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Effect of heating condition and starch concentration on the structure and properties of freeze-dried rice starch paste
Food Research International 40 (2007) 215–223 www.elsevier.com/locate/foodres
Effect of heating condition and starch concentration on the structure and properties of freeze-dried rice starch paste Seog-Won Lee a, Chul Rhee b
b,*
a Department of Food & Nutrition, Yuhan College, 185-34, Goean-dong, Sosa-gu, Bucheon, Kyeonggi-do 422-749, Republic of Korea Division of Food Science, College of Life and Environmental Sciences, Korea University, 1, 5-Ka, Anam-dong Sungbuk-ku, Seoul 136-701, Republic of Korea
Received 17 January 2006; accepted 22 May 2006
Abstract The structure and characteristics of freeze dried rice starch paste were investigated at low starch concentrations (1%, 3% and 5%). The microstructure of samples made at 1% of starch concentration showed a loose filamentous network compared to other concentrations (3% and 5%). Especially the samples prepared at processing conditions above 110 °C and 3% were appeared a coarse honeycomb-like structure. Crystallinity of samples calculated by intensities of X-ray diffraction peaks was very low values (3.3–9.8%). The mechanical properties (hardness, cohesiveness and springiness) of the samples were highly influenced by starch concentration and heating temperature. The relative increment of water absorption index (WAI) according to increasing of heating temperature showed the highest value at 1% of starch concentration, and the heating temperature was analyzed to the important factor affecting WAI and WSI (water solubility index). The digestibility of samples showed an increasing trend with heating temperature regardless of starch concentration. Overall, the important factors influencing the properties of freeze dried rice starch paste were starch concentration and heating temperature. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Rice starch matrix; Freeze drying; Microstructure of rice starch matrix; Mechanical property; Water absorption index; Digestibility
1. Introduction Pregelatinized starches are widely used in the food industry because of their functionality to form viscous solutions, gels, films and encapsulating matrices. The functionalities depend on the physicochemical properties of gelatinized starches, and by changing of raw materials and process parameters it is possible to obtain pregelatinized starches with different functional characteristics. Among the modification methods, a physical modification is one of the important treatments for preparing of starch with functionally desirable attributes. This physical modification has been studied by several methods such as extrusion (Alves, Grossmann, & Silva, 1999), drum drying, spray drying (Herman, Remon, & De Vilder, 1989) and freeze drying technique after heat treatment (Rassis, Saguy, & *
Corresponding author. Tel.: +82 2 3290 3023; fax: +82 2 928 1351. E-mail address: [email protected] (C. Rhee).
0963-9969/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2006.05.005
Nussinovitch, 2002; Sa´nchez, Torrado, & Lastres, 1995). The structure and physicochemical properties of solid starch matrix formed by these methods is determined by the starch concentration, the structure of the swollen starch granule, and heating condition (Lii, Shao, & Tseng, 1995). Morphological and structural changes during starch gelatinization have been observed by scanning electron microscopy (SEM) in various cereals (Atkin, Abeysekera, & Robards, 1998; Garcia, Colonna, Bouchet, & Gallant, 1997; Varriano-Marston, Zeleznak, & Nowontna, 1985). The product needs to satisfy the various physicochemical properties such as mechanical properties, water absorption index, water solubility index, inclusion complexes formation with various molecules, and digestibility. To date, most of previous studies have reported for the solid starch matrix prepared at concentrations where gelation occurs (normally >6%) (Ahmad & Williams, 1998; Carr, Wing, & Doane, 1991; Clark & Ross-Murphy, 1987; Lauro, Ring, Bull, & Poutanen, 1997; Morris, 1998), and under these
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conditions, formation of a partially crystalline precipitate produces a gel during storage. A large amount of knowledge about the structure and physical properties of solid starch matrix prepared at sufficiently high concentrations (>6%) has been known (Li, Vasanthan, Hoover, & Rossnagel, 2004), but the report is not sufficient to understand the characteristics of solid starch matrix at concentrations <6%. Hydrocolloid-based solid matrices can be produced by drying the gel immediately after their production (Nussinovitch, Corradini, Normand, & Peleg, 2000), but this operation is difficult under gel-forming concentration (normally <6%). The choice of drying method and control of processing conditions are very important in the preparation of dried starch matrices because it may influence the characteristics of starch-containing products due to the variation in structure and physical properties. Therefore, the objective of this work were to study the freezing/freeze-drying technique for the production of solid starch matrices under gel-forming concentration, and to investigate the differences in the solid starch matrices’ microstructure and physical properties according to various process parameters (rice starch concentration, heating temperature and time). The influence of process parameters on the structure and physical properties of samples was analyzed by Surface Response Methodology. 2. Materials and methods 2.1. Experimental design and statistical analysis The experiment was performed using an incomplete factorial design with three independent variables at three levels of variation: starch concentration (1%, 3% and 5%), temperature (90, 110 and 130 °C) and holding time (20, 40 and 60 min). The three levels of each variable were coded as 1, 0 and +1 for statistical analysis. A factorial experimental design, consisting of 15 treatments, which included triplication of the center point (Table 1), was used. The dependent variables were mechanical properties (hardness, cohesiveness and springiness), water absorption index, water solubility index, and digestibility. Experimental data analysis were performed by multiple regression analysis using Statistical Analysis System (SAS, 1989) to fit second-order polynomial equations to response variables. 2.2. Preparation of solid rice starch matrices To prepare the solid rice starch matrices, rice starch (S7260, Sigma Chemical Co., USA) suspensions (1%, 3% and 5%, w/v) were prepared by combining of starch and distilled water in a glass bottle. Each starch suspension was heated on a hot plate and agitation during heating was achieved using a magnetic stirrer. Continuous stirring was maintained until a temperature of 80 °C was reached. Samples were immediately transferred to an autoclave
Table 1 Experimental design for solid rice starch matrix Run
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Coded variables
Real variables
X1
X2
X3
Starch (%)
Temperature (°C)
Time (min)
1 +1 1 +1 1 +1 1 +1 0 0 0 0 0 0 0
1 1 +1 +1 0 0 0 0 1 +1 1 +1 0 0 0
0 0 0 0 1 1 +1 +1 1 1 +1 +1 0 0 0
1 5 1 5 1 5 1 5 3 3 3 3 3 3 3
90 90 130 130 110 110 110 110 90 130 90 130 110 110 110
40 40 40 40 20 20 60 60 20 20 60 60 40 40 40
and then heated to 90, 110 and 130 °C and held at these temperatures for 20, 40 or 60 min according to the experimental design. The samples were subsequently cooled at 80 °C and 20 ml of the formed pastes were poured into plastic cylinder (3 cm diameter and 5 cm height). The samples were frozen in a freezer at a temperature of 70 °C and then freeze-dried in a freeze dryer (Ilshin Co., Gyeonggi-do, Korea) operating at 50 °C and 1.33 Pa. The moisture content of the samples was measured with an infrared auto-moisture analyzer (Precisa HA300, Swiss). The freeze-dried samples had low water content (4–5% (w/w)) and elastic structure. The samples were either tested immediately or stored in desiccators until testing. 2.3. Scanning electron microscopy (SEM) The freeze-dried starch matrices were directly mounted on circular aluminum stubs with double-sided sticky tape, coated with 10 nm gold, then examined and photographed in a JEOL (JSM-840 A) scanning electron microscope (JEOL, Ltd., Tokyo, Japan) at an accelerating voltage of 20 kV. The samples were measured with the use of the calibrated scale bar on the micrograph. 2.4. Mechanical test of rice starch matrices The mechanical properties (hardness, cohesiveness and springiness) of rice starch matrices were determined using a Texture Analyzer (TA-XT2, Stable Micro Systems, Surrey, England). The samples stored in desiccators until testing were equilibrated for 2 h at 25 °C before measurement. Analysis of textural characteristics is based on the procedure of Kortstee et al. (1998). The products were compressed to 25% deformation of their original height with a 25 mm diameter probe at a constant speed (0.5 mm/s). Each test product was compressed twice, each compression being followed by decompression. The time interval between the end of the first compression and the second
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compression was 30 s. Results reported here are averages of the measurements. 2.5. Water absorption index and water solubility index Water absorption index (WAI) and water solubility index (WSI) were based on the procedure of Anderson, Conway, Pfeifer, and Griffin (1969). The concentration used for the various starch matrices was 1.0% (w/v). The samples were dispersed in 10 ml of distilled water in 15 ml centrifuge tubes. The contents of the tubes were mixed with a vortex mixer; then the tubes were kept in a water bath at 25 °C for 1 h with shaking at regular intervals. The tubes were centrifuged at 3000 rpm for 15 min. The supernatants were removed and their solids content determined. The sediments were dried and weighed. The WAI and WSI were calculated by: WAI ¼ Weight of sediment=weight of dry sample WSI ð%Þ ¼ ðWeight of dissolved solid in supernatant= weight of dry solidsÞ 100
217
in ice water. The absorbance of the samples was measured at 550 nm. The digestibility of samples was calculated by: Digestibilityð%Þ glucose contentðmgÞin supernatant solution 100 ¼ weightðmgÞof dried sample 2.7. X-ray diffraction X-ray diffraction patterns of rice starch matrices were obtained by using a high resolution X-ray diffractometer (D8 Discover, Bruker, Germany). The scanning speed and diffraction range of 2h were 1°/min and 10–35°, respectively. As the water content of starch is known to have an influence on its crystallinity, the samples were used after placing them in a desiccator over a silica gel for two weeks. The crystallinity of rice starch matrices was calculated as the proportion of crystalline area to total area at angles between 10 and 35°2h (Cairns, Bogracheva, Ring, Hedley, & Morris, 1997). 3. Results and discussion
2.6. Digestibility by enzyme 3.1. SEM micrographs and photographs The digestibility of freeze-dried starch matrices was determined by measuring the reducing sugar corresponding to the degradation by enzyme (Guraya, Kadan, & Champagne, 1997; Tsuge, Hishida, Iwasaki, Watanabe, & Goshima, 1990). The dried samples (20 mg) were vigorously mixed with 5 ml of 0.02 M acetate buffer (pH 5.4) in a 15 ml centrifuge tube, and then after 30 min added 7 ml glucoamylase solution (about 20 U) to each tube. After incubation for 1 h at 40 °C, the reaction was stopped by adding 2 ml of 25% trichloroacetic acid (TCA). After centrifugation of the solution at 3000 rpm for 15 min, a 0.5 ml of a diluted supernatant was transferred to a test tube and 1.5 ml of DNS reagent (1000 ml distilled water + 7.5 g dinitrosalicylic acid + 14 g NaOH + 216 g potassium sodium tartrate + 5.4 ml phenol + 5.9 g Na2S2O5) was added. The tubes were allowed to stand for 5 min in boiling water and cooled to room temperature
The typical appearance of freeze-dried rice starch matrices had a cylindrical shape as shown in Fig. 1. All samples showed a stable appearance regardless of the processing conditions with the exception of the sample prepared at 1%, 90 °C and 40 min. The sample made at condition of 1%, 90 °C and 40 min was not cylindrical shape but shrinkage state in plastic cylinder mold. This result means that the rice starch solution frozen after heat treatment at this condition was not a continuous phase in consist of amylose and amylopectin (Fig. 2(a)) and the severe shrinkage and collapse in inner structure of frozen samples occurred during freeze-drying. The other samples having a stable cylindrical shape showed the continuous network composed of amylose and amylopectin (Figs. 2(b)–(k)). This indicates that at these preparing conditions the frozen rice starch solution contains a mixture of dispersed gelatinized
Fig. 1. The typical appearance of freeze dried rice starch paste.
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Fig. 2. Scanning electron micrographs of the rice starch matrices formed by freeze drying after thermal treatment at various conditions.
amylose and amylopectin in a continuous phase with fragmented ghost structures in an excluded phase. This continuous network structure composed of amylose and amylopectin will be also affected by freezing rate. Mashl, Flores, and Trivedi (1996) reported that the freezing rate influences on redistribution of starch particles within a frozen sample. But the correlation between microstructure of starch matrix and freezing rate need a further study. The solid rice starch matrices appeared various microstructures according to the heating conditions and starch concentration (Fig. 2). At 1% of starch concentration, all
samples formed a loose filamentous network compared to those prepared at 3% and 5%. Especially the solid starch matrices made at 3% and 5% formed a coarse honeycomb-like structure above 110 °C and 3%, which may have been due to the ice crystal formation during freezing and the sufficient leaching of amylose and amylopectin during heat treatment (Figs. 2(g)–(h) and (j)–(l)). Li et al. (2004) reported that a coarse honeycomb-like network structure in starch solution can be formed below 100 °C. However, they have studied at 10% of starch concentration, and this concentration is above gel-forming concentration (nor-
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219
Fig. 2 (continued)
mally >6%) (Morris, 1998). We think that the formation of honeycomb-like and filamentous network structure below gel-forming concentration needs to a leaching of amylose and amylopectin by a sufficient thermal treatment. 3.2. Mechanical property All regression models fitted to the experimental results of the mechanical properties (hardness, cohesiveness and springiness) of rice starch matrices showed both high correlation coefficients (R2 > 0.95) and significant F values
(Table 2). This indicates that the regression models can be considered adequate to study response tendencies. Overall the rice starch concentration and heating temperature was analyzed to the important factors affecting the mechanical properties compared to holding time (Table 2). The hardness of samples was influenced significantly by the linear and quadratic effects for rice starch concentration and heating temperature and by the interaction effects for these two variables (Table 2). The hardness of samples prepared at 3% and 5% of starch concentration showed an increasing trend as the heating temperature increased
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Table 2 Coefficient of regression models (model: Y = b0 + b1X1 + b2X2 + b3X3 + b11X11 + b22X22 + b33X33 + b12X1X2 + b13X1X3 + b23X2X3 + e; X1, rice starch concentration; X2, heating temperature; X3, holding time) for dependent variables (*P < 0.05; **P < 0.01, respectively) Coefficient
Response variables Hardness
Cohesiveness
b0
409.686
0.747
Linear b1 b2 b3
359.333** 7.166* 0.213
Quadratic b11 b22 b33
5.472* 0.031** 0.004
Interaction b12 b13 b23 R2 p Variation
6.139** 0.110 0.002 0.9997 0.002 5.25
Springiness
WSI
WAI
1.153
276.839
180.147
0.920** 0.017 0.012*
1.307* 0.033 0.016
53.930 4.447* 1.721*
48.955* 3.050* 0.326
174.737* 5.055 0.373
0.071** 0.0001 0.00001**
0.162* 0.0003 0.00008
1.312* 0.020* 0.011
1.851* 0.011* 0.003
4.603* 0.026 0.015*
0.009** 0.001 0.000 0.9991 0.005 1.00
0.003* 0.0008 0.00006 0.9898 0.060 4.59
0.740 0.285 0.006 0.9953 0.028 3.53
0.676* 0.028 0.001 0.9972 0.016 7.89
2.432 0.551 0.019* 0.9956 0.026 2.12
(Fig. 3). But at lower concentration (1%) the hardness of samples did not show a regular trend according to heating temperature and the maximum hardness appeared at heating condition of 110 °C. We assume that the lower hardness value at higher temperature (130 °C) may be attributed to the forming of weaker structure due to starch fragmentation. Alves et al. (1999) also reported that the gel strength formed with greater starch fragmentation at high temperature showed low value compared to those of samples at intermediate temperatures. The cohesiveness of solid rice starch matrices was affected mainly by starch concentration and holding time.
Digestibility 362.423
The regression model for cohesiveness showed linear and quadratic effects for these two variables, whereas the hardness of samples was influenced by starch concentration and heating temperature (Table 2). But the interaction effect of starch concentration and heating temperature was appeared high significant like a hardness of sample. The variation coefficient for regression model of cohesiveness was the lowest value (about 1.00) among response variables tested. This indicates that the data variations were adequately explained. The effects of holding time and starch concentration for the cohesiveness of rice starch matrices prepared at 110 °C are shown in Fig. 4. The cohesiveness
60 50 0.66 0.63
Cohesiveness
Hardness (N)
40 30 20
0.60 0.57 0.54
10
0.51
0
0.48 60
5
5 130
%)
Tem p
e ra t
3
110
u re
( oC)
90 1
ra ent
onc
hc
rc Sta
( tion
Fig. 3. Effect of rice starch concentration and heating temperature on hardness of the solid rice starch matrices prepared at a constant holding time of 40 min.
3
Ho
ldi ng
40
tim e(
S
mi n) 20
c ch tar
ce on
) % n( o i at ntr
1
Fig. 4. Effect of rice starch concentration and holding time on cohesiveness of the rice starch matrices prepared at a constant heating temperature of 110 °C.
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1.0
Springiness
0.9 0.8 0.7 0.6 0.5
5
130
Tem p
3
110
e rat
u re
( oC)
rch
90 1
ion
trat
en onc
) (%
c
Sta
Fig. 5. Effect of rice starch concentration and heating temperature on springiness of the rice starch matrices prepared at a constant holding time of 40 min.
of samples showed a reducing trend regardless of holding time as the starch concentration increased. However, cohesiveness of samples as a function of starch concentration exhibited different trends as the holding time increased. The starch concentration showed the linear (p = 0.025) and quadratic (p = 0.015) effects for springiness of samples, and it also had the interaction (p = 0.039) effects with heating temperature (Table 2). The other dependent variables did not show a significant effect for the springiness. This means that among rice starch concentration, heating temperature and holding time, the rice starch concentration is the most important variable affecting the springiness of sample. The effects of starch concentration and heating temperature for the springiness of samples are shown in Fig. 5. At 1% of starch concentration, springiness of samples showed predominantly an increasing trend with heating temperature, whereas those of samples made at 3% and 5% relatively exhibited a small change as the heating temperature increased. Correlation between springiness and hardness of samples prepared at high temperature (130 °C) showed an inverse behavior (Figs. 3 and 5). This inverse mechanical behavior may be attributed to the structure collapse in sample showing high hardness during the first compression. Keetels, Visser, van Vliet, Jurgens, and Walstra (1996) reported that the resistance to mechanical collapse during the first compression was decreased according to the increase of firmness of bread. We assume that these mechanical properties can be due to the difference of microstructures of samples (Fig. 2). 3.3. Water absorption and water solubility Water absorption index (WAI) and water solubility index (WSI) depend on several factors such as starch ori-
gin, amylose and amylopectin content, isolation procedure and thermal history (Singh & Smith, 1997). The important factors influencing the WAI of solid rice starch matrices were starch concentration and heating temperature among three dependent variables (Table 2). Fig. 6 gives the effect of the concentration and temperature on WAI of samples. The WAI of samples is similar to springiness, and the relative increment according to increasing of heating temperature showed the highest value at 1% of starch concentration. The regression equation of WSI showed highly significant as revealed by R2 (Table 2). The regression coefficients presented in Table 2 showed that WSI was affected by the linear (p = 0.040 and p = 0.027, respectively) effects for holding time and heating temperature and the quadratic (p = 0.037 and p = 0.035, respectively) effects for starch concentration and heating temperature. These results indicate that the most important factor affecting the WSI of samples is a heating temperature. Overall the increase of WSI with increasing temperature is consistent with the results reported by Mercier, Charbonniere, Grebaut, and Gueriviere (1980), but it did not revealed a regular trend according to starch concentration at same heating temperature (Fig. 7). 3.4. Digestibility The regression model showed that starch concentration and holding time in interaction with heating temperature had a significant effect on the digestibility of solid starch matrices (Table 2). Overall the effect of heating temperature and starch concentration on the digestibility was similar to the WSI as shown in Figs. 7 and 8. The digestibility of
20
18
16
WAI
1.1
221
14
12
10 130
Te mp era 110 tur e( o C)
5
90
1
(%) 3 ration ncent o c h c Star
Fig. 6. Effect of rice starch concentration and heating temperature on water absorption index (WAI) of the rice starch matrices prepared at a constant holding time of 40 min.
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7000
Run No. 12 (3%, 130oC, 60 min)
Run No. 4 (5%, 130oC, 40 min) Run No. 3 (1%, 130oC, 40 min)
Intensity [Arbitrary Units]
6000 28
24
WSI (%)
20
5000 4000 3000
Run No. 8 (5%, 110oC, 60 min)
2000
16
Run No. 7 (1%, 110oC, 60 min)
1000 12
0
8 130
Run No. 13 (3%, 110oC, 40 min) 10
15
5
Tem p
e ra t
3
110
u re
( oC)
90 1
rc S ta
hc
on c
en
io trat
n(
20
25
30
35
Diffraction Angle, 2θ (o)
Fig. 9. X-ray diffraction patterns of the solid rice starch matrices prepared at various processing conditions.
%)
Fig. 7. Effect of rice starch concentration and heating temperature on water solubility index (WSI) of the rice starch matrices prepared at a constant holding time of 40 min.
samples revealed an increasing trend with heating temperature regardless of starch concentration, and especially was predominant at high concentration (5%) (Fig. 8). Enhancement of digestibility of samples prepared at higher temperature (130 °C) has been attributed to the high solubility and rupturing of starch granules, which facilitate an easy access for the enzyme to effect hydrolysis (Lee, Brooks, Kim, Heitlinger, & Lebenthal, 1985). Madhusudhan and Tharanathan (1995) also reported that the greater digestibility of rice starch compared to that of ragi starch can be correlated with its high solubility in water.
Table 3 Crystallinity of the solid rice starch matrices prepared at various thermal treatment conditions Run
1 2 3 4 5 6 7 8 9 10 11 12 13
Preparation condition
Crystallinity (%)
Starch (%)
Temperature (°C)
Time (min)
1 5 1 5 1 5 1 5 3 3 3 3 3
90 90 130 130 110 110 110 110 90 130 90 130 110
40 40 40 40 20 20 60 60 20 20 60 60 40
6.1 7.8 5.5 6.7 4.9 7.3 3.3 7.8 7.9 9.2 4.9 9.8 4.3
3.5. X-ray diffraction
100
Digestibility (%)
95 90 85 80 75 70 5 130
Tem 110 p era t u re o ( C)
3
rch
90 1
%)
n(
atio
tr cen
con
Sta
Fig. 8. Effect of rice starch concentration and heating temperature on digestibility of the rice starch matrices prepared at a constant holding time of 40 min.
X-ray diffraction patterns and percentage crystallinity of solid rice starch matrices are shown in Fig. 9 and Table 3. The intensities of X-ray diffraction peaks of samples showed very low values due to the loss of crystallinity during thermal treatment (Fig. 9), and the calculated crystallinity of samples was a range of 3.3–9.8% (Table 3). This result may be attributed to both insufficient gelatinization during heating and partial recrystallization during cooling and freezing. In general, the extent of retrogradation of gelatinized starch depends on cooling rate, and decreased with rapid cooling rate. We assume that the important factors influencing the crystallinity of freeze-dried rice starch matrices are heating conditions, starch concentration, and freezing rate. 4. Conclusions In this paper we have described the characteristics of solid rice starch matrices prepared after thermal treatment of rice starch followed by freeze drying. The present work
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shows correlation between microstructures and physical properties of the samples. The microstructures of samples made at low starch concentration (1–5%) below gel-forming showed a honeycomb-like and filamentous network at a sufficient thermal treatment above 90 °C and 60 min, and these microstructures affected the physical properties of solid rice starch matrices. The regression models on the properties of solid rice starch matrices showed a high correlation coefficients (R2 > 0.95) and significant F values. Overall, among the tested processing conditions, the important factors influencing the physical properties of solid rice starch matrices were starch concentration and heating temperature. Prediction equations for the properties of solid rice starch matrices can be used in selection of processing condition for specific industrial applications. Using adequate combination of processing conditions, it can be possible to obtain a solid rice starch matrix of desirable properties. Acknowledgement We would like to thank the financial support by Korea University Post-Doc. grant in 2003–2004 year. References Ahmad, F. B., & Williams, P. A. (1998). Rheological properties of sago starch. Journal of Agricultural and Food Chemistry, 46, 4060–4065. Alves, R. M. L., Grossmann, M. V. E., & Silva, R. S. S. F. (1999). Gelling properties of extruded yam (Dioscorea alata) starch. Food Chemistry, 67, 123–127. Anderson, R. A., Conway, H. F., Pfeifer, V. F., & Griffin, E. L. Jr., (1969). Gelatinization of corn grits by roll and extrusion cooking. Cereal Science Today, 14, 4–12. Atkin, N. J., Abeysekera, R. M., & Robards, A. W. (1998). The events leading to the formation of ghost remnants from the starch granule surface and the contribution of the granule surface to the gelatinization endotherm. Carbohydrate Polymers, 36, 193–204. Cairns, P., Bogracheva, T. Y., Ring, S. G., Hedley, C. L., & Morris, V. J. (1997). Determination of the polymorphic composition of smooth pea starch. Carbohydrate Polymers, 32, 275–282. Carr, M. E., Wing, R. E., & Doane, W. M. (1991). Encapsulation of atrazine within a starch matrix by extrusion processing. Cereal Chemistry, 68, 262–266. Clark, A. H., & Ross-Murphy, S. B. (1987). Structural and mechanical properties of biopolymer gels. Advances in Polymer Science, 83, 57–192. Garcia, V., Colonna, P., Bouchet, B., & Gallant, D. J. (1997). Structural changes of cassava starch granules after heating at intermediate water contents. Starch/Sta¨rke, 49, 171–179. Guraya, H. S., Kadan, R. S., & Champagne, E. T. (1997). Effect of rice starch-lipid complexes on in vitro digestibility, complexing index, and viscosity. Cereal Chemistry, 74, 561–565.
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Effect of heating condition and starch concentration on the structure and properties of freeze-dried rice starch paste Seog-Won Lee a, Chul Rhee b
b,*
a Department of Food & Nutrition, Yuhan College, 185-34, Goean-dong, Sosa-gu, Bucheon, Kyeonggi-do 422-749, Republic of Korea Division of Food Science, College of Life and Environmental Sciences, Korea University, 1, 5-Ka, Anam-dong Sungbuk-ku, Seoul 136-701, Republic of Korea
Received 17 January 2006; accepted 22 May 2006
Abstract The structure and characteristics of freeze dried rice starch paste were investigated at low starch concentrations (1%, 3% and 5%). The microstructure of samples made at 1% of starch concentration showed a loose filamentous network compared to other concentrations (3% and 5%). Especially the samples prepared at processing conditions above 110 °C and 3% were appeared a coarse honeycomb-like structure. Crystallinity of samples calculated by intensities of X-ray diffraction peaks was very low values (3.3–9.8%). The mechanical properties (hardness, cohesiveness and springiness) of the samples were highly influenced by starch concentration and heating temperature. The relative increment of water absorption index (WAI) according to increasing of heating temperature showed the highest value at 1% of starch concentration, and the heating temperature was analyzed to the important factor affecting WAI and WSI (water solubility index). The digestibility of samples showed an increasing trend with heating temperature regardless of starch concentration. Overall, the important factors influencing the properties of freeze dried rice starch paste were starch concentration and heating temperature. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Rice starch matrix; Freeze drying; Microstructure of rice starch matrix; Mechanical property; Water absorption index; Digestibility
1. Introduction Pregelatinized starches are widely used in the food industry because of their functionality to form viscous solutions, gels, films and encapsulating matrices. The functionalities depend on the physicochemical properties of gelatinized starches, and by changing of raw materials and process parameters it is possible to obtain pregelatinized starches with different functional characteristics. Among the modification methods, a physical modification is one of the important treatments for preparing of starch with functionally desirable attributes. This physical modification has been studied by several methods such as extrusion (Alves, Grossmann, & Silva, 1999), drum drying, spray drying (Herman, Remon, & De Vilder, 1989) and freeze drying technique after heat treatment (Rassis, Saguy, & *
Corresponding author. Tel.: +82 2 3290 3023; fax: +82 2 928 1351. E-mail address: [email protected] (C. Rhee).
0963-9969/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2006.05.005
Nussinovitch, 2002; Sa´nchez, Torrado, & Lastres, 1995). The structure and physicochemical properties of solid starch matrix formed by these methods is determined by the starch concentration, the structure of the swollen starch granule, and heating condition (Lii, Shao, & Tseng, 1995). Morphological and structural changes during starch gelatinization have been observed by scanning electron microscopy (SEM) in various cereals (Atkin, Abeysekera, & Robards, 1998; Garcia, Colonna, Bouchet, & Gallant, 1997; Varriano-Marston, Zeleznak, & Nowontna, 1985). The product needs to satisfy the various physicochemical properties such as mechanical properties, water absorption index, water solubility index, inclusion complexes formation with various molecules, and digestibility. To date, most of previous studies have reported for the solid starch matrix prepared at concentrations where gelation occurs (normally >6%) (Ahmad & Williams, 1998; Carr, Wing, & Doane, 1991; Clark & Ross-Murphy, 1987; Lauro, Ring, Bull, & Poutanen, 1997; Morris, 1998), and under these
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conditions, formation of a partially crystalline precipitate produces a gel during storage. A large amount of knowledge about the structure and physical properties of solid starch matrix prepared at sufficiently high concentrations (>6%) has been known (Li, Vasanthan, Hoover, & Rossnagel, 2004), but the report is not sufficient to understand the characteristics of solid starch matrix at concentrations <6%. Hydrocolloid-based solid matrices can be produced by drying the gel immediately after their production (Nussinovitch, Corradini, Normand, & Peleg, 2000), but this operation is difficult under gel-forming concentration (normally <6%). The choice of drying method and control of processing conditions are very important in the preparation of dried starch matrices because it may influence the characteristics of starch-containing products due to the variation in structure and physical properties. Therefore, the objective of this work were to study the freezing/freeze-drying technique for the production of solid starch matrices under gel-forming concentration, and to investigate the differences in the solid starch matrices’ microstructure and physical properties according to various process parameters (rice starch concentration, heating temperature and time). The influence of process parameters on the structure and physical properties of samples was analyzed by Surface Response Methodology. 2. Materials and methods 2.1. Experimental design and statistical analysis The experiment was performed using an incomplete factorial design with three independent variables at three levels of variation: starch concentration (1%, 3% and 5%), temperature (90, 110 and 130 °C) and holding time (20, 40 and 60 min). The three levels of each variable were coded as 1, 0 and +1 for statistical analysis. A factorial experimental design, consisting of 15 treatments, which included triplication of the center point (Table 1), was used. The dependent variables were mechanical properties (hardness, cohesiveness and springiness), water absorption index, water solubility index, and digestibility. Experimental data analysis were performed by multiple regression analysis using Statistical Analysis System (SAS, 1989) to fit second-order polynomial equations to response variables. 2.2. Preparation of solid rice starch matrices To prepare the solid rice starch matrices, rice starch (S7260, Sigma Chemical Co., USA) suspensions (1%, 3% and 5%, w/v) were prepared by combining of starch and distilled water in a glass bottle. Each starch suspension was heated on a hot plate and agitation during heating was achieved using a magnetic stirrer. Continuous stirring was maintained until a temperature of 80 °C was reached. Samples were immediately transferred to an autoclave
Table 1 Experimental design for solid rice starch matrix Run
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Coded variables
Real variables
X1
X2
X3
Starch (%)
Temperature (°C)
Time (min)
1 +1 1 +1 1 +1 1 +1 0 0 0 0 0 0 0
1 1 +1 +1 0 0 0 0 1 +1 1 +1 0 0 0
0 0 0 0 1 1 +1 +1 1 1 +1 +1 0 0 0
1 5 1 5 1 5 1 5 3 3 3 3 3 3 3
90 90 130 130 110 110 110 110 90 130 90 130 110 110 110
40 40 40 40 20 20 60 60 20 20 60 60 40 40 40
and then heated to 90, 110 and 130 °C and held at these temperatures for 20, 40 or 60 min according to the experimental design. The samples were subsequently cooled at 80 °C and 20 ml of the formed pastes were poured into plastic cylinder (3 cm diameter and 5 cm height). The samples were frozen in a freezer at a temperature of 70 °C and then freeze-dried in a freeze dryer (Ilshin Co., Gyeonggi-do, Korea) operating at 50 °C and 1.33 Pa. The moisture content of the samples was measured with an infrared auto-moisture analyzer (Precisa HA300, Swiss). The freeze-dried samples had low water content (4–5% (w/w)) and elastic structure. The samples were either tested immediately or stored in desiccators until testing. 2.3. Scanning electron microscopy (SEM) The freeze-dried starch matrices were directly mounted on circular aluminum stubs with double-sided sticky tape, coated with 10 nm gold, then examined and photographed in a JEOL (JSM-840 A) scanning electron microscope (JEOL, Ltd., Tokyo, Japan) at an accelerating voltage of 20 kV. The samples were measured with the use of the calibrated scale bar on the micrograph. 2.4. Mechanical test of rice starch matrices The mechanical properties (hardness, cohesiveness and springiness) of rice starch matrices were determined using a Texture Analyzer (TA-XT2, Stable Micro Systems, Surrey, England). The samples stored in desiccators until testing were equilibrated for 2 h at 25 °C before measurement. Analysis of textural characteristics is based on the procedure of Kortstee et al. (1998). The products were compressed to 25% deformation of their original height with a 25 mm diameter probe at a constant speed (0.5 mm/s). Each test product was compressed twice, each compression being followed by decompression. The time interval between the end of the first compression and the second
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compression was 30 s. Results reported here are averages of the measurements. 2.5. Water absorption index and water solubility index Water absorption index (WAI) and water solubility index (WSI) were based on the procedure of Anderson, Conway, Pfeifer, and Griffin (1969). The concentration used for the various starch matrices was 1.0% (w/v). The samples were dispersed in 10 ml of distilled water in 15 ml centrifuge tubes. The contents of the tubes were mixed with a vortex mixer; then the tubes were kept in a water bath at 25 °C for 1 h with shaking at regular intervals. The tubes were centrifuged at 3000 rpm for 15 min. The supernatants were removed and their solids content determined. The sediments were dried and weighed. The WAI and WSI were calculated by: WAI ¼ Weight of sediment=weight of dry sample WSI ð%Þ ¼ ðWeight of dissolved solid in supernatant= weight of dry solidsÞ 100
217
in ice water. The absorbance of the samples was measured at 550 nm. The digestibility of samples was calculated by: Digestibilityð%Þ glucose contentðmgÞin supernatant solution 100 ¼ weightðmgÞof dried sample 2.7. X-ray diffraction X-ray diffraction patterns of rice starch matrices were obtained by using a high resolution X-ray diffractometer (D8 Discover, Bruker, Germany). The scanning speed and diffraction range of 2h were 1°/min and 10–35°, respectively. As the water content of starch is known to have an influence on its crystallinity, the samples were used after placing them in a desiccator over a silica gel for two weeks. The crystallinity of rice starch matrices was calculated as the proportion of crystalline area to total area at angles between 10 and 35°2h (Cairns, Bogracheva, Ring, Hedley, & Morris, 1997). 3. Results and discussion
2.6. Digestibility by enzyme 3.1. SEM micrographs and photographs The digestibility of freeze-dried starch matrices was determined by measuring the reducing sugar corresponding to the degradation by enzyme (Guraya, Kadan, & Champagne, 1997; Tsuge, Hishida, Iwasaki, Watanabe, & Goshima, 1990). The dried samples (20 mg) were vigorously mixed with 5 ml of 0.02 M acetate buffer (pH 5.4) in a 15 ml centrifuge tube, and then after 30 min added 7 ml glucoamylase solution (about 20 U) to each tube. After incubation for 1 h at 40 °C, the reaction was stopped by adding 2 ml of 25% trichloroacetic acid (TCA). After centrifugation of the solution at 3000 rpm for 15 min, a 0.5 ml of a diluted supernatant was transferred to a test tube and 1.5 ml of DNS reagent (1000 ml distilled water + 7.5 g dinitrosalicylic acid + 14 g NaOH + 216 g potassium sodium tartrate + 5.4 ml phenol + 5.9 g Na2S2O5) was added. The tubes were allowed to stand for 5 min in boiling water and cooled to room temperature
The typical appearance of freeze-dried rice starch matrices had a cylindrical shape as shown in Fig. 1. All samples showed a stable appearance regardless of the processing conditions with the exception of the sample prepared at 1%, 90 °C and 40 min. The sample made at condition of 1%, 90 °C and 40 min was not cylindrical shape but shrinkage state in plastic cylinder mold. This result means that the rice starch solution frozen after heat treatment at this condition was not a continuous phase in consist of amylose and amylopectin (Fig. 2(a)) and the severe shrinkage and collapse in inner structure of frozen samples occurred during freeze-drying. The other samples having a stable cylindrical shape showed the continuous network composed of amylose and amylopectin (Figs. 2(b)–(k)). This indicates that at these preparing conditions the frozen rice starch solution contains a mixture of dispersed gelatinized
Fig. 1. The typical appearance of freeze dried rice starch paste.
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Fig. 2. Scanning electron micrographs of the rice starch matrices formed by freeze drying after thermal treatment at various conditions.
amylose and amylopectin in a continuous phase with fragmented ghost structures in an excluded phase. This continuous network structure composed of amylose and amylopectin will be also affected by freezing rate. Mashl, Flores, and Trivedi (1996) reported that the freezing rate influences on redistribution of starch particles within a frozen sample. But the correlation between microstructure of starch matrix and freezing rate need a further study. The solid rice starch matrices appeared various microstructures according to the heating conditions and starch concentration (Fig. 2). At 1% of starch concentration, all
samples formed a loose filamentous network compared to those prepared at 3% and 5%. Especially the solid starch matrices made at 3% and 5% formed a coarse honeycomb-like structure above 110 °C and 3%, which may have been due to the ice crystal formation during freezing and the sufficient leaching of amylose and amylopectin during heat treatment (Figs. 2(g)–(h) and (j)–(l)). Li et al. (2004) reported that a coarse honeycomb-like network structure in starch solution can be formed below 100 °C. However, they have studied at 10% of starch concentration, and this concentration is above gel-forming concentration (nor-
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219
Fig. 2 (continued)
mally >6%) (Morris, 1998). We think that the formation of honeycomb-like and filamentous network structure below gel-forming concentration needs to a leaching of amylose and amylopectin by a sufficient thermal treatment. 3.2. Mechanical property All regression models fitted to the experimental results of the mechanical properties (hardness, cohesiveness and springiness) of rice starch matrices showed both high correlation coefficients (R2 > 0.95) and significant F values
(Table 2). This indicates that the regression models can be considered adequate to study response tendencies. Overall the rice starch concentration and heating temperature was analyzed to the important factors affecting the mechanical properties compared to holding time (Table 2). The hardness of samples was influenced significantly by the linear and quadratic effects for rice starch concentration and heating temperature and by the interaction effects for these two variables (Table 2). The hardness of samples prepared at 3% and 5% of starch concentration showed an increasing trend as the heating temperature increased
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Table 2 Coefficient of regression models (model: Y = b0 + b1X1 + b2X2 + b3X3 + b11X11 + b22X22 + b33X33 + b12X1X2 + b13X1X3 + b23X2X3 + e; X1, rice starch concentration; X2, heating temperature; X3, holding time) for dependent variables (*P < 0.05; **P < 0.01, respectively) Coefficient
Response variables Hardness
Cohesiveness
b0
409.686
0.747
Linear b1 b2 b3
359.333** 7.166* 0.213
Quadratic b11 b22 b33
5.472* 0.031** 0.004
Interaction b12 b13 b23 R2 p Variation
6.139** 0.110 0.002 0.9997 0.002 5.25
Springiness
WSI
WAI
1.153
276.839
180.147
0.920** 0.017 0.012*
1.307* 0.033 0.016
53.930 4.447* 1.721*
48.955* 3.050* 0.326
174.737* 5.055 0.373
0.071** 0.0001 0.00001**
0.162* 0.0003 0.00008
1.312* 0.020* 0.011
1.851* 0.011* 0.003
4.603* 0.026 0.015*
0.009** 0.001 0.000 0.9991 0.005 1.00
0.003* 0.0008 0.00006 0.9898 0.060 4.59
0.740 0.285 0.006 0.9953 0.028 3.53
0.676* 0.028 0.001 0.9972 0.016 7.89
2.432 0.551 0.019* 0.9956 0.026 2.12
(Fig. 3). But at lower concentration (1%) the hardness of samples did not show a regular trend according to heating temperature and the maximum hardness appeared at heating condition of 110 °C. We assume that the lower hardness value at higher temperature (130 °C) may be attributed to the forming of weaker structure due to starch fragmentation. Alves et al. (1999) also reported that the gel strength formed with greater starch fragmentation at high temperature showed low value compared to those of samples at intermediate temperatures. The cohesiveness of solid rice starch matrices was affected mainly by starch concentration and holding time.
Digestibility 362.423
The regression model for cohesiveness showed linear and quadratic effects for these two variables, whereas the hardness of samples was influenced by starch concentration and heating temperature (Table 2). But the interaction effect of starch concentration and heating temperature was appeared high significant like a hardness of sample. The variation coefficient for regression model of cohesiveness was the lowest value (about 1.00) among response variables tested. This indicates that the data variations were adequately explained. The effects of holding time and starch concentration for the cohesiveness of rice starch matrices prepared at 110 °C are shown in Fig. 4. The cohesiveness
60 50 0.66 0.63
Cohesiveness
Hardness (N)
40 30 20
0.60 0.57 0.54
10
0.51
0
0.48 60
5
5 130
%)
Tem p
e ra t
3
110
u re
( oC)
90 1
ra ent
onc
hc
rc Sta
( tion
Fig. 3. Effect of rice starch concentration and heating temperature on hardness of the solid rice starch matrices prepared at a constant holding time of 40 min.
3
Ho
ldi ng
40
tim e(
S
mi n) 20
c ch tar
ce on
) % n( o i at ntr
1
Fig. 4. Effect of rice starch concentration and holding time on cohesiveness of the rice starch matrices prepared at a constant heating temperature of 110 °C.
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1.0
Springiness
0.9 0.8 0.7 0.6 0.5
5
130
Tem p
3
110
e rat
u re
( oC)
rch
90 1
ion
trat
en onc
) (%
c
Sta
Fig. 5. Effect of rice starch concentration and heating temperature on springiness of the rice starch matrices prepared at a constant holding time of 40 min.
of samples showed a reducing trend regardless of holding time as the starch concentration increased. However, cohesiveness of samples as a function of starch concentration exhibited different trends as the holding time increased. The starch concentration showed the linear (p = 0.025) and quadratic (p = 0.015) effects for springiness of samples, and it also had the interaction (p = 0.039) effects with heating temperature (Table 2). The other dependent variables did not show a significant effect for the springiness. This means that among rice starch concentration, heating temperature and holding time, the rice starch concentration is the most important variable affecting the springiness of sample. The effects of starch concentration and heating temperature for the springiness of samples are shown in Fig. 5. At 1% of starch concentration, springiness of samples showed predominantly an increasing trend with heating temperature, whereas those of samples made at 3% and 5% relatively exhibited a small change as the heating temperature increased. Correlation between springiness and hardness of samples prepared at high temperature (130 °C) showed an inverse behavior (Figs. 3 and 5). This inverse mechanical behavior may be attributed to the structure collapse in sample showing high hardness during the first compression. Keetels, Visser, van Vliet, Jurgens, and Walstra (1996) reported that the resistance to mechanical collapse during the first compression was decreased according to the increase of firmness of bread. We assume that these mechanical properties can be due to the difference of microstructures of samples (Fig. 2). 3.3. Water absorption and water solubility Water absorption index (WAI) and water solubility index (WSI) depend on several factors such as starch ori-
gin, amylose and amylopectin content, isolation procedure and thermal history (Singh & Smith, 1997). The important factors influencing the WAI of solid rice starch matrices were starch concentration and heating temperature among three dependent variables (Table 2). Fig. 6 gives the effect of the concentration and temperature on WAI of samples. The WAI of samples is similar to springiness, and the relative increment according to increasing of heating temperature showed the highest value at 1% of starch concentration. The regression equation of WSI showed highly significant as revealed by R2 (Table 2). The regression coefficients presented in Table 2 showed that WSI was affected by the linear (p = 0.040 and p = 0.027, respectively) effects for holding time and heating temperature and the quadratic (p = 0.037 and p = 0.035, respectively) effects for starch concentration and heating temperature. These results indicate that the most important factor affecting the WSI of samples is a heating temperature. Overall the increase of WSI with increasing temperature is consistent with the results reported by Mercier, Charbonniere, Grebaut, and Gueriviere (1980), but it did not revealed a regular trend according to starch concentration at same heating temperature (Fig. 7). 3.4. Digestibility The regression model showed that starch concentration and holding time in interaction with heating temperature had a significant effect on the digestibility of solid starch matrices (Table 2). Overall the effect of heating temperature and starch concentration on the digestibility was similar to the WSI as shown in Figs. 7 and 8. The digestibility of
20
18
16
WAI
1.1
221
14
12
10 130
Te mp era 110 tur e( o C)
5
90
1
(%) 3 ration ncent o c h c Star
Fig. 6. Effect of rice starch concentration and heating temperature on water absorption index (WAI) of the rice starch matrices prepared at a constant holding time of 40 min.
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7000
Run No. 12 (3%, 130oC, 60 min)
Run No. 4 (5%, 130oC, 40 min) Run No. 3 (1%, 130oC, 40 min)
Intensity [Arbitrary Units]
6000 28
24
WSI (%)
20
5000 4000 3000
Run No. 8 (5%, 110oC, 60 min)
2000
16
Run No. 7 (1%, 110oC, 60 min)
1000 12
0
8 130
Run No. 13 (3%, 110oC, 40 min) 10
15
5
Tem p
e ra t
3
110
u re
( oC)
90 1
rc S ta
hc
on c
en
io trat
n(
20
25
30
35
Diffraction Angle, 2θ (o)
Fig. 9. X-ray diffraction patterns of the solid rice starch matrices prepared at various processing conditions.
%)
Fig. 7. Effect of rice starch concentration and heating temperature on water solubility index (WSI) of the rice starch matrices prepared at a constant holding time of 40 min.
samples revealed an increasing trend with heating temperature regardless of starch concentration, and especially was predominant at high concentration (5%) (Fig. 8). Enhancement of digestibility of samples prepared at higher temperature (130 °C) has been attributed to the high solubility and rupturing of starch granules, which facilitate an easy access for the enzyme to effect hydrolysis (Lee, Brooks, Kim, Heitlinger, & Lebenthal, 1985). Madhusudhan and Tharanathan (1995) also reported that the greater digestibility of rice starch compared to that of ragi starch can be correlated with its high solubility in water.
Table 3 Crystallinity of the solid rice starch matrices prepared at various thermal treatment conditions Run
1 2 3 4 5 6 7 8 9 10 11 12 13
Preparation condition
Crystallinity (%)
Starch (%)
Temperature (°C)
Time (min)
1 5 1 5 1 5 1 5 3 3 3 3 3
90 90 130 130 110 110 110 110 90 130 90 130 110
40 40 40 40 20 20 60 60 20 20 60 60 40
6.1 7.8 5.5 6.7 4.9 7.3 3.3 7.8 7.9 9.2 4.9 9.8 4.3
3.5. X-ray diffraction
100
Digestibility (%)
95 90 85 80 75 70 5 130
Tem 110 p era t u re o ( C)
3
rch
90 1
%)
n(
atio
tr cen
con
Sta
Fig. 8. Effect of rice starch concentration and heating temperature on digestibility of the rice starch matrices prepared at a constant holding time of 40 min.
X-ray diffraction patterns and percentage crystallinity of solid rice starch matrices are shown in Fig. 9 and Table 3. The intensities of X-ray diffraction peaks of samples showed very low values due to the loss of crystallinity during thermal treatment (Fig. 9), and the calculated crystallinity of samples was a range of 3.3–9.8% (Table 3). This result may be attributed to both insufficient gelatinization during heating and partial recrystallization during cooling and freezing. In general, the extent of retrogradation of gelatinized starch depends on cooling rate, and decreased with rapid cooling rate. We assume that the important factors influencing the crystallinity of freeze-dried rice starch matrices are heating conditions, starch concentration, and freezing rate. 4. Conclusions In this paper we have described the characteristics of solid rice starch matrices prepared after thermal treatment of rice starch followed by freeze drying. The present work
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shows correlation between microstructures and physical properties of the samples. The microstructures of samples made at low starch concentration (1–5%) below gel-forming showed a honeycomb-like and filamentous network at a sufficient thermal treatment above 90 °C and 60 min, and these microstructures affected the physical properties of solid rice starch matrices. The regression models on the properties of solid rice starch matrices showed a high correlation coefficients (R2 > 0.95) and significant F values. Overall, among the tested processing conditions, the important factors influencing the physical properties of solid rice starch matrices were starch concentration and heating temperature. Prediction equations for the properties of solid rice starch matrices can be used in selection of processing condition for specific industrial applications. Using adequate combination of processing conditions, it can be possible to obtain a solid rice starch matrix of desirable properties. Acknowledgement We would like to thank the financial support by Korea University Post-Doc. grant in 2003–2004 year. References Ahmad, F. B., & Williams, P. A. (1998). Rheological properties of sago starch. Journal of Agricultural and Food Chemistry, 46, 4060–4065. Alves, R. M. L., Grossmann, M. V. E., & Silva, R. S. S. F. (1999). Gelling properties of extruded yam (Dioscorea alata) starch. Food Chemistry, 67, 123–127. Anderson, R. A., Conway, H. F., Pfeifer, V. F., & Griffin, E. L. Jr., (1969). Gelatinization of corn grits by roll and extrusion cooking. Cereal Science Today, 14, 4–12. Atkin, N. J., Abeysekera, R. M., & Robards, A. W. (1998). The events leading to the formation of ghost remnants from the starch granule surface and the contribution of the granule surface to the gelatinization endotherm. Carbohydrate Polymers, 36, 193–204. Cairns, P., Bogracheva, T. Y., Ring, S. G., Hedley, C. L., & Morris, V. J. (1997). Determination of the polymorphic composition of smooth pea starch. Carbohydrate Polymers, 32, 275–282. Carr, M. E., Wing, R. E., & Doane, W. M. (1991). Encapsulation of atrazine within a starch matrix by extrusion processing. Cereal Chemistry, 68, 262–266. Clark, A. H., & Ross-Murphy, S. B. (1987). Structural and mechanical properties of biopolymer gels. Advances in Polymer Science, 83, 57–192. Garcia, V., Colonna, P., Bouchet, B., & Gallant, D. J. (1997). Structural changes of cassava starch granules after heating at intermediate water contents. Starch/Sta¨rke, 49, 171–179. Guraya, H. S., Kadan, R. S., & Champagne, E. T. (1997). Effect of rice starch-lipid complexes on in vitro digestibility, complexing index, and viscosity. Cereal Chemistry, 74, 561–565.
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