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Journal of Food Engineering 88 (2008) 186–192

A study on freeze–thaw characteristics and microstructureof Chinese water chestnut starch gels

Wang Lan, Yin Zhihua, Wu Jia, Sun Zhida, Xie Bijun *

College of Food Science and Technology, Huazhong Agriculture University, Wuhan, Hubei 430070, People’s Republic of China

Received 22 October 2007; received in revised form 16 December 2007; accepted 3 February 2008Available online 9 February 2008

Abstract

The influence of the repeatedly freeze-thawed (FT) treatment on the microstructure, crystallinity, thermal properties, textural prop-erties and resistant starch content of Chinese water chestnut starch (CWCS) gels were investigated, using scanning electron microscopy(SEM), X-ray diffractometry, differential scanning calorimetry (DSC) and textural analysis (TA). The microstructure of the native starchgel was a compact and random phase. Freeze-thawed starch gels formed a honeycomb-like network structure, that was almost broken atthe 7th FT cycle. The freeze-thawed starch gels showed a typical B-type crystal structure in the X-ray diffractogram, significantly differentfrom native starch (A-type crystal structure). The crystallinity was increased by repeating the FT cycles. The thermodynamic parametervalues (To, Tp, Tc and DH) of the freeze-thawed starch gels were also increased with the number of freeze–thaw cycles, and theretrogradation ratio reached 30% at the 7th FT cycle. Textural properties (hardness and springiness) of freeze-thawed starch gels weresignificantly influenced by repeating the FT cycles. The change of hardness corresponded with micro-structural transformation. Byrepeated freeze-thawing, the resistance to digestive enzyme of the starch gels increased, which was due to the rapid retrogradation ofleached amylose.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Freeze-thawing cycle; Chinese water chestnut starch gels; Morphology; Thermal properties; Textural properties; Resistant starch content

1. Introduction

As a thickening and gelling agent, starch is an importanttexture modifier in foods. When starch-containing foodssuch as sauces, soups, ice-creams and desserts, are sub-jected to repeated freeze-thawed (FT) cycles, their texturesand other physicochemical properties may be extensivelychanged. Gelatinized starch gel is a continuous homoge-neous system. During freezing, the starch gel systembecomes heterogeneous and separates into starch-rich andstarch-deficient ice phases. Low temperature or repeatedFT cycle treatment of concentrated pastes gives rise tocryotropic gel formation, the final products of which arein the form of sponge-like textures (Lozinsky et al.,

0260-8774/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2008.02.005

* Corresponding author. Tel.: +86 27 87283201; fax: +86 27 87282966.E-mail address: bijunxie@sina.com.cn (B. Xie).

2000). Repeating the FT cycle enforces the phase separa-tion and ice growth (Eliasson and Kim, 1992), becausethe starch gel is syneresised and the water is separated fromthe gel.

The micro-structural change and physicochemical prop-erties of freeze-thawed starch gel had been studied by manyresearchers. The micro-structural change during therepeated FT cycle consists of morphological and crystalstructure alteration. The particular spongy structure offreeze-thawed starch gel was observed by scanning electronmicroscopy, showing membranes that holds the ice phaseand the ice cells (Jeong and Lim, 2003; Sae-kang andSuphantharika, 2006). The starch gel was recrystallizedby FT treatment although the crystallinity was lower thannative starch. As the ice crystals become larger, the syner-esis and sponge formation occur more readily. This partic-ular sponge structure was formed by retrogradation ofamylose and amylopectin. The crystallinity of high amylose

L. Wang et al. / Journal of Food Engineering 88 (2008) 186–192 187

maize starch gels was decreased by repeating the FT cycles,since repeating the FT cycles disrupt the crystal structure(Jeong and Lim, 2003). Numerous publications havereported physical changes of starch gels by freezing orrepeated FT treatment. The changes included syneresis,thermal properties, textural properties and digestion andso on. The difference in thermal properties depends onbotanical source and the granular composition of starch.White et al. (1989) measured the retrogradation enthalpyof starch gels subjected to 10 FT cycles and found thatwaxy maize and regular maize starch recovered 58% and59% of the initial gelatinization, respectively. A freeze-thawed waxy maize starch gel (20% solids) displayed anendotherm for amylopectin crystals which was similar tothat found for an isothermal retrogradation (Jacobsonand BeMiller, 1998). The melting temperatures of freeze-thawed (five cycles) tapioca starch were 43.1–64.9 �C andthe enthalpy was 1.2 J/g, about 8% of initial gelatinization(Sae-kang and Suphantharika, 2006). Regarding texturalproperties, the starch gels with greater hardness were pro-duced from tuber starches and a significant increase ingel hardness was observed from the 1st or 2nd FT cycle(Takeiti et al., 2007). Lo and Ramsden (2000) found thatthe hardness of rice starch gels on thawing after storageat �20 �C for 7 days higher than when stored at 15 �C.Starch resistant to digestive enzymes was enhanced byretrogradation, and the resistant starch type 3 (RS3) iscommercially prepared by retrogradation. Sievert andPomeranz (1989) raised the RS level to over 40% by per-forming 20 autoclaving and cooling cycles with amyloma-ize starch. Jeong and Lim (2003) reported the resistanceof high amylose maize starch to digestive enzymes wasslightly raised by FT treatment.

In the process of repeated FT cycles, many commercialstarches have been studied in previous reports (Eliassonand Kim, 1992; Jeong and Lim, 2003; Sae-kang andSuphantharika, 2006), but Chinese water chestnut starchhas not been investigated. Furthermore, there are few stud-ies on the relationship between structure and physicochem-ical properties in the process of repeated FT cycles, andhow textural properties are changed. Chinese water chest-nut (CWC) is eaten fresh; and can also be canned orground to make flour. CWC cake with high elasticity is atraditional dessert in the south of China. But CWC cakewas easy to delaminate and break in the storage at low tem-perature. In the present study, the microstructure of CWCstarch gels were observed, and the physical properties char-acterized during repeated FT cycles to know if CWC can bea useful source of starch for further application in foodproduction.

2. Materials and methods

2.1. Materials

Chinese water chestnut was bought at a local market inGuangzhou province. Fresh Chinese water chestnuts were

rinsed, peeled and wet grinded. After the tubers were grin-ded, starch was separated from pulp using a filtrating cloth.Starch was washed to remove any traces of adhering pulpand then dried at 40 �C in a hot cabinet drier for 48 h.Nitrogen, fat, crude fiber, ash and moisture content weredeterminations with AOAC official procedures (methods954.01, 920.39, 962.09, 923.03 and 925.09) (AOAC,1997). The protein content was assumed to be related tonitrogen content by a factor of 6.25. Amylose content ofthe isolated starch was determined according to the color-imetric procedure (Juliano, 1985). Ash, lipid, fiber, proteinand amylose content of CWCS were 0.4%, 0.9%, 0.3%,0.5% and 32.1%, respectively.

2.2. Freeze-thawing cycles

The starch (10%, DW) in distilled water was cooked for30 min in a boiling water bath with moderate mechanicalagitation and cooled in molds (2 cm height � 4 cm diame-ter). The molds were placed in a freezer at �20 �C for22 h and placed in a 30 �C water bath for 1.5 h to thawand equilibrate. The properties of samples were measuredafter repeating 1, 2, 4 and 7 FT cycles.

2.3. Scanning electron microscopy

The starch gels were fixed and dried following themethod of Anderson (1951). Scanning electron micro-graphs of gels were obtained with scanning electron micro-scope (HITACHIX-650, Japan). The samples were coatedwith gold–palladium (60:40). An acceleration potential of20 kV was used during scanning.

2.4. X-ray diffraction analysis

X-ray diffraction analysis was performed with a D/max-RA X-ray diffractometer (Rigaku Corporation, Tokyo,Japan). Diffractograms of the freeze-dried samples wereobtained from 3� to 50� (2h) at a speed of 8�/min.

2.5. Thermal properties

Thermal characteristics of freeze-dried samples werestudied using a differential scanning calorimeter (DSC204F, Netzsch, Germany) with a thermal analysis datastation. About 15 mg of starch solution (1:2; starch:water) was hermetically sealed in an aluminum pan.The pan was equilibrated for 1 h at room temperaturebefore heating in the DSC. The samples were heatedfrom 25 �C to 110 �C at a rate of 5 �C/min. Onset (To),peak (Tp) and conclusion (Tc) temperatures together withgelatinization enthalpy (DH) were evaluated automati-cally and percentage of retrogradation (R%) was calcu-lated as

R% ¼ Enthalpy of retrogradation

Enthalpy of gelatinization

� �� 100

188 L. Wang et al. / Journal of Food Engineering 88 (2008) 186–192

2.6. Textural analysis

Samples for stress analysis were in the shape of a cylinder(with the diameter of 4 cm and height of 2 cm) and measure-ments were performed in triplicate. The texture of starchgels was determined in a TA.XT2 Stable Micro Systems tex-ture analyzer (Texture Technologies Corp., Scarsdale, NY,USA) with a P/36R cylinder probe, 40% strain, 1.0 mm/sspeed, 1.0 mm/s pre-test speed, 10.0 mm/s post-test speedand 5 g test sensitivity.

2.7. Resistant starch content

All the gels were homogenized at 11,000 rpm for 30 s.The resistant starch (RS) content was determined enzymat-ically using the method of Goni et al. (1996).

2.8. Statistical analysis

The data obtained for textural properties were subjectedto a one way analysis of variance (ANOVA) followed byDuncan’s multiple range test using the SAS system for win-

Fig. 1. Effect of repeated freeze–thaw treatment on the structure of CWCS ge

dows, version 9.00 (SAS Institute Inc., Cary, NC, USA)and the significance level was P < 0.05.

3. Results and discussion

3.1. Scanning electron microcopy

The starch gels, freshly cooked and treated with differentFT cycles, were observed by scanning electron microscope.Freshly cooked starch gel was compact and random(Fig. 1a), showed a homogenous phase. Fig. 1b–e showsthe freeze-thawed starch gel matrix consists of ice cellsand concentrated starch phase. The ice cells in the gelmatrix were significantly changed in size when differentFT cycles applied. The starch gel treated with one FT cycleshowed a honeycomb structure and the size of ice cellsranged from 10 to 45 lm (Fig. 1b), indicated that starchmolecules rearranged in order. In the following two andfour repeatedly FT cycles, the membranes surroundingthe ice cells became thicker and the ice cells became largerand fewer (Fig. 1c and d). The concentration of starch pro-ceeded and enforced the separation of starch phase and ice

ls: (a) control; (b) 1st cycle; (c) 2nd cycle; (d) 4th cycle; and (e) 7th cycle.

L. Wang et al. / Journal of Food Engineering 88 (2008) 186–192 189

phase. In the end, the network structure of starch gel wasbroken after seven repeatedly FT cycles (Fig. 1e).

The morphology of other starch gels such as potato,maize and tapioca, showed thick, large and loose spongystructures (Ferrero et al., 1993; Jeong and Lim, 2003; Leeet al., 2002). Compared with previous reports, the orderednetwork structure of CWCS gel in present study was firstobserved. The high degree of order in starch gels suggestedthat the CWCS molecular rearrangements were regular,possibly due to samples from different sources as well asthe amylose content, the chain length and the degree ofbranching. And the repeatedly FT treatment of the CWCstarch gels finally resulted in the collapse of the networkstructure.

3.2. X-ray diffraction pattern

X-ray diffraction patterns of native starch and freeze-dried starch at different periods of FT treatment are givenin Fig. 2. The profile for native starch had the distinctivefeatures of A-type starch, with a medium intensity of the15.26�, strong intensities for the 17.42�, 18.06� and 23.18�peaks. The freeze-thawed starch showed diffraction pat-terns with peaks at 5.6�, 16.72�, 21.78� and 23.76� (2h), adiffraction pattern typical for B-type crystals. Obviouslythe re-crystallization of CWCS began from the 2nd FTcycle and crystallinity peaks were very clear at the 7thFT cycle. Before 4th FT cycle, the relative crystallinity ofstarch gel was increased quickly. From 4th FT cycle to7th FT cycle, the relative crystallinity was increased slowly.The results of relative crystallinity indicated different re-crystallization speed of starch components. The X-ray dif-fraction pattern of starch gels at the 7th FT cycle was sim-ilar to maize starch with 40% amylose observed by NormanCheetham and Tao (1998). Jeong and Lim (2003) foundthat amylomaize starch treated by repeated FT cyclesshowed combined B- and V-type crystalline patterns.

0 10 20 30

Diffraction angle (2

16.72

14.28 19.7621.70

15.32

17.52 23.28

20.04

5.6

23.76

Fig. 2. X-ray diffractions patterns of native CWCS and freeze

As the starch gel was subjected to seven FT cycles, theoverall peak intensity displayed an increase in contrast tothe 1st, 2nd and 4th FT cycle. The crystallization was accel-erated by the repeated FT treatment, although the crystal-linity of freeze-thawed starch gels was lower than that ofnative starch. The variation from A- to B-type diffractionpattern indicated that the stress caused the crystal structureformation during the repeated FT processing because ofcryotropic gel formation. Typical A-type pattern starchmolecular chains were thought to have shorter chains,higher branching density and shorter distance betweenbranching points than B-type starches. Therefore, theincrease in relative crystallinity among the freeze-thawedstarch gels could be attributed to the interplay of the fol-lowing factors: (1) crystal size (since distances of lamellasincreased), (2) amount of crystalline regions (influencedby amylopectin content and amylopectin chain length),and (3) orientation of double helices (since the enthalpyincreased as shown in Table 1).

3.3. Thermal properties

The results of DSC analysis of starches during differentFT cycles are summarized in Table 1. Native starch showedthe highest gelatinization temperature and enthalpy (DHgel)with a value of 10.77 J/g. At 7th FT cycle, retrogradedstarch enthalpy cycle was about 70% smaller than gelatini-zation enthalpy and transition temperatures (To, Tp andTc) were about 15 �C lower than those for gelatinizationof native starch granules. During the repeated FT cycles,the transition temperatures (To, Tp and Tc) and enthalpiesof gelatinization of the starches increased with the numberof FT cycles, in accordance with the increase of crystallin-ity. A slow increase in enthalpies was observed between the1st and the 2nd FT cycle and their R% were 4.18% and10.03%, respectively, but the R% was 27.58% at the 4thFT cycle and then a slight increase occurred between the

40 50

)

7th FT cycle

4th FT cycle

2nd FT cycle

1st FT cycle

Native starch

-dried CWCS gels treated by repeated freeze–thaw cycles.

Table 1Thermal properties of native CWCS and CWCS samples treated byrepeated freeze–thaw cycles

Sample To

(�C)Tp

(�C)Tc

(�C)DHgel

(J/g)DHreg

(J/g)R%

Native starch 58.54 64.13 68.43 10.77 – –1st Cycle 41.37 45.85 52.85 10.77 0.45 4.182nd Cycle 42.46 49.23 53.30 10.77 1.08 10.034th Cycle 44.03 51.61 58.72 10.77 2.97 27.587th Cycle 44.50 51.76 58.11 10.77 3.3 30.64

CWCS = Chinese water chestnut starch; To = onset temperature,Tp = peak temperature, Tc = conclusion temperature; R = gelatinizationrange (Tc � To); DHgel = enthalpy of gelatinization (DW, based on starchweight), DHret = enthalpy of retrogradation, R% = percentage of retro-gradation (ratio of enthalpy of retrogradation to enthalpy ofgelatinization).

190 L. Wang et al. / Journal of Food Engineering 88 (2008) 186–192

4th and the 7th cycle (about 3%). Since the gelatinizationenthalpy reflected the disassociation of double helices(Cooke and Gidley, 1992), a higher enthalpy in freeze-thawed starch gels indicated a higher level of double helixcontent or more ordered double helices after four repeatedFT cycles. The amylose content has been reported to beone of the influential factors for starch retrogradation,but amylopectins and intermediate materials also play animportant role in starch retrogradation (Yamin et al.,1999). The amylose content of CWCS was 30.1%, but theR% at 1st and 2nd FT cycle was less than 10.03%, indicat-ing that the amylose retrogradation was low before 2nd FTcycle. The amylose retrogradation accounted for R% in ashort time, but amylopectin retrogradation predominatedin a long term. The retrogradation could be acceleratedby amylopectin with longer branching chains (Kalichevskyet al., 1990; Yuan et al., 1993). The rapid increase of R%might be ascribed to the formation of ordered double heli-ces and the acceleration of amylopectin retrogradation.

3.4. Textural properties

The textural properties of freeze-thawed CWCS gel sub-mitted to the 10 FT cycles are shown in Table 2. Hardnesswas defined as the force required to compress the productby a pre-set strain of 40%. A significant increase in gelhardness was observed at the 1st cycle compared with con-trol and a gradual decline of hardness appeared from the2nd to 7th FT cycle. There were two drastic declines instarch gel hardness during the experiment: one was

Table 2Textural properties of CWCS gels treated by repeated freeze–thaw cycles

Textual parameters FT cycles

Control 1st Cycle

Hardness (g) 326.3 ± 24.3a 6401.3 ± 263.0b

Springiness (%) 54.1 ± 4.1a 36.2 ± 1.8b

CWCS = Chinese water chestnut starch.Means and standard deviations of three replicates.a–d Mean values with different letters in the same column are significantly diff

between the 1st and 2nd FT cycle and another was betweenthe 4th and 7th FT cycle. The starch gels treated byrepeated FT cycles had higher hardness; the hardness ofstarch gel at the 7th FT cycle was 6.8 times higher than thatof the freshly cooked starch gel. Since initial developmentin firmness was due to amylose gelation, subsequent net-work rigidity increase was due to starch re-crystallization.The gel firmness was mainly caused by retrogradation ofstarch gels, which associated with the syneresis of watergels and crystallization of amylopectin, leading to hardergels (Miles et al., 1985). The hypotheses to explain thechanges in hardness could be related to syneresis, the asso-ciation of starch molecules and the network structure of thegels formed, influenced by the FT cycles, and were appar-ently not related only to retrogradation. The increase ingel hardness could be due to the formation of a networkstructure whereas the first decline could be ascribed to largeice cells of starch gels; or to the cooperation of rearrangedstarch molecules and the collapsed starch gels structure. Asimple way of measuring the springiness was to record theforce after 60 s divide this by the maximum force and thenmultiply by 100%. Higher springiness appeared when thegel structure was compact and random, whereas lowerspringiness resulted from the honeycomb-like networkstructure of gels. The change of gel springiness might beascribed to increased aggregation of starch molecules anddecreased plasticization of water. Repeated FT treatmentdid not significantly affect (P > 0.05) the springiness ofstarch gel except for the 4th FT cycle.

3.5. Resistant starch content

Resistant starch has been defined as the fraction ofstarch not absorbed in small intestine, but digested inthe large intestine (Englyst et al., 1992). RS3 representedthe most resistant starch fraction and is mainly retrogra-ded amylose formed during cooling of gelatinized starch(Sajilata et al., 2006). The RS content in this experiment,measured by amylolytic treatment with porcine pancreatica-amylase increased significantly (Fig. 3). RS content of thecontrol was 6.0%, while RS contents at the 1st, 2nd, 4thand 7th cycle were 6.7%, 7.6%, 9.2% and 10.0%, respec-tively. The curve of RS content growth was not linearand there was an inflexion at the 4th FT cycle as shownin Fig. 3. These results might be related to the differentretrogradation characteristics of two starch components

2nd Cycle 4th Cycle 7th Cycle

4443.7 ± 281.2c 4240.4 ± 381.7c 2219.9 ± 256.2d

34.7 ± 3.3b 25.0 ± 1.3c 37.0 ± 3.1b

erent (P < 0.05).

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8

Number of freeze-thaw cycles

Res

ista

nt s

tarc

h co

nten

t/%

Fig. 3. Resistant starch contents of CWCS gels treated by repeated freeze–thaw cycles.

L. Wang et al. / Journal of Food Engineering 88 (2008) 186–192 191

according to X-ray diffraction and DSC data. The RScontent was positively correlated with relative crystallinity.The re-crystallized starch molecules were resistant to diges-tive enzyme. Relating RS content and retrogradationenthalpy values, positive relationship was found for thefreeze-thawed starches tested. The R% and relative crystal-linity increased quickly before 4th FT cycle, and thenincreased slowly, suggested different retrogradation speeds.Amylose retrogrades quickly and was responsible for RScontent changes occurring in starch gels before the 4thFT cycle, whereas the aggregation of amylopectin mighthave an influence on the RS content over longer periods.

4. Conclusions

Repeated FT treatment of CWCS leaded to two mainobservations. First, a honeycomb-like gel network struc-ture of starch gels was observed and the structure wasdestroyed at 7th FT cycle. The results showed that thestarch gel structure of CWC was not stable during the stor-age at lower temperature. Second, there was a turningpoint of the physicochemical properties and microstructurechanges at 4th FT cycle, including the appearance of X-raydiffraction peaks, high retrogradation enthalpy and lowhardness. The gelatinization of native starch involved ran-dom entanglements of starch molecules and the interactionof starch with water. But during the repeated FT cycles,amylose and amylopectin were rearranged in an orderedmanner. The results of X-ray diffraction analysis, thermalanalysis and textural analysis indicated that amylose rear-rangement mainly induced the changes of physicochemicalproperties and microstructure for starch gels before 4th FTcycle, while amylose rearrangement was interfered by amy-lopectin re-association after 4th FT cycle. In addition, thepresent study showed that the transformation of starchgel structure was in accordance with the change of texturalanalysis. The freeze-thawed starch showed a typical B-typecrystal structure, as compared to native starch (A-typecrystal structure). The crystallinity, retrograded enthalpy

and RS content of freeze-thawed starch gel increased withthe number of FT cycles. Although the harder texturelimits the application for fast food, freeze-thawed CWCScould be used in food products where a small sponge-likestructure is required, or could probably be used at lowerconcentration. The transformation of the X-ray diffractionpattern suggested that freeze-thawed CWCS can be utilizedin food manufactures without chemical modification. Fur-ther work is needed to understand the physicochemicalproperties to develop insights to potential application offreeze-thawed CWCS.

Acknowledgements

This work was supported by the National Great Projectof Scientific and Technical Supporting Programs Fundedby Ministry of Science & Technology of China during the11th five-year Plan (No. 2006BAD27B09).

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