Kinetics of Sucrose Hydrolysis in a Subcritical Water-ethanol Mixture

Abstract

The kinetics of sucrose hydrolysis was investigated in a water-ethanol mixture with ethanol concentrations of 0–80% (v/v) under subcritical conditions in the 160–190°C range. Sucrose underwent autocatalytic hydrolysis in the subcritical mixtures. The rate of sucrose hydrolysis to glucose and fructose was reduced with increasing ethanol concentration, and ethanol showed a dilution effect on the conversion. The temperature dependence of the reaction rate constant for sucrose hydrolysis obeyed the Arrhenius equation. The fructose and glucose products underwent further decomposition, and the yield of fructose was much lower than that of glucose when the ethanol concentration increased. Thus, ethanol exerted other effects on the reaction in the subcritical water-ethanol mixture.

INTRODUCTION

Water can play several roles in chemical reactions: as medium, reactant, product and catalyst.¹⁾ Subcritical water (or compressed hot water) has been used recently in many chemical processes because of its two distinct characteristics—a low dielectric constant and a high ion product. With these two properties, water can act as an extractant for hydrophobic functional compounds, such as radical scavenging agents from defatted rice bran,²⁾ phenolic substances from food wastes,^{3) 4)} and essential oils from plants.⁵⁾ Subcritical water has been also used to produce amino acids from fish protein⁶⁾ and poultry waste,⁷⁾ and to produce reducing sugars from seaweed.⁸⁾ We used a subcritical water-ethanol mixture to effectively extract functional substances such as phenolic compounds from defatted rice bran.⁹⁾

In these treatments, chemical reactions occur simultaneously with extraction. Several reaction models have been proposed and the kinetics analyzed for the dehydration of 1- and 2-propanol,¹⁰⁾ the decomposition of methyl tert-butyl ether,¹¹⁾ and the hydrolyses of, for example, benzonitrile,¹²⁾ fatty acid esters,^{13) 14)} and disaccharides such as maltose and sucrose.^{15) 16) 17)}

In our previous studies, sucrose was hydrolyzed in an autocatalytic mode most easily among several tested disaccharides in subcritical water.^{15) 16) 17)} Sucrose was first hydrolyzed to fructose and glucose, which further decomposed to small acidic compounds.^{15) 16) 18)}

However, the reactions that occur during the extraction process in a water-ethanol mixture under subcritical conditions are still unclear. In this study, we investigated the influence of the addition of ethanol into water on the kinetics of sucrose hydrolysis under subcritical conditions.

Materials. Sucrose (purity, >97%), D-fructose (>99%) and D-glucose (>98%) were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan).

Hydrolysis of sucrose in a subcritical water-ethanol mixture. Sucrose was dissolved in distilled water and then mixed with ethanol to produce solutions with a final sucrose concentration of 0.5% (w/v) and water concentrations of 20-100% (v/v). The solutions were sonically degassed under reduced pressure before the subcritical treatment. The solution reservoir was connected to a helium gasbag to prevent re-dissolution of atmospheric oxygen. The solution was delivered into a coiled stainless steel tubular reactor (0.8 mmφ × 1.0 m) immersed in an SRX 310 silicone oil bath (Toray-Dow-Corning Silicone Co., Ltd., Tokyo, Japan), with a residence time of 10-240 s, by an LC-10ADVP HPLC pump (Shimadzu Corp., Kyoto, Japan). The reaction was conducted in the temperature range of 160-190°C. The reactor effluent was directly introduced to a stainless steel tube immersed in an ice bath to terminate the reaction. The pressure inside the tube was regulated at ca. 10 MPa by a back-pressure valve (Upchurch Scientific, Inc., Oak Harbor, USA). The effluent was collected in a test tube for HPLC analysis. The experiments were carried out in triplicate, and the obtained values were averaged.

The residence time was calculated according to the inner diameter, length of the stainless steel tube, and density of the water-ethanol mixture under subcritical conditions. The density of the mixture was calculated according to the densities of water and ethanol,^{19) 20)} assuming that additivity of the volume holds even for the mixture of ethanol and water under subcritical conditions.

Analysis. The concentrations of the residual sucrose and the glucose and fructose products were determined by HPLC. The system consisted of an LC-10ADVP HPLC pump, an RI-101 refractometer (Showa Denko K.K., Tokyo, Japan) and a Supelcogel Ca column (7.8 mmφ × 300 mm, Sigma-Aldrich Japan K.K., Tokyo, Japan) with a guard column (4.6 mmφ × 50 mm, Sigma-Aldrich Japan K.K.). The columns were kept at 60°C in a CTO-10AVP column oven (Shimadzu Corp.).

The pH of the reactor effluent was measured using a D-51 pH meter (Horiba Ltd., Kyoto, Japan) with a 6377-10D pH electrode (Horiba) at room temperature.

RESULTS AND DISCUSSION

Effects of temperature and water concentration on the rate of sucrose hydrolysis.

Figure 1 shows the changes in the remaining fraction of sucrose with residence time at different water concentrations and temperatures. Sucrose was hydrolyzed faster at higher temperature for all the water concentrations. For instance, the half-lives of sucrose at 190 and 160°C in subcritical water were ca. 45 and 180 s, respectively.

Fig. 1.

Change in the remaining fraction of sucrose, C_s/C_St, with residence time at (a) 160°C, (b) 170°C, (c) 180°C and (d) 190°C and different water-ethanol concentrations (v/v): (◇) 100%, (□) 80%, (△) 60%, (○) 40% and (▽) 20%.

Curves were drawn based on Eq. (3).

The sucrose hydrolysis rate decreased with decreasing water concentration. For example, the half-life of sucrose at 190°C was almost 4 times longer in 20% (v/v) water than in subcritical water alone. In addition, the half-life at 180°C in 40% (v/v) water was near to that in subcritical water at 160°C.

Because sucrose consumes water during its hydrolysis, water can be considered as not only a solvent but also a substrate in the reaction. Therefore, sucrose would be hydrolyzed more slowly at lower water concentration.

Kinetic analysis of sucrose hydrolysis.

Since sucrose was hydrolyzed through the autocatalytic mode in subcritical water,¹⁵⁾ the same mode was assumed for sucrose hydrolysis in the subcritical water-ethanol mixture. In the autocatalytic mode, the hydrolysis rate is proportional to the concentrations of sucrose, water and resulting products, such as acidic compounds. Therefore, the hydrolysis rate can be expressed as a function of the molar concentrations of water, C_W; the remaining sucrose, C_S; the total sucrose, C_St and the consumed sucrose, C_St – C_S:

dC_S/dt = －kC_SC_W(C_St－C_S) (1)

where k is the rate constant. Although water is consumed during sucrose hydrolysis, its concentration is significantly higher than that of sucrose. Therefore, C_W can be considered as a constant in the kinetic equation. Equation (1) can then be rewritten as follows:

dY / dt = －k′Y(1－Y) (2)

where Y is the remaining fraction of sucrose, C_S/C_St, and k′ is the apparent rate constant, which is equal to kC_wC_St. Equation (2) can be integrated with the initial condition Y = Y₀ at t = 0 as follows:

ln[(1－Y)/Y = k′t + ln[(1－Y₀)/Y₀] (3)

Based on Eq. (3), ln[(1 − Y)/Y] was plotted against t. Figure 2 shows typical plots for the case of sucrose hydrolysis in 40% (v/v) water in ethanol. The straight-line plots indicated that sucrose was hydrolyzed autocatalytically in the subcritical water-ethanol mixture, similarly to the hydrolysis in subcritical water. The apparent rate constant k′ could be calculated from the slope of the line. The k′ value declined not only with decreasing temperature but also with decreasing water concentration (Fig. 3). Due to the difficulty in estimating the concentration of ionized water, the k′ value was plotted against the water concentration in Fig. 3. The apparent rate constants k′ had linear relationships with the water concentrations, indicating that the addition of ethanol resulted in a dilution effect on sucrose hydrolysis. The curves in Fig. 1 were drawn by substituting the estimated k′ and Y₀ values in Eq. (3).

Fig. 2.

Estimation of the apparent rate constants, k′, of sucrose hydrolysis at (○) 160°C, (△) 170°C, (□) 180°C and (◇) 190°C in a 40% (v/v) water-ethanol mixture.

Fig. 3.

Dependence of apparent reaction rate constants, k′, of sucrose hydrolysis on water concentration in the 160−190°C range.

Symbols are the same as in Fig. 2.

The rate constant k was then obtained from the equation k′ = kC_StC_W. The k values did not depend on the water concentration. To investigate the dependence of the rate constant on temperature, the k values at different water concentrations were averaged and plotted against the reciprocal of absolute temperature (Arrhenius plot, Fig. 4). The plot was linear with a high correlation coefficient (R² = 0.985). From the line, the frequency factor and activation energy were evaluated to be 1.7 × 10⁹ (L/mol)²/s and 90 kJ/mol, respectively. The activation energy was similar to that of acid-catalyzed hydrolysis of sucrose (ca. 95 kJ/mol)²¹⁾ and those of lactose and D-melezitose hydrolyses in subcritical water (ca. 85 kJ/mol for both saccharides).^{16) 22)} On the other hand, it was a little smaller than those of other disaccharides, such as maltose (ca.118 kJ/mol)¹⁶⁾, turanose (ca. 152 kJ/mol)¹⁶⁾ and melibiose (ca. 132 kJ/mol)¹⁶⁾ etc., indicating that the hydrolysis rate constant of sucrose had similar sensitivity toward temperature to lactose and D-melezitose.

Fig. 4.

Arrhenius plot for the rate constant, k, of sucrose hydrolysis.

The k values at different water concentrations were averaged.

Yields of glucose and fructose during sucrose hydrolysis.

Figure 5 shows the typical dependence of the yields of glucose and fructose on the conversion of sucrose at water concentrations of 100, 60 and 20% (v/v). When only hydrolysis occurs, equimolar amounts of glucose and fructose should be produced. The glucose and fructose yields were lower than the conversion of sucrose, indicating that the monosaccharides also underwent decomposition. It was reported that, in subcritical water or in a water-alcohol mixture under alkali conditions at 60°C,^{23) 24)} the monosaccharides decomposed to various compounds, such as 5-hydroxymethylfurfural and acids,^{25) 26) 27)} or isomerized into each other or another saccharide such as mannose. However, mannose was not detected in this study. Therefore, it can be considered that sucrose was first hydrolyzed to glucose and fructose, followed by the decomposition and isomerization of these two monosaccharides.

Fig. 5.

Typical relationships between the yields of glucose and fructose and conversion of sucrose at (●, ○) 160°C, (▲, △) 170°C, (■, □) 180°C and (◆, ◇) 190°C and different concentrations of water in ethanol (v/v): (a) 100%, (b) 80%, (c) 60%, (d) 40% and (e) 20%.

Closed and open symbols represent glucose and fructose, respectively. The lines were drawn under the assumption that decomposition of glucose and fructose did not occur.

In subcritical water, fructose and glucose were formed in almost the same yields, and the yields in the 40-80% (v/v) water mixtures were higher than those in subcritical water alone. This indicates that the decomposition rates of fructose and glucose would be faster in subcritical water. On the other hand, 20% (v/v) water mixture gave the lower yield of fructose. Although its reason is unclear, the presence of ethanol at moderate concentration may promote the stabilization of glucose and fructose.

In the subcritical water-ethanol mixtures, the fructose yields were lower than those of glucose, and these differences became greater with decreasing water concentration. Possible reasons for the difference are as follows: 1) the decomposition rate constant of fructose was greater than that of glucose; 2) isomerization between fructose and glucose occurred; 3) the addition of ethanol may have promoted the decomposition and isomerization of glucose and fructose and 4) a combination of more than one of these factors may be in play.

pH change during sucrose hydrolysis.

When sucrose was treated in subcritical water or water-ethanol mixtures, the pH decreased due to the decomposition of the glucose and fructose products into acidic compounds.^{15) 28) 29)} Figure 6 shows the relationship between the pH of the effluent measured at room temperature and the conversion of sucrose in water concentrations of 100, 60 and 20% (v/v). Changes in the pH were larger at longer residence times with higher conversions of sucrose (Figs. 1 and 6). In subcritical water, the pH decreased further after the sucrose was completely hydrolyzed. This corresponded to the decreases in the fructose and glucose yields at the sucrose conversion of 1.0 in Fig. 5, indicating that weak acids were produced through the decomposition of the monosaccharides.²⁷⁾ In 60 and 20% (v/v) water mixtures, changes in the pH were ca. 2, and these values were smaller than that in subcritical water (ΔpH = ca. 3). Therefore, the hydrolysis would be suppressed in 20–80% water mixture due to the lower concentration of hydrogen ion (Fig. 1).

Fig. 6.

Typical relationships between pH and conversion of sucrose at (◇) 190°C, (□) 180°C, (△) 170°C and (○) 160°C at different water-ethanol concentrations (v/v): (a) 100%, (b) 60% and (c) 20%.

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