Accepted Manuscript Title: Preparation of hydroxypropyl agars and their properties Author: Xiaodong Zhang Xin Liu Mingzhao Cao Kai Xia Yuqiao Zhang PII: DOI: Reference:
S0144-8617(15)00372-0 http://dx.doi.org/doi:10.1016/j.carbpol.2015.04.056 CARP 9887
To appear in: Received date: Revised date: Accepted date:
2-2-2015 8-4-2015 18-4-2015
Please cite this article as: Zhang, X., Liu, X., Cao, M., Xia, K., and Zhang, Y.,Preparation of hydroxypropyl agars and their properties, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.04.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
1
The physicochemical properties of hydroxypropyl agar were studied. The gel skeleton structure of hydroxypropyl agar was characterized by Cyro-SEM. Verified that the hygroscopicity of agar was increased after hydroxypropylation. TPA of hydroxypropyl agar gel was carried out.
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Preparation of hydroxypropyl agars and their properties
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Xiaodong Zhang*, Xin Liu, Mingzhao Cao, Kai Xia, Yuqiao Zhang
9
College of Chemical Science and Engineering, Qingdao University, Qingdao 266071, China
10
*Corresponding author: Tel.: +86 532 85955589; fax: +86 532 85950518. E-mail address: [email protected].
11
Abstract: A series of hydroxypropyl agars (HPAs) with different hydroxypropyl molar substitution (MS) were
12
prepared and their physicochemical properties were characterized. After hydroxypropylation, the dissolving
13
temperature, the gelling temperature, the gel melting temperature, the gel strength, and the thermal stability of agar all
14
decreased except that its hygroscopicity increased. The gel skeleton structures of raw agar and HPAs were all of the
15
porous network structures, but the pores of gel skeleton structure of HPAs became smaller and denser.
16
Key words:
17
1. Introduction
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Agar; Hydroxypropylation; Gelling temperature; Thermal stability; Optical rotation.
Hydroxypropylation is a method which is often used to modify the properties of the polysaccharides, and there have
19
been a large number of hydroxypropyl polysaccharides which have been reported, such as hydroxypropyl starch
20
(El-Rafie et al., 2011), hydroxypropyl cellulose (Abdel-Halim & Al-Deyab, 2011), hydroxypropyl methylcellulose
21
(White, McBride, Pate, Tieppo, & Byrne, 2011), hydroxypropyl β-cyclodextrin (Zeng, Ren, Zhou, Yu, & Chen, 2011)
22
and hydroxypropyl chitosan (Wang, Tao, Zhang, Wei, & Ren, 2010). Agar, as a quality natural polysaccharide, has a
23
large range of applications, especially in food, medical and biological industries, and its properties have been
24
researched in detail (Arnott et al., 1974; Sousa, Borges, Silva, & Gonçalves, 2013). However, there have been few
25
literatures and reports on the physicochemical properties of the modified agars except for agar acetate (Xia, Liu, Zhao,
26
& Zhang, 2014), oxidized agar (Zhang, Liu, Xia, & Luan, 2014) and carboxymethyl agar (Cao, Liu, Luan, & Zhang,
27
2014), which have been reported by our research group recently. In this paper, a series of hydroxypropyl agars with
28
different MSs were prepared and their physicochemical properties were characterized.
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2. Experiment
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2.1 Materials Gracilaria agar (3,6-anhydro-L-galactose (3,6-AG) content: 31.60%; sulfate content: 3.72%; gel strength: 1070
32
g/cm2) was purchased from Qingdao Agar Manufacturing Company, Shandong Province, China. All reagent used in
33
this research were all of reagent grade and purchased from Laiyang Chemical Reagent Plant, Shandong Province,
34
China.
35
2.2 The preparation of a series of HPAs with different MSs
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The reaction solution containing agar (30.0 g), sodium hydroxide (0.3 g), deionized water (210 g) and a certain
37
amount of epoxy propane was added in a sealed 500 mL flask equipped with a magnetic stirrer and a temperature
38
controller, and kept at 45°C to react for 12 h, then cooled, neutralized, filtered, and washed three times with 60%
39
ethanol. The product was dried in an oven at 60°C. The MS of HPA was determined by the method described by
40
Lemieux and Purves (1947).
41
2.3 Characterizations
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The gelling temperatures, gel melting temperatures and gel strength were measured based on the methods described
43
by Freile-Pelegrín and Robledo (1997). The measurements above were all performed on a 1.5 wt% sample solution.
44
Sulfate content and 3,6-AG content were determined based on the methods described by Matos, Fortunato, Velizarov,
45
Reis, and Crespo (2008) and Yaphe and Arsenault (1965), respectively. The dissolving temperature, the FT-IR spectra,
46
rheological properties, optical rotation, cryo scanning electron microscopy (Cryo-SEM), textural profile analysis (TPA)
47
and thermogravimetric analyses were all measured as described by Cao, Liu, Luan, and Zhang (2014). The
48
measurement of optical rotation was performed on a 0.05 wt% sample solution. The measurement of TPA was
49
performed at 25°C.
50
3. Results and discussion
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Analysis of IR and Cryo-SEM
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Fig.1 The FT-IR spectra of raw agar and HPA, (a): raw agar; (b): HPA (MS=0.557)
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Fig.2 Cryo-SEM images of the gel skeleton structures of raw agar and HPAs, (a1, a2): raw agar; (b1, b2): HPA 4
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(MS=0.075); (c1, c2): HPA (MS=0.305) Fig.1 indicated that there appeared the new absorption peaks at 2968 cm−1 and at 1195 cm-1 in the FT-IR spectrum
59
of HPA (curve b) as compared with the FT-IR spectrum of raw agar (curve a), which indicated the presence of methyl
60
group (-CH3) and the hydroxypropyl ether bond in HPA molecule respectively. Fig.2 indicated that the gel skeleton
61
structures of raw agar and HPAs were all of the porous network microstructures, and the pores of gel skeleton structure
62
of raw agar tended to be smaller and denser after hydroxypropylation.
63
Physicochemical properties
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Table 1 and Fig.3 showed that the dissolving temperature, the gelling temperature, the gel melting temperature,
65
the gel strength, sulfate content and 3,6-AG content of HPA were all decreased with the increase of MS. Especially,
66
the MS values dependence of dissolving temperature, gelling temperature and gel melting temperature of HPA were
67
in good linear correlation in the range from 0 to 0.557. This result was not completely the same as that of
68
carboxymethyl agars (Cao, Liu, Luan, & Zhang, 2014) and agar acetates (Xia, Liu, Zhao, & Zhang, 2014). According
69
to the gelling mechanism of agar solution (Arnott et al., 1974), the gel strength of agar decreased after
70
hydroxypropylation may be due to the introduction of hydroxypropyl group into agar molecules, which may hinder
71
the gathering between two agar molecules during the gelling process.
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Table 1 Physicochemical properties of HPAs with different MSs MS of HPA
Physicochemical properties
0
0.075
0.176
0.305
0.419
0.557
Dissolving temperature (°C)
90
88
86
84
82
80
Gelling temperature (°C)
38
36
34
32
30
28
Gel melting temperature (°C)
78
77
76
74
73
72
Gel strength (g/cm2)
1070
670
510
400
300
200
5
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3,6-AG content (%)
31.60
31.26
30.58
29.87
29.27
28.58
Sulfate content (%)
3.72
3.68
3.60
3.51
3.44
3.36
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Optical rotation and rheology
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Fig.3 MS values dependence of gelling temperature, melting temperature, dissolving temperature and gel strength
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78 79 80
Fig.4 Temperature dependence of the optical rotation and apparent viscosity for solutions of raw agar and HPAs on cooling
6
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Fig.4 shows that the values of solution optical rotation and apparent viscosity of HPA all decreased with the
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increase of MS, and the variation trend of temperature dependence of the optical rotation and apparent viscosity of
83
HPA solutions were also similar to that of agar, carboxymethyl agars and agar acetates. The temperatures of gel
84
formation point of samples, calculated from Fig.4 (Plashchina et al., 1986), were about 39°C (raw agar), 37°C
85
(MS=0.075), 34°C (MS=0.176), 32°C (MS=0.305), 30°C (MS=0.419), 27°C (MS=0.557) respectively, which were
86
consistent with corresponding values of gelling temperature in Table 1.
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Textural properties
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88 89 90
Fig.5. MS dependence of the TPA parameters of HPA gels at 25°C As shown in Fig.5, the gel TPA parameters including fracturability, adhesiveness, hardness, springiness, 7
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chewiness and gumminess of HPA gels all decreased with the increase of MS of HPA, and the MS dependence of the
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gel adhesiveness, the gel springiness and gel gumminess of HPA gels were in good linear correlation (their correlation
93
coefficients were RA =-0.9966, RS =-0.9971 and RG =-0.9941 respectively). These results were quite different from that
94
of carboxymethyl agars (Cao, Liu, Luan, & Zhang, 2014), which DS dependence of the gel hardness was in good linear
95
correlation, and the gel springiness increased with the increase of its DS. Moreover, the results that the gel springiness
96
of agar became small after hydroxypropylation meant that the gel of HPA was easy to be broken into many small
97
pieces, and could be supported by Fig.2, which showed that the pores of porous skeleton structures of HPA gels tended
98
to be smaller and denser after hydroxypropylation. The reason why the gel hardness decreased following the
99
hydroxypropylation was same as that of the gel strength decreased after hydroxypropylation.
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Thermogravimetric analysis (TG)
101 102
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Fig.6. The thermogravimetric analysis of raw agar and HPA (MS＝0.305)
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TG curves of agar and HPA (MS=0.305) are shown in Fig.6. The initial slight loss in weight, due to the moisture
104
drying, was 18.63% and 19.88% for raw agar and HPA respectively, which indicated that the hygroscopicity of HPA
105
was better than that of raw agar. Then a rather sharp break in raw agar and HPA thermogram, meaning the onset of the
106
decomposition process, were at 280°C and 260°C for agar and HPA respectively, which indicated that the thermal 8
Page 8 of 13
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stability of raw agar decreased after hydroxypropylation.
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4. Conclusions Hydroxypropyl agars with different degree of molar substitution were prepared, and their properties were
110
determined and analyzed. The physicochemical properties of agar, including the gelling temperature, the melting
111
temperature, the gel strength and the thermal stability, all decreased except that the hygroscopicity of agar increased
112
after hydroxypropylation. Analysis of optical rotation and rheology showed that the temperature of the transition end
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point and the solution apparent viscosity values of hydroxypropyl agar were all lower than that of raw agar under the
114
same conditions. The cryo-SEM showed that the gel skeleton microstructures of HPA were of porous network structure.
115
TPA parameters (the hardness, the fracturability, the springiness, the chewiness, the adhesiveness and the gumminess)
116
of HPAs gel were all decreased with the increase of its MS. Thermogravimetric analysis showed that the thermal
117
stability of agar decreased after hydroxypropylation.
118
References
119
Abdel-Halim, E. S., & Al-Deyab, S. S. (2011). Utilization of hydroxypropyl cellulose for green and efficient synthesis
121 122 123 124 125
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of silver nanoparticles. Carbohydrate Polymers, 86(4), 1615-1622.
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Arnott, S., Fulmer, A., Scott, W. E., Dea, I. C. M., Moorhouse, R., & Rees, D. A. (1974). The agarose double helix and its function in agarose gel structure. Journal of Molecular Biology, 90, 269-284. Cao, M., Liu, X., Luan, J., & Zhang, X. (2014). Characterization of physicochemical properties of carboxymethyl agar. Carbohydrate Polymers, 111, 449-455. El-Rafie, M. H., El-Naggar, M. E., Ramadan, M. A., Fouda, M. M., Al-Deyab, S. S., & Hebeish, A. (2011).
126
Environmental
synthesis
of
silver
nanoparticles
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characterization. Carbohydrate Polymers, 86(2), 630-635.
using
hydroxypropyl
starch
and
their
9
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129 130 131
Freile-Pelegrín, Y., & Robledo, D. (1997). Influence of alkali treatment on agar from Gracilaria cornea from Yucatan, Mexico. Journal of applied Phycology, 9(6), 533-539. Lemieux, R. U., & Purves, C. B. (1947). Quantitative estimation as acetic acid of acetyl, ethylidene, ethoxy, and
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α-hydroxyethyl groups. Canadian journal of research, 25(5), 485-489.
Matos, C. T., Fortunato, R., Velizarov, S., Reis, M. A., & Crespo, J. G. (2008). Removal of mono-valent oxyanions
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from water in an ion exchange membrane bioreactor: Influence of membrane permselectivity. Water
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research, 42(6), 1785-1795.
138 139 140
us
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Sousa, A. M., Borges, J., Silva, A. F., & Gonçalves, M. P. (2013). Influence of the extraction process on the rheological
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κ-carrageenan above the coil-helix transition temperature range. Carbohydrate polymers, 6(1), 15-34.
and structural properties of agars. Carbohydrate polymers, 96(1), 163-171. Wang, J., Tao, X., Zhang, Y., Wei, D., & Ren, Y. (2010). Reversion of multidrug resistance by tumor targeted delivery
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Plashchina, I. G., Muratalieva, I. R., Braudo, E. E., & Tolstoguzov, V. B. (1986). Studies of the gel formation of
te
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cr
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of antisense oligodeoxynucleotides in hydroxypropyl-chitosan nanoparticles. Biomaterials, 31(15), 4426-4433. White, C. J., McBride, M. K., Pate, K. M., Tieppo, A., & Byrne, M. E. (2011). Extended release of high molecular
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weight hydroxypropyl methylcellulose from molecularly imprinted, extended wear silicone hydrogel contact
143
lenses. Biomaterials, 32(24), 5698-5705.
144 145 146 147 148 149
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Xia, K., Liu, X., Zhao, J., & Zhang, X. (2014). The physicochemical property characterization of agar acetate. Carbohydrate Polymers, 110, 32-37.
Yaphe, W., & Arsenault, G. P. (1965). Improved resorcinol reagent for the determination of fructose, and of 3, 6-anhydrogalactose in polysaccharides. Analytical Biochemistry, 13(1), 143-148. Zeng, J., Ren, Y., Zhou, C., Yu, S., & Chen, W. H. (2011). Preparation and physicochemical characteristics of the complex of edaravone with hydroxypropyl-β-cyclodextrin. Carbohydrate Polymers, 83(3), 1101-1105.
10
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150 151
Zhang, X., Liu, X., Xia, K., & Luan, J. (2014). Preparation of oxidized agar and characterization of its properties. Carbohydrate polymers, 112, 583-586.
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Page 11 of 13
Figure captions
152
Fig.1 The FT-IR spectra of raw agar and HPA, (a): raw agar; (b): HPA (MS=0.557)
154
Fig.2 Cryo-SEM images of the gel skeleton structures of raw agar and HPAs, (a1, a2): raw agar; (b1, b2): HPA
155
(MS=0.075); (c1, c2): HPA (MS=0.305)
156
Fig.3 MS values dependence of gelling temperature, melting temperature, dissolving temperature and gel strength
157
Fig.4 Temperature dependence of the optical rotation and apparent viscosity for solutions of raw agar and HPAs on
158
cooling
159
Fig.5 MS dependence of the TPA parameters of HPA gels at 25°C
160
Fig.6 The thermogravimetric analysis of raw agar and HPA (MS=0.305)
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Table 1 Physicochemical properties of HPAs with different MSs MS of HPA Physicochemical properties 0.176
0.305
Dissolving temperature (°C)
90
88
86
84
Gelling temperature (°C)
38
36
34
32
Gel melting temperature(°C)
78
76
75
74
Gel strength (g/cm2)
1070
670
510
3,6-AG content (%)
31.60
31.26
Sulfate content (%)
3.72
3.68
0.557
82
80
30
28
72
400
300
200
30.58
29.87
29.27
28.58
an
3.51
3.44
3.36
3.60
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0.419
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0.057
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0
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S0144-8617(15)00372-0 http://dx.doi.org/doi:10.1016/j.carbpol.2015.04.056 CARP 9887
To appear in: Received date: Revised date: Accepted date:
2-2-2015 8-4-2015 18-4-2015
Please cite this article as: Zhang, X., Liu, X., Cao, M., Xia, K., and Zhang, Y.,Preparation of hydroxypropyl agars and their properties, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.04.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
1
The physicochemical properties of hydroxypropyl agar were studied. The gel skeleton structure of hydroxypropyl agar was characterized by Cyro-SEM. Verified that the hygroscopicity of agar was increased after hydroxypropylation. TPA of hydroxypropyl agar gel was carried out.
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Page 1 of 13
Preparation of hydroxypropyl agars and their properties
8
Xiaodong Zhang*, Xin Liu, Mingzhao Cao, Kai Xia, Yuqiao Zhang
9
College of Chemical Science and Engineering, Qingdao University, Qingdao 266071, China
10
*Corresponding author: Tel.: +86 532 85955589; fax: +86 532 85950518. E-mail address: [email protected].
11
Abstract: A series of hydroxypropyl agars (HPAs) with different hydroxypropyl molar substitution (MS) were
12
prepared and their physicochemical properties were characterized. After hydroxypropylation, the dissolving
13
temperature, the gelling temperature, the gel melting temperature, the gel strength, and the thermal stability of agar all
14
decreased except that its hygroscopicity increased. The gel skeleton structures of raw agar and HPAs were all of the
15
porous network structures, but the pores of gel skeleton structure of HPAs became smaller and denser.
16
Key words:
17
1. Introduction
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Agar; Hydroxypropylation; Gelling temperature; Thermal stability; Optical rotation.
Hydroxypropylation is a method which is often used to modify the properties of the polysaccharides, and there have
19
been a large number of hydroxypropyl polysaccharides which have been reported, such as hydroxypropyl starch
20
(El-Rafie et al., 2011), hydroxypropyl cellulose (Abdel-Halim & Al-Deyab, 2011), hydroxypropyl methylcellulose
21
(White, McBride, Pate, Tieppo, & Byrne, 2011), hydroxypropyl β-cyclodextrin (Zeng, Ren, Zhou, Yu, & Chen, 2011)
22
and hydroxypropyl chitosan (Wang, Tao, Zhang, Wei, & Ren, 2010). Agar, as a quality natural polysaccharide, has a
23
large range of applications, especially in food, medical and biological industries, and its properties have been
24
researched in detail (Arnott et al., 1974; Sousa, Borges, Silva, & Gonçalves, 2013). However, there have been few
25
literatures and reports on the physicochemical properties of the modified agars except for agar acetate (Xia, Liu, Zhao,
26
& Zhang, 2014), oxidized agar (Zhang, Liu, Xia, & Luan, 2014) and carboxymethyl agar (Cao, Liu, Luan, & Zhang,
27
2014), which have been reported by our research group recently. In this paper, a series of hydroxypropyl agars with
28
different MSs were prepared and their physicochemical properties were characterized.
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Page 2 of 13
29
2. Experiment
30
2.1 Materials Gracilaria agar (3,6-anhydro-L-galactose (3,6-AG) content: 31.60%; sulfate content: 3.72%; gel strength: 1070
32
g/cm2) was purchased from Qingdao Agar Manufacturing Company, Shandong Province, China. All reagent used in
33
this research were all of reagent grade and purchased from Laiyang Chemical Reagent Plant, Shandong Province,
34
China.
35
2.2 The preparation of a series of HPAs with different MSs
us
cr
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The reaction solution containing agar (30.0 g), sodium hydroxide (0.3 g), deionized water (210 g) and a certain
37
amount of epoxy propane was added in a sealed 500 mL flask equipped with a magnetic stirrer and a temperature
38
controller, and kept at 45°C to react for 12 h, then cooled, neutralized, filtered, and washed three times with 60%
39
ethanol. The product was dried in an oven at 60°C. The MS of HPA was determined by the method described by
40
Lemieux and Purves (1947).
41
2.3 Characterizations
te
d
M
an
36
The gelling temperatures, gel melting temperatures and gel strength were measured based on the methods described
43
by Freile-Pelegrín and Robledo (1997). The measurements above were all performed on a 1.5 wt% sample solution.
44
Sulfate content and 3,6-AG content were determined based on the methods described by Matos, Fortunato, Velizarov,
45
Reis, and Crespo (2008) and Yaphe and Arsenault (1965), respectively. The dissolving temperature, the FT-IR spectra,
46
rheological properties, optical rotation, cryo scanning electron microscopy (Cryo-SEM), textural profile analysis (TPA)
47
and thermogravimetric analyses were all measured as described by Cao, Liu, Luan, and Zhang (2014). The
48
measurement of optical rotation was performed on a 0.05 wt% sample solution. The measurement of TPA was
49
performed at 25°C.
50
3. Results and discussion
Ac ce p
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3
Page 3 of 13
Analysis of IR and Cryo-SEM
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52
Fig.1 The FT-IR spectra of raw agar and HPA, (a): raw agar; (b): HPA (MS=0.557)
M
53
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55 56
Fig.2 Cryo-SEM images of the gel skeleton structures of raw agar and HPAs, (a1, a2): raw agar; (b1, b2): HPA 4
Page 4 of 13
57
(MS=0.075); (c1, c2): HPA (MS=0.305) Fig.1 indicated that there appeared the new absorption peaks at 2968 cm−1 and at 1195 cm-1 in the FT-IR spectrum
59
of HPA (curve b) as compared with the FT-IR spectrum of raw agar (curve a), which indicated the presence of methyl
60
group (-CH3) and the hydroxypropyl ether bond in HPA molecule respectively. Fig.2 indicated that the gel skeleton
61
structures of raw agar and HPAs were all of the porous network microstructures, and the pores of gel skeleton structure
62
of raw agar tended to be smaller and denser after hydroxypropylation.
63
Physicochemical properties
us
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Table 1 and Fig.3 showed that the dissolving temperature, the gelling temperature, the gel melting temperature,
65
the gel strength, sulfate content and 3,6-AG content of HPA were all decreased with the increase of MS. Especially,
66
the MS values dependence of dissolving temperature, gelling temperature and gel melting temperature of HPA were
67
in good linear correlation in the range from 0 to 0.557. This result was not completely the same as that of
68
carboxymethyl agars (Cao, Liu, Luan, & Zhang, 2014) and agar acetates (Xia, Liu, Zhao, & Zhang, 2014). According
69
to the gelling mechanism of agar solution (Arnott et al., 1974), the gel strength of agar decreased after
70
hydroxypropylation may be due to the introduction of hydroxypropyl group into agar molecules, which may hinder
71
the gathering between two agar molecules during the gelling process.
M
d
te
Ac ce p
72 73
an
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Table 1 Physicochemical properties of HPAs with different MSs MS of HPA
Physicochemical properties
0
0.075
0.176
0.305
0.419
0.557
Dissolving temperature (°C)
90
88
86
84
82
80
Gelling temperature (°C)
38
36
34
32
30
28
Gel melting temperature (°C)
78
77
76
74
73
72
Gel strength (g/cm2)
1070
670
510
400
300
200
5
Page 5 of 13
3,6-AG content (%)
31.60
31.26
30.58
29.87
29.27
28.58
Sulfate content (%)
3.72
3.68
3.60
3.51
3.44
3.36
an
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Optical rotation and rheology
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Fig.3 MS values dependence of gelling temperature, melting temperature, dissolving temperature and gel strength
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75
78 79 80
Fig.4 Temperature dependence of the optical rotation and apparent viscosity for solutions of raw agar and HPAs on cooling
6
Page 6 of 13
Fig.4 shows that the values of solution optical rotation and apparent viscosity of HPA all decreased with the
82
increase of MS, and the variation trend of temperature dependence of the optical rotation and apparent viscosity of
83
HPA solutions were also similar to that of agar, carboxymethyl agars and agar acetates. The temperatures of gel
84
formation point of samples, calculated from Fig.4 (Plashchina et al., 1986), were about 39°C (raw agar), 37°C
85
(MS=0.075), 34°C (MS=0.176), 32°C (MS=0.305), 30°C (MS=0.419), 27°C (MS=0.557) respectively, which were
86
consistent with corresponding values of gelling temperature in Table 1.
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Textural properties
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M
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87
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88 89 90
Fig.5. MS dependence of the TPA parameters of HPA gels at 25°C As shown in Fig.5, the gel TPA parameters including fracturability, adhesiveness, hardness, springiness, 7
Page 7 of 13
chewiness and gumminess of HPA gels all decreased with the increase of MS of HPA, and the MS dependence of the
92
gel adhesiveness, the gel springiness and gel gumminess of HPA gels were in good linear correlation (their correlation
93
coefficients were RA =-0.9966, RS =-0.9971 and RG =-0.9941 respectively). These results were quite different from that
94
of carboxymethyl agars (Cao, Liu, Luan, & Zhang, 2014), which DS dependence of the gel hardness was in good linear
95
correlation, and the gel springiness increased with the increase of its DS. Moreover, the results that the gel springiness
96
of agar became small after hydroxypropylation meant that the gel of HPA was easy to be broken into many small
97
pieces, and could be supported by Fig.2, which showed that the pores of porous skeleton structures of HPA gels tended
98
to be smaller and denser after hydroxypropylation. The reason why the gel hardness decreased following the
99
hydroxypropylation was same as that of the gel strength decreased after hydroxypropylation.
cr
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Thermogravimetric analysis (TG)
101 102
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Fig.6. The thermogravimetric analysis of raw agar and HPA (MS＝0.305)
103
TG curves of agar and HPA (MS=0.305) are shown in Fig.6. The initial slight loss in weight, due to the moisture
104
drying, was 18.63% and 19.88% for raw agar and HPA respectively, which indicated that the hygroscopicity of HPA
105
was better than that of raw agar. Then a rather sharp break in raw agar and HPA thermogram, meaning the onset of the
106
decomposition process, were at 280°C and 260°C for agar and HPA respectively, which indicated that the thermal 8
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stability of raw agar decreased after hydroxypropylation.
108
4. Conclusions Hydroxypropyl agars with different degree of molar substitution were prepared, and their properties were
110
determined and analyzed. The physicochemical properties of agar, including the gelling temperature, the melting
111
temperature, the gel strength and the thermal stability, all decreased except that the hygroscopicity of agar increased
112
after hydroxypropylation. Analysis of optical rotation and rheology showed that the temperature of the transition end
113
point and the solution apparent viscosity values of hydroxypropyl agar were all lower than that of raw agar under the
114
same conditions. The cryo-SEM showed that the gel skeleton microstructures of HPA were of porous network structure.
115
TPA parameters (the hardness, the fracturability, the springiness, the chewiness, the adhesiveness and the gumminess)
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of HPAs gel were all decreased with the increase of its MS. Thermogravimetric analysis showed that the thermal
117
stability of agar decreased after hydroxypropylation.
118
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Figure captions
152
Fig.1 The FT-IR spectra of raw agar and HPA, (a): raw agar; (b): HPA (MS=0.557)
154
Fig.2 Cryo-SEM images of the gel skeleton structures of raw agar and HPAs, (a1, a2): raw agar; (b1, b2): HPA
155
(MS=0.075); (c1, c2): HPA (MS=0.305)
156
Fig.3 MS values dependence of gelling temperature, melting temperature, dissolving temperature and gel strength
157
Fig.4 Temperature dependence of the optical rotation and apparent viscosity for solutions of raw agar and HPAs on
158
cooling
159
Fig.5 MS dependence of the TPA parameters of HPA gels at 25°C
160
Fig.6 The thermogravimetric analysis of raw agar and HPA (MS=0.305)
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Table 1 Physicochemical properties of HPAs with different MSs MS of HPA Physicochemical properties 0.176
0.305
Dissolving temperature (°C)
90
88
86
84
Gelling temperature (°C)
38
36
34
32
Gel melting temperature(°C)
78
76
75
74
Gel strength (g/cm2)
1070
670
510
3,6-AG content (%)
31.60
31.26
Sulfate content (%)
3.72
3.68
0.557
82
80
30
28
72
400
300
200
30.58
29.87
29.27
28.58
an
3.51
3.44
3.36
3.60
us
73
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0.419
ip t
0.057
cr
0
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