Article pubs.acs.org/JPCB
In Situ Observations of Thermoreversible Gelation and Phase Separation of Agarose and Methylcellulose Solutions under High Pressure Noritsugu Kometani,*,† Masahiro Tanabe,† Lei Su,‡ Kun Yang,‡ and Katsuyoshi Nishinari§ †
Department of Applied Chemistry & Bioengineering, Graduate School of Engineering, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan ‡ The High Pressure Research Center of Science and Technology, Zhengzhou University of Light Industry, Zhengzhou 450002, China § Glyn O. Phillips Hydrocolloids Research Centre, School of Food and Pharmaceutical Engineering, Faculty of Light Industry, Hubei University of Technology, Wuchang, Wuhan 430068, China ABSTRACT: Thermoreversible sol−gel transitions of agarose and methylcellulose (MC) aqueous solutions on isobaric cooling or heating under high pressure up to 400 MPa have been investigated by in situ observations of optical transmittance and falling-ball experiments. For agarose, which undergoes the gelation on cooling, the application of pressure caused a gradual rise in the cloud-point temperature over the whole pressure range examined, which is almost consistent with the pressure dependence of gelling temperature estimated by falling-ball experiments, suggesting that agarose gel is stabilized by compression and that the gelation occurs nearly in parallel with phase separation under ambient and high-pressure conditions. For MC, which undergoes the gelation on heating, the cloud-point temperature showed a slight rise with an initial elevation of pressure up to ∼150 MPa, whereas it showed a marked depression above 200 MPa. In contrast, the gelling temperature of MC, which is nearly identical to the cloud-point temperature at ambient pressure, showed a monotonous rise with increasing pressure up to 350 MPa, which means that MC undergoes phase separation prior to gelation on heating under high pressure above 200 MPa. Similar results were obtained for the melting process of MC gel on cooling. The unique behavior of the sol−gel transition of MC under high pressure has been interpreted in terms of the destruction of hydrophobic hydration by compression.
1. INTRODUCTION Agarose is a natural polysaccharide polymer known as a primary component of agar and has been used widely in foods and separation technologies. The repeating unit of agarose typically consists of (1,4)-linked 3,6-anhydro-α-L-galactose and (3,6)linked β-D-galactose. The aqueous solution of agarose undergoes gelation when it is cooled below ∼35 °C. The gel formation results from the development of an infinite 3D network of agarose fibers held together by hydrogen bonding.1 The sol−gel transition of agarose is thermoreversible and therefore the agarose gel can be melted back to an initial sol state by heating. Methylcellulose (MC) is a compound derived from cellulose in which some of hydroxyl residues on the chain are substituted by hydrophobic methoxy groups. MC has found wide applications in foods, cosmetics, and pharmaceutical industries. Unlike cellulose, MC is soluble in cold water because the interand intrachain hydrogen bonds are broken by methoxy substituents. A more intriguing feature of MC is the thermoreversible gelation of its aqueous solution by “heating”, that is, the inverse behavior of agarose gelation. The gelation mechanism of MC has been extensively studied. In a sol state, © 2015 American Chemical Society
methyl groups of MC are surrounded by structured water molecules.2−5 As temperature is raised, such hydration structure is disrupted while the hydrophobic interaction between methyl groups is enhanced, leading to the development of the crosslinked network of MC chains and resulting in the gel formation. As the high-pressure processing technique has been widely utilized for sterilizing and preserving foods in a past decade,6 it is increasingly important to understand the pressure effects on the properties of food macromolecules; however, the sol−gel transition of food macromolecules such as agarose and MC under high pressure has been poorly examined. In 1972, Suzuki et al.7 studied for the first time the pressure effects on the sol− gel transition of gelatin, poly(vinyl alcohol), and MC, in which the changes in volume (ΔV) and enthalpy (ΔH) accompanied by the gel formation were estimated from the T−P phase diagrams; however, in that work, the sol−gel transition point under high pressure was determined not in situ but after pressure release. First, in situ observations of sol−gel transitions Received: April 15, 2015 Revised: May 12, 2015 Published: May 18, 2015 6878
DOI: 10.1021/acs.jpcb.5b03632 J. Phys. Chem. B 2015, 119, 6878−6883
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The Journal of Physical Chemistry B
a direct indication of gelling or melting of the sample.8−10,18 The same high-pressure system as for the optical transmittance measurement was utilized for the falling-ball experiments except that a transparent flexible tube made of polyvinyl chloride was used in place of the inner quartz cell, in which the sample solution and a tin−lead ball (∼3 mm in diameter) were encapsulated. In the course of isobaric cooling or heating the sample under high pressure, the high-pressure optical cell was turned upside down every 1 °C to check visually if the ball in the sample moved or not. Nominal heating or cooling rate was controlled to be 0.1 °C/min. The gelling temperature, Tgel, was determined by the temperature at which the ball was completely frozen. The melting temperature, Tmelt, was estimated by the temperature at which the ball began to move.
of food macromolecules under high pressure have been conducted by Gekko and coworkers.8−10 They measured the melting temperatures of agarose, gelatin, and carrageenan gels at high pressures by the falling-ball method and estimated ΔV and ΔH accompanied by the sol−gel transitions. Very recently, we examined the pressure-induced gelation of MC by in situ fluorescence measurements using a fluorescent probe, 9(dicyanovinyl)-julolidine, which is a well-established indicator of solvent microviscosity.11 It was indicated that the microviscosity showed a dramatic change in the vicinity of the sol− gel transition point and the T−P phase diagram of 2 wt % MC solution was constructed up to 200 MPa. In this study, we have examined the thermoreversible sol−gel transitions of agarose and MC solutions on isobaric cooling or heating under high pressure up to 400 MPa by in situ observations of optical transmittance and by the falling-ball experiments. Recently, the interplay between the thermoreversible gelation and phase separation has been attracting much attention in both agarose12 and MC.13,14 The measurements of optical transmittance and the falling ball have proved to be simple yet useful and powerful to understand this interplay. Agarose and MC were employed as a contrasting pair of food macromolecules having completely different gelation mechanisms. The results are discussed in terms of the pressure effects on ΔV and ΔH associated with the sol−gel transitions along with the transformation of the hydration structure by compression.
3. RESULTS AND DISCUSSION 3.1. Agarose. Figure 1 shows the cooling and heating curves of optical transmittance for 1 wt % agarose solution at
2. EXPERIMENTAL METHODS Agarose powder (agarose XP) was purchased from Nippon Gene (Japan) and used without further purification. MC powder (SM4000) supplied from Shin-Etsu Chemical (Japan) was also used as received, the average molecular weight and the average degree of substitution (DS) of which were 380 000 and 1.8, respectively.15 1 wt % agarose solution was prepared in the following manner. First, a 0.1 g of agarose XP powder was swollen in a 9.9 g of deionized distilled water at 40 °C for 2 h. Then, this suspension was preheated to 70 °C for 30 min and finally heated to 90 °C for 30 min with stirring. 1 wt % MC solution was prepared by mixing a 0.1 g of MC powder in a 9.9 g of deionized distilled water, followed by stirring at 4 °C for 24 h. In situ measurements of optical transmittance of sample solutions under high pressure up to 400 MPa were performed utilizing a combination of an inner quartz cell (the optical path length = 5 mm), a stainless-steel high-pressure optical cell equipped with four sapphire windows, a pressure generating system (type KP-5B, Hikari Koatsu, Japan), and a UV-3600 spectrophotometer (Shimadzu, Japan). The details of the highpressure system have already been described elsewhere.16,17 The sample solution was encapsulated within the inner quartz cell and then put inside the high-pressure optical cell. Pressure was applied to the sample through silicon oil serving as a pressure medium, which was injected to the high-pressure cell by the pressure-generating system. Temperature was controlled by circulating water through a tube-like flow channel installed to the high-pressure cell using an F25 refrigerated/heating circulator (JULABO, Germany). The optical transmittance at a wavelength of 350 nm was recorded as a function of temperature in the course of isobaric heating or cooling at a rate of 0.1 °C/min under a desired pressure (0.1−400 MPa). The sol−gel transition under high pressure was also investigated by the falling-ball experiments, which can provide
Figure 1. Cooling and heating curves of optical transmittance for 1 wt % agarose solution at different pressures.
different pressures. Heating and cooling rates are both 0.1 °C/ min. It has been known that the aqueous solution of agarose undergoes gelation almost in parallel with phase separation to yield a translucent gel under ambient condition,12 although the interplay between gelation and phase separation has not been fully understood. An abrupt decrease in transmittance observed around 35−30 °C in cooling curves and a sharp rise around 60−65 °C in heating curves are therefore attributed to gelling or melting as well as phase separation of agarose. It is found that both cooling and heating curves show a gradual shift toward higher temperature with elevated pressure, suggesting the rise of transition temperatures because of compression. To discuss the pressure effects in further detail, we determined the transition temperatures from observed curves in the following way. The cloud-point temperature, Tcloud, was determined from cooling curves by the temperature 6879
DOI: 10.1021/acs.jpcb.5b03632 J. Phys. Chem. B 2015, 119, 6878−6883
Article
The Journal of Physical Chemistry B of 75% transmittance because it corresponds to the steepest point of decreasing transmittance in all curves. In the same way, the “transparency temperature”, Ttrans, at which the solution returns to a transparent sol state was determined from heating curves by the temperature of 75% transmittance. Figure 2
Table 2. Cloud-Point (Tcloud), Transparency (Ttrans), Gelling (Tgel), and Melting (Tmelt) Temperatures of MC at Different Pressures pressures (MPa) 0.1 100 150 200 250 300 350 380 390 400
shows the plots of Tcloud and Ttrans thus obtained as a function of pressure, and the actual data are summarized in Table 1. The Table 1. Cloud-Point (Tcloud), Transparency (Ttrans), and Gelling (Tgel) Temperatures of Agarose at Different Pressures 0.1 200 300 400
Tcloud (°C) 30.0 32.1 34.6 35.8
± ± ± ±
2.4 3.1 3.1 2.9
Ttrans (°C)
Tgel (°C)
± ± ± ±
31
60.7 69.4 74.9 76.6
3.0 4.5 3.6 1.7
58.5 61.3 63.0 59.9 55.5 48.5 40.8 33.5 30.5 −
± ± ± ± ± ± ± ± ±
1.4 1.9 1.2 1.5 1.5 1.7 1.7 3.4 4.1
Ttrans (°C)
Tgel (°C)
Tmelt (°C)
± ± ± ± ± ± ± ± ± ±
56 59
33 34
59
38
32.7 35.9 36.7 36.0 36.0 33.7 32.4 29.9 28.7 23.4
1.1 1.7 1.1 1.2 1.5 1.3 1.6 2.1 2.6 5.2
59 61
helices through enhanced hydrogen bonding under high pressure. The slope of the phase boundary curve can be described generally by Clausius−Clapeyron relation, dT/dP = TΔV/ΔH, with which the changes in volume and enthalpy, ΔV and ΔH, associated with the phase transition can be deduced. A good linearity seen in the plots of Figure 2 implicates that ΔV and ΔH that accompany the sol−gel transition of agarose are almost independent of pressure. It is also suggested that there is little difference in the gelation behavior of agarose between under ambient and high-pressure conditions. From the Ttrans−P plots between 0.1 and 200 MPa in Figure 2, the slope of sol− gel phase boundary was determined to be dT/dP = 4.35 × 10−2 K/MPa. In conjunction with the data of ΔH = −35.1 kJ/kg (kJ per 1 kg of agarose) reported for the gelation of agarose at ambient pressure19 and Ttrans at 0.1 MPa (60.2 °C), the value of ΔV = −4.57 × 10−6 m3/kg has been calculated, which means that the volume is reduced when the agarose solution undergoes gelation. The reduction in volume has been reported for the gelation of gelatin as well (ΔV = −0.52 × 10−6 m3/ kg);20 however, the value of ΔV is an order of magnitude smaller than that for agarose. This means that the gels of agarose and gelatin are stabilized under high pressure, but their stabilization mechanism may be different. In addition, it should be noted that the pressure stabilization is not common to hydrogen-bonding gels because carrageenan gel is known to be destabilized by compression.8 3.2. MC. The gelation mechanism of MC is completely different from that of agarose, so it is expected that the compression may have distinct effects on MC. Figure 3 shows the cooling and heating curves of optical transmittance for 1 wt % MC solution at different pressures. It has been known that MC also undergoes gelation almost in parallel with phase separation on heating to yield the translucent gel at ambient pressure.14 The abrupt change in optical transmittance observed at 0.1 MPa is therefore indicative of gelling or melting of MC as well as phase separation boundary. As seen in Figure 3, the elevation of pressure has a dramatic impact on heating curves and somewhat moderate one on cooling curves. Because both curves show the steepest change at 60% transmittance, the transition temperatures, Tcloud and Ttrans, have been estimated by the temperature of this transmittance from each curve. In addition, Tgel and Tmelt at each pressure have been determined by the falling-ball experiments conducted under high pressure. Figure 4 and Table 2 show the summary of the T−P phase diagram thus obtained for 1 wt % MC solution. Note that Tmelt was obtained
Figure 2. Plots of cloud-point temperature, Tcloud (square) and transparency temperature, Ttrans (diamond) against pressure for 1 wt % agarose solution. Triangles show the gelling temperature, Tgel, measured by the falling-ball method.
pressures (MPa)
Tcloud (°C)
40
values of Tcloud and Ttrans at 0.1 MPa are 30.0 and 60.7 °C, respectively, which are almost identical to gelling and melting temperatures of 1 wt % agarose solution at ambient pressure. The elevation of pressure causes a monotonous rise in both Tcloud and Ttrans to reach Tcloud = 35.8 °C and Ttrans = 76.6 °C at 400 MPa. On the whole, this observation is in conformity with the results of previous study8 and suggests the stabilization of agarose gel by compression. To validate the rise of the sol−gel transition temperature by compression, we have determined the gelling temperature of agarose under high pressure by the falling-ball experiments. The gelling temperatures, Tgel, measured at 0.1 and 400 MPa are shown in Figure 2 (filled triangles) and Table 2. It is evident that Tgel at each pressure is approximately consistent with Tcloud within the range of experimental error, which reveals that the gelation of agarose occurs almost simultaneously with phase separation, even under high pressure. It is also found that Tgel shows a rise with elevated pressure, substantiating that agarose gel is stabilized by compression. The stabilization is probably caused by the increased cross-link formation between agarose 6880
DOI: 10.1021/acs.jpcb.5b03632 J. Phys. Chem. B 2015, 119, 6878−6883
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undergoes phase separation prior to gelation on heating under high pressure above 200 MPa, while they occur almost simultaneously below 150 MPa. In other words, the gelation behavior of MC under high pressure considerably differs from that under ambient pressure. Another notable feature of Figure 4 is that Tcloud and Ttrans approach each other rapidly with increasing pressure over 200 MPa. Because the difference between Tcloud and Ttrans arises from the hysteresis characteristic of sol−gel transition, it is indicated that there is a significant effect of pressure on the phase transition dynamics of MC. Figure 5 shows the loops of
Figure 3. Heating and cooling curves of the optical transmittance for 1 wt % MC aqueous solution measured at different pressures.
Figure 5. Heating and cooling curves for the optical transmittance of 1 wt % MC aqueous solution measured at 0.1 (a) and 380 MPa (b) with different heating and cooling rates.
Figure 4. T−P phase diagram of 1 wt % MC aqueous solution.
only at pressures below 200 MPa due to the unexpected instability of pressure during the measurement for cooling under higher pressure. Figure 4 and Table 2 reveal that Tcloud and Ttrans initially rise with elevation of pressure from Tcloud = 58.5 °C and Ttrans = 32.7 °C at 0.1 MPa to Tcloud = 63.0 °C and Ttrans = 36.7 °C at 150 MPa; however, both of them begin to shift to lower temperatures with further elevation of pressure and finally fall below those at 0.1 MPa. This suggests that the phase separation boundary of MC forms a unique convex curve having a maximum at ∼150 MPa. It is found that Tgel, which is approximately identical to Tcloud at 0.1−200 MPa, continues to rise over the whole range of pressure examined here. Tmelt also shows a monotonous rise with pressure up to 200 MPa in harmony with Ttrans. It is therefore indicated that MC
optical transmittance measured at 0.1 and 380 MPa on isobaric heating and cooling with different rates, which elaborates the tapering of thermal hysteresis under elevated pressure. The increase in heating rate at 0.1 MPa is found to cause a shift of heating curve toward higher temperature, suggesting that the phase separation is a nonequilibrium process. Arvidson et al.14 shows that the gel formation begins; that is, the storage modulus begins to increase, at lower temperatures with lowering heating rate in their figure 10. This is in good agreement with the present Figure 5 for the data taken at 0.1 MPa. As previously mentioned, the phase separation of MC is closely coupled to gelation process under ambient pressure, and thereby its dynamics could be a highly nonequilibrium process 6881
DOI: 10.1021/acs.jpcb.5b03632 J. Phys. Chem. B 2015, 119, 6878−6883
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The Journal of Physical Chemistry B
alternation of hydration structure surrounding MC by compression. Finally, it is worth noting the similarities of MC gel under high pressure and hydroxypropyl methylcellulose (HPMC) gel under ambient pressure. HPMC is a derivative of MC in which hydroxypropyl groups are introduced partially in place of methyl groups. Interestingly, it has been known that gelation of HPMC takes place after phase separation to yield a loose gel.13 This behavior is similar to MC gel under high pressure. Because of the presence of additional hydroxyl groups in HPMC, its hydrophobicity should be reduced relative to that of MC, which will lead to the formation of incomplete hydrophobic hydration around HPMC. This situation has something in common with the disrupted hydrophobic hydration of MC under high pressure and may be responsible for the similarities of gelation behavior between them.
under the present conditions. In fact, it has been known that it takes ∼1 week to achieve the complete equilibrium in the course of gelation of MC.21 Meanwhile, cooling curves at 0.1 MPa are negligibly affected by cooling rate, which suggests that melting of MC gel is nearly an equilibrium process. At 380 MPa, both heating and cooling curves are almost independent of the sweep rate, which imply the dramatic change in the phase separation dynamics. As previously discussed, the phase separation of MC occurs without being accompanied by gelation under high pressure above 200 MPa; therefore, its dynamics approaches an equilibrium process like a usual liquid−liquid phase separation. This fact is also responsible for the tapering of thermal hysteresis observed at elevated pressures. The data of ΔH for the gelation of MC at ambient pressure are available in the literature.22 Because the molar mass and DS of MC sample used in ref 22 are 310 000 and DS = 1.8 and are similar to those of the present sample with molar mass 380 000 and DS = 1.8, the ΔH value in ref 22 will be used for the comparison. The value reported in ref 22, ΔH = 3.67 × 10−3 kJ/mol for 0.03 mM (0.93 wt %) MC (table 2 in ref 22), corresponds to ΔH = 11.8 kJ/kg. The data of ΔV = 0.80 × 10−6 m3/kg can be found in ref 20. Because the Tgel for the present sample is 330 K, as shown in Figure 4 (this value in ref 22 is 334 K closer to the present value), TΔV/ΔH is given as 330 K × 0.80 × 10−6 m3/kg/11.8 kJ/kg = 2.24 × 10−2 K/MPa, which is in good agreement with dT/dP = 2.97 × 10−2 K/MPa, determined from the slope of Tgel−P plots between 0.1 and 150 MPa. A similar result has been also obtained for the slope of Tmelt−P plots, leading to the conclusion that MC gel is destabilized by compression as contrasted with agarose. The phase-separation boundary of MC (Tcloud−P and Ttrans− P plots in Figure 4) forms a convex curve having a peak at ∼150 MPa, which is suggestive that the sign of either ΔV or ΔH associated with phase separation is inverted by compression. Suzuki et al. has reported that the gelation (or phase separation) of MC is an endothermic reaction at any pressures examined from 0.1 to 507 MPa.7 Consequently, the inversion in the slope of phase separation boundary should be attributed solely to the sign inversion of ΔV by compression. The volume change associated with phase separation of MC can be understood in view of the nature of hydration water surrounding the hydrophobic moieties of MC (methyl groups). In the sol sate, water molecules surrounding methyl groups form the hydrophobic hydration with a cage-like structure, which is more compact than free water. As temperature is raised, such a cage-like structure is disrupted due to the thermal dissociation of hydrogen bonding, leading to the expansion of hydration water as well as phase separation of MC. As a result, the volume is expected to increase when MC undergoes phase separation; however, the compression makes a drastic change in the situation of hydrophobic hydration. Sawamura et al. examined the pressure effects on the volume change accompanied by the dissolution of alkylbenene in water (ΔVsol).23−25 They found that the sign of ΔVsol was inverted from negative to positive by compression, which is indicative that the hydration water surrounding alkyl groups of solutes becomes less compact than free water under high pressure. The inversion of ΔVsol was seen at 100−200 MPa, which is in good accordance with the present observations, and it has been attributed to the destruction of hydrophobic hydration by compression. Accordingly, it is suggested that the unique behavior of phase separation boundary of MC results from the
4. CONCLUSIONS In this study, we have examined the thermoreversible sol−gel transitions of agarose and MC on isobaric cooling or heating under high pressure by in situ observations of optical transmittance and falling-ball experiments. P−T phase diagrams for sol−gel transition and phase separation were constructed, which revealed that agarose gel was stabilized while MC was destabilized by compression. In addition, it was found that the gelation of MC took place after phase separation on heating under high pressure above 200 MPa, whereas agarose underwent gelation almost in parallel with phase separation on cooling under both ambient and high-pressure conditions. The phase-separation boundary for MC showed a convex curve having a maximum at ∼150 MPa, which is indicative that the sign of volume change accompanied by phase separation was inverted by compression. It was suggested that this sign inversion was due to the disrupted hydrophobic hydration surrounding methyl groups of MC by compression.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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