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

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Journal of Food Engineering 87 (2008) 436–444 www.elsevier.com/locate/jfoodeng

Effect of high-pressure homogenization on the structure and 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, China College of Food Science and Nutritional Engineering, China Agricultural University, 17 Qinghua Donglu, Beijing 100083, China c Department of Chemical and Materials Engineering, University of Auckland, Private Bag 92019, Auckland City, New Zealand

b

Received 9 September 2007; received in revised form 18 December 2007; accepted 22 December 2007 Available 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 diffraction technique, laser scattering, and microscope, with native maize starch (suspended in water, but not homogenized) as a control sample. DSC analysis showed a decrease in gelatinization temperatures (To, Tp) and gelatinization enthalpy (DHgel) with increasing homogenizing pressure. No noticeable effect of high-pressure homogenization on the retrogradation of maize starch was observed. Laser scattering measurements of particle size demonstrated an increase in the granule size at a homogenizing pressure of 140 MPa. This was attributed to the gelatinization and aggregation of the starch granules. X-ray diffraction patterns showed that there was an evident loss of crystallinity after homogenization at 140 MPa. Microscopy studies showed that the maize starch was partly gelatinized after high-pressure homogenization, and the gelatinized granules were prone to aggregate with each other, resulting in an increase of granule size. Ó 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 in nature. As reproducible biomass, starch has been widely used in paper, textile, adhesive, sweetener, and food industries (Che et al., 2007a). Among all kinds of starches, maize starch 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, and adhesive in industry (Singh et al., 2003). High-pressure technology has been used as a novel method to obtain special denatured starches. Many papers have reported that high hydrostatic pressure (HHP) could

*

Corresponding authors. Tel./fax: +86 10 62737351. E-mail addresses: [email protected] (D. Li), [email protected]. edu.au (X.D. Chen). 0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.12.027

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; Stolt et al., 2001; Stute et al., 1996). The extent of gelatinization depended on the pressure applied, moisture content of the starch–water suspension, treatment time, temperature, starch concentration, and the type of starch (Bauer and Knorr, 2004; Stute et al., 1996). Kawai et al. (2007) demonstrated that the values of DHgel and DHret (enthalpies of gelatinization and retrogradation of starch, respectively) of potato starch–water (10–70% w/w) were affected significantly by the treatment pressure and starch content, and little effect of treatment time. They found that the DHgel values of 10–50% (w/w) mixtures decreased with increasing treatment pressure and decreasing starch content. The 30–60% (w/w) mixtures retrograded, and the DHret values

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increased with decreasing DHgel values and increasing starch content. Although HHP treatment causes starch gelatinization, the mechanism of pressure-induced gelatinization is significantly different from that of heat-induced gelatinization (Rubens and Heremans, 2000; Stute et al., 1996). For example, during pressure-induced gelatinization, starch can retain granular structures, with little amylose leaching out from the granules (Douzals et al., 1998; Stolt et al., 2001; Stute et al., 1996). Also, the starch granules gelatinized by high pressure contain two different zones, the outer zone of which remained unchanged while the inner zone was completely destroyed and formed gel-like structures (Błaszczak et al., 2005b). Using X-ray diffraction studies, Katopo et al. (2002) showed that when the starch and water mixtures in 1/1 and 2/1 (water/starch, v/w) was pressurized under the ultra-high pressure of 690 MPa, the pressure converted A-type starches (normal maize starch and waxy maize starch, rice starch) to B-type-like pattern. However, the B-type pattern starches (potato starch and high amylose maize starch) were not changed by pressure treatment in a water suspension. It is also reported that B-type starches were more resistant to pressure gelatinization than A-type starches (Ezaki and Hayashi, 1992; Katopo et al., 2002; Muhr and Blanshard, 1982; Rubens et al., 1999), due to the different crystalline structure formed by amylopectin. The B-type crystallite has more combined water molecules which fill up the channel in the cell unit of the crystallite and play a role as a stabilizer, while the amylopectin of the A-type starch has more scattered branching structure (Jane et al., 1997). Hence, the B-type starches are more stable, and the A-type starches with more flexible scattered branching structures are more active and tend to be rearranged or destroyed by water under high pressure. Furthermore, the amylose content of starch affects the susceptibility of high-pressure treatment. Starches with high amylose contents often resist high-pressure treatment better than low amylose content starches. Błaszczak et al. (2007) indicated that HHP treatment could cause the loss of crystalline structure of starches. Waxy maize starch completely lost crystalline structure and formed a gel-like structure after treated with high pressure at 650 MPa for 9 min, while high amylose maize starch under the same treatment still retained its granule structure with a decreased crystallinity. This is because amylose can form complexes with lipids present in maize starch, stabilizing the structure of starch granules to restrict the swelling of granules. Chen et al. (2007) reported that the diameter growth rate and final accretion ratio sequence of different maize starches during heating under shearless conditions were negatively correlated with amylose/amylopectin ratio and the higher the melting temperature of the amylose– lipid complex, the higher the gelatinization temperature. Such results indicated the stabilization effect of amylose in starch.

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To date, most research has focused on the HHP treatment of starch, while the knowledge on the dynamic high-pressure treatment of starch is needed. Long-time HHP treatment is economically undesirable in starch industry and has not come into real industrial production, while high-pressure homogenization treatment that produces dynamic pressure has already been used in chemical, pharmaceutical, specialty food and biotechnology industries (Pandolf and Kinney, 1998). In the industry, homogenization pressures normally used are between 20 and 50 MPa. When the homogenizing pressure is over 100 MPa, it is usually called ultra-high-pressure homogenization (Tribst et al., 2007). During high-pressure homogenization, liquids experience high pressure, high shear, turbulence, and cavitation caused by rapid change in pressure (Hayes and Kelly, 2003). Although high-pressure homogenizers are now rarely utilized in native starch processing, it is of fundamental importance to obtain a detailed understanding of its effects on the structure and thermal properties of starches. Che et al. (2007b) have investigated the effect of high-pressure homogenization on the structure of cassava starch. However, knowledge of the effect of high-pressure homogenization of starches is still very limited. Among all kinds of starches, maize starch is the most used in the industries. Also, it is necessary to extend our research from a tuber starch (cassava) to a grain starch (maize). In this study, the maize starch–water suspension was homogenized at different pressures using a high-pressure homogenizer. The effect of high-pressure homogenization on both the structure and thermal characteristics of normal maize starch was investigated. 2. Materials and methods 2.1. Materials Commercial maize starch was purchased from Beijing Quanfeng Starch Company, China. The moisture content of the starch was determined by drying measurement in an air-oven at 105 °C for 24 h, and the average value was determined to be 11.99% (w/w). An analytical grade anhydrous alcohol (95%, w/w) was purchased from Beijing Lanyi Chemical Company. 2.2. High-pressure homogenization of maize starch suspension Normal maize starch suspension (1.0%, w/w) was prepared by adding maize starch in de-ionized water at 17 °C. The well mixed suspension was homogenized in a high-pressure homogenizer (NS1001L-PANDA 2K, Niro Soavi S.p.A., Italy) for one pass at 60, 100, and 140 MPa, respectively. Five hundred milliliters of suspension was processed at each pressure level. The PANDA 2K homogenizer used in the present study is a two-stage homogenizer with two high-pressure valves. The

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homogenizing pressure range of the PANDA 2K homogenizer is from 0 MPa to 150 MPa. In industrial practice, the pressure of the second stage high pressure should be adjusted to about 1/10 of that of the first high-pressure stage in order to achieve better homogenization. In this study, only the first stage high pressure was used. The samples treated with high-pressure homogenization were vacuum filtered and dehydrated with anhydrous alcohol, and then were dried in an oven at 40 °C for 24 h in order to obtain dry starch samples (Katopo et al., 2002). Dried starch samples were carefully pulverized with a mortar and pestle, and then stored in a desiccator for later studies (Che et al., 2007b). The native maize starch was soaked in de-ionized water for 10 min and then treated using the method described above. 2.3. Thermal properties of gelatinization and retrogradation The gelatinization properties of the native maize starch and high-pressure homogenized maize starch in the presence of excess water was conducted using a differential scanning calorimeter (DSC-Q10, TA Instruments, New Castle, USA) equipped with a thermal analysis data station. The DSC analyzer was calibrated using indium and an empty aluminum pan was used as a reference. Starch (2.5 mg) was directly measured into the aluminum DSC pan and distilled water (7.5 lL) was added with a microsyringe. Pans were then immediately hermetically sealed and equilibrated for 3 h at room temperature before heating in the DSC. The samples were then heated from 20 °C to 120 °C at 10 °C/min. The onset, peak, and conclusion temperatures (To, Tp, and Tc) together with gelatinization enthalpy (DHgel) were quantified. After conducting thermal analysis, the gelatinized samples were stored at 4 °C for 7 days and then rescanned to determine the temperature and enthalpy changes due to retrogradation. For retrogradation studies, the temperature range and heating rate were set as 20–120 °C and 10 °C/min, respectively. The values of gelatinization degree (GD) was calculated using the following equation (Błaszczak et al., 2007): GD ¼ fðDH ns  DH ts ÞDH 1 ns g  100% where DHns and DHts are the gelatinization enthalpies of native and homogenization treated starches, respectively. The values of retrogradation degree (RD) was calculated as (Sandhu and Singh, 2007) RD ¼ DH ret =DH gel  100% where DHgel and DHret are the gelatinization enthalpies of native and retrograded starches, respectively. The peak height index (PHI) was calculated as (Krueger et al., 1987) PHI ¼ DH =ðT p  T o Þ The temperature range for gelatinization (R) was calculated as (Sandhu and Singh, 2007) R ¼ 2  ðT p  T o Þ

2.4. Laser scattering measurement The particle size distributions of the high-pressure homogenized samples and the native sample were determined using the laser scattering method. The equipment used was a Mastersizer 2000 laser diffractometer (Malvern Instruments, UK) equipped with a He–Ne laser with wavelength of 632.8 nm. The dry starch samples were dispersed in anhydrous alcohol in the diffractometer cell before measurements. The refractive indices of anhydrous alcohol and the starch used were 1.32 and 1.53, respectively. The absorbance of starch granules was taken as 0.1 (Singh et al., 2006; Zhou et al., 2006). The density of maize starch was determined using the following method: maize starch was filled into a graduated cylinder and then the cylinder was dropped from a constant height. The jolting action consolidates the starch to the required bulk density which was determined from the weight and volume of starch in the cylinder (Muramatsu et al., 2005). Every sample was replicated thrice. The special surface area (SSA) was calculated as (Zhou et al., 2006): SSA = 6/ d3,2, where d3,2 is the area mean diameter (Sauter diameter) of maize starch samples. 2.5. X-ray diffraction analysis of maize starch An XD-2 X-ray diffractometer (Beijing Purkinje General Instrument Co. Ltd., China) was used for the X-ray diffraction analysis of maize starch. The starch samples were first pulverized to pass 360 mesh using a carnelian mortar. X-ray powder diffraction analysis were then performed at 36 kV and 20 mA using nickel-filtered Cu Ka ˚ ) radiation. The 2h scan was done (wavelength 1.5405 A from 5° to 40° with a scanning speed of 0.25°/min and sampling interval of 0.02°. 2.6. Microscopy study of maize starch granules To investigate the effect of high-pressure homogenization on the structure of maize starch, both treated and native 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 homogenizing pressure of 60, 100, and 140 MPa, respectively. DSC was used to investigate the effect of homogenization on the gelatinization and retrogradation properties of maize starch. Laser scattering measurement, X-ray diffraction analysis and optical microscope were used to investigate the structure properties of maize starch after homogenization. Results from DSC measurement were presented as mean values with standard deviations. The Student’s t test was

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used to estimate significant differences among means at a probability level of 5% (p < 0.05). The experimental data were statistically calculated by SAS system (release 8.2, SAS Institute Inc., Cary, NC, USA). Particle size distributions were summarized by the characteristic volume-based d-values that correspond to volume mean diameter (d4,3), area mean diameter (d3,2), and 10%, 50%, and 90% of the total particle population (d(0.1), d(0.5), and d(0.9)), respectively. Particle size distributions and X-ray diffraction analyses were both replicated twice for every sample. 3. Results and discussion 3.1. Gelatinization properties of high-pressure homogenized maize starch DSC thermograms obtained for the gelatinization of all the starch samples are shown in Fig. 1. The enthalpy of gelatinization (DHgel), gelatinization temperatures (onset,To; peak, Tp; and conclusion, Tc), peak height index (PHI), gelatinization temperature range (R), and the degree of gelatinization (GD) are shown in Table 1. The To, Tp, and DHgel of high-pressure homogenized maize starch decreased significantly with increasing processing pressure, with the lowest values observed at 140 MPa. When the homogenizing pressure was set at 60 MPa, the difference ofTo, Tp, and DHgel between native and 60 MPa treated samples was relatively small. When the homogenizing pres-

Heat Flow (W/g)

0

A

-1

B C

-2 D

-3 20 Exo Up

40

60

80

100

Temperature (°C)

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

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sure was set at 100 and 140 MPa, the differences between native and homogenization treated samples became much greater. The Tc of high-pressure homogenized maize starch increased with higher homogenizing pressure. According to Błaszczak 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 necessary to reduce significantly the onset temperature of gelatinization of maize starch, but the present work indicates that the high-pressure homogenization treatment could decrease the 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-pressure homogenization at 100 MPa or higher homogenizing pressure destroyed the compact arrangements of molecules in the crystalline regions of maize starch, thus during heating in DSC, the water molecules in the aluminum pan could react with the molecules in the crystalline region more easily, and therefore decreased the onset temperature (To) of gelatinization. The high-pressure homogenization also increased the gelatinization temperature range, and the value reached a maximum at the homogenizing pressure of 100 MPa. Also, the peak height index (PHI) decreased with increasing homogenizing pressure. Compared with native maize starch, the To of maize starch after homogenization decreased significantly with higher homogenizing pressure, while the Tp decreased relatively small, and thus the temperature range of gelatinization increased obviously. Also, the DHgel of homogenized maize starch decreased with higher pressure. Thus, the thermogram curves became flatter at higher pressure, as shown in Fig. 1, resulting in the decreased peak height index (PHI) with higher pressure. The degree of gelatinization (GD) was very small at 60 MPa, and reached 12.9 ± 3.2% at 100 MPa and 26.8 ± 1.8% at 140 MPa, respectively. Che et al. (2007b) reported that high-pressure homogenization linearly increased the water temperature by 0.187 °C/MPa, and attributed the gelatinization of starch to the increase of the water temperature. But it should be noted that the time of high-pressure homogenization treatment was very short (<30 s), and the temperature of starch–water suspension after treatment was not high (43.1 °C when homogenization pressure was at 140 MPa in this study). Hence temperature increase is unlikely to be solely responsible for the gelatinization of maize starch. As indicated previously, high-pressure treatment contributes to the gelatinization

Table 1 DSC measurements for gelatinization properties of high-pressure homogenized maize starch and the native maize starch Starch

To (°C)

Tp (°C)

Tc (°C)

DHgel (J/g)

PHI

R (°C)

GD (%)

Native sample 60 MPa treated 100 MPa treated 140 MPa treated

65.6 ± 0.1a 64.3 ± 1.1b 60.1 ± 0.8c 58.7 ± 0.2d

70.4 ± 0.1a 70.4 ± 0.4ab 69.1 ± 0.8b 67.2 ± 0.7c

75.2 ± 0.5a 75.8 ± 0.1b 77.2 ± 0.3c 78.0 ± 0.4c

12.5 ± 0.7a 12.4 ± 0.2a 10.9 ± 0.2b 9.2 ± 0.2c

2.6 ± 0.2a 1.8 ± 0.2b 1.2 ± 0.1c 1.0 ± 0.1c

9.6 ± 0.3a 12.1 ± 1.5b 17.9 ± 1.4c 17.1 ± 1.0c

0 0.8 ± 1.6a 12.9 ± 3.2b 26.8 ± 1.8c

To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; DHgel, enthalpy of gelatinization; PHI, peak height index; R, temperature range for gelatinization; and GD, degree of gelatinization. Values represent the means ± standard deviation; n = 3. Values with the same superscript in a column do not differ significantly (p < 0.05).

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of starch, and the higher the pressure is, the more easily the gelatinization of starch will happen (Błaszczak et al., 2007; Buckow et al., 2007). Chen et al. (2007) indicated that the shear stress could enhance swelling and therefore promote the gelatinization of starch granules during high-pressure homogenization treatment. When the homogenizing pressure is higher, the shear stress produced during homogenization will become larger, and both the increased homogenizing pressure and increased shear stress during homogenization can make the gelatinization of maize starch much easier. Also, higher homogenizing pressure produced higher water temperature, which can help to the gelatinization of maize starch. As a result, the gelatinization of maize starch might be caused by the combined effect of the temperature rise and the dynamic high pressure during the treatment. 3.2. Retrogradation properties of high-pressure homogenized maize starch Starch retrogradation is the process that occurs when the molecular chains in gelatinized starches begin to reassociate into an ordered structure as a result of hydrogen bonding between starch chains (Atwell et al., 1988; Hoover, 2001). The DSC thermograms and thermal properties of retrograded starch samples are presented in Fig. 2 and Table 2, respectively. By comparing Table 2 to Table 1, it

0.0

Heat Flow (W/g)

-0.5 A

-1.0

B C

-1.5

D

-2.0

-2.5 Exo Up

30

40

50

60

70

80

90

Temperature (°C)

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

can be seen that the transition temperatures of retrograded starch were significantly lower than the corresponding gelatinization 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 retrogradation. This may be because the recrystallized amylopectin that formed during retrogradation was less ordered than the native form (Sandhu and Singh, 2007). The difference between peak and onset temperatures (Tp  To) for the retrograded starch was found to be greater than the corresponding values for high-pressure homogenized starch during its first DSC scan, and the peak height index (PHI) of retrograded maize starch was less than the corresponding values for high-pressure homogenized starch during its first DSC scan. Similar observations have been reported (Karim et al., 2000; Sandhu and Singh, 2007). From the data listed in Table 2, a conclusion could be drawn that there was no obvious effect of high-pressure homogenization on the retrogradation property for maize starch. 3.3. Particle size distributions of maize starch granules The results of particle size measurements are listed in Tables 3 and 4. The histogram of the size distributions for maize starch granules treated with different homogenizing pressure is presented in Fig. 3. As shown in Table 3, the volume mean diameter of the native maize starch granules used in this study was 13.5 lm, which was similar to the values obtained in other studies (Chen et al., 2007a; Gregorova et al., 2006). From the histogram we know that homogenization pressure at 60 MPa had no obvious effect on the particle size. While at 100 MPa, it could be seen in Fig. 4 that the d(0.9) has a slight increase from 20.8 lm of the native sample to 22.7 lm, indicating that the number of big granules increased, which could be proved by Table 4. Such a result was due to the partial gelatinization of starch granules, which aggregated with others, as suggested by the micrographs. There was a significant increase in granule size at 140 MPa. The volume mean diameter reached 26.9 lm. The d(0.5) and d(0.9) increased from 13.1 lm and 22.7 lm at 100 MPa to 22.7 lm and 52.6 lm at 140 MPa, respectively. While the d(0.1) changed very little, from 7.7 lm to 8.7 lm. The results could demonstrate that when

Table 2 DSC measurements for retrogradation properties of high-pressure homogenized maize starch and the native maize starch Starch

To (°C)

Tp (°C)

Tc (°C)

DHret (J/g)

PHI

R (°C)

RD (%)

Native sample 60 MPa treated 100 MPa treated 140 MPa treated

46.4 ± 0.9a 46.5 ± 0.5a 45.5 ± 0.5a 45.3 ± 1.5a

57.0 ± 1.4a 57.9 ± 0.1a 56.9 ± 0.8a 57.8 ± 0.8a

69.9 ± 1.0a 69.7 ± 0.5a 69.8 ± 2.3a 70.0 ± 1.4a

5.4 ± 0.4a 5.5 ± 1.1a 5.5 ± 0.6a 5.4 ± 0.5a

0.5 ± 0.1a 0.5 ± 0.1a 0.5 ± 0.1a 0.4 ± 0.1a

22.1 ± 2.1a 22.3 ± 0.1a 23.0 ± 0.4a 25.1 ± 1.6a

43.0 ± 3.3a 43.8 ± 3.6a 44.2 ± 5.1a 43.3 ± 4.3a

To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; D Hret, enthalpy of retrogradation; PHI, peak height index; R, temperature range for gelatinization in retrograded starch; and RD, degree of retrogradation. Values represent the means ± standard deviation; n = 3. Values with the same superscript in a column do not differ significantly (p < 0.05).

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

d4,3a

d3,2a

d(0.1)b

d(0.5)b

d(0.9)b

Density (kg/m3)

SSA (m2/g)c

Native sample 60 MPa treated 100 MPa treated 140 MPa treated

13.5 13.0 14.3 26.9

11.7 11.2 12.1 9.8

7.6 7.2 7.7 8.7

12.6 12.1 13.2 22.7

20.8 20.2 22.7 52.6

649.3 766.2 685.8 627.3

0.51 0.54 0.50 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 4 Granule size distribution of maize starch Sample name

Granule size distribution (%)

Native sample 60 MPa treated 100 MPa treated 140 MPa treated

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

0 0 0 4.8

28.8 32.6 26.5 8.8

38.2 37.7 35.8 16.0

20.6 19.1 20.7 13.8

12.3 10.6 15.9 21.5

0.1 0 1.1 13.6

0 0 0 11.5

0 0 0 4.3

0 0 0 5.0

0 0 0 0.7

Values are the means of duplicate.

Diameter (μm) 60 50 40 30 20 140MPa treated 100MPa treated 60MPa treated Native starch

10 0

d(0.1)

d(0.5)

d(0.9)

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

homogenized at 140 MPa, a lot of granules of maize starch were wholly or partially gelatinized and aggregated with each other, as a result increased the d(0.5) and d(0.9). Also, homogenized at 140 MPa formed many small granules with diameters less than 4 lm, which represented 4.8% of all particles, as shown in Table 4. Thus, the d(0.1) at 140 MPa increased very limited comparing with the corresponding value of 100 MPa. During the homogenization treatment, starch granules were simultaneously subjected to high-temperature, highpressure, and high shearing-stress in the presence of water molecules. As a result of these conditions, the starch granules swelled rapidly into gel-like material. To the native starch granules, the compact arrangements of molecules in the crystalline regions inhibit water or chemical reagents from making contact with the molecules in the crystalline region, and thus the chemical reactivity of starch is decreased. As a result, the surface of the gelatinized gran-

ules would possess higher chemical reactivity, and the strength of van der Waal’s force and electrostatic force between the gelatinized granules would be large enough to penetrate the boundaries of each other (Huang et al., 2007). Thus, the gelatinized granules aggregate with each other. Some small granules could not resist the van der Waal’s force and electrostatic force of the big congregations, and were also absorbed into the aggregates, as shown in Fig. 3. As a result, the gel-like granules and small grains all together formed congregations which increased the granule size. The specific surface area (SSA) was calculated from the area mean diameter (d3,2) of maize starch. As can be seen in Table 4, the size distribution broadened greatly after the homogenization treatment at 140 MPa. Particles less than 4 lm represented 4.8% of all particles, which could be fragments of the gelatinized starch granules. These small particles might be responsible for the increase in the SSA of maize starch when homogenized at 140 MPa homogenization. 3.4. X-ray diffraction analysis of maize starch treated under different pressures The X-ray diffraction patterns of maize starch under different 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 did not change the diffraction pattern much because of the limited loss of crystalline structure. At 140 MPa, the intensity of the peaks around 15.1°, 17.2°, 18.0°, and 23.1° decreased obviously, indicating the obvious loss of the crystalline structure, which resulted in the decrease of To and Tp

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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.

greatly, as indicated previously. However, we also found that the peak intensity around 20.0° increased at 100 MPa, and when the homogenizing pressure was set at 140 MPa, such a trend became more obvious, as shown in Fig. 5. No similar results have been reported before, so it was hard to explain such a phenomenon. Katopo et al. (2002) reported that maize starch mixed with water under HHP would transform from A-type pattern to Btype pattern, with the double peak around 17° turned into a single peak and the peak intensity around 20° increased. It is therefore likely that the high-pressure homogenization provided a chance for maize starch granules to react with water molecules activated by heat and dynamic high-pres-

sure, and finally induced a weak trend for the maize starch granules to transform from A-type to B-type pattern. Certainly, such an inference is not sufficient because of the very limited change in the diffraction patterns, and more investigation 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 this study have irregular shapes. When the homogenizing pressure was set at 60 MPa, most starch granules retained their granule structure. However, there were some very small

Fig. 5. Comparison of X-ray diffraction patterns for native maize starch and maize starch homogenized at 140 MPa.

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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).

ones together with a few big granules, formed during the 60 MPa homogenization treatment, indicating that the granule structure was changed by the high-pressure homogenization. After the treatment at 100 MPa, the maize starch was characterized by significant deformations and partial gelatinization. Many big granules lost granule structure and ruptured into fragments. The high-pressure homogenization treatment linearly increases water temperature (Che et al., 2007b), thus the fragments could absorb water and swell in warm water (Jyothi et al., 2005; Spigno and De Faveri, 2004). Similar granule swelling could also be induced by HHP as reported by Stolt et al. (2001). It seems the fragments were often very big compared with starch granules, which may be due to the loss of the starch granule envelope. Błaszczak et al. (2003) reported that the external part of starch granule differs significantly from the interior in terms of uniformity, and is likely to be composed mainly of amylopectin having also a wide range of high molecular mass fragments. Błaszczak et al. (2005b) suggested that the outer part of the starch granule has a very dense layer which is more resistant to any changes. Thus, the fragments which lost the protection of the dense outer part of starch granules swelled into much bigger size, suggesting that the inner zone of starch granule could more effectively absorb water and swell. Furthermore, some of the fragments showed gel-like structures, and aggregated with each other or with other starch granules. After being treated at 140 MPa, it can be seen that almost all the relatively big starch granules have lost their normal granule structure, having broke up into fragments

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

4. Conclusion In the current study, we investigated the effect of highpressure homogenization (up to 140 MPa) on the structure and thermal properties of maize starch. DSC analysis of high-pressure homogenized starch showed a distinct decrease in gelatinization temperatures (To, Tp) and gelatinization enthalpy (DHgel) with increasing homogenizing pressure. High-pressure homogenization induced the gelatinization of maize starch, and the GD increased with increasing homogenizing pressure. However, high-pressure homogenization has no effect on the retrogradation property of maize starch. The results from laser scattering measurements suggested a significant increase in granule size when the homogenizing pressure was at 140 MPa as a result of granule aggregation. The X-ray diffraction pattern showed a loss of crystalline structure after homogenization treatment at 140 MPa. Microscopy studies showed that maize starch was partly gelatinized after high-pressure homogenization, and the gelatinized granules were prone

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