Continuous production of tagatose through isomerization of galactose using a tubular reactor filled with eggshell powder

Abstract

Galactose solution was passed through a tubular reactor filled with eggshell powder, and isomerization of galactose to rare sugars was continuously performed under subcritical conditions at 120–150°C with the reaction time (RT) between 3–37 min. Isomerization produced rare sugars (tagatose, talose, and sorbose). At 135°C and RT= 6 min, the maximum yield of tagatose (14.5 %) was achieved. Compared to this result, the yield of tagatose was lower at 150°C or 120°C, and the yield of byproducts was also especially lower at 120°C. However, when RT was prolonged to 37 min at 120°C, the yield of tagatose improved to about 10 %, while the formation of byproducts was kept under control. Furthermore, during the continuous use of the reactor (up to 30 h), the yield of byproducts significantly decreased with increasing usage time. Therefore, it was suggested that continuous contact of eggshell with the reaction mixture may change properties of the powder and affect the isomerization behavior of galactose. Although the eggshell powder became light brown after use for the reaction, an apparent change of the surface could not be observed by scanning electron microscope.