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Journal of Food Engineering 87 (2008) 436–444

Effect of high-pressure homogenization on the structureand thermal properties of maize starch

Bao Wang a, Dong Li a,*, Li-jun Wang b, Yu Lung Chiu c, Xiao Dong Chen a,*, Zhi-huai Mao a

a College of Engineering, China Agricultural University, P.O. Box 50, 17 Qinghua Donglu, Beijing 100083, Chinab College of Food Science and Nutritional Engineering, China Agricultural University, 17 Qinghua Donglu, Beijing 100083, Chinac Department of Chemical and Materials Engineering, University of Auckland, Private Bag 92019, Auckland City, New Zealand

Received 9 September 2007; received in revised form 18 December 2007; accepted 22 December 2007Available online 8 January 2008

Abstract

Maize starch–water suspensions (1.0%) were subjected to single-pass high-pressure homogenization treatment at 60 MPa, 100 MPa,and 140 MPa. The structure and thermal properties of the high-pressure homogenized starches were investigated using DSC, X-ray dif-fraction technique, laser scattering, and microscope, with native maize starch (suspended in water, but not homogenized) as a controlsample. DSC analysis showed a decrease in gelatinization temperatures (To,Tp) and gelatinization enthalpy (DHgel) with increasinghomogenizing pressure. No noticeable effect of high-pressure homogenization on the retrogradation of maize starch was observed. Laserscattering measurements of particle size demonstrated an increase in the granule size at a homogenizing pressure of 140 MPa. This wasattributed to the gelatinization and aggregation of the starch granules. X-ray diffraction patterns showed that there was an evident loss ofcrystallinity after homogenization at 140 MPa. Microscopy studies showed that the maize starch was partly gelatinized after high-pres-sure homogenization, and the gelatinized granules were prone to aggregate with each other, resulting in an increase of granule size.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Maize starch; High pressure; Homogenization; Structure; DSC

1. Introduction

Starch is one of the most abundant biotic resources innature. As reproducible biomass, starch has been widelyused in paper, textile, adhesive, sweetener, and food indus-tries (Che et al., 2007a). Among all kinds of starches, maizestarch is a valuable ingredient in the production of food,and has been widely used as thickener, colloidal stabilizer,gelling agent, bulking agent, water retention agent, andadhesive in industry (Singh et al., 2003).

High-pressure technology has been used as a novelmethod to obtain special denatured starches. Many papershave reported that high hydrostatic pressure (HHP) could

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

doi:10.1016/j.jfoodeng.2007.12.027

* Corresponding authors. Tel./fax: +86 10 62737351.E-mail addresses: dongli@cau.edu.cn (D. Li), dong.chen@eng.monash.

edu.au (X.D. Chen).

result in the gelatinization of starch granules in starch–water suspension at room temperature (Błaszczak et al.,2007, 2005a,b; Buckow et al., 2007; Douzals et al., 1996,1998; Kawai et al., 2007; Muhr and Blanshard, 1982;Rubens and Heremans, 2000; Rubens et al., 1999; Stoltet al., 2001; Stute et al., 1996). The extent of gelatinizationdepended on the pressure applied, moisture content of thestarch–water suspension, treatment time, temperature,starch concentration, and the type of starch (Bauer andKnorr, 2004; Stute et al., 1996). Kawai et al. (2007) demon-strated that the values of DHgel and DHret (enthalpies ofgelatinization and retrogradation of starch, respectively)of potato starch–water (10–70% w/w) were affected signif-icantly by the treatment pressure and starch content, andlittle effect of treatment time. They found that the DHgel

values of 10–50% (w/w) mixtures decreased with increasingtreatment pressure and decreasing starch content. The30–60% (w/w) mixtures retrograded, and the DHret values

B. Wang et al. / Journal of Food Engineering 87 (2008) 436–444 437

increased with decreasing DHgel values and increasingstarch content.

Although HHP treatment causes starch gelatinization,the mechanism of pressure-induced gelatinization is signif-icantly different from that of heat-induced gelatinization(Rubens and Heremans, 2000; Stute et al., 1996). Forexample, during pressure-induced gelatinization, starchcan retain granular structures, with little amylose leachingout from the granules (Douzals et al., 1998; Stolt et al.,2001; Stute et al., 1996). Also, the starch granules gelati-nized by high pressure contain two different zones, theouter zone of which remained unchanged while the innerzone was completely destroyed and formed gel-like struc-tures (Błaszczak et al., 2005b).

Using X-ray diffraction studies, Katopo et al. (2002)showed that when the starch and water mixtures in 1/1and 2/1 (water/starch, v/w) was pressurized under theultra-high pressure of 690 MPa, the pressure convertedA-type starches (normal maize starch and waxy maizestarch, rice starch) to B-type-like pattern. However, theB-type pattern starches (potato starch and high amylosemaize starch) were not changed by pressure treatmentin a water suspension. It is also reported that B-typestarches were more resistant to pressure gelatinizationthan A-type starches (Ezaki and Hayashi, 1992; Katopoet al., 2002; Muhr and Blanshard, 1982; Rubens et al.,1999), due to the different crystalline structure formedby amylopectin. The B-type crystallite has more com-bined water molecules which fill up the channel in the cellunit of the crystallite and play a role as a stabilizer, whilethe amylopectin of the A-type starch has more scatteredbranching structure (Jane et al., 1997). Hence, the B-typestarches are more stable, and the A-type starches withmore flexible scattered branching structures are moreactive and tend to be rearranged or destroyed by waterunder high pressure.

Furthermore, the amylose content of starch affects thesusceptibility of high-pressure treatment. Starches withhigh amylose contents often resist high-pressure treatmentbetter than low amylose content starches. Błaszczak et al.(2007) indicated that HHP treatment could cause the lossof crystalline structure of starches. Waxy maize starchcompletely lost crystalline structure and formed a gel-likestructure after treated with high pressure at 650 MPa for9 min, while high amylose maize starch under the sametreatment still retained its granule structure with adecreased crystallinity. This is because amylose can formcomplexes with lipids present in maize starch, stabilizingthe structure of starch granules to restrict the swelling ofgranules. Chen et al. (2007) reported that the diametergrowth rate and final accretion ratio sequence of differentmaize starches during heating under shearless conditionswere negatively correlated with amylose/amylopectin ratioand the higher the melting temperature of the amylose–lipid complex, the higher the gelatinization temperature.Such results indicated the stabilization effect of amylosein starch.

To date, most research has focused on the HHP treat-ment of starch, while the knowledge on the dynamichigh-pressure treatment of starch is needed. Long-timeHHP treatment is economically undesirable in starchindustry and has not come into real industrial production,while high-pressure homogenization treatment that pro-duces dynamic pressure has already been used in chemical,pharmaceutical, specialty food and biotechnology indus-tries (Pandolf and Kinney, 1998). In the industry, homog-enization pressures normally used are between 20 and50 MPa. When the homogenizing pressure is over100 MPa, it is usually called ultra-high-pressure homogeni-zation (Tribst et al., 2007). During high-pressure homoge-nization, liquids experience high pressure, high shear,turbulence, and cavitation caused by rapid change in pres-sure (Hayes and Kelly, 2003). Although high-pressurehomogenizers are now rarely utilized in native starchprocessing, it is of fundamental importance to obtain adetailed understanding of its effects on the structure andthermal properties of starches. Che et al. (2007b) haveinvestigated the effect of high-pressure homogenizationon the structure of cassava starch. However, knowledgeof the effect of high-pressure homogenization of starchesis still very limited.

Among all kinds of starches, maize starch is the mostused in the industries. Also, it is necessary to extend ourresearch from a tuber starch (cassava) to a grain starch(maize). In this study, the maize starch–water suspensionwas homogenized at different pressures using a high-pres-sure homogenizer. The effect of high-pressure homogeniza-tion on both the structure and thermal characteristics ofnormal maize starch was investigated.

2. Materials and methods

2.1. Materials

Commercial maize starch was purchased from BeijingQuanfeng Starch Company, China. The moisture contentof the starch was determined by drying measurement inan air-oven at 105 �C for 24 h, and the average value wasdetermined to be 11.99% (w/w). An analytical grade anhy-drous alcohol (95%, w/w) was purchased from BeijingLanyi Chemical Company.

2.2. High-pressure homogenization of maize starchsuspension

Normal maize starch suspension (1.0%, w/w) wasprepared by adding maize starch in de-ionized water at17 �C. The well mixed suspension was homogenized in ahigh-pressure homogenizer (NS1001L-PANDA 2K, NiroSoavi S.p.A., Italy) for one pass at 60, 100, and140 MPa, respectively. Five hundred milliliters of suspen-sion was processed at each pressure level. The PANDA2K homogenizer used in the present study is a two-stagehomogenizer with two high-pressure valves. The

438 B. Wang et al. / Journal of Food Engineering 87 (2008) 436–444

homogenizing pressure range of the PANDA 2K homoge-nizer is from 0 MPa to 150 MPa. In industrial practice, thepressure of the second stage high pressure should beadjusted to about 1/10 of that of the first high-pressurestage in order to achieve better homogenization. In thisstudy, only the first stage high pressure was used.

The samples treated with high-pressure homogenizationwere vacuum filtered and dehydrated with anhydrous alco-hol, and then were dried in an oven at 40 �C for 24 h inorder to obtain dry starch samples (Katopo et al., 2002).Dried starch samples were carefully pulverized with amortar and pestle, and then stored in a desiccator for laterstudies (Che et al., 2007b). The native maize starch wassoaked in de-ionized water for 10 min and then treatedusing the method described above.

2.3. Thermal properties of gelatinization and retrogradation

The gelatinization properties of the native maize starchand high-pressure homogenized maize starch in the pres-ence of excess water was conducted using a differentialscanning calorimeter (DSC-Q10, TA Instruments, NewCastle, USA) equipped with a thermal analysis data sta-tion. The DSC analyzer was calibrated using indium andan empty aluminum pan was used as a reference. Starch(2.5 mg) was directly measured into the aluminum DSCpan and distilled water (7.5 lL) was added with a microsy-ringe. Pans were then immediately hermetically sealed andequilibrated for 3 h at room temperature before heating inthe DSC. The samples were then heated from 20 �C to120 �C at 10 �C/min. The onset, peak, and conclusion tem-peratures (To, Tp, and Tc) together with gelatinizationenthalpy (DHgel) were quantified. After conducting thermalanalysis, the gelatinized samples were stored at 4 �C for 7days and then rescanned to determine the temperatureand enthalpy changes due to retrogradation. For retrogra-dation studies, the temperature range and heating rate wereset as 20–120 �C and 10 �C/min, respectively.

The values of gelatinization degree (GD) was calculatedusing the following equation (Błaszczak et al., 2007):

GD ¼ fðDH ns � DH tsÞDH�1ns g � 100%

where DHns and DHts are the gelatinization enthalpies ofnative and homogenization treated starches, respectively.

The values of retrogradation degree (RD) was calcu-lated as (Sandhu and Singh, 2007)

RD ¼ DH ret=DH gel � 100%

where DHgel and DHret are the gelatinization enthalpies ofnative and retrograded starches, respectively.

The peak height index (PHI) was calculated as (Kruegeret al., 1987)

PHI ¼ DH=ðT p � T oÞThe temperature range for gelatinization (R) was calcu-

lated as (Sandhu and Singh, 2007)

R ¼ 2� ðT p � T oÞ

2.4. Laser scattering measurement

The particle size distributions of the high-pressurehomogenized samples and the native sample were deter-mined using the laser scattering method. The equipmentused was a Mastersizer 2000 laser diffractometer (MalvernInstruments, UK) equipped with a He–Ne laser with wave-length of 632.8 nm. The dry starch samples were dispersedin anhydrous alcohol in the diffractometer cell before mea-surements. The refractive indices of anhydrous alcohol andthe starch used were 1.32 and 1.53, respectively. The absor-bance of starch granules was taken as 0.1 (Singh et al.,2006; Zhou et al., 2006).

The density of maize starch was determined using thefollowing method: maize starch was filled into a graduatedcylinder and then the cylinder was dropped from a constantheight. The jolting action consolidates the starch to therequired bulk density which was determined from theweight and volume of starch in the cylinder (Muramatsuet al., 2005). Every sample was replicated thrice.

The special surface area (SSA) was calculated as (Zhouet al., 2006):

SSA = 6/ d3,2, where d3,2 is the area mean diameter (Sau-ter diameter) of maize starch samples.

2.5. X-ray diffraction analysis of maize starch

An XD-2 X-ray diffractometer (Beijing Purkinje Gen-eral Instrument Co. Ltd., China) was used for the X-raydiffraction analysis of maize starch. The starch sampleswere first pulverized to pass 360 mesh using a carnelianmortar. X-ray powder diffraction analysis were then per-formed at 36 kV and 20 mA using nickel-filtered Cu Ka(wavelength 1.5405 A) radiation. The 2h scan was donefrom 5� to 40� with a scanning speed of 0.25�/min and sam-pling interval of 0.02�.

2.6. Microscopy study of maize starch granules

To investigate the effect of high-pressure homogeniza-tion on the structure of maize starch, both treated andnative samples were observed using an optical microscope(CX31 Biological Microscope, Olympus Corporation,Japan) equipped with a CCD camera module.

2.7. Experimental design and statistical analysis

Maize starch samples were homogenized under homog-enizing pressure of 60, 100, and 140 MPa, respectively.DSC was used to investigate the effect of homogenizationon the gelatinization and retrogradation properties ofmaize starch. Laser scattering measurement, X-ray diffrac-tion analysis and optical microscope were used to investi-gate the structure properties of maize starch afterhomogenization.

Results from DSC measurement were presented as meanvalues with standard deviations. The Student’s t test was

B. Wang et al. / Journal of Food Engineering 87 (2008) 436–444 439

used to estimate significant differences among means at aprobability level of 5% (p < 0.05). The experimental datawere statistically calculated by SAS system (release 8.2,SAS Institute Inc., Cary, NC, USA). Particle size distribu-tions were summarized by the characteristic volume-basedd-values that correspond to volume mean diameter (d4,3),area mean diameter (d3,2), and 10%, 50%, and 90% of thetotal particle population (d(0.1), d(0.5), and d(0.9)), respec-tively. Particle size distributions and X-ray diffraction anal-yses were both replicated twice for every sample.

3. Results and discussion

3.1. Gelatinization properties of high-pressure homogenizedmaize starch

DSC thermograms obtained for the gelatinization of allthe starch samples are shown in Fig. 1. The enthalpy ofgelatinization (DHgel), gelatinization temperatures(onset,To; peak, Tp; and conclusion, Tc), peak height index(PHI), gelatinization temperature range (R), and the degreeof gelatinization (GD) are shown in Table 1. The To, Tp,and DHgel of high-pressure homogenized maize starchdecreased significantly with increasing processing pressure,with the lowest values observed at 140 MPa. When thehomogenizing pressure was set at 60 MPa, the differenceofTo, Tp, and DHgel between native and 60 MPa treatedsamples was relatively small. When the homogenizing pres-

A

B

C

D

-3

-2

-1

0

Hea

t Flo

w (W

/g)

20 40 60 80 100Temperature (°C)Exo Up

Fig. 1. DSC thermograms of gelatinization properties for high-pressurehomogenized maize starch and the native maize starch: (A) native; (B)60 MPa; (C) 100 MPa; and (D) 140 MPa.

Table 1DSC measurements for gelatinization properties of high-pressure homogenize

Starch To (�C) Tp (�C) Tc (�C)

Native sample 65.6 ± 0.1a 70.4 ± 0.1a 75.2 ± 0.5a

60 MPa treated 64.3 ± 1.1b 70.4 ± 0.4ab 75.8 ± 0.1b

100 MPa treated 60.1 ± 0.8c 69.1 ± 0.8b 77.2 ± 0.3c

140 MPa treated 58.7 ± 0.2d 67.2 ± 0.7c 78.0 ± 0.4c

To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; DHge

for gelatinization; and GD, degree of gelatinization.Values represent the means ± standard deviation; n = 3. Values with the same

sure was set at 100 and 140 MPa, the differences betweennative and homogenization treated samples became muchgreater. The Tc of high-pressure homogenized maize starchincreased with higher homogenizing pressure. According toBłaszczak et al. (2005b), HHP treatment decreased the To,Tp, and DHgel of tomato starch. Buckow et al. (2007)pointed out that HHP higher than 300 MPa is necessaryto reduce significantly the onset temperature of gelatiniza-tion of maize starch, but the present work indicates that thehigh-pressure homogenization treatment could decreasethe To of maize starch from 65.6 ± 0.1 �C to 60.1 ±0.8 �C even at 100 MPa. It is likely that the high-pressurehomogenization at 100 MPa or higher homogenizing pres-sure destroyed the compact arrangements of molecules inthe crystalline regions of maize starch, thus during heatingin DSC, the water molecules in the aluminum pan couldreact with the molecules in the crystalline region more eas-ily, and therefore decreased the onset temperature (To) ofgelatinization.

The high-pressure homogenization also increased thegelatinization temperature range, and the value reached amaximum at the homogenizing pressure of 100 MPa. Also,the peak height index (PHI) decreased with increasinghomogenizing pressure. Compared with native maizestarch, the To of maize starch after homogenizationdecreased significantly with higher homogenizing pressure,while the Tp decreased relatively small, and thus the tem-perature range of gelatinization increased obviously. Also,the DHgel of homogenized maize starch decreased withhigher pressure. Thus, the thermogram curves became flat-ter at higher pressure, as shown in Fig. 1, resulting in thedecreased peak height index (PHI) with higher pressure.

The degree of gelatinization (GD) was very small at60 MPa, and reached 12.9 ± 3.2% at 100 MPa and26.8 ± 1.8% at 140 MPa, respectively. Che et al. (2007b)reported that high-pressure homogenization linearlyincreased the water temperature by 0.187 �C/MPa, andattributed the gelatinization of starch to the increase ofthe water temperature. But it should be noted that the timeof high-pressure homogenization treatment was very short(<30 s), and the temperature of starch–water suspensionafter treatment was not high (43.1 �C when homogeniza-tion pressure was at 140 MPa in this study). Hence temper-ature increase is unlikely to be solely responsible for thegelatinization of maize starch. As indicated previously,high-pressure treatment contributes to the gelatinization

d maize starch and the native maize starch

DHgel (J/g) PHI R (�C) GD (%)

12.5 ± 0.7a 2.6 ± 0.2a 9.6 ± 0.3a 012.4 ± 0.2a 1.8 ± 0.2b 12.1 ± 1.5b 0.8 ± 1.6a

10.9 ± 0.2b 1.2 ± 0.1c 17.9 ± 1.4c 12.9 ± 3.2b

9.2 ± 0.2c 1.0 ± 0.1c 17.1 ± 1.0c 26.8 ± 1.8c

l, enthalpy of gelatinization; PHI, peak height index; R, temperature range

superscript in a column do not differ significantly (p < 0.05).

440 B. Wang et al. / Journal of Food Engineering 87 (2008) 436–444

of starch, and the higher the pressure is, the more easily thegelatinization of starch will happen (Błaszczak et al., 2007;Buckow et al., 2007). Chen et al. (2007) indicated that theshear stress could enhance swelling and therefore promotethe gelatinization of starch granules during high-pressurehomogenization treatment. When the homogenizing pres-sure is higher, the shear stress produced during homogeni-zation will become larger, and both the increasedhomogenizing pressure and increased shear stress duringhomogenization can make the gelatinization of maizestarch much easier. Also, higher homogenizing pressureproduced higher water temperature, which can help tothe gelatinization of maize starch. As a result, the gelatini-zation of maize starch might be caused by the combinedeffect of the temperature rise and the dynamic high pressureduring the treatment.

3.2. Retrogradation properties of high-pressure homogenized

maize starch

Starch retrogradation is the process that occurs whenthe molecular chains in gelatinized starches begin to re-associate into an ordered structure as a result of hydrogenbonding between starch chains (Atwell et al., 1988; Hoo-ver, 2001). The DSC thermograms and thermal propertiesof retrograded starch samples are presented in Fig. 2 andTable 2, respectively. By comparing Table 2 to Table 1, it

A

B

C

D

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Hea

t Flo

w (W

/g)

30 40 50 60 70 80 90Temperature (°C)

Exo Up

Fig. 2. DSC thermograms of retrogradation properties for high-pressurehomogenized maize starch and the native maize starch: (A) native; (B)60 MPa; (C) 100 MPa; and (D) 140 MPa.

Table 2DSC measurements for retrogradation properties of high-pressure homogeniz

Starch To (�C) Tp (�C) Tc (�C)

Native sample 46.4 ± 0.9a 57.0 ± 1.4a 69.9 ± 1.0a

60 MPa treated 46.5 ± 0.5a 57.9 ± 0.1a 69.7 ± 0.5a

100 MPa treated 45.5 ± 0.5a 56.9 ± 0.8a 69.8 ± 2.3a

140 MPa treated 45.3 ± 1.5a 57.8 ± 0.8a 70.0 ± 1.4a

To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; Drange for gelatinization in retrograded starch; and RD, degree of retrogradatiValues represent the means ± standard deviation; n = 3. Values with the same

can be seen that the transition temperatures of retrogradedstarch were significantly lower than the corresponding gela-tinization temperatures; for example, To, Tp, and Tc

decreased from 65.6 ± 0.1 �C, 70.4 ± 0.1 �C, and 75.2 ±0.5 �C to 46.4 ± 0.9 �C, 57.0 ± 1.4 �C, and 69.9 ± 1.0 �C,respectively, for native maize starch before and after retro-gradation. This may be because the recrystallized amylo-pectin that formed during retrogradation was lessordered than the native form (Sandhu and Singh, 2007).The difference between peak and onset temperatures(Tp � To) for the retrograded starch was found to begreater than the corresponding values for high-pressurehomogenized starch during its first DSC scan, and the peakheight index (PHI) of retrograded maize starch was lessthan the corresponding values for high-pressure homoge-nized starch during its first DSC scan. Similar observationshave been reported (Karim et al., 2000; Sandhu and Singh,2007). From the data listed in Table 2, a conclusion couldbe drawn that there was no obvious effect of high-pressurehomogenization on the retrogradation property for maizestarch.

3.3. Particle size distributions of maize starch granules

The results of particle size measurements are listed inTables 3 and 4. The histogram of the size distributionsfor maize starch granules treated with different homogeniz-ing pressure is presented in Fig. 3.

As shown in Table 3, the volume mean diameter of thenative maize starch granules used in this study was13.5 lm, which was similar to the values obtained in otherstudies (Chen et al., 2007a; Gregorova et al., 2006). Fromthe histogram we know that homogenization pressure at60 MPa had no obvious effect on the particle size. Whileat 100 MPa, it could be seen in Fig. 4 that the d(0.9) hasa slight increase from 20.8 lm of the native sample to22.7 lm, indicating that the number of big granulesincreased, which could be proved by Table 4. Such a resultwas due to the partial gelatinization of starch granules,which aggregated with others, as suggested by the micro-graphs. There was a significant increase in granule size at140 MPa. The volume mean diameter reached 26.9 lm.The d(0.5) and d(0.9) increased from 13.1 lm and22.7 lm at 100 MPa to 22.7 lm and 52.6 lm at 140 MPa,respectively. While the d(0.1) changed very little, from7.7 lm to 8.7 lm. The results could demonstrate that when

ed maize starch and the native maize starch

DHret (J/g) PHI R (�C) RD (%)

5.4 ± 0.4a 0.5 ± 0.1a 22.1 ± 2.1a 43.0 ± 3.3a

5.5 ± 1.1a 0.5 ± 0.1a 22.3 ± 0.1a 43.8 ± 3.6a

5.5 ± 0.6a 0.5 ± 0.1a 23.0 ± 0.4a 44.2 ± 5.1a

5.4 ± 0.5a 0.4 ± 0.1a 25.1 ± 1.6a 43.3 ± 4.3a

Hret, enthalpy of retrogradation; PHI, peak height index; R, temperatureon.superscript in a column do not differ significantly (p < 0.05).

Table 3Diameters of maize starch granules treated under different homogenizing pressure (lm)

Sample name d4,3a d3,2

a d(0.1)b d(0.5)b d(0.9)b Density (kg/m3) SSA (m2/g)c

Native sample 13.5 11.7 7.6 12.6 20.8 649.3 0.5160 MPa treated 13.0 11.2 7.2 12.1 20.2 766.2 0.54100 MPa treated 14.3 12.1 7.7 13.2 22.7 685.8 0.50140 MPa treated 26.9 9.8 8.7 22.7 52.6 627.3 0.61

Values are the means of duplicate.a d4,3 is the volume mean diameter (De Brouckere diameter). d3,2 is the area mean diameter (Sauter diameter).b d(0.1), d(0.5), and d(0.9) are the particle sizes at which 10%, 50%, and 90% of all the particles by volume are smaller, respectively.c SSA is the specific surface area of all the particles.

Table 4Granule size distribution of maize starch

Sample name Granule size distribution (%)

0–4 lm 4–10 lm 10–15 lm 15–20 lm 20–30 lm 30–40 lm 40–50 lm 50–60 lm 60–80 lm 80–100 lm

Native sample 0 28.8 38.2 20.6 12.3 0.1 0 0 0 060 MPa treated 0 32.6 37.7 19.1 10.6 0 0 0 0 0100 MPa treated 0 26.5 35.8 20.7 15.9 1.1 0 0 0 0140 MPa treated 4.8 8.8 16.0 13.8 21.5 13.6 11.5 4.3 5.0 0.7

Values are the means of duplicate.

0

10

20

30

40

50

60

Diameter (μm)

d(0.1) d(0.5) d(0.9)Native starch

60MPa treated100MPa treated

140MPa treated

Fig. 3. Particle size distributions histograms for maize starch granulestreated under different homogenizing pressure.

B. Wang et al. / Journal of Food Engineering 87 (2008) 436–444 441

homogenized at 140 MPa, a lot of granules of maize starchwere wholly or partially gelatinized and aggregated witheach other, as a result increased the d(0.5) and d(0.9). Also,homogenized at 140 MPa formed many small granules withdiameters less than 4 lm, which represented 4.8% of allparticles, as shown in Table 4. Thus, the d(0.1) at140 MPa increased very limited comparing with the corre-sponding value of 100 MPa.

During the homogenization treatment, starch granuleswere simultaneously subjected to high-temperature, high-pressure, and high shearing-stress in the presence of watermolecules. As a result of these conditions, the starch gran-ules swelled rapidly into gel-like material. To the nativestarch granules, the compact arrangements of moleculesin the crystalline regions inhibit water or chemical reagentsfrom making contact with the molecules in the crystallineregion, and thus the chemical reactivity of starch isdecreased. As a result, the surface of the gelatinized gran-

ules would possess higher chemical reactivity, and thestrength of van der Waal’s force and electrostatic forcebetween the gelatinized granules would be large enoughto penetrate the boundaries of each other (Huang et al.,2007). Thus, the gelatinized granules aggregate with eachother. Some small granules could not resist the van derWaal’s force and electrostatic force of the big congrega-tions, and were also absorbed into the aggregates, as shownin Fig. 3. As a result, the gel-like granules and small grainsall together formed congregations which increased thegranule size.

The specific surface area (SSA) was calculated from thearea mean diameter (d3,2) of maize starch. As can be seen inTable 4, the size distribution broadened greatly after thehomogenization treatment at 140 MPa. Particles less than4 lm represented 4.8% of all particles, which could befragments of the gelatinized starch granules. These smallparticles might be responsible for the increase in the SSAof maize starch when homogenized at 140 MPahomogenization.

3.4. X-ray diffraction analysis of maize starch treated under

different pressures

The X-ray diffraction patterns of maize starch underdifferent homogenizing pressure are presented in Fig. 4.The main peaks at about 15.1�, 17.2�, 18.0�, and 23.1�(2h) indicate that the structure of maize starch is A pattern.Homogenization treatments at 60 MPa and 100 MPa didnot change the diffraction pattern much because of the lim-ited loss of crystalline structure. At 140 MPa, the intensityof the peaks around 15.1�, 17.2�, 18.0�, and 23.1� decreasedobviously, indicating the obvious loss of the crystallinestructure, which resulted in the decrease of To and Tp

Fig. 4. X-ray powder diffraction patterns of maize starch treated at different homogenizing pressures: r native; s 60 MPa; t 100 MPa; and u 140 MPa.

442 B. Wang et al. / Journal of Food Engineering 87 (2008) 436–444

greatly, as indicated previously. However, we also foundthat the peak intensity around 20.0� increased at100 MPa, and when the homogenizing pressure was set at140 MPa, such a trend became more obvious, as shownin Fig. 5. No similar results have been reported before,so it was hard to explain such a phenomenon. Katopoet al. (2002) reported that maize starch mixed with waterunder HHP would transform from A-type pattern to B-type pattern, with the double peak around 17� turned intoa single peak and the peak intensity around 20� increased.It is therefore likely that the high-pressure homogenizationprovided a chance for maize starch granules to react withwater molecules activated by heat and dynamic high-pres-

Fig. 5. Comparison of X-ray diffraction patterns for native m

sure, and finally induced a weak trend for the maize starchgranules to transform from A-type to B-type pattern. Cer-tainly, such an inference is not sufficient because of the verylimited change in the diffraction patterns, and more inves-tigation will be done in the future.

3.5. Micrographs of maize starch granules

The micrographs of starch granules are shown in Fig. 6.The granules of the native maize starch (A) used in thisstudy have irregular shapes. When the homogenizing pres-sure was set at 60 MPa, most starch granules retained theirgranule structure. However, there were some very small

aize starch and maize starch homogenized at 140 MPa.

Fig. 6. Micrographs of maize starch granules treated under different conditions: native (A), homogenized at 60 MPa (B), 100 MPa (C), and 140 MPa (D).

B. Wang et al. / Journal of Food Engineering 87 (2008) 436–444 443

ones together with a few big granules, formed during the60 MPa homogenization treatment, indicating that thegranule structure was changed by the high-pressurehomogenization.

After the treatment at 100 MPa, the maize starch wascharacterized by significant deformations and partial gela-tinization. Many big granules lost granule structure andruptured into fragments. The high-pressure homogeniza-tion treatment linearly increases water temperature (Cheet al., 2007b), thus the fragments could absorb water andswell in warm water (Jyothi et al., 2005; Spigno and DeFaveri, 2004). Similar granule swelling could also beinduced by HHP as reported by Stolt et al. (2001). It seemsthe fragments were often very big compared with starchgranules, which may be due to the loss of the starch granuleenvelope. Błaszczak et al. (2003) reported that the externalpart of starch granule differs significantly from the interiorin terms of uniformity, and is likely to be composed mainlyof amylopectin having also a wide range of high molecularmass fragments. Błaszczak et al. (2005b) suggested that theouter part of the starch granule has a very dense layerwhich is more resistant to any changes. Thus, the fragmentswhich lost the protection of the dense outer part of starchgranules swelled into much bigger size, suggesting that theinner zone of starch granule could more effectively absorbwater and swell. Furthermore, some of the fragmentsshowed gel-like structures, and aggregated with each otheror with other starch granules.

After being treated at 140 MPa, it can be seen thatalmost all the relatively big starch granules have lost theirnormal granule structure, having broke up into fragments

and formed gel-like structures by aggregating with eachother. Many micrographs showed cloudiness around thestarch particles (photos not shown). This can be attributedto the gelatinization of starch grains. Many small starchparticles still retained their granule structure, although theywere deformed by homogenization at 140 MPa, suggestingthat small particles were more resistant to high-pressurehomogenization, which is in consistent with Che et al.(2007b).

4. Conclusion

In the current study, we investigated the effect of high-pressure homogenization (up to 140 MPa) on the structureand thermal properties of maize starch. DSC analysis ofhigh-pressure homogenized starch showed a distinctdecrease in gelatinization temperatures (To, Tp) and gelati-nization enthalpy (DHgel) with increasing homogenizingpressure. High-pressure homogenization induced the gela-tinization of maize starch, and the GD increased withincreasing homogenizing pressure. However, high-pressurehomogenization has no effect on the retrogradation prop-erty of maize starch. The results from laser scattering mea-surements suggested a significant increase in granule sizewhen the homogenizing pressure was at 140 MPa as aresult of granule aggregation. The X-ray diffraction patternshowed a loss of crystalline structure after homogenizationtreatment at 140 MPa. Microscopy studies showed thatmaize starch was partly gelatinized after high-pressurehomogenization, and the gelatinized granules were prone

444 B. Wang et al. / Journal of Food Engineering 87 (2008) 436–444

to aggregate with each other, resulting in an increase ofgranule size.

Acknowledgements

Research support was provided by the Key Project ofChinese Ministry of Education (No. 105014), the FundingSystem for Scientific Research Projects of Doctor Subjectof Chinese Advanced University (No. 20050019029), andthe Funding System for Scientific Research Projects of Chi-na Agricultural University (No. 2004010).

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