Production of tagatose and talose through isomerization of galactose in a buffer solution under subcritical water conditions

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

“…In contrast, approximately 6% of tagatose was formed at 300 s in 50 wt % ethanol; this yield was lower than that reported previously (22% in 80 wt % ethanol at 180 °C), 22 which is mainly attributed to the low treatment temperature and higher galactose concentration. In buffer− subcritical water, the yield of tagatose soared within the first 90 s. 24 Then, the yield leveled off at approximately 12% between 90 and 120 s. After 120 s, the yield of tagatose barely changed at approximately 13%, and no remarkable tagatose decomposition was observed.…”

“…This result is consistent with those reported by a previous study, which required a temperature of 180 °C or higher for the isomerization or epimerization in pure subcritical water. 18 However, in the presence of ethanol in pure water, the galactose content decreased significantly even at 140 °C; approximately 94% of galactose remained at 300 s. From previous studies, it is reported that the decrease in galactose content was further promoted in subcritical water containing 10 mmol/L sodium phosphate buffer (hereafter, the solution is referred to as buffer−subcritical water) 24,25 and 79% of galactose remained at 120 s. After 120 s, the decrease in galactose content leveled off, and the remaining fraction of galactose was approximately 75% at 300 s in buffer−subcritical water. These results suggest that both ethanol and phosphate buffer promote the reaction of galactose.…”

“…In our previous study in buffer−subcritical water, isomerization rarely occurred when the pH dropped below 6.3. 24 Similarly, it is necessary to maintain the pH at higher than 6.5, even in the presence of ethanol, for the efficient production of tagatose.…”

“…0.8 mm; inner volume approximately 1.9 mL) immersed in a silicone oil bath, which was similar to those reported in previous studies. 24,26,27 The starting mixture was pumped into the tubular reactor by using an HPLC pump (LC-10ADVP, Shimadzu, Kyoto, Japan) at a flow rate of 0.35−3.5 mL/min. These flow rates corresponded to the reaction times (residence times) of 30− 300 s. The pressure inside the tubular reactor was maintained at approximately 5 MPa with a back-pressure regulator (P-880, Upchurch Scientific, Oak Harbor, WA, USA).…”

“…However, in pure subcritical water, most sugars are usually decomposed into organic acids, and the yields of isomerized sugars are generally very low. 18−20 Therefore, to overcome the low yields, we attempted subcritical water treatment of common sugars in aqueous ethanol 19,21−23 or in sodium phosphate buffer 24,25 to demonstrate the efficiency of these solutions. However, the synergistic effects in the presence of both ethanol and phosphate buffer have not been clarified.…”