D-tagatose is what chemists call a rare sugar, a hexose isomer found only in trace amounts in nature. It carries roughly the same sweetness as sucrose but with a very low caloric value, and a growing body of nutritional literature has linked it to beneficial effects on blood glucose control, making it attractive for diabetic-friendly foods and beverages. It has also drawn attention for its antimicrobial potential against oral bacteria, a property that could position it as a tooth-friendlier alternative to conventional sweeteners. The catch is supply. Chemical synthesis routes to rare carbohydrates exist but tend to be multistep, wasteful and difficult to scale cleanly. Biocatalysis, in which an enzyme performs a single precise isomerization, is the greener route, and the enzyme of choice for converting abundant D-fructose into D-tagatose is tagatose 4-epimerase.

The problem with that enzyme class has always been durability. Free enzymes in solution are delicate protein machines; at the elevated temperatures that favor the fructose-to-tagatose equilibrium and suppress microbial contamination, they gradually unfold, aggregate and lose catalytic power. The research team, led by Xue Cai, Yuanshan Wang and Yuguo Zheng of Zhejiang University of Technology together with collaborators at Jiangxi Alpha Hi-tech Pharmaceutical, started from a thermostable tagatose 4-epimerase called TpT4Ease, originally identified in Thermotoga petrophila, a bacterium that thrives in hot environments. Even a thermostable enzyme, however, degrades at industrial operating temperatures over time, and a dissolved enzyme cannot simply be fished out of a reaction broth and used again.

The team’s solution was a two-step immobilization strategy. First, they treated the enzyme with glutaraldehyde, a bifunctional crosslinker that reacts with amino groups on the protein surface, creating covalent bridges both within and between enzyme molecules. Then they deposited the crosslinked enzyme onto LX-1000HA, a macroporous resin whose surface is decorated with amino groups. The glutaraldehyde chemistry locks the enzyme onto the resin through stable covalent bonds, while the resin’s large, open pores provide an enormous internal surface area for enzyme loading and allow sugar solutions to flow freely through the material. The resulting hybrid biocatalyst, designated TpT4Ease@LX-1000HA-GA, was characterized by circular dichroism spectroscopy and scanning electron microscopy, which confirmed that the protein retained its secondary structure and that the enzyme was successfully distributed across the resin matrix.

What makes the study particularly interesting is the kinetic analysis that followed. Immobilization is often a trade-off: anchoring a protein can stabilize it, but the same conformational constraints that prevent unfolding can also stiffen the enzyme’s natural breathing motions, slowing the catalytic cycle. That is precisely what the researchers observed. The conformational constraint imposed by the porous support slightly compromised the enzyme’s intrinsic turnover number, meaning each immobilized enzyme molecule worked a bit more slowly than its free counterpart. But the compensation was dramatic. By effectively preventing thermal unfolding, the support extended the enzyme’s half-life at seventy degrees Celsius from 3.3 hours for the free enzyme to 15.4 hours for the immobilized version, nearly a fivefold improvement in operational lifetime at the process temperature.

Storage stability improved as well, which matters enormously for any commercial deployment. A biocatalyst that loses activity while sitting on a warehouse shelf is a liability regardless of how well it performs on day one. The immobilized preparation retained its catalytic capacity far better than the free enzyme over storage, consistent with a well-established principle in enzyme technology: multipoint covalent attachment rigidifies a protein’s structure and raises the energetic barrier to the unfolding events that initiate irreversible deactivation. The finding echoes a broader literature on immobilization strategies, which has repeatedly shown that covalent attachment to appropriately functionalized supports is among the most reliable ways to engineer robust industrial biocatalysts.

The real test came in continuous operation. Rather than running batch reactions, the team packed the immobilized enzyme into a flow reactor and pumped fructose solution through it. Under optimized conditions, a 200 gram per liter fructose feed at seventy degrees Celsius and a flow rate of 0.5 milliliters per minute, the system achieved a D-tagatose yield of 17.3 percent. That number may look modest, but it must be read in context: the fructose-to-tagatose conversion is an equilibrium reaction, and the epimerization of one sugar isomer into another is inherently limited by thermodynamics. Continuous-flow operation offers a way to push past the limitations of batch processing, because the product is continuously removed from the catalyst bed, shifting the effective equilibrium and protecting the enzyme from prolonged exposure to reaction conditions.

Even more striking was the operational stability. After five consecutive cycles of continuous-flow bioconversion, the immobilized enzyme retained 90.6 percent of its original catalytic activity. In industrial terms, that is the difference between a laboratory curiosity and a process candidate. Reusability is the entire economic argument for immobilization: the enzyme is a cost, and every cycle of reuse dilutes that cost across more kilograms of product. A catalyst that surrenders less than ten percent of its activity over five full production cycles, while operating at seventy degrees Celsius in a concentrated sugar stream, demonstrates the kind of durability that process engineers need before they will commit to a biocatalytic route.

The study also fits into a larger shift in how bioprocesses are designed. Continuous processing has transformed pharmaceutical manufacturing over the past two decades, replacing large batch reactors with smaller, more controllable flow systems, and biocatalysis is following the same trajectory. Immobilized enzymes are the natural partners of flow chemistry, because they allow a solid catalyst phase to be held in place while liquid substrate streams through. The integrated immobilization-plus-continuous-flow architecture described in this work is a template that could extend beyond tagatose to other rare sugars and value-added carbohydrates, including D-allulose and related functional sweeteners that the food industry is actively pursuing.

There remain hurdles between this bench-scale system and a full industrial process. The yield of 17.3 percent per pass means that product recovery and substrate recycling will be essential to overall process economics, and longer-term deactivation studies beyond five cycles will be needed to establish realistic catalyst lifetimes. The work was supported by the National Natural Science Foundation of China, and the authors declare no competing financial interests. Still, the core message is hard to ignore: by understanding exactly what immobilization does to an enzyme, constraining its motion to buy thermal stability at a modest cost in turnover, the researchers turned a fragile protein into a reusable industrial catalyst. For a low-calorie sugar that could reshape how the food industry sweetens its products, that is a meaningful step from promise to production.

Subject of Research: Enzyme immobilization on amino-functionalized macroporous resin for continuous-flow biocatalytic production of D-tagatose from D-fructose

Article Title: Immobilization of a tagatose 4-epimerase on amino-functionalized macroporous resin and its application in bioconversion of D-fructose to D-tagatose

Article References: Cai, X., Wu, Y., Wu, A., Li, Q., Wang, C., Mi, W., Miao, Z., Wang, X., Liu, Z., Wang, Y., & Zheng, Y. (2026). Immobilization of a tagatose 4-epimerase on amino-functionalized macroporous resin and its application in bioconversion of D-fructose to D-tagatose. 3 Biotech, 16(9), Article 398. https://doi.org/10.1007/s13205-026-05035-z

Image Credits: AI Generated

DOI: 10.1007/s13205-026-05035-z

Keywords: D-tagatose, tagatose 4-epimerase, enzyme immobilization, macroporous resin, glutaraldehyde crosslinking, continuous flow, biocatalysis, D-fructose, rare sugar, thermal stability, Thermotoga petrophila, biomanufacturing