Production of tagatose from galactose using environmentally friendly catalyst

Abstract

Tagatose is a rare sugar with health benefits, including low calories, a low glycemic index, and prebiotic properties. This study explored environmentally friendly catalysts, including arginine and buffers, for producing tagatose. The production of tagatose by heating galactose with arginine in aqueous solution under various conditions was investigated. The effects of temperature (90-120 °C), reaction time (0-20 min), arginine concentration (0.01-0.15 mol/mol of galactose), and galactose concentration (5-20% w/v) were evaluated. The highest yields of tagatose (16.8%), talose (2.7%), and sorbose (3.3%) were achieved at 120 °C after 20 min. Talose and sorbose are by-products of this process. The reaction at 120 °C for 4 min provided the highest productivity of 92.4 g/(L·h). Increasing the arginine concentration enhanced the isomerization and the Maillard reaction, turning the solution from clear to yellow-brown and lowering the pH. Higher galactose concentrations reduced tagatose yield but increased productivity to 278 g/(L·h) at an initial galactose concentration of 20% w/v, which is favorable for large-scale production. Additionally, the potential use of lactose, which is much cheaper than galactose, as a raw material for the direct production of tagatose was also investigated. Experiments were conducted at 100-120 °C revealed that lactose primarily converted to lactulose, achieving a maximum yield of approximately 26%. The formation of galactose, however, was almost undetectable. The kinetics of tagatose formation were then studied when galactose underwent isomerization with arginine at temperatures ranging from 90 to 120 °C over a period of 15 to 120 min. The initial rate of tagatose formation increased significantly with temperature, with an activation energy of 66.24 kJ/mol. Arginine decomposed during the reaction, with more degradation occurring at higher temperatures. Furthermore, ethanol (0-40% w/w) was tested as a modifier to enhance the isomerization. Adding 10% ethanol to the reaction solution increased the yield from 17% to 19%, whereas a 40% concentration of ethanol resulted in significant reduction of the yield (15%). Lastly, the isomerization of galactose in buffer systems was explored. Various buffers were tested for their catalytic activity, including CAPS, carbonate, triethylamine, quinuclidine, and L-arginine. The maximum yields of tagatose were 15.0% with CAPS, 15.2% with carbonate, 19.3% with triethylamine, 19.6% with quinuclidine, and 18.1% with L-arginine. However, L-arginine resulted in highest browning reaction. Notably, the initial rate of tagatose formation with carbonate buffer was 3 to 8 times higher than with CAPS, despite both having the same pH. For catalysis involving carbonate buffer, the reaction orders for hydroxide anions and carbonate species were approximately first and zeroth, respectively. Operando NMR studies of deuterated galactose isomerization in both carbonate and CAPS buffers indicated similar tautomeric distributions. The deuterium kinetic isotope effect study suggested that carbonate facilitates isomerization through a proton transfer mechanism, with hydroxide anions acting as the catalytically active species, while carbonate anions stabilize the enediolate anion and/or the transition state.

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