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Rheological characterization of corn starch isolated by alkali method
Food Hydrocolloids 24 (2010) 172–177
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Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd
Rheological characterization of corn starch isolated by alkali method Ljubica Dokic´ a, Tamara Dapcˇevic´ b, Veljko Krstonosˇic´ c, *, Petar Dokic´ b, Miroslav HadnaCev b a
Faculty of Technology, University of Novi Sad, Bul. cara Lazara 1, 21000 Novi Sad, Serbia Institute for Food Technology, University of Novi Sad, Bul. cara Lazara 1, 21000 Novi Sad, Serbia c Faculty of Medicine, Department of Pharmacy, University of Novi Sad, Hajduk Veljkova 3, 21000 Novi Sad, Serbia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2009 Accepted 1 September 2009
The pasting properties of alkali-isolated corn starches obtained from corn grits, as well as the rheological behaviour of their 8% gels were investigated. The alkali isolation was performed using different conditions concerning alkali concentration (0.15% and 0.30%), steeping time (30 min and 90 min) and temperature (25 C and 50 C). Alkali-isolated starches exhibited lower pasting temperatures and higher peak viscosities than the wet-milled starch. With respect to the steady shear measurements, alkali starch gels showed higher viscosities and greater extent of thixotropy. Changes in alkali isolation conditions did not influence the changes in gelatinization and peak temperature values. On contrary, peak viscosity was significantly influenced by these factors. Steady shear, as well as dynamic shear tests, showed that gels formed from starch isolated using higher alkali concentration were stronger than those obtained from starch isolated at lower alkali concentration. However, isolation at higher steeping temperature and longer steeping time resulted in starch with weak gel-like behaviour. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Alkali isolation method Corn starch Pasting Rheology
1. Introduction Starch is the major product from corn wet milling industry and is used for various purposes in food, feed, cosmetics and pharmaceutical industrial processes. Because of the increasing market demand in starches and especially in its derivates, which are produced mainly from corn, tapioca or potato starch (Lineback, 1999), there is an increased interest in obtaining starch using methods other than conventional SO2 wet milling. The alkali isolation method was pioneered by Dimler, Davis, Rist, and Hilbert (1944) and modified by Mistry (1991). It has become important because of economic and ecological reasons (Mistry & Eckhoff, 1992). According to that method, starch is isolated from corn flour with alkali solution of low concentration. Compared to wet milling process, steeping time is shorter and steeping temperature is lower (Eckhoff et al., 1999). Optimal alkali concentration, steeping time and temperature resulted in starch with a high yield and degree of purity (Mistry & Eckhoff, 1992). Alkali concentration used for starch isolation (0.05–0.5%) is lower (Thys et al., 2008; Verwimp, Vandeputte, Marrant, & Delcour, 2004)
* Corresponding author. Tel.: þ44 381641861004. E-mail address: [email protected] (V. Krstonosˇic´). 0268-005X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2009.09.002
than that required for production of starch derivates, (5–10%) (McClain, 2001) so it does not provoke granule damage. The alkaliisolated starch granules have X-ray diffraction pattern, birefringence and morphology similar to commercial starch (Mistry & Eckhoff, 1992). These findings suggest that the alkali preferentially attacks the amorphous rather than the crystalline regions (Karim et al., 2008). Viscosity curves obtained by Brabender method showed that alkali starches have lower pasting temperature and higher peak viscosity (Collado & Corke, 1998; Mistry & Eckhoff, 1992). Although many investigations have been made about the properties of alkali-isolated starches, the recent ones are more focused to rice starches (Cardoso, Putaux, Samios, & Silveira, 2007; Cardoso, Samios, & Silveira, 2006; Han & Hamaker, 2002). Among that, much attention was paid on analyzing the granular structure and composition by optical techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (Cardoso et al., 2006; Lee, Htoon, & Paterson, 2007; Mistry & Eckhoff, 1992). Very little work has been done on characterizing the structural changes in alkali-isolated starch responsible for the specific rheological behaviour. In this research starch was isolated under various sets of alkali conditions from degerminated corn grits. The objective was to examine pasting properties of alkali-isolated starches and to compare them with wet-milled starch which served as
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a benchmark. Rheological properties of alkali starch gels were investigated by flow and oscillatory measurements, in order to identify the influence of alkali isolation, as well as the conditions during the isolation, on the changes in gel structure. Moreover, effects of temperature and mechanical force on rheological behaviour of alkali starch gels were followed. 2. Materials and methods
173
Table 1 Conditions of alkali treatment used for obtaining the different alkali starches. Starch sample
Alkali conditions
Alkali starch-SA1 Alkali starch-SA2 Alkali starch-SA3
NaOH concentration [%]
Steeping temperature [ C]
Steeping time [min]
0.15 0.30 0.15
25 25 50
30 30 90
2.1. Materials Wet-milled corn starch (food grade, 10.7% moisture content) was obtained from IPOK, Zrenjanin, Serbia. Corn grits (food grade) were supplied by Corn Flakes, S. Mitrovica, Serbia. Corn used for obtaining wet-milled corn starch and corn grits represented the average sample obtained during harvest 2007. Sodium hydroxide (reagent grade, pallets) was obtained from Zorka, Sˇabac, Serbia. 2.2. Methods 2.2.1. Starch isolation Alkali isolation of corn starch was performed in laboratory condition according to the procedure given by Mistry, Schmidt, Eckhoff, and Sutherlan (1992). Corn grits were used instead of corn flour. Different conditions of alkali treatment are given in Table 1. As it can be seen from the Table 1 influence of steeping temperature and time was monitored as mutual. The reason was the investigation done by Mistry and Eckhoff (1992) according to which these factors do not have significant effect when examined separately. 2.2.2. Preparation of starch gels Gels (8% w/w, dry weight basis) of alkali-isolated starches, as well as of wet-milled corn starch, were prepared using Brabender Viscoamylograph. The samples were heated from 25 C at a rate of 1.5 C/min, at 75 rpm, until the peak temperatures (Tp) were reached. After reaching peak temperature, one portion of paste was taken out (Tp), the second portion of starch paste was homogenized for 10 min with homogenizer (Ultra – Turrax T25 basic, IKA – Werke, Germany), at 9500 r/min and then taken out (Thg), the third portion was prolonged heated for 30 min at Tp (Tht), while the fourth portion was heated up to 95 C (Tm). So obtained starch pastes were cooled to 20 C in order to form gels. The samples which were analyzed after 24 h were kept at 4 C. Parameters obtained from the recorded viscosity profile were: pasting temperature (Tg), peak temperature (Tp) and peak viscosity. 2.2.3. Rheological measurements Rheological measurements were carried out using a HAAKE RheoStress 600 rheometer (Thermo Scientific, Germany) equipped with Z20 cylinder measuring geometry for steady shear tests and with plate/plate, PP60 (diameter: 60 mm) measuring geometry for dynamic shear tests. In order to avoid sample destruction during the loading, gels were cut using a special geometry mould (diameter: 60 mm, height: 20 mm) which corresponded to the shape of measuring geometry. All measurements (steady and dynamic shear) were performed at 20 0.1 C. 2.2.3.1. Determination of flow properties. The flow curves were obtained by registering shear stress at shear rate which was increased from 0 to 700 s1 in 4.0 min, held constant at 700 s1 untill total system destruction, and decreased from 700 to 0 s1 in 4 min. So obtained data were described using Herschel–Bulkley model (Eq.(1)):
s ¼ s0 þ K g_ n
(1)
where s is the shear stress (Pa), s0 is the yield stress (Pa), K is the consistency index (Pa sn), and n is the flow behaviour index. 2.2.3.2. Determination of dynamic properties. The linear viscoelastic range of each paste was identified by oscillatory stress sweep test which was conducted by increasing the stress from 1 to 200 Pa at a constant frequency of 1 Hz. The parameters obtained from dynamic test data were: storage modulus G0 , loss modulus G00 , G0 /G00 , loss tangent tan d(G00 /G0 ) and yield stress (s0 ). They were determined as the means of values that were within the linear viscoelastic range of each gel. 2.2.4. Statistical analysis In order to include the variations between the properties of the starches obtained from different batches, three batches for each isolation method were run and the results were expressed as mean standard deviations for three batches. Significant differences between the results obtained with different isolation methods were analyzed by the Analysis of Variance (ANOVA) and Tukey Test, both using the software Statistica 8.0 (Statsoft, Tulsa, USA). The rheological data were fitted to models using commercial software (RheoWin 4.0 Data Manager, Thermo Scientific, Karlsruhe, Germany). 3. Results and discussion 3.1. Gelatinization properties of alkali starches The characteristic temperatures and viscosities of alkali-isolated starches (SA1, SA2 and SA3) and wet-milled starch (SWM), deduced from the viscoamylograms, are summarized in Table 2. According to these data the pasting temperatures of all alkali starches were lower than that of the wet-milled starch, whereas the peak viscosity of alkali starches was higher than the peak viscosity of wet-milled starch. These results corresponded with the results of the studies done by Mistry and Eckhoff (1992). As reported by Mistry and Eckhoff (1992) alkali increases swelling and hydration of
Table 2 Viscoamylograph data of wet-milled starch (SWM) and alkali starches (SA).a Sample
Pasting temperature [ C]
SWM SA1 SA2 SA3
75 68.5 68.5 71.5
0.5(c) 0.5(a) 0.5(a) 0.5(b)
Peak temperature [ C] 95 80.5 79 80.5
0.5 (2)b(c) 0.5(b) 0.5(a) 0.5(b)
Peak viscosity [BU] 780 860 800 1140
9(a) 11(b) 12(a) 12(c)
a Assays were performed in triplicate. Mean standard deviation values in the same column followed by the same alphabetical superscripts in parenthesis are not significantly different (p > 0.05). b Value in parentheses indicates time (in min) that sample maintained peak temperature.
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174
200 180 160
Shear stress, τ (Pa)
140 120 100 80 60 40
SWM SA1 SA3
20
100
200
300
400
500
. -1 Shear rate, γ (s )
600
700
800
Fig. 1. Flow curves of wet-milled (SWM) and alkali (SA) starch gels prepared by heating the starch suspension to peak temperature Tp.
During gelatinization and pasting, alkali starches, isolated under different conditions, presented similar pasting (Tg) and peak temperatures (Tp). Tg and Tp varied between 68.5–71.5 and 79–80.5 C, respectively. At the same time the extraction conditions used had significant effect on the peak viscosity values. The influence of alkali concentration on the peak viscosity has differed among reports. Mistry and Eckhoff (1992) reported higher peak viscosities for corn starches isolated using higher alkali concentrations, while Zhong, Ibanz, Oh, McKenzie, and Shoemaker (2009) reported lower peak viscosities for rice starches isolated using higher alkali concentrations. Our results supported the report given by Zhong et al. (2009). According to the results starch isolated at lower alkali concentration showed higher peak viscosity than the starch isolated at higher alkali concentration. These results indicated that granule integrity was more weakened when higher concentration of NaOH was used during the treatment. Influence of synergic effect of isolation time and temperature on peak viscosity was even more pronounced. Starch isolated at higher temperature and longer treatment time exhibited significantly higher peak viscosity than those subjected to lower temperature and shorter isolation time. 3.2. Rheological behaviour of alkali starches
starch granules. Karim et al. (2008) have suggested that high swelling capacities of alkali starches can be attributed to the ionization of the hydroxyl group during the alkali treatment. According to their explanation the presence of negative charge on starch molecules results in repulsion and thus facilitates the increase in water percolation within the granule leading to increase in swelling volumes. Moreover, the alkali treatment causes lipids removal (Liukkonen, Kaukovirta-Norja, & Laakso, 1992) and it is well known that lipids have a tendency to interact with the starch granule, preventing complete hydration, which results in lower viscosity development (Thomas & Atwell, 1999).
The influence of alkali treatment on granule structure was more noticeable when gels prepared from alkali and wet-milled starches were subjected to shear induced flow. The rheological behaviour of SA1, SA3 and SWM gels, prepared by heating the starch suspension to Tp, is shown in Fig. 1. Appearance of a hysteresis area in the plot of shear stress versus shear rate meant that all gels exhibited time-dependent (thixotropic) behaviour. The magnitude of the gels thixotropy was estimated as the coefficient of thixotropic breakdown, Kd (Eq.(2)) which is defined as the ratio of the hysteresis area to the area
Fig. 2. Flow curves of alkali (SA) starch gels prepared by heating the starch suspension to peak temperature (Tp), followed by prolonged heating at Tp for 30 min; measured after 1 h and 24 h.
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Table 3 Parameters of alkali starch gels obtained from flow measurements, performed after 1 h and 24 h.a Alkali starch SA1, SA2, SA3, SA1, SA2, SA3,
1h 1h 1h 24h 24h 24h
s0 [Pa] (H.B.)b 42.69 58.59 22.70 10.56 18.94 –
(c)
2.13 3.52(d) 1.82(b) 0.63(a) 1.33(b)
Kb 7.71 5.65 16.50 15.67 9.94 26.75
nb
(ab)
0.46 0.45(a) 1.48(c) 1.57(c) 0.89(b) 2.67(d)
0.54 0.56 0.40 0.44 0.48 0.33
s0 [Pa] (S.P.)c
(e)
0.011 0.008(e) 0.006(b) 0.007(c) 0.006(d) 0.005(a)
78.12 94.24 49.60 82.83 100.44 62.00
Kdd (bc)
8.59 8.48(bd) 3.97(a) 7.04(ce) 8.33(de) 5.62(ab)
0.208 0.274 0.133 0.257 0.365 0.169
0.019(bc) 0.019(d) 0.011(a) 0.021(cd) 0.034(e) 0.002(ab)
a Assays were performed in triplicate. Mean standard deviation values in the same column followed by the same alphabetical superscripts in parenthesis are not significantly different (p > 0.05). b Parameters obtained by fitting the descending curve using Herschel–Bulkley equation. c Yield stress obtained as the start point of ascending curve. d Coefficient of thixotropic breakdown.
beneath the ascending shear curve (Dokic´, Sovilj, Sˇefer, & Rasulic´, 1999):
Kd ¼
Aup Adown Aup
(2)
where Aup and Adown are the areas under ascending and descending flow curves, respectively. Dolz, Gonza´lez, Delgido, Herna´ndez, and Pellicer (2000) used the same parameter, but they called it the relative hysteresis area. According to Dolz et al. (2000) usage of relative hysteresis area instead of the absolute thixotropic areas produces better results in comparatively investigation of systems with highly different viscosities. Coefficient of thixotropic breakdown for wet-milled starch was 0.089 0.008, while for alkali starches SA1 and SA3 was 0.111 0.009 and 0.154 0.014, respectively. Since a coefficient of thixotropic breakdown is an index of the energy needed to destroy the structure of the system, the experimental data indicated that SA1 and SA3 gels needed higher energy to breakdown the structure. That increase in Kd, as well as in apparent viscosity was due to enhanced swelling and hydration of alkali starch granule which resulted in tightly packing of the granules. These changes in structure of SA starches can also be directly ascribed to the increased leaching of amylose from the granule caused by disruption of the granular amorphous region during the alkaline treatment (Karim et al., 2008). The rheological properties of gels prepared from all three alkali starches are presented in Fig. 2. The samples were prepared by heating the suspensions until Tp followed by prolonged heating at Tp for 30 min (Tht). The measurements were performed after 1 h and 24 h in order to get some insights about delayed reordering of the structure segments and new bonds formation. The fact that gels prepared using SA2 starch exhibited the highest coefficient of thixotropic breakdown (Table 3), confirmed the conclusion about the influence of high concentrated alkaline solution on granule integrity. Namely, high concentrated alkaline solution reduced the rigidity of the molecular organization of granule, which led to increased leaching of amylose chains during the prolonged heating (Cardoso et al., 2007). Therefore, the number of hydrogen bonds formed between the amylose molecules during the cooling phase (retrogradation process) was increased. SA2 starch, containing the highest amount of solubilised amylose, exhibited rapid retrogradation, as it can be noticed from the appearance of the characteristics drop in shear stress at the ascending curve, which indicated the abrupt destruction of the system. The gel prepared from SA1 starch, having less amount of amylose spreaded around the granule, needed more time for reordering of polymer chains and formation of more complex structure. Therefore, only the SA1 system recorded after 24 h has shown the drop in shear stress at the beginning of the shearing. The least amount of structuration was recorded for SA3 gels. Their flow curves did not exhibit the characteristic drop in shear stress.
The shape of the hysteresis loop obtained for gel prepared from SA3 starch (Fig. 2) was even similar to that obtained by heating the suspension until Tp (Fig. 1), which indicated that prolonged heating for 30 min, was not enough to destroy completely the integrity of granules. The increase in apparent viscosity (Fig. 2) after prolonged heating and resting for 24 h was due to swelling of granules to a greater extent. Namely, the method of preparing SA3 starch involved heating of the corn grits at a temperature below starch pasting point for prolonged period of time, where small Naþ ions diffused into the granules, thus causing the enhancement of the granule stability through electrostatic interactions between the cations and hydroxyl groups of starch (Oosten, 1979). On the other hand, the remained hydroxyl anions promoted charge screening of the available groups on the starch molecules. As a result of that hydrogen bond formation between starch molecules was hindered. Therefore, the gel formed from SA3 starch was weaker than the gels obtained from SA1 and SA2 starches. The consistency index, the flow behaviour index and the yield stress were derived from the flow curves of three different alkali starches (Table 3) by fitting the descending flow curve with the Herschel–Bulkley model (R2 > 0.99). While the gels exhibited extremely thixotropic behaviour, which manifested as a peak in the ascending curve of hysteresis loop, neither the Herschel–Bulkley nor any other typical viscous model could fit to the ascending flow curve. Therefore, only the parameters obtained by fitting the descending flow curve were reported. Alkali starch isolated at lower NaOH concentration (SA1) had insignificantly lower (p > 0.05) n value than that of starch isolated
Fig. 3. Storage, G0 (full symbols) and loss, G00 (open symbols) modulus as a function of shear stress for alkali (SA) starch gels measured after 1 h of preparation.
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Table 4 Parameters of alkali starch gels obtained from dynamic measurements, performed after 1 h.a Alkali starch SA1, 1h SA2, 1h SA3, 1h
Storage modulus G0 [Pa] (b)
425.3 4.26 531.9 3.19(c) 249.6 1.99(a)
Loss modulus G00 [Pa] (c)
27.3 0.27 22.8 0.14(b) 20.1 0.16(a)
G0 /G00 (b)
15.6 23.3(c) 12.4(a)
Loss tangent tan(d) (b)
0.064 0.043(a) 0.081(c)
Yield stress s0 [Pa] (S.S.)b 61.3 1.226(b) 84.0 3.36(c) 34.1 1.23(a)
a Assays were performed in triplicate. Mean standard deviation values in the same column followed by the same alphabetical superscripts in parenthesis are not significantly different (p > 0.05). b Yield stress obtained from stress sweep measurements.
at higher lime concentration (SA2); the significantly lower (p < 0.05) n value was obtained for starch isolated using higher steeping temperature and longer steeping time (SA3). Besides that, the consistency index value of SA1 was also insignificantly higher (p < 0.05) than n value of SA2, while that of SA3 was the highest. With ageing (samples measured after 24 h) gels exhibited significant decrease (p < 0.05) in value of coefficient of non-Newtonian behaviour (n). Gels prepared of SA3 starch, measured after 24 h showed the highest shear-thinning behaviour. As a result of that, SA3 gels produced flow curve having characteristic shape, which fitting with Herschel–Bulkley model led to illogical (negative) yield stress value. Therefore, for a simple qualitative comparison, the values of start points of ascending flow curves were considered as yield stress values (Table 3). Comparing the values of yield stress (the minimum stress required to make the material flow) and coefficient of thixotropic breakdown, it was observed that alkali starches characterized with higher yield stress values showed greater thixotropies. Oscillatory (dynamic) rheological properties were used along with flow tests to provide insight on the structure of the sample. The obtained rheograms (Fig. 3) showed that the storage modulus (G0 ) was much larger than the loss modulus (G00 ) for all the alkali starch samples, which is typical of gels. This behaviour may be classified rheologically as a strong gel structure (Clark & RossMurphy, 1987). It can be clearly seen that the dynamic tests (Table 4) supported the results of the flow tests. Samples having higher values of the end point of the linear viscoelastic area, obtained by oscillation stress sweep test, also had higher yield points derived from the flow measurements. Values which indicate the gel strength were also in accordance. Namely, alkali starches characterized with
higher G0 /G00 ratio or lower loss tangent (tan d) had higher coefficients of thixotropic breakdown. 3.3. Thermo-mechanical resistance of alkali starches The other effect which determines the physical characteristics of the starch granules, besides the isolation conditions, is the influence of food processing operations, such as cooking and mixing. The method and intensity of these operations determine the structural, sensory and functional properties (Valetudie, Gallant, Bouchet, Colonna, & Champ, 1999) of final starch-based products. To evaluate the influence of cooking and mixing on the rheological properties of alkali starch gel, the SA1 starch was subjected to different thermo-mechanical treatments. The SA1 starch served as a model system, because it was prepared according to the procedure which makes it comparable to the other two gels. In order to perform these experiments, alkali starch–water suspension was heated to Tp (80.5 C), to Tm (95 C), to Tht (to Tp with prolonged heating at Tp for 30 min) and to Thg (to Tp with homogenization at Tp for 10 min). The obtained pastes were cooled down to 20 C and so formed gels were subjected to shear induced force in order to measure their rheological properties. All samples exhibited thixotropic behaviour (Fig. 4). Coefficients of thixotropic breakdown for the samples marked as Tp, Tht, Thg and Tm were: 0.143 0.007, 0.210 0.011, 0.251 0.013 and 0.415 0.054, respectively. Sample which was heated to 95 C (Tm) showed the highest Kd value as well as the highest apparent viscosity. As expected, the increase in both apparent viscosity and Kd was also noticeable in the sample which was prolonged heated (Tht), but the intensity of the increase was less significant. These results indicated that increase in temperature and prolonged heating induced an increase in granule rupturing and amylose and amylopectin leaching. That resulted in an increased number of hydrogen bonds between the polymers and formation of threedimensional network. That occurrence could also be noticed following the shape of the ascending curve which, at shear rate of 212 s1, exhibited significant drop in shear stress indicating the beginning of the system rheodestruction. The system in which the granule rupturing was provoked by mechanical force showed almost the same value of the apparent viscosity as the prolonged heated system. However, Thg gel showed higher value of Kd which indicated that mixing of the sample for 10 min, resulted in better granule rupturing, amylose and amylopectin releasing and distribution and thus, the stronger structure formation than prolonged heating for 30 min. The results for SA2 and SA3 gels were alike. 4. Conclusions
Fig. 4. Flow curves of alkali starch (SA1) subjected to different thermo-mechanical treatments.
The present study has investigated the influence of alkali isolation method, as well as the conditions during the alkali treatment on the properties of obtained corn starches by monitoring the pasting properties and the rheological behaviour of their gels.
L. Dokic´ et al. / Food Hydrocolloids 24 (2010) 172–177
In general, all alkali starches exhibited lower pasting temperatures, higher viscosity and greater thixotropy than the wet-milled starch. It was due to their enhanced swelling capacities which resulted in close-packing of the system. Starch isolated using more concentrated alkali solution had lower peak viscosity and formed stronger gels, which indicated that high lime levels weakened the granule structure, thus provoking the enhanced leakage of the amylose during the starch pasting and greater amount of hydrogen bonds during gelation. On contrary, gels prepared from alkali starch which was obtained using higher steeping temperature and longer steeping time and which had higher peak viscosity were the weak ones. This was a result of ions diffusion within the granule where the cations act as granule stabilizers and protectors, while anions act as charge screening agents thus hindering the structure formation. During examination of thermo-mechanical resistance of the alkali starch, it was determined that heating of the sample at higher temperatures resulted in gel of the greater extent of thixotropy and the highest apparent viscosity. On the other hand, prolonged heated samples (at the same temperature) produced weaker gels than the homogenized samples. Acknowledgements This work was financially supported by Ministry of Science and Technological Development, Republic of Serbia (project no. 20066). References Cardoso, M. B., Putaux, J. L., Samios, D., & Silveira, N. P. (2007). Influence of alkali concentration on the deproteinization and/or gelatinization of rice starch. Carbohydrate Polymers, 70, 160–165. Cardoso, M. B., Samios, D., & Silveira, N. P. (2006). Study of protein detection and ¨rke, ultrastructure of brazilian rice starch during alkaline extraction. Starch/Sta 58, 345–352. Clark, A. H., & Ross-Murphy, S. B. (1987). Structural and mechanical properties of biopolymer gels. Advances in Polymer Science, 83, 57–192.
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Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd
Rheological characterization of corn starch isolated by alkali method Ljubica Dokic´ a, Tamara Dapcˇevic´ b, Veljko Krstonosˇic´ c, *, Petar Dokic´ b, Miroslav HadnaCev b a
Faculty of Technology, University of Novi Sad, Bul. cara Lazara 1, 21000 Novi Sad, Serbia Institute for Food Technology, University of Novi Sad, Bul. cara Lazara 1, 21000 Novi Sad, Serbia c Faculty of Medicine, Department of Pharmacy, University of Novi Sad, Hajduk Veljkova 3, 21000 Novi Sad, Serbia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2009 Accepted 1 September 2009
The pasting properties of alkali-isolated corn starches obtained from corn grits, as well as the rheological behaviour of their 8% gels were investigated. The alkali isolation was performed using different conditions concerning alkali concentration (0.15% and 0.30%), steeping time (30 min and 90 min) and temperature (25 C and 50 C). Alkali-isolated starches exhibited lower pasting temperatures and higher peak viscosities than the wet-milled starch. With respect to the steady shear measurements, alkali starch gels showed higher viscosities and greater extent of thixotropy. Changes in alkali isolation conditions did not influence the changes in gelatinization and peak temperature values. On contrary, peak viscosity was significantly influenced by these factors. Steady shear, as well as dynamic shear tests, showed that gels formed from starch isolated using higher alkali concentration were stronger than those obtained from starch isolated at lower alkali concentration. However, isolation at higher steeping temperature and longer steeping time resulted in starch with weak gel-like behaviour. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Alkali isolation method Corn starch Pasting Rheology
1. Introduction Starch is the major product from corn wet milling industry and is used for various purposes in food, feed, cosmetics and pharmaceutical industrial processes. Because of the increasing market demand in starches and especially in its derivates, which are produced mainly from corn, tapioca or potato starch (Lineback, 1999), there is an increased interest in obtaining starch using methods other than conventional SO2 wet milling. The alkali isolation method was pioneered by Dimler, Davis, Rist, and Hilbert (1944) and modified by Mistry (1991). It has become important because of economic and ecological reasons (Mistry & Eckhoff, 1992). According to that method, starch is isolated from corn flour with alkali solution of low concentration. Compared to wet milling process, steeping time is shorter and steeping temperature is lower (Eckhoff et al., 1999). Optimal alkali concentration, steeping time and temperature resulted in starch with a high yield and degree of purity (Mistry & Eckhoff, 1992). Alkali concentration used for starch isolation (0.05–0.5%) is lower (Thys et al., 2008; Verwimp, Vandeputte, Marrant, & Delcour, 2004)
* Corresponding author. Tel.: þ44 381641861004. E-mail address: [email protected] (V. Krstonosˇic´). 0268-005X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2009.09.002
than that required for production of starch derivates, (5–10%) (McClain, 2001) so it does not provoke granule damage. The alkaliisolated starch granules have X-ray diffraction pattern, birefringence and morphology similar to commercial starch (Mistry & Eckhoff, 1992). These findings suggest that the alkali preferentially attacks the amorphous rather than the crystalline regions (Karim et al., 2008). Viscosity curves obtained by Brabender method showed that alkali starches have lower pasting temperature and higher peak viscosity (Collado & Corke, 1998; Mistry & Eckhoff, 1992). Although many investigations have been made about the properties of alkali-isolated starches, the recent ones are more focused to rice starches (Cardoso, Putaux, Samios, & Silveira, 2007; Cardoso, Samios, & Silveira, 2006; Han & Hamaker, 2002). Among that, much attention was paid on analyzing the granular structure and composition by optical techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (Cardoso et al., 2006; Lee, Htoon, & Paterson, 2007; Mistry & Eckhoff, 1992). Very little work has been done on characterizing the structural changes in alkali-isolated starch responsible for the specific rheological behaviour. In this research starch was isolated under various sets of alkali conditions from degerminated corn grits. The objective was to examine pasting properties of alkali-isolated starches and to compare them with wet-milled starch which served as
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a benchmark. Rheological properties of alkali starch gels were investigated by flow and oscillatory measurements, in order to identify the influence of alkali isolation, as well as the conditions during the isolation, on the changes in gel structure. Moreover, effects of temperature and mechanical force on rheological behaviour of alkali starch gels were followed. 2. Materials and methods
173
Table 1 Conditions of alkali treatment used for obtaining the different alkali starches. Starch sample
Alkali conditions
Alkali starch-SA1 Alkali starch-SA2 Alkali starch-SA3
NaOH concentration [%]
Steeping temperature [ C]
Steeping time [min]
0.15 0.30 0.15
25 25 50
30 30 90
2.1. Materials Wet-milled corn starch (food grade, 10.7% moisture content) was obtained from IPOK, Zrenjanin, Serbia. Corn grits (food grade) were supplied by Corn Flakes, S. Mitrovica, Serbia. Corn used for obtaining wet-milled corn starch and corn grits represented the average sample obtained during harvest 2007. Sodium hydroxide (reagent grade, pallets) was obtained from Zorka, Sˇabac, Serbia. 2.2. Methods 2.2.1. Starch isolation Alkali isolation of corn starch was performed in laboratory condition according to the procedure given by Mistry, Schmidt, Eckhoff, and Sutherlan (1992). Corn grits were used instead of corn flour. Different conditions of alkali treatment are given in Table 1. As it can be seen from the Table 1 influence of steeping temperature and time was monitored as mutual. The reason was the investigation done by Mistry and Eckhoff (1992) according to which these factors do not have significant effect when examined separately. 2.2.2. Preparation of starch gels Gels (8% w/w, dry weight basis) of alkali-isolated starches, as well as of wet-milled corn starch, were prepared using Brabender Viscoamylograph. The samples were heated from 25 C at a rate of 1.5 C/min, at 75 rpm, until the peak temperatures (Tp) were reached. After reaching peak temperature, one portion of paste was taken out (Tp), the second portion of starch paste was homogenized for 10 min with homogenizer (Ultra – Turrax T25 basic, IKA – Werke, Germany), at 9500 r/min and then taken out (Thg), the third portion was prolonged heated for 30 min at Tp (Tht), while the fourth portion was heated up to 95 C (Tm). So obtained starch pastes were cooled to 20 C in order to form gels. The samples which were analyzed after 24 h were kept at 4 C. Parameters obtained from the recorded viscosity profile were: pasting temperature (Tg), peak temperature (Tp) and peak viscosity. 2.2.3. Rheological measurements Rheological measurements were carried out using a HAAKE RheoStress 600 rheometer (Thermo Scientific, Germany) equipped with Z20 cylinder measuring geometry for steady shear tests and with plate/plate, PP60 (diameter: 60 mm) measuring geometry for dynamic shear tests. In order to avoid sample destruction during the loading, gels were cut using a special geometry mould (diameter: 60 mm, height: 20 mm) which corresponded to the shape of measuring geometry. All measurements (steady and dynamic shear) were performed at 20 0.1 C. 2.2.3.1. Determination of flow properties. The flow curves were obtained by registering shear stress at shear rate which was increased from 0 to 700 s1 in 4.0 min, held constant at 700 s1 untill total system destruction, and decreased from 700 to 0 s1 in 4 min. So obtained data were described using Herschel–Bulkley model (Eq.(1)):
s ¼ s0 þ K g_ n
(1)
where s is the shear stress (Pa), s0 is the yield stress (Pa), K is the consistency index (Pa sn), and n is the flow behaviour index. 2.2.3.2. Determination of dynamic properties. The linear viscoelastic range of each paste was identified by oscillatory stress sweep test which was conducted by increasing the stress from 1 to 200 Pa at a constant frequency of 1 Hz. The parameters obtained from dynamic test data were: storage modulus G0 , loss modulus G00 , G0 /G00 , loss tangent tan d(G00 /G0 ) and yield stress (s0 ). They were determined as the means of values that were within the linear viscoelastic range of each gel. 2.2.4. Statistical analysis In order to include the variations between the properties of the starches obtained from different batches, three batches for each isolation method were run and the results were expressed as mean standard deviations for three batches. Significant differences between the results obtained with different isolation methods were analyzed by the Analysis of Variance (ANOVA) and Tukey Test, both using the software Statistica 8.0 (Statsoft, Tulsa, USA). The rheological data were fitted to models using commercial software (RheoWin 4.0 Data Manager, Thermo Scientific, Karlsruhe, Germany). 3. Results and discussion 3.1. Gelatinization properties of alkali starches The characteristic temperatures and viscosities of alkali-isolated starches (SA1, SA2 and SA3) and wet-milled starch (SWM), deduced from the viscoamylograms, are summarized in Table 2. According to these data the pasting temperatures of all alkali starches were lower than that of the wet-milled starch, whereas the peak viscosity of alkali starches was higher than the peak viscosity of wet-milled starch. These results corresponded with the results of the studies done by Mistry and Eckhoff (1992). As reported by Mistry and Eckhoff (1992) alkali increases swelling and hydration of
Table 2 Viscoamylograph data of wet-milled starch (SWM) and alkali starches (SA).a Sample
Pasting temperature [ C]
SWM SA1 SA2 SA3
75 68.5 68.5 71.5
0.5(c) 0.5(a) 0.5(a) 0.5(b)
Peak temperature [ C] 95 80.5 79 80.5
0.5 (2)b(c) 0.5(b) 0.5(a) 0.5(b)
Peak viscosity [BU] 780 860 800 1140
9(a) 11(b) 12(a) 12(c)
a Assays were performed in triplicate. Mean standard deviation values in the same column followed by the same alphabetical superscripts in parenthesis are not significantly different (p > 0.05). b Value in parentheses indicates time (in min) that sample maintained peak temperature.
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174
200 180 160
Shear stress, τ (Pa)
140 120 100 80 60 40
SWM SA1 SA3
20
100
200
300
400
500
. -1 Shear rate, γ (s )
600
700
800
Fig. 1. Flow curves of wet-milled (SWM) and alkali (SA) starch gels prepared by heating the starch suspension to peak temperature Tp.
During gelatinization and pasting, alkali starches, isolated under different conditions, presented similar pasting (Tg) and peak temperatures (Tp). Tg and Tp varied between 68.5–71.5 and 79–80.5 C, respectively. At the same time the extraction conditions used had significant effect on the peak viscosity values. The influence of alkali concentration on the peak viscosity has differed among reports. Mistry and Eckhoff (1992) reported higher peak viscosities for corn starches isolated using higher alkali concentrations, while Zhong, Ibanz, Oh, McKenzie, and Shoemaker (2009) reported lower peak viscosities for rice starches isolated using higher alkali concentrations. Our results supported the report given by Zhong et al. (2009). According to the results starch isolated at lower alkali concentration showed higher peak viscosity than the starch isolated at higher alkali concentration. These results indicated that granule integrity was more weakened when higher concentration of NaOH was used during the treatment. Influence of synergic effect of isolation time and temperature on peak viscosity was even more pronounced. Starch isolated at higher temperature and longer treatment time exhibited significantly higher peak viscosity than those subjected to lower temperature and shorter isolation time. 3.2. Rheological behaviour of alkali starches
starch granules. Karim et al. (2008) have suggested that high swelling capacities of alkali starches can be attributed to the ionization of the hydroxyl group during the alkali treatment. According to their explanation the presence of negative charge on starch molecules results in repulsion and thus facilitates the increase in water percolation within the granule leading to increase in swelling volumes. Moreover, the alkali treatment causes lipids removal (Liukkonen, Kaukovirta-Norja, & Laakso, 1992) and it is well known that lipids have a tendency to interact with the starch granule, preventing complete hydration, which results in lower viscosity development (Thomas & Atwell, 1999).
The influence of alkali treatment on granule structure was more noticeable when gels prepared from alkali and wet-milled starches were subjected to shear induced flow. The rheological behaviour of SA1, SA3 and SWM gels, prepared by heating the starch suspension to Tp, is shown in Fig. 1. Appearance of a hysteresis area in the plot of shear stress versus shear rate meant that all gels exhibited time-dependent (thixotropic) behaviour. The magnitude of the gels thixotropy was estimated as the coefficient of thixotropic breakdown, Kd (Eq.(2)) which is defined as the ratio of the hysteresis area to the area
Fig. 2. Flow curves of alkali (SA) starch gels prepared by heating the starch suspension to peak temperature (Tp), followed by prolonged heating at Tp for 30 min; measured after 1 h and 24 h.
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175
Table 3 Parameters of alkali starch gels obtained from flow measurements, performed after 1 h and 24 h.a Alkali starch SA1, SA2, SA3, SA1, SA2, SA3,
1h 1h 1h 24h 24h 24h
s0 [Pa] (H.B.)b 42.69 58.59 22.70 10.56 18.94 –
(c)
2.13 3.52(d) 1.82(b) 0.63(a) 1.33(b)
Kb 7.71 5.65 16.50 15.67 9.94 26.75
nb
(ab)
0.46 0.45(a) 1.48(c) 1.57(c) 0.89(b) 2.67(d)
0.54 0.56 0.40 0.44 0.48 0.33
s0 [Pa] (S.P.)c
(e)
0.011 0.008(e) 0.006(b) 0.007(c) 0.006(d) 0.005(a)
78.12 94.24 49.60 82.83 100.44 62.00
Kdd (bc)
8.59 8.48(bd) 3.97(a) 7.04(ce) 8.33(de) 5.62(ab)
0.208 0.274 0.133 0.257 0.365 0.169
0.019(bc) 0.019(d) 0.011(a) 0.021(cd) 0.034(e) 0.002(ab)
a Assays were performed in triplicate. Mean standard deviation values in the same column followed by the same alphabetical superscripts in parenthesis are not significantly different (p > 0.05). b Parameters obtained by fitting the descending curve using Herschel–Bulkley equation. c Yield stress obtained as the start point of ascending curve. d Coefficient of thixotropic breakdown.
beneath the ascending shear curve (Dokic´, Sovilj, Sˇefer, & Rasulic´, 1999):
Kd ¼
Aup Adown Aup
(2)
where Aup and Adown are the areas under ascending and descending flow curves, respectively. Dolz, Gonza´lez, Delgido, Herna´ndez, and Pellicer (2000) used the same parameter, but they called it the relative hysteresis area. According to Dolz et al. (2000) usage of relative hysteresis area instead of the absolute thixotropic areas produces better results in comparatively investigation of systems with highly different viscosities. Coefficient of thixotropic breakdown for wet-milled starch was 0.089 0.008, while for alkali starches SA1 and SA3 was 0.111 0.009 and 0.154 0.014, respectively. Since a coefficient of thixotropic breakdown is an index of the energy needed to destroy the structure of the system, the experimental data indicated that SA1 and SA3 gels needed higher energy to breakdown the structure. That increase in Kd, as well as in apparent viscosity was due to enhanced swelling and hydration of alkali starch granule which resulted in tightly packing of the granules. These changes in structure of SA starches can also be directly ascribed to the increased leaching of amylose from the granule caused by disruption of the granular amorphous region during the alkaline treatment (Karim et al., 2008). The rheological properties of gels prepared from all three alkali starches are presented in Fig. 2. The samples were prepared by heating the suspensions until Tp followed by prolonged heating at Tp for 30 min (Tht). The measurements were performed after 1 h and 24 h in order to get some insights about delayed reordering of the structure segments and new bonds formation. The fact that gels prepared using SA2 starch exhibited the highest coefficient of thixotropic breakdown (Table 3), confirmed the conclusion about the influence of high concentrated alkaline solution on granule integrity. Namely, high concentrated alkaline solution reduced the rigidity of the molecular organization of granule, which led to increased leaching of amylose chains during the prolonged heating (Cardoso et al., 2007). Therefore, the number of hydrogen bonds formed between the amylose molecules during the cooling phase (retrogradation process) was increased. SA2 starch, containing the highest amount of solubilised amylose, exhibited rapid retrogradation, as it can be noticed from the appearance of the characteristics drop in shear stress at the ascending curve, which indicated the abrupt destruction of the system. The gel prepared from SA1 starch, having less amount of amylose spreaded around the granule, needed more time for reordering of polymer chains and formation of more complex structure. Therefore, only the SA1 system recorded after 24 h has shown the drop in shear stress at the beginning of the shearing. The least amount of structuration was recorded for SA3 gels. Their flow curves did not exhibit the characteristic drop in shear stress.
The shape of the hysteresis loop obtained for gel prepared from SA3 starch (Fig. 2) was even similar to that obtained by heating the suspension until Tp (Fig. 1), which indicated that prolonged heating for 30 min, was not enough to destroy completely the integrity of granules. The increase in apparent viscosity (Fig. 2) after prolonged heating and resting for 24 h was due to swelling of granules to a greater extent. Namely, the method of preparing SA3 starch involved heating of the corn grits at a temperature below starch pasting point for prolonged period of time, where small Naþ ions diffused into the granules, thus causing the enhancement of the granule stability through electrostatic interactions between the cations and hydroxyl groups of starch (Oosten, 1979). On the other hand, the remained hydroxyl anions promoted charge screening of the available groups on the starch molecules. As a result of that hydrogen bond formation between starch molecules was hindered. Therefore, the gel formed from SA3 starch was weaker than the gels obtained from SA1 and SA2 starches. The consistency index, the flow behaviour index and the yield stress were derived from the flow curves of three different alkali starches (Table 3) by fitting the descending flow curve with the Herschel–Bulkley model (R2 > 0.99). While the gels exhibited extremely thixotropic behaviour, which manifested as a peak in the ascending curve of hysteresis loop, neither the Herschel–Bulkley nor any other typical viscous model could fit to the ascending flow curve. Therefore, only the parameters obtained by fitting the descending flow curve were reported. Alkali starch isolated at lower NaOH concentration (SA1) had insignificantly lower (p > 0.05) n value than that of starch isolated
Fig. 3. Storage, G0 (full symbols) and loss, G00 (open symbols) modulus as a function of shear stress for alkali (SA) starch gels measured after 1 h of preparation.
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Table 4 Parameters of alkali starch gels obtained from dynamic measurements, performed after 1 h.a Alkali starch SA1, 1h SA2, 1h SA3, 1h
Storage modulus G0 [Pa] (b)
425.3 4.26 531.9 3.19(c) 249.6 1.99(a)
Loss modulus G00 [Pa] (c)
27.3 0.27 22.8 0.14(b) 20.1 0.16(a)
G0 /G00 (b)
15.6 23.3(c) 12.4(a)
Loss tangent tan(d) (b)
0.064 0.043(a) 0.081(c)
Yield stress s0 [Pa] (S.S.)b 61.3 1.226(b) 84.0 3.36(c) 34.1 1.23(a)
a Assays were performed in triplicate. Mean standard deviation values in the same column followed by the same alphabetical superscripts in parenthesis are not significantly different (p > 0.05). b Yield stress obtained from stress sweep measurements.
at higher lime concentration (SA2); the significantly lower (p < 0.05) n value was obtained for starch isolated using higher steeping temperature and longer steeping time (SA3). Besides that, the consistency index value of SA1 was also insignificantly higher (p < 0.05) than n value of SA2, while that of SA3 was the highest. With ageing (samples measured after 24 h) gels exhibited significant decrease (p < 0.05) in value of coefficient of non-Newtonian behaviour (n). Gels prepared of SA3 starch, measured after 24 h showed the highest shear-thinning behaviour. As a result of that, SA3 gels produced flow curve having characteristic shape, which fitting with Herschel–Bulkley model led to illogical (negative) yield stress value. Therefore, for a simple qualitative comparison, the values of start points of ascending flow curves were considered as yield stress values (Table 3). Comparing the values of yield stress (the minimum stress required to make the material flow) and coefficient of thixotropic breakdown, it was observed that alkali starches characterized with higher yield stress values showed greater thixotropies. Oscillatory (dynamic) rheological properties were used along with flow tests to provide insight on the structure of the sample. The obtained rheograms (Fig. 3) showed that the storage modulus (G0 ) was much larger than the loss modulus (G00 ) for all the alkali starch samples, which is typical of gels. This behaviour may be classified rheologically as a strong gel structure (Clark & RossMurphy, 1987). It can be clearly seen that the dynamic tests (Table 4) supported the results of the flow tests. Samples having higher values of the end point of the linear viscoelastic area, obtained by oscillation stress sweep test, also had higher yield points derived from the flow measurements. Values which indicate the gel strength were also in accordance. Namely, alkali starches characterized with
higher G0 /G00 ratio or lower loss tangent (tan d) had higher coefficients of thixotropic breakdown. 3.3. Thermo-mechanical resistance of alkali starches The other effect which determines the physical characteristics of the starch granules, besides the isolation conditions, is the influence of food processing operations, such as cooking and mixing. The method and intensity of these operations determine the structural, sensory and functional properties (Valetudie, Gallant, Bouchet, Colonna, & Champ, 1999) of final starch-based products. To evaluate the influence of cooking and mixing on the rheological properties of alkali starch gel, the SA1 starch was subjected to different thermo-mechanical treatments. The SA1 starch served as a model system, because it was prepared according to the procedure which makes it comparable to the other two gels. In order to perform these experiments, alkali starch–water suspension was heated to Tp (80.5 C), to Tm (95 C), to Tht (to Tp with prolonged heating at Tp for 30 min) and to Thg (to Tp with homogenization at Tp for 10 min). The obtained pastes were cooled down to 20 C and so formed gels were subjected to shear induced force in order to measure their rheological properties. All samples exhibited thixotropic behaviour (Fig. 4). Coefficients of thixotropic breakdown for the samples marked as Tp, Tht, Thg and Tm were: 0.143 0.007, 0.210 0.011, 0.251 0.013 and 0.415 0.054, respectively. Sample which was heated to 95 C (Tm) showed the highest Kd value as well as the highest apparent viscosity. As expected, the increase in both apparent viscosity and Kd was also noticeable in the sample which was prolonged heated (Tht), but the intensity of the increase was less significant. These results indicated that increase in temperature and prolonged heating induced an increase in granule rupturing and amylose and amylopectin leaching. That resulted in an increased number of hydrogen bonds between the polymers and formation of threedimensional network. That occurrence could also be noticed following the shape of the ascending curve which, at shear rate of 212 s1, exhibited significant drop in shear stress indicating the beginning of the system rheodestruction. The system in which the granule rupturing was provoked by mechanical force showed almost the same value of the apparent viscosity as the prolonged heated system. However, Thg gel showed higher value of Kd which indicated that mixing of the sample for 10 min, resulted in better granule rupturing, amylose and amylopectin releasing and distribution and thus, the stronger structure formation than prolonged heating for 30 min. The results for SA2 and SA3 gels were alike. 4. Conclusions
Fig. 4. Flow curves of alkali starch (SA1) subjected to different thermo-mechanical treatments.
The present study has investigated the influence of alkali isolation method, as well as the conditions during the alkali treatment on the properties of obtained corn starches by monitoring the pasting properties and the rheological behaviour of their gels.
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In general, all alkali starches exhibited lower pasting temperatures, higher viscosity and greater thixotropy than the wet-milled starch. It was due to their enhanced swelling capacities which resulted in close-packing of the system. Starch isolated using more concentrated alkali solution had lower peak viscosity and formed stronger gels, which indicated that high lime levels weakened the granule structure, thus provoking the enhanced leakage of the amylose during the starch pasting and greater amount of hydrogen bonds during gelation. On contrary, gels prepared from alkali starch which was obtained using higher steeping temperature and longer steeping time and which had higher peak viscosity were the weak ones. This was a result of ions diffusion within the granule where the cations act as granule stabilizers and protectors, while anions act as charge screening agents thus hindering the structure formation. During examination of thermo-mechanical resistance of the alkali starch, it was determined that heating of the sample at higher temperatures resulted in gel of the greater extent of thixotropy and the highest apparent viscosity. On the other hand, prolonged heated samples (at the same temperature) produced weaker gels than the homogenized samples. Acknowledgements This work was financially supported by Ministry of Science and Technological Development, Republic of Serbia (project no. 20066). References Cardoso, M. B., Putaux, J. L., Samios, D., & Silveira, N. P. (2007). Influence of alkali concentration on the deproteinization and/or gelatinization of rice starch. Carbohydrate Polymers, 70, 160–165. Cardoso, M. B., Samios, D., & Silveira, N. P. (2006). Study of protein detection and ¨rke, ultrastructure of brazilian rice starch during alkaline extraction. Starch/Sta 58, 345–352. Clark, A. H., & Ross-Murphy, S. B. (1987). Structural and mechanical properties of biopolymer gels. Advances in Polymer Science, 83, 57–192.
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