at room temperature orspray-dried with a mobile small spray dryer (Gea-Niro,Denmark) at an inlet temperature of 160 °C and an outlettemperature of 60 °C. The dried powder was finally sieved

Napaporn Atichokudomchaia, Sujin Shobsngobb, Saiyavit Varavinita

a Department of Biotechnology, Faculty of Science, Mahidol University, Rama 6 Road,Bangkok 10400, Thailand

b Department of Chemistry, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand

Morphological Properties of Acid-modifiedTapioca StarchHighly crystalline tapioca starch was prepared by partial hydrolysis of tapioca starch inhydrochloric acid at room temperature for various lengths of time. The crystallinity ofthe starch increased while its amylose content decreased with increasing reactiontime. Scanning electron micrographs of these natural and highly crystalline starcheswere taken in order to study the morphological changes and mode of acid attack dur-ing hydrolysis. Exocorrosion all over the surface was observed after 96 h of hydrolysis.Further hydrolysis caused the outer layer of the starch grain surface to erode away. After complete erosion was accomplished, an inner layer or underlayer was revealedwith a smooth surface similar to that of the native starch grain surface. Endocorrosionwas not observed at any time in this study.

Keywords: Scanning electron microscopy; High crystallinity; Acid-modified starch;Tapioca starch

Correspondence: Saiyavit Varavinit, Department of Biotechnol-ogy, Faculty of Science, Mahidol University, Rama 6 Road,Bangkok 10400, Thailand. Phone: 662-2460063 ext. 2715, Fax: 662-2463026, E-mail: grsvr@mahidol.ac.th.

Tab. 1. Composition of native and acid modified tapiocastarches after various times (h) of acid hydrolysis.

Reaction time % Amylose % Relative [h] content* crystallinity**

0 28.80 39.53

12 27.20 41.59

24 26.81 45.60

48 22.82 46.55

96 16.00 47.89

192 6.01 51.01

384 3.06 53.28

768 1.30 57.75

* Measurement by iodine affinity method [8].**Measurement according to [2].

284 Atichokudomchai et al. Starch/Stärke 52 (2000) 283–289

Fig. 1. Scanning electron micrographs (magnification 1500 ×) of air-dried native (A) and acid-modified tapioca starchesafter various hydrolysis times: (B) 12 h; (C) 24 h; (D) 48 h; (E) 96 h; (F) 192 h; (G) 384 h and (H) 768 h.

Starch/Stärke 52 (2000) 283–289 Morphological Properties of Acid-modified Tapioca Starch 285

Fig. 2. Scanning electron micrographs (magnification 5000 × and 7500 ×) of air-dried native (A) and acid-modified tapiocastarches after various hydrolysis times: (B) 12 h; (C) 24 h; (D) 48 h; (E) 96 h; (F) 192 h; (G) 384 h and (H) 768 h.

286 Atichokudomchai et al. Starch/Stärke 52 (2000) 283–289

through a 100-mesh sifter to obtain acid-modified starchpowder.

2.2 Scanning electron microscopy

Native and acid-modified tapioca starch samples weremounted on SEM stubs with double sided adhesive tapeand coated with gold. Scanning electron micrographswere taken using a JEOL JSM-5410LV microscope (JEOL, Tokyo, Japan). The accelerating voltage and themagnification are shown on the micrographs.

3 Results and Discussion

For easy comparisons in size and morphology, all of theair-dried native and acid-modified tapioca starch sam-ples were photographed at a magnification of 1500 ×(Fig. 1A–1H). For more detailed morphology, further mi-crographs were taken at magnifications of either 5000 ×or 7500 × depending on the size of the focused granules(Fig. 2A–2H).

Native tapioca starch granules had irregular or truncatedshapes with diameters ranging from 5 to 25 µm (Fig. 1A).The surface of native starch granules was smooth withoutobservable pores (Fig. 2A). After 12 h of hydrolysis, thesurface of most of the starch granules was still smoothand there were no visible imperfections at 1500 × magni-fication (Fig. 1B). However, at 5000 × magnification(Fig. 2B), a slightly roughened surface was observed.Similar observations were recorded for the surface ofstarch granules hydrolyzed for 24 h (Fig. 1C and 2C). Arough surface due to exocorrosion was clearly visible af-ter hydrolysis for 48 h (Fig. 1D and 2D). At 96 h of hydrol-ysis, severe exocorrosion had taken place all over thegranule surface (Fig. 1E and 2E). After prolonged hydrol-ysis for 192 h, the outer layer (or upper layer) surface ofthe tapioca starch granules was eroded. The erosionprocess proceeded from one side of the surface (Fig. 1Fand 2F). Erosion continued until the entire outer surfacewas destroyed (Fig. 1G) and the granule surface againbecame smooth, similar to the surface of a native starchgranule (Fig. 2G). However, as hydrolysis proceeded fur-ther, part of the grain was destroyed (Fig. 1H–2H).

Spray-dried native and acid-modified tapioca starch gran-ules are shown in Fig. 3 at a magnification of 1500 ×. Theshape of the granules was spherical as a result of the

aggregation of the smaller starch granules during spraydrying. At higher magnification (7500 ×), the surfaces ofthe composite starch granules (Fig. 4) showed the samesurface erosion as the starch hydrolyzed for similarlengths of time.

The crystallinity of starch granules increases as the amy-lose content decreases. The amylose content can be de-creased by acid hydrolysis, because amylose moleculesare cleaved by hydrolysis with acid more easily easilythan amylopectin molecules (Tab. 1) [1]. The exocorro-sion, which appears all over the starch granules surfaceas detected by SEM, suggested that amylose was distrib-uted evenly over the entire surface. This would lead to the formation of evenly distributed pores on the surface ofthe acid-modified starch granules as shown in Fig. 2E.The hydrolysis of amylopectin might then occur after mostof the amylose molecules have been degraded. After all amylose and amylopectin molecules on the outer layer had been destroyed, the inner layer (or underlayer)of the granule again became smooth, as can be seen inFig. 2G.

From the SEM micrograph it was clear that the very shorttime of heating during spray drying of the wet modifiedstarch did not affect the surface of the granule. The mi-crographs of the acid-modified starch granules dried ei-ther by air or spray-drying had similar morphology. How-ever, spray drying did cause gelatinization of starch, al-though this was restricted to less than 1% (w/w). Thegelatinized starch held the starch grains together inspherical granules. The spherical shape of the granulescomprised of agglomerated starch grains provides goodflowability, which is essential in preparing tablets.

4 Conclusion

From scanning electron micrographs, acid hydrolysis at-tack on starch granules was evident in the form of super-ficial surface erosion, without the apparent formation ofsurface pores. However, our previous work had shownthat amylose content was reduced from 28.80% to 1.30%in native tapioca starch after 768 h of hydrolysis. This re-sult suggested that endocorrosion of amylose had oc-curred in the starch grains. Thus, further investigation onthe internal morphology of starch grains is needed for abetter understanding of endocorrosion by acid hydrolysis.

Acknowledgements

The authors would like to thank Thailand Research Fund(TRF) for financial support via The Royal Golden JubileePh.D. Program and Prof. T. W. Flegel for reviewing themanuscript.

Starch/Stärke 52 (2000) 283–289 Morphological Properties of Acid-modified Tapioca Starch 287

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Fig. 3. Scanning electron micrographs (magnification1500 ×) of spray-dried native (A) and acid-modified tapio-ca starches after various hydrolysis times: (B) 12 h; (C)24 h; (D) 48 h; (E) 96 h; (F) 192 h; (G) 384 h and (H) 768 h.

288 Atichokudomchai et al. Starch/Stärke 52 (2000) 283–289

Fig. 4. Scanning electron micrographs (magnification 7500 ×) of spray-dried native (A) and acid-modified tapioca starchesafter various hydrolysis times: (B) 12 h; (C) 24 h; (D) 48 h; (E) 96 h; (F) 192 h; (G) 384 h and (H) 768 h.

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[4] S. Vasudeva, S. Zakiuddin Ali, and S. Divakar: 13C CP/MASNMR Spectroscopy of Native and Acid Modified Starches.Starch/Stärke 45 (1993), 59–62.

[5] P. J. Jenkins and A. M. Donald: The Effect of Acid Hydrolysison Native Starch Granule Structure. Starch/Stärke 49(1997), 262–267.

[6] L. E. Fitt and E. M. Snyder: Photomicrographs of Starches.In: Starch Chemistry and Technology. Ed. R. L. Whistler, Academic Press, New York 1984, pp. 675–689.

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[8] C. A. Knutson: A Simple Colorimetric Procedure for Determi-nation of Amylose in Maize Starch. Cereal Chem. 63 (1986),89–92.

(Received: July 1, 2000)

Starch/Stärke 52 (2000) 283–289 Morphological Properties of Acid-modified Tapioca Starch 289