Cheaper, more efficient alternative to traditional sugar: Researchers unlock improved tagatose production

05 Dec 2019 --- Researchers from Tufts University, US, have developed a process that may unlock the commercial potential of tagatose, a low-calorie, low-glycemic sugar. By using bacteria as tiny bioreactors that encapsulate the enzymes and reactants, the scientists were able to increase yields from 30 to 85 percent, which they say could lead to large-scale manufacturing and getting tagatose on every supermarket shelf. The US Food and Drug Administration (FDA)-approved natural sugar has only 38 percent of the calories of traditional table sugar, is safe for diabetics and does not cause cavities, meaning it could be a key tool in reformulation targets.

“Although other commercial syntheses of tagatose are available, they result in a supply priced at about US$35 per pound, which is very expensive. It is hoped that this method of synthesis could bring that price down significantly, making it more widely available,” Mike Silver, Associate Director of Public Relations at Tufts, tells NutritionInsight.

While tagatose is derived from fruits and dairy products, it is not abundant and can be difficult to extract. The current manufacturing process involves a conversion from more easily obtained galactose to tagatose and is highly inefficient.

Thanks to its good browning ability, tagatose is very suitable for baking purposes, where a caramelized flavor is desirable. Additionally, Silver hopes that its natural taste could give it an edge over artificial sweeteners. “It only slightly raises the level of glucose in the blood after ingesting, with a glycemic index of 3, in comparison to 99 for conventional sugar.”

However, the method has not yet been developed to commercial scale, and the scientists have not pursued a patent for their system. “There is always room to improve processes. Although this is the most productive system compared to other published studies, it is still quite slow, so we can improve rates and productivity. Higher conversions – up to 100 percent – would be even more desirable,” says Nikhil U. Nair, Assistant Professor in Chemical & Biological Engineering at Tufts and who partook in devising the system.

The team is now interested in exploring if a similar approach can be used to address issues with other hard-to-make sugars.An innovative method
Now published in Nature Communications, Nair and postdoctoral fellow Josef Bober used Lactobacillus plantarum – a food-safe bacterium – to make large quantities of an enzyme called L-arabinose isomerase (LAI), which makes tagatose from galactose. As galactose is not the main target for the enzyme, the rates and yields of the reaction with galactose are less than optimal. 

Additionally, LAI is not very stable in a solution, and the reaction can only push forward until about 39 percent of the sugar is converted to tagatose at 37 °C, and only up to 16 percent at 50 °C before the enzyme begins to degrade.

However, L. plantarum can keep LAI safe and stable within the confines of the bacterial cell wall. The researcher found that when expressed in L. plantarum, the enzyme kept converting galactose to tagatose and pushed the yield to 47 percent at 37 °C. With LAI stabilized within the cell, it increases the yield to 83 percent at the higher temperature of 50 °C without degrading significantly, while producing tagatose at a much faster rate.

The researchers also found evidence that the transport of the starting material, galactose, into the cell was a limiting factor to speed. To resolve that issue, they treated the bacteria with just enough detergents to make their cell walls leaky. The galactose was able to get in and tagatose released from the cells, allowing the enzyme to convert galactose to tagatose at a faster rate, shaving a couple of hours off the time needed to get to 85 percent yield at 50 °C. 

Nair notes that the team is now interested in exploring if a similar approach can be used to address issues with other hard-to-make sugars that are of interest. This comes at a key time as the industry is facing increased reformulation pressure. 

Earlier this week, it was found that that one in four UK festive hot drinks had more sugar than directly comparable hot beverages, while dairy alternatives harbored excess sugar to just as great an extent. Action on Sugar is now calling for the highly successful Soft Drink Industry Levy (SDIL) to be extended to sugary-milk based drinks.

By Katherine Durrell