The study, published in MicrobiologyOpen, was led by scientists at Teagasc, the Irish agriculture and food development authority, working with strains from their culture collection and the German Collection of Microorganisms and Cell Cultures. Their motivation was twofold. First, the global mannitol market, valued at roughly 451 million US dollars in 2024 and growing at about 5 percent annually, depends on a process that consumes energy and generates an unwanted co-product. Second, microbial fermentation offers an inherently gentler alternative: it runs at atmospheric pressure, at mild temperatures between 30 and 37 degrees Celsius, and can in principle use cheap agricultural feedstocks instead of pure sugars. The catch has always been that complex plant carbohydrates usually need to be broken down into fructose before microbes can turn them into mannitol, adding a costly processing step.

The Irish team’s insight was to pick a feedstock whose fructose chains are short enough for the bacteria to handle on their own. Yacon, Smallanthus sonchifolius, is a tuber native to the Andes that stores much of its carbohydrate as fructooligosaccharides, or FOS, polymers of fructose with a degree of polymerization typically no greater than ten. The researchers obtained yacon root powder, prepared by spray-drying yacon syrup from a grower in County Kilkenny, Ireland, where the crop has been trialed successfully in temperate conditions. Chemical analysis showed the powder contained 54.8 percent FOS by weight, along with 31.1 percent free sugars, mostly fructose, glucose and sucrose. Chromatographic profiling revealed that most of the FOS chains carried between four and six fructose units, making them considerably less complex than the inulin found in Jerusalem artichoke or chicory.

To ferment this material, the team screened ten strains of fructophilic lactic acid bacteria, or FLAB, belonging mainly to the genera Leuconostoc and Fructobacillus, all previously isolated from flowers on an Irish farm, including cotoneaster, dandelion and white clover blossoms. These bacteria thrive in fructose-rich environments such as nectar and are known to tolerate fructose concentrations of up to 40 percent. Crucially, genome analysis of one strain, Leuconostoc mesenteroides DPC7261, identified two genes encoding glycoside hydrolases carrying the GH32 domain, the enzymatic signature of invertases and inulinases, suggesting the bacteria could cleave yacon FOS into fructose themselves. That endogenous enzymatic toolkit is what makes a single-step, consolidated bioprocess possible.

The screening results were striking. When grown for 20 hours in laboratory medium containing 1 percent yacon root powder at 30 degrees Celsius, all ten flower-derived FLAB strains produced between 4 and 5 grams of mannitol per liter. The official type strains of the same species fared noticeably worse, producing less than 3 grams per liter, and the gap was especially pronounced for Fructobacillus fructosus, where the flower isolates yielded roughly twice as much mannitol as the type strain. A comparison strain, Limisolactobacillus fermentum, which has been reported to make mannitol from pure fructose, managed less than 3 grams per liter on yacon powder, even though it produced over 10 grams per liter when given plain fructose. The flower isolates, in other words, were uniquely suited to the untreated FOS substrate.

From the screen, the researchers focused on two strains: Fructobacillus fructosus DPC 7237, from white clover flowers, and Leuconostoc mesenteroides DPC 7246, from cotoneaster flowers. Although the Fructobacillus strains produced slightly higher peak titers, they flocculated strongly regardless of agitation, complicating cell density measurements and consistent inoculation. Leuconostoc mesenteroides DPC 7246 grew more robustly and was less demanding about oxygen levels, so it became the workhorse for optimization and scale-up. Time-course experiments showed mannitol production peaked between 16 and 20 hours, reaching 5.1 grams per liter for DPC 7246 and 5.5 grams per liter for DPC 7237, before declining after 24 hours. The researchers suggest that because the mannitol dehydrogenase reaction is reversible, the bacteria may reconsume mannitol to support central metabolism or regenerate cofactors once fructose runs low.

The team then tested whether temperature, agitation or substrate loading could push titers higher. Raising the temperature to 37 degrees Celsius or removing agitation brought no significant improvement, so subsequent work proceeded at 30 degrees Celsius with mild shaking. Doubling the yacon powder concentration to 2 percent increased peak mannitol by only about a third, and the extra substrate shifted the bacterium’s metabolism: ethanol appeared as a by-product, lactic acid rose, acetic acid fell, and the yield of mannitol per gram of FOS dropped from 0.92 to 0.635 grams. This overflow metabolism, in which surplus fructose is routed through the pentose phosphate pathway and ultimately into ethanol, suggests that simply loading more substrate is not enough; future processes will likely need fed-batch strategies to keep fructose flowing at a rate the cells can channel into mannitol.

The critical test came at scale. Moving from 15-milliliter tubes to 200-milliliter benchtop bioreactors, with controlled heating and mixing but no aeration and no pH adjustment, the researchers found that mannitol production peaked at 6.7 grams per liter, essentially unchanged from small-scale results, and actually arrived earlier, at 24 to 28 hours rather than 32. A post-hoc comparison across volumes showed titers of 6.55 grams per liter at 15 milliliters and 6.38 grams per liter at 200 milliliters at the 24-hour mark, both exceeding the 5.73 grams per liter seen in 40-milliliter shake flasks, where extra aeration appears to have altered the cells’ redox balance and reduced their need to regenerate NAD+ via mannitol. Accounting for the FOS content of the powder, the peak yields at bioreactor scale were 0.61 to 0.62 grams of mannitol per gram of FOS at both temperatures tested.

Those yields stand up well against the published literature on complex substrates. Fermentation of hydrolyzed inulin has delivered 0.58 grams per gram, chicory-derived inulin hydrolysate 0.69 grams per gram, and Jerusalem artichoke extract 0.68 grams per gram, but all of those required the feedstock to be saccharified before fermentation. Uncracked, unhydrolyzed substrates have generally performed far worse: Jerusalem artichoke juice fermented directly by Lactobacillus casei strains yielded only about 3.6 grams of mannitol per liter, compared with the 4 to 5 grams per liter the Irish strains achieved from yacon powder at screening concentrations. The researchers attribute this advantage to the short chain length of yacon FOS, which allows the bacteria’s own enzymes to hydrolyze and ferment the substrate simultaneously, a genuine consolidated bioprocess.

The work remains a proof of concept, and the authors are candid about the hurdles ahead. Titer and productivity must rise substantially to compete industrially, which will demand higher substrate concentrations, strains with greater fructose tolerance, cheaper nitrogen sources such as tryptone or corn steep liquor to replace the complex laboratory medium, and feeding strategies that suppress ethanol formation. Yet the underlying proposition is compelling: a natural, plant-based fructooligosaccharide from a tuber that grows well in temperate climates, converted by food-safe bacteria at mild temperatures into a sweetener with a low glycaemic index and applications spanning food and pharmaceuticals, all in one vessel and one step. As demand grows for sugar substitutes that do not carry the metabolic baggage of sucrose, the humble yacon tuber and its flower-borne microbial partners may have earned a place in the industrial biotechnology spotlight.

Subject of Research: Microbial production of mannitol from yacon fructooligosaccharides using fructophilic lactic acid bacteria

Article Title: Single‐Step Production of Mannitol From Yacon Fructo‐Oligosaccharides Using Fructophilic Lactic Acid Bacteria

Article References: Rajkumar, A. S., Leech, J., & McAuliffe, O. (2026). Single‐Step Production of Mannitol From Yacon Fructo‐Oligosaccharides Using Fructophilic Lactic Acid Bacteria. MicrobiologyOpen, 15(5), Article e70387. https://doi.org/10.1002/mbo3.70387

Image Credits: AI Generated

DOI: 10.1002/mbo3.70387

Keywords: mannitol, yacon, fructooligosaccharides, fructophilic lactic acid bacteria, Leuconostoc mesenteroides, Fructobacillus fructosus, fermentation, biotechnology, sweeteners, consolidated bioprocessing, prebiotics, sustainable food production