Conversion of D-galactose and guar gum hydrolysate to D-tagatose using L-arabinose isomerase immobilized in sodium alginate-silica hydrogel

D-tagatose is an attractive functional sweetener, and its production via enzymatic synthesis is simple and environmentally friendly; however, it faces challenges related to enzyme and substrate costs. In this study, a novel and cost-effective method for synthesizing D-tagatose was established using L-arabinose isomerase (L-AI) immobilized within a sodium alginate (SA) composite hydrogel, with the low-cost guar gum as the starting material. Eight inorganic or organic materials were individually combined with SA to encapsulate L-AI. The immobilized enzyme prepared with SA and 50-nm SiO₂ particles demonstrated superior properties, including enhanced mechanical strength and improved reusability, compared to enzymes immobilized with other carriers. The structure of the immobilized enzyme was characterized by scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction. The D-tagatose synthesis reaction using the immobilized enzyme was optimized through both single-factor and statistical methods. The maximum yield of D-tagatose reached 30.12 % from 1 M D-galactose at 68°C, with a productivity of 6.78 g/L/h. The immobilized enzyme was reusable for 20 cycles of reaction at 60 °C, retaining over 55% of its initial activity and accumulating a high amount of 863.74 g/L D-tagatose. Furthermore, it efficiently converted guar gum hydrolysate into D-tagatose, retaining over 50% enzyme activity and yielding 114.32 g/L of D-tagatose after 10 cycles. The excellent enzyme reusability and the exploration of an economic substrate would significantly reduce production costs in large-scale D-tagatose synthesis in the future.