Polym. Bull. (2013) 70:3331–3346 DOI 10.1007/s00289-013-1025-x ORIGINAL PAPER
Preparation and properties of partially hydrolyzed cross-linked guar gum Tang Hongbo • Sun Min • Li Yanping • Dong Siqing
Received: 4 February 2013 / Revised: 1 June 2013 / Accepted: 16 August 2013 / Published online: 28 September 2013 Ó Springer-Verlag Berlin Heidelberg 2013
Abstract There has been a growing interest in the modification of guar gum to improve its properties and enlarge its application. The aim of this study was to prepare partially hydrolyzed cross-linked guar gum with a composite modification method, based on guar gum, hydrochloric acid, epichlorohydrin and solvent. To obtain a product with suitable properties for other fields, the effects of various factors such as reaction time, reaction temperature, pH, the amount of ethanol and cross-linking agent on the cross-linking degree of partially hydrolyzed cross-linked guar gum were studied. The stability of the hot paste viscosity, cold paste viscosity, acid resistance, alkali resistance, retrogradation, freeze–thaw stability and swelling power were determined. The product was characterized by differential scanning calorimetry. The sedimentation volume method was selected to determine the crosslinking degree of the product. The best conditions for preparing partially hydrolyzed cross-linked guar gum were: reaction temperature 50 °C, reaction time 4 h, the amount of ethanol 58 %, pH 10.5. After guar gum was modified by acid, or crosslinked by epichlorohydrin, its freeze–thaw stability and expansion capability decreased, but its acid resistance, alkali resistance, cold paste stability and hot paste viscosity stability were improved. Keywords Property
Guar gum Acidolysis Cross-linking Epichlorohydrin
T. Hongbo (&) L. Yanping D. Siqing Science School, Shenyang University of Technology, Shenyang 110870, People’s Republic of China e-mail:
[email protected];
[email protected] S. Min Library, Shenyang Aerospace University, Shenyang 110136, People’s Republic of China
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Introduction Guar gum (GG), a naturally occurring galactomannan polysaccharide, is derived from the ground endosperms of Cyamopsis tetragonolobus. It is made up of a linear chain of b-D-mannopyranose joined by b-(1–4) linkage with a-D-galactopyranosyl units attached by 1, 6-links in the ratio of 1:2 [1, 2]. GG is soluble in cold water without heating to form a highly viscous solution at relatively low concentrations, but it is insoluble in most organic solvents. It has excellent thickening, emulsifying, stabilizing and film forming properties as well as excellent abilities to control the rheology by water phase management. The viscosity of guar gum is influenced by temperature, pH, presence of salts and other solids [3, 4]. GG is widely used as a binding agent or a thickening agent for the food, cosmetics and pharmaceutical industry [5–7]. These applications of guar gum evidence some drawbacks, such as uncontrolled rates of hydration, pH-dependent solubility and high susceptibility to microbial attack. The chemical modification provides an efficient route not only for removing some drawbacks of GG, but also for improving its expansibility and solubility. Guar gum can be modified by the etherification, esterification, oxidation, graft and enzymolysis [8–12]. The derivatization of GG leads to some subtle changes in properties, such as decreasing hydrogen bonding, increasing solubility in water–alcohol mixture and improving electrolyte compatibility [13, 14]. These changes in properties result in increased use in the different fields such as textile printing, eco-friendly water-saving materials, ion-exchange resins, oil–water fracturing agents and so on [15–17]. However, little research has been focused on the partially hydrolyzed crosslinked guar gum (PHCGG). On the basis of the above background, in this work PHCGG was synthesized by the solvent method, characterized by a differential scanning calorimeter (DSC) and a thermogravimetric analyzer (TGA). PHCGG is better used as a food additive due to its very low viscosity in the aqueous media, even at the high concentrations, for ice creams, baked goods, sauces and beverages as fat replacers, binders and water-reducing agents. Guar gum is more soluble than other gums in water. Furthermore, it is also a better emulsifier because it has more galactose branch points. GG shows high lowshear viscosity, but it is strongly shear thinning.
Materials and methods Materials Guar gum was purchased from Binzhou Zhongbo Chemical Co., Ltd. Sodium hydroxide and epichlorohydrin were purchased from Tianjin Bodi Chemical Co., Ltd. Ethanol and hydrochloric acid were purchased from Shenyang City Huadong Reagent Factory and Economic and Technological Development Zone Reagent Factory, respectively. All the above reagents were of analytical grade.
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Preparation of partially hydrolyzed cross-linked guar gum Partially hydrolyzed guar gum A quantity of guar gum powders was weighed, and then ethanol aqueous solution with a concentration of 85 % was added to produce the guar gum slurry with a concentration of 30 % (w/w) by mass basis. The slurry was put into the flask, stirred and heated. A certain quantity of hydrochloric acid with a concentration of 36 % (w/w) (the mass ratio of hydrogen chloride to dry guar gum 1:100) was added after the resultant slurry was heated to the required temperature. Guar gum was hydrolyzed by acid for 4 h at a constant temperature. After the hydrolysis was complete, the slurry was neutralized by sodium hydroxide ethanol aqueous solution with a concentration of 16.7 % (the mass ratio of sodium hydroxide to ethanol 2:5) to a pH of 6.0*7.0. The vacuum filtration of slurry was completed. The obtained filtered cake was washed four times with ethanol aqueous solution at a concentration of 95 %. The resultant cake was dried at 100 °C for about 4 h in a 1010-2 electrothermal constant-temperature dry box (Jintan City Dadi Automation Instrument Factory) so the moisture content could be \12 %. The dried cake was ground and screened. Finally, the partially hydrolyzed guar gum was obtained. Partially hydrolyzed cross-linked guar gum 40 g of precisely weighed partially hydrolyzed guar gum powders (its water content is 7.23 %, its fluidity is 39.8 mL) was put into a four-mouth flask with constant stirring by a mechanical stirrer, and then a quantity of alcohol and water was added to produce the slurry with a concentration of 30 % (w/w) by mass basis. The resultant slurry was heated to the required temperature. After about 5 min, saturated oxyhydrogen sodium ethanol water solution was added to adjust the pH value of the slurry to 10.5. Then a quantity of the cross-linking agent was dropped into the slurry. At the same time, the pH of the slurry was kept constant using a saturated oxyhydrogen sodium ethanol water solution. After the reaction was complete, the slurry was neutralized using 0.5 mol/L hydrochloric acid solution to a pH of 6.0*7.0. The vacuum filtration of the slurry was completed by an SHZ-C vacuum pump with the circulated water system (Gongyi City Yuhua Instrument Co., Ltd.). The filtered cake was then washed with ethanol aqueous solution at a concentration of 95 %. The resultant cake was dried at 100 °C for about 4 h so that its moisture content could be \12 %. The dry cake was ground and screened. Finally, the partially hydrolyzed cross-linked guar gum was obtained. The formula of partially hydrolyzed guar gum (PHGG-OH) reaction with a cross-linking agent (epichlorohydrin) in alkaline catalytic condition was as follows: OH
O 2 PHGG
OH + CH3
CH
CHCl
NaOH
PHGG
O
CH2
CH
CH2
O
PHGG + NaCl
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Determination of fluidity The fluidity of samples was determined by a fluidity meter (Shanghai Hongfu Instrument and Apparatus Co., Ltd.). 0.5 g of dry partially hydrolyzed guar gum was placed in a 125 mL conical flask, and then distilled water was added to bring the total weight to 100 g. The paste was heated in a boiling bath for 5 * 10 min. At the same time, the volume of the paste would be kept at this level during heating. After the paste was cooled to 25 °C, 100 mL of the sample paste was transferred to a funnel preheated to 25 °C, whose tip was closed with a finger. The finger was removed and the stopwatch started. The paste of the sample was collected in a measuring beaker. The volume V in mL was recorded after time T s. The fluidity of samples was represented by this volume V. 100 mL of water was added into the above funnel at 25 °C. The time in seconds for 100 mL of water to run out of the funnel preheated to 25 °C was exactly measured and expressed as T. The greater the fluidity, the greater was the acidolysis degree of guar gum [18]. Sedimentation volume The smaller the sedimentation volume, the higher was the cross-linking degree. 1 g of dry sample was weighed and taken in a 100 mL beaker and distilled water was then added to bring the total weight to 50 g. The paste was heated in a water bath at temperature ranging from 82 to 85 °C for 2 min. The mixture was then stirred thoroughly and cooled to 25 °C. 10 mL of the mixture was transferred to two 10 mL graduated cylinders which were sealed with a cap and centrifuged at 4,000 rpm for 15 min in a TDL80-2B desk centrifuge (ShangHai Anting Scientific Instrument Factory). The clear liquid was decanted and the volume of the clear liquid was determined. The sedimentation volume of the sample was determined to be 10 mL, the point at which the sample completely dissolved in water. The sedimentation volume was expressed by the calculation as [19]: sedimentation volume ¼ 10 V where V is the volume of the clear liquid (mL). Swelling power 0.05 g of precisely weighed dry sample was added to a pre-weighed 10 mL centrifugal tube, and then distilled water was added to give a total volume of water equivalent to 9.95 g. The centrifugal tube was placed in a water bath whose temperature was controlled at 85 °C and shaken continuously for 30 min. The paste was cooled to 25 °C. After capping, the centrifugal tube was centrifuged for 5 min at 3,500 r/min in a TDL80-2B desk centrifuge (ShangHai Anting Scientific Instrument Factory). After that, the supernatant was transferred into an evaporating Petri dish and dried for 180 min at 105 °C in a 1010-2 electrothermal constanttemperature dry box (Jintan City Dadi Automation Instrument Factory). The dried residue was then cooled in the desiccator and weighed for the soluble sample. To
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measure the swelling power, the residual supernatant was carefully removed and discarded. The bottle with the sediment paste was then weighed to give the weight of the swollen sample granules. The result was expressed by the calculation as [19, 20]: Swelling power ð%Þ ¼ Solubility ð%Þ ¼
weight of sediment paste 100 : weight of sample on dry basis (100-% solubility)
weight of dried residue of supernatant 100 : weight of sample on dry basis
Freeze–thaw stability The sample paste was prepared by mixing 0.5 g of dry guar gum or its derivative with 99.5 g of distilled water and then heated up to 95 °C in a 100 mL beaker and kept at this temperature for 10 min before cooling to the ambient temperature. 10 g of the sample paste which was precisely weighed was added to each of the preweighed 10 mL centrifuge tubes, and then the sample paste was frozen at -18 °C for 24 h in a BCD-177A refrigerator (Meiling Group). All tubes were removed from the freezer and thawed at 25 °C in a water bath for 2 h. Three tubes from each thawing cycle of these samples were centrifuged at 3,500 rpm for 15 min. The clear liquid was decanted and the residue was weighed. The separated water percentage was then calculated as the ratio of the mass of the liquid decanted to the total mass of the paste before the centrifugation and multiplied by 100. The lower the separated water percentage, the higher was the freeze–thaw stability [21–23]. Retrogradation The sample paste, with a mass concentration of 0.3 % (w/w) by dry guar gum or its derivative, was produced with distilled water. 50 mL of the paste was taken in a 100 mL beaker and heated in a boiling bath for 10 min. The volume of the paste was kept at this level during heating. The sample paste was then cooled to 25 °C. The transparency of the supernatant was measured, respectively, at the different standing times. The more transparent the gum paste, the stronger was the retrogradation and vice versa [24–28]. Acid resistance and alkali resistance The sample paste with a mass concentration of 1 % (w/w) with dry guar gum derivative was produced with distilled water. The sample paste was heated in a boiling bath under constant agitation until it became a complete paste. Then the paste was cooled to the required temperature in cold water. The pH value of the paste was adjusted to 10 or 3, respectively. The paste viscosity was then measured by an NDJ-1A rotary viscometer at constant temperature (25 °C). The viscosity of the sample changed slightly, which meant strong acid resistance and alkali resistance. The measurement method of acid and alkali resistance of guar gum was similar to that of guar gum derivative [29–32].
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Stability of hot paste viscosity and cold paste viscosity An NDJ-1A rotational viscometer (Shanghai Changji Geological Instrument Company Limited) was used to measure the viscosity of the samples. The hot paste viscosity and the cold paste viscosity referred to the viscosity of a sample at 95 and 50 °C, respectively. The mass concentration of the sample paste was 1 % by dry guar gum or its dry derivative. The stability of the viscosity was determined from Stability of viscosity ð%Þ ¼ 100 fluctuation ratio of viscosity ð%Þ: The fluctuation ratio of the viscosity was expressed as Fluctuation ratio of viscosity ð%Þ ¼
maxjg g0 j 100 g00
where maxjg g0 jis the maximum viscosity difference measured by keeping temperature constant for 60, 90, 120, 150 and 180 min, respectively, at 95 or 50 °C (the paste was cooled to 50 °C after a sample was completely turned into paste at 95 °C, and then its viscosity was measured by a rotational viscometer at 50 °C). g00 is the viscosity measured by keeping temperature constant for 1 h at 95 or 50 °C [33–35]. Thermal analysis The thermal analysis of guar gum or its derivative was carried out with a TGA Q50 V20.10 Build 36 thermogravimetric analyzer and a DSC Q20 V24.4 Build 116 differential scanning calorimeter (TA Instruments) in a nitrogen atmosphere. To properly characterize the thermal properties of guar gum, partially hydrolyzed guar gum and partially hydrolyzed cross-linked guar gum, the mixture needed to be analyzed in a sealed pan to prevent the loss of water from the formulation during heating. Analysis conditions of DSC: sample mass 3.0 * 5.5 mg, heating rate 10 °C/min and temperature range 10 * 200 °C. Analysis conditions of TGA: sample mass 15.0 * 16.0 mg, heating rate 10 °C/ min and temperature range 10 * 900 °C. Particle morphology The particle morphology was observed by an XPL-2 transflective polarizing microscope (Nanjing Jiangnan Yongxin Optics Company Limited). About 5 mg of the sample was placed on a clean slide. A few drops of ethanol were added to the slide. A cover glass was covered on the sample particles and then moved back and forth until these particles could be uniformly dispersed on the slide. The cover glass was removed. The lighting power of the polarizing microscope was opened. An appropriate magnification was selected and then the light source was adjusted and focused. The slide with the sample particles was placed and moved under the object lens of the polarizing microscope. The appropriate viewing area was selected to observe the size and the shape of the sample particles.
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Statistical analysis Data were expressed as means of triplicate determinations. The statistical significance was assessed with the one-way analysis of variance using ORIGIN 7.5 for Windows program. The treatment means were considered significantly different at P B 0.05.
Results and discussion The effect of reaction temperature on sedimentation volume The effect of reaction temperature on the sedimentation volume of partially hydrolyzed cross-linked guar gum is shown in Fig. 1. Reaction conditions: the required amount of epichlorohydrin 5 % (by dry partially hydrolyzed guar gum, w/w), reaction time 4 h, the amount of ethanol 58 % (on the basis of the initial slurry of partially hydrolyzed guar gum, the following just the same) and pH 10.5. From Fig. 1, the sedimentation volume of PHCGG was markedly influenced by the reaction temperature. The sedimentation volume of PHCGG decreased with increasing reaction temperature when the reaction temperature was \50 °C, but the sedimentation volume increased with increasing reaction temperature when the reaction temperature was more than 50 °C. So, the best reaction temperature was 50 °C.
Sedimentation Volume/mL
4.5
4.0
3.5
3.0
2.5 30.0
40.0
50.0
60.0
70.0
Reaction Temperature/°C Fig. 1 Effect of reaction temperature on sedimentation volume of partially hydrolyzed cross-linked guar gum
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The effect of reaction time on sedimentation volume The effect of reaction time on the sedimentation volume of partially hydrolyzed crosslinked guar gum is shown in Fig. 2. Reaction conditions: the required amount of epichlorohydrin 5 %, reaction temperature 50 °C, amount of ethanol 58 % and pH 10.5. From Fig. 2, the sedimentation volume of PHCGG decreased with increasing reaction time when the reaction time was \4 h. The cross-linking reaction was somewhat rapid within the range of 2 h. So, the best reaction time was 4 h. The effect of pH on sedimentation volume The effect of pH on the sedimentation volume of partially hydrolyzed cross-linked guar gum is shown in Fig. 3. Reaction conditions: the required amount of epichlorohydrin 5 %, reaction temperature 50 °C, amount of ethanol 58 % and reaction time 4 h. The cross-linking reaction of guar gum should be in alkaline environments. Sodium hydroxide acted as a catalyst and an expansive agent during the crosslinking reaction. From Fig. 3, the sedimentation volume of PHCGG reduced with increasing pH when pH was\10.5, but increased when pH was more than 10.5. The slight expansion of guar gum particles was beneficial to the diffusion of the crosslinking agent into particles. However, the excessive expansion of guar gum particles derived from high pH retarded the diffusion of the cross-linking agent into the particles. So, the best pH was 10.5. The effect of the amount of ethanol (solvent) on sedimentation volume The effect of the amount of ethanol on the sedimentation volume of partially hydrolyzed cross-linked guar gum is shown in Fig. 4. Reaction conditions: the
Sedimentation Volume/mL
4.5
4.0
3.5
3.0
2.5 0.0
1.0
2.0
3.0
4.0
5.0
6.0
Reaction Time/h Fig. 2 Effect of reaction time on sedimentation volume of partially hydrolyzed cross-linked guar gum
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Sedimentation Volume/mL
5.0
4.5
4.0
3.5
3.0
2.5 9.5
10.0
10.5
11.0
11.5
pH Fig. 3 Effect of pH on sedimentation volume of partially hydrolyzed cross-linked guar gum
Sedimentation Volume/mL
5.0
4.5
4.0
3.5
3.0
2.5 46
48
50
52
54
56
58
60
62
64
Amount of Ethanol/% Fig. 4 Effect of amount of ethanol on sedimentation volume of partially hydrolyzed cross-linked guar gum
required amount of epichlorohydrin 5 %, reaction temperature 50 °C, pH 10.5 and reaction time 4 h. Guar gum powder was dispersed by ethanol to keep guar gum in the original particle state during the cross-linking reaction. Ethanol as a solvent did not participate in the reaction. The addition of the solvent prevents the expansion of guar gum particles. From Fig. 4, the sedimentation volume of partially hydrolyzed
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cross-linked guar gum reduced with increase in the amount of ethanol when the amount of ethanol was \58 %, but increased when the amount of ethanol was more than 58 %. It proved that increasing the amount of ethanol effectively reduced guar gum molecules reacting with the cross-linking agent. More ethanol means that more cross-linking agent and water were evaporated so that the molecular collision probability between the guar gum and the cross-linking agent was lowered. So, the best amount of ethanol was 58 %. The effect of the amount of epichlorohydrin on sedimentation volume The effect of the amount of epichlorohydrin on the sedimentation volume of partially hydrolyzed cross-linked guar gum is shown in Fig. 5. Reaction conditions: amount of ethanol 58 %, reaction temperature 50 °C, pH 10.5 and reaction time 4 h. From Fig. 5 the sedimentation volume of PHCGG decreased with increase in the amount of epichlorohydrin. So, the amount of epichlorohydrin was selected to be 5 % by considering the cross-linking degree of guar gum in the use. Freeze–thaw stability, acid resistance and alkali resistance The freeze–thaw stability, acid resistance and alkali resistance of guar gum, partially hydrolyzed guar gum (fluidity 39.8 mL, the same below) and partially hydrolyzed cross-linked guar gum (fluidity 39.8 mL, sedimentation volume 2.8 mL; the fluidity was referred to that of partially hydrolyzed guar gum during preparing partially hydrolyzed cross-linked guar gum, the same as below.) are shown in Table 1.
Sedimentation Volume/mL
6.0 5.0 4.0 3.0 2.0 1.0 0.0 2.0
4.0
6.0
8.0
10.0
12.0
Amount of Epichlorohydrin/% Fig. 5 Effect of amount of epichlorohydrin on sedimentation volume of partially hydrolyzed crosslinked guar gum
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From Table 1, the viscosity of 1 % aqueous solution of guar gum was up to 9,480 mPa s, whereas that of a 1 % solution of partially hydrolyzed guar gum or partially hydrolyzed cross-linked guar gum was\120 mPa s, where this value was an ideal viscosity for increasing the dietary fiber content in beverages. Its freeze–thaw stability became poor after guar gum was partially hydrolyzed by hydrochloric acid, or cross-linked with epichlorohydrin. However, acidolysis and cross-linking increased the alkali resistance and acid resistance of guar gum. The viscosity of guar gum, partially hydrolyzed guar gum and partially hydrolyzed cross-linked guar gum increased in acidic or alkaline environment. The viscosity change rate of partially hydrolyzed crosslinked guar gum was less than that of partially hydrolyzed guar gum. Swelling power and viscosity stability The swelling power and the viscosity stability of guar gum, partially hydrolyzed guar gum and partially hydrolyzed cross-linked guar gum are shown in Table 2. Table 2 shows that the swelling power of guar gum is decreased by acidolysis and cross-linking. However, the viscosity stability of guar gum was increased due to acidolysis and cross-linking. The fluctuation ratio of the cold paste viscosity of guar gum, partially hydrolyzed guar gum and partially hydrolyzed cross-linked guar gum was more than that of the hot paste viscosity. Cross-linking further improved the viscosity stability of partially hydrolyzed guar gum. Retrogradation The retrogradation of guar gum, partially hydrolyzed guar gum and partially hydrolyzed cross-linked guar gum are shown in Fig. 6. Table 1 Freeze–thaw stability, alkali resistance and acid resistance Sample
Separated water percentage/%
Viscosity/mPa s (pH = 6.5)
Viscosity/mPa s (pH = 3)
Viscosity/mPa s (pH = 10)
Guar gum
86.2 ± 1
9,480 ± 280
10,500 ± 310
10,900 ± 320
Partially hydrolyzed guar gum
89.9 ± 1
90.2 ± 3.2
94.0 ± 3.3
98.6 ± 3.4
Partially hydrolyzed cross -linked guar gum
93.2 ± 1
116.6 ± 4.0
120.8 ± 4.2
125.0 ± 4.3
Table 2 Swelling power and viscosity stability Sample
Swelling power/%
Fluctuation ratio of hot viscosity/%
Fluctuation ratio of cold viscosity/%
Guar gum
53.4 ± 0.4
32.0 ± 0.5
47.2 ± 0.5
Partially hydrolyzed guar gum
46.5 ± 0.5
28.2 ± 0.5
29.6 ± 0.5
Partially hydrolyzed cross-linked guar gum
42.7 ± 0.6
22.8 ± 0.6
25.7 ± 0.5
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From Fig. 6, the retrogradation of guar gum was strengthened after guar gum was modified by acid and epichlorohydrin. The retrogradation of partially hydrolyzed cross-linked guar gum was that between guar gum and partially hydrolyzed guar gum. It was shown that acidolysis cut the long chains of guar gum molecules into short chains. Thermal analysis The DSC and TGA curves of guar gum, partially hydrolyzed guar gum and partially hydrolyzed cross-linked guar gum are shown in Figs. 7 and 8. The DSC curve of partially hydrolyzed cross-linked guar gum is located at the top in Fig. 7. The thermal property of guar gum was changed obviously by acidolysis and cross-linking. From Fig. 7, the corresponding onset temperature (To), peak temperature (Tp), end temperature (Te) and melting enthalpy (DH) were obtained by an analysis. The results are listed in Table 3. Acidolysis resulted in the increase of the onset temperature, peak temperature and end temperature of guar gum, but reduced its melting enthalpy. The cross-linking reduced obviously the end temperature of partially hydrolized guar gum. However, the cross-linking increased evidently its peak temperature. The melting enthalpy of guar gum was decreased by acidolysis and cross-linking, and this melting enthalpy of partially hydrolyzed crosslinked guar gum reached 124.3 J/g. It was proved that the structure of the guar gum particles was destroyed owing to acidolysis and cross-linking. From Fig. 8, the corresponding onset decomposition, the end decomposition temperature and the rate of weight loss of guar gum, partially hydrolyzed guar gum and partially hydrolyzed cross-linked guar gum were obtained by an analysis; the results are listed in Table 4. Table 4 shows that the onset decomposition temperature, the end decomposition temperature and the rate of weight loss of guar gum were influenced by acidolysis and cross-linking. The onset decomposition temperature, end decomposition
100.0
Transparency/%
90.0 80.0 70.0 60.0 50.0 40.0 30.0
0.0
1.0
2.0
3.0
4.0
5.0
Sedimentation Time/h Fig. 6 Retrogradation of guar gum (a), partially hydrolyzed guar gum (b) and partially hydrolyzed crosslinked guar gum (c)
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2.0
Heat Flow/mW
0.0
c
-2.0 b
a
-4.0
-6.0
40
80
120
160
200
Temperature/°C Fig. 7 DSC curve of guar gum (a), partially hydrolyzed guar gum (b) and partially hydrolyzed crosslinked guar gum (c)
Rate of Residual Mass/%
100
80
b
a 60
40
c 20
0 0
100
200
300
400
500
600
700
800
900
Temperature/°C Fig. 8 TGA curve of guar gum (a), partially hydrolyzed guar gum (b) and partially hydrolyzed crosslinked guar gum (c)
temperature of partially hydrolyzed guar gum and partially hydrolyzed cross-linked guar gum were less than those of guar gum, but the rate of weight loss was more than that of guar gum. The thermal characteristic parameters were influenced significantly by acidolysis, compared with cross-linking. The thermal stability of guar gum was reduced by acidolysis and cross-linking. Particle morphology The particle morphology of the samples was observed with a polarizing microscope. The polarizing microscope photos of guar gum, partially hydrolyzed guar gum and
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Table 3 Onset temperature, peak temperature, end temperature and melting enthalpy To/°C
Sample
Tp/°C
DH/J g-1
Te/°C
Guar gum
27.9 ± 0.1
106.8 ± 0.1
164.0 ± 0.1
227.6 ± 0.5
Partially hydrolyzed guar gum
44.6 ± 0.1
110.7 ± 0.1
168.8 ± 0.1
216.2 ± 0.5
Partially hydrolyzed cross-linked guar gum
43.7 ± 0.1
113.0 ± 0.1
165.7 ± 0.1
124.3 ± 0.5
Table 4 Onset decomposition temperature, end decomposition temperature and rate of weight loss Sample
Onset decomposition temperature/°C
End decomposition temperature/°C
Rate of weight loss/%
Guar gum
227.6 ± 0.1
440.4 ± 0.2
60.7 ± 0.1
Partially hydrolyzed guar gum
220.1 ± 0.1
428.0 ± 0.2
69.8 ± 0.1
Partially hydrolyzed crosslinked guar gum
219.0 ± 0.1
422.4 ± 0.2
70.6 ± 0.1
Fig. 9 Polarizing microscope photos of guar gum (a), partially hydrolyzed guar gum (b) and partially hydrolyzed cross-linked guar gum (c)
partially hydrolyzed cross-linked guar gum are shown in Fig. 9. From Fig. 9, there was no Maltese cross on the surface of the particles of guar gum, which were very different from the particles of starch. The shape of guar gum particles was irregular
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and thick. After guar gum was partially hydrolyzed or partially hydrolyzed crosslinked, however, its appearance was changed to a fine strip and its size was also reduced. The structure of guar gum granules was apparently destroyed by this acid hydrolysis.
Conclusions The method of preparing partially hydrolyzed cross-linked guar gum with low viscosity was practical using hydrochloric acid as the acidolysis agent, epichlorohydrin as the cross-linking agent and ethanol as the solvent. The best reaction conditions for preparing partially hydrolyzed cross-linked guar gum were: reaction temperature 50 °C, reaction time 4 h, pH 10.5 and the amount of ethanol 58 %. The viscosity stability, alkali resistance and acid resistance of guar gum were improved by acidolysis and cross-linking. However, the freeze–thaw stability became poor after guar gum was modified by acidolysis or acidolysis cross-linking. The swelling power of guar gum was reduced by acidolysis and cross-linking. The retrogradation of partially hydrolyzed cross-linked guar gum was between guar gum and partially hydrolyzed guar gum. The melting enthalpy and thermal stability of guar gum were decreased by acidolysis and cross-linking. The effect of crosslinking on the thermal properties of guar gum was different from acidolysis. The slight cross-linking of guar gum was not able to make up for the destruction from acidolysis owing to guar gum being modified by acid. The structure of guar gum granules was apparently destroyed by acid hydrolysis.
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