Isomerization and Epimerization of Galactose to Tagatose and Talose in a Phosphate Buffer Containing Organic Solvents under Subcritical Water Conditions

Abstract

In this study, galactose was treated under subcritical water conditions at 140 °C and 5 MPa in a mixture of 10 mmol/L sodium phosphate buffer (pH 7.0) and ethanol using a tubular reactor. The effects of ethanol concentration (0−60 wt %) on the isomerization and epimerization of galactose, respectively, to tagatose and talose were investigated. Both ethanol and buffer synergistically improved the isomerization behavior. Increasing the ethanol concentration up to 60 wt % improved the tagatose yield from 13 to 16% at 300 s. In contrast, the yield of talose slightly decreased from 1.5 to 1.2% at 300 s when ethanol was added to the phosphate buffer. The pH of treated solution at 25 °C decreased with an increase in the duration of treatment under each treatment condition. However, the decrease in pH was moderately suppressed by the addition of ethanol, which could be one of the reasons for the improvement in tagatose yield. The addition of other organic solvents, such as alkanols, polyols, acetonitrile, and pyridine, also improved the tagatose yield.

Isomerization of Galactose to Tagatose: Recent Advances in Non-enzymatic Isomerization

“…Nitrogen was adequately blown into the system to avoid oxidative degradation reactions. Results showed that the increase in the ethanol concentration up to 60 wt % enhanced the tagatose yield from 13 to 16% but decreased the talose yield from 1.5 to 1.2% at a reaction time of 300 s. The improvement of the tagatose yield could be attributed to the addition of ethanol into the supercritical system, which prevented the reduction in pH …”

“…In this work, galactose and tagatose are both d types, unless otherwise mentioned. From a processing point of view, enzymatic or non-enzymatic isomerization of galactose is the common pathway for tagatose production. − The usage of l -arabinose isomerase for galactose isomerization is of great interest, particularly because the hydrolysis process can be operated at relatively mild pH and temperature conditions without forming side products . Several reviews have compiled the isomerization of galactose to tagatose by l -arabinose isomerase. − , Herein, we will not extend enzymatic functionality and molecular evolution but focus on its challenges for industrial application.…”

Toward Understanding Base-Catalyzed Isomerization of Saccharides

“…The isomerization was quicker in ethanol–water mixture compared to pure water, and the maximum Tag yield increased from 13% in water to 16% in 60% EtOH–40% water. The authors explained the observed effect by a more thermodynamically favorable formation of Tag in aqueous-ethanolic mixtures than in aqueous solutions . Moreover, base-catalyzed isomerization of Glc into Fru was reported to accelerate markedly as a function of alcohol content in EtOH-water solution.…”

“…The authors explained the observed effect by a more thermodynamically favorable formation of Tag in aqueousethanolic mixtures than in aqueous solutions. 75 Moreover, basecatalyzed isomerization of Glc into Fru was reported to accelerate markedly as a function of alcohol content in EtOHwater solution. The isomerization rate was 2.4 times the corresponding value in water solution.…”

“…In the presence of bases, Tag undergoes 3-epimerization to produce d -sorbose (Sorb). In recent years, catalytic activity of Mg/SiO 2 , arginine, phosphate buffer at pH 7, − Mg–Al hydrotalcite, , MgO, and Na 2 HPO 4 +NaOH at pH 0 12.0 was explored. Figure shows selectivity–conversion plots for Tag and Sorb obtained in the presence of various bases.…”