• Xylitol
    from Biomass
    Bioprocess Development

Biobased Chemicals - Background

Rationale for Biobased Chemicals Production

The production of chemicals from biomass, also known as bio-based chemicals, plays a critical role in creating a sustainable and environmentally friendly future, particularly as the world strives to reduce dependence on fossil fuels. Some of the advantages of biobased chemicals are listed below:
  • Climate Change Mitigation - Unlike fossil fuels, bio-based chemicals are made from plant materials that recently absorbed carbon dioxide from the atmosphere.
  • Security of Supply - Fossil fuel supply often is reliant on geopolitically unstable regions. Developing domestic sources of biomass and the infrastructure to convert such feedstocks to chemicals, can help improve national chemicals security.
  • Sustainable Development - Biomass can often be produced and processed locally, promoting rural development and creating jobs in agriculture, industry, and research.
  • Waste Management - Biomass for bio-based chemicals can come from waste residues from agriculture, forestry, or even municipal waste. Using these waste streams for biobased chemicals can help solve waste disposal problems.
  • Biodegradability - Many bio-based chemicals and the products made from them are biodegradable, avoiding environmental pollution associated with many fossil-derived resources.
  • Resource Efficiency - The use of biomass as a raw material can contribute to a more circular economy, where waste from one process becomes the feedstock for another. This approach increases resource efficiency and reduces environmental impact compared to linear models of production.

Approaches for the Production of Biobased Chemicals

There are two main ways in which biobased chemicals can be obtained from biomass feedstocks:

  1. Direct Extraction from Biomass - In this approach the target chemicals already exist within the feedstock. Hence, the focus of the bioprocess is on the extraction of the target chemical and then on subsequent separation and purification steps. CBD (cannabidiol), an alkaloid obtained from extracts of the hemp (cannabis) plant, is one example, among thousands, of a biobased chemical obtained this way.
  2. Production from Biomass or Biomass-Derived Compounds - Here the biobased chemical does not exist natively in the feedstock but is produced from it. This conversion can involve chemical, thermal, catalytic, and biological approaches or a combination of these. It is usually the case that the key stage of the bioprocess, where the biobased chemical is produced, works on a fraction, or derivative, of the original biomass feedstock. For example, ethanol can be produced via fermentation of the monomeric sugars obtained when the lignocellulosic polysaccharides (cellulose and/or hemicellulose) are hydrolysed. Alternatively, ethanol can also be produced via catalytic reforming of the syngas produced in the gasification of biomass.
The viability of obtaining a specific biobased chemical from biomass depends on a wide variety of factors, including the chemical composition of the feedstock and its suitability for different bioprocessing technologies. In some cases a feedstock may not be a good match for a particular biobased chemical but may be more suitable for the production of other types of biobased chemicals.

How Celignis Can Help

At Celignis our multidisciplinary team has strong understanding of: biomass chemistry, bioprocessing technologies, and the mechanisms and challenges involved in producing a wide variety of biobased chemicals. We are ready to work with you on developing a suitable bioprocess to either obtain your targeted biobased chemical from biomass or to obtain the most appropriate biobased chemicals from a given feedstock.

Xylitol - Background

Xylitol Chemistry and Applications

Xylitol is a five-carbon sugar alcohol, chemically classified as a polyol. It is a white crystalline solid that is soluble in water, with a sweet taste similar to that of sucrose. It has the chemical formula C5H12O5.

As a sugar alcohol, xylitol is neither a sugar nor an alcohol in the traditional sense, but shares some properties with both. It has the same sweetness as sucrose, but with approximately 40% fewer calories, which makes it a popular choice as a sugar substitute in many "sugar-free" and "low-calorie" products.

Xylitol has many important applications, a few of these are listed below:
  • Food Industry - Xylitol is used extensively in the food industry as a sugar substitute. It is used in a variety of products including chewing gum, candy, baked goods, and other food items, particularly those marketed as "sugar-free" or "low-calorie."
  • Dental Health - One of the unique benefits of xylitol is its ability to help prevent tooth decay. Unlike sugars, which can contribute to tooth decay, xylitol is not fermented by oral bacteria, and thus does not produce the acids that lead to tooth decay. It can even inhibit the growth of certain bacteria, such as Streptococcus mutans, which are known to cause tooth decay.
  • Pharmaceuticals - Xylitol is used in pharmaceuticals and personal care products like toothpaste, mouthwash, and certain over-the-counter drugs due to its antibacterial properties and its ability to increase the bioavailability of certain drugs.
  • Nutraceuticals - Due to its low calorie content and low glycemic index (it is metabolized independently of insulin), xylitol is popular in nutraceuticals and food products designed for diabetics and those on calorie-restricted diets.

History of Xylitol Production

The history of xylitol production goes back to the late 19th and early 20th centuries. Xylitol was first discovered and isolated by German and French chemists in the late 19th century, though it remained a relatively obscure compound for several decades due to the lack of efficient production methods.

The interest in xylitol production increased during World War II due to sugar shortages. Finland, a country rich in birch trees, began to investigate the production of xylitol from the xylan-rich hemicellulose fraction of birch wood as a substitute for sugar. This marked the beginning of large-scale xylitol production.

Early xylitol production processes were based on the acid hydrolysis of hardwood xylan to yield xylose, which was then reduced to xylitol by catalytic hydrogenation. However, these early processes were energy-intensive and had relatively low yields.

In the second half of the 20th century, more efficient processes were developed. A significant advance was the development of the so-called "sugar platform" approach, which involves the enzymatic hydrolysis of hemicellulose to produce xylose, followed by microbial fermentation to convert the xylose into xylitol. This process, although more efficient than acid hydrolysis, is still challenging due to the relatively low yields and the need for extensive purification of the final product.

Most commercial xylitol is still produced by the catalytic hydrogenation of xylose derived from hardwoods or corncobs. These processes can be considered "biobased" in the sense that they start from renewable feedstocks, but they still rely on industrial chemical processes for the actual production of xylitol.

There's currently interest in producing xylitol via microbial fermentation directly from lignocellulosic biomass. However, this approach presents its own challenges, including the need to develop more efficient microbial strains and processes.

Xylitol from Lignocellulosic Feedstocks

Biomass Hydrolysis to Xylose

Xylitol is currently mostly produced from the pentose sugars (primarily xylose) present in the hemicellulose polysaccharides of lignocellulosic feedstocks.

The first step is to hydrolyse hemicellulose in order to obtain these monomers. This hydrolysis can involve enzymes and be undertaken alongside the hydrolysis of cellulose. However, depending on the pretreatment method used, hemicellulose may end up in the liquid output stream of that pretreatment. For example, hemicelluloses tend to be well hydrolysed in dilute-acid pretreatments and, depending on the process conditions, in hydrothermal (Liquid Hot Water) pretreatments. Hence, the production of xylitol can be a means of valorising the pretreatment liquid sidestreams in bioprocesses involving the pretreatment of biomass for subsequent enzymatic hydrolysis of cellulose.

Catalytic Production of Xylitol from Xylose

The chemical approach to xylitol production from xylose generally involves a process known as catalytic hydrogenation. The main steps and parameters of the process are described below:

  1. Feeding the Reactants: Xylose and hydrogen are fed into a reaction vessel. The xylose can be in a pure form or as part of a solution, whilst the hydrogen is typically provided as a gas.
  2. Catalysis: The reaction is facilitated by a catalyst, typically a metal such as nickel, ruthenium, or palladium supported on a carrier like silica or activated carbon. The catalyst accelerates the hydrogenation process, helping the hydrogen atoms to be added across the carbonyl group in xylose, reducing it to the alcohol group in xylitol.
  3. Reaction conditions: The reaction is typically carried out at elevated temperatures (100-160 °C) and pressures (20-50 bar), although this can vary depending on the specific process and catalyst used. The pH of the reaction is also controlled, as it can affect the selectivity of the hydrogenation reaction towards xylitol.
  4. Product Separation: After the reaction, the product mixture typically needs to be separated and purified. This can involve steps such as filtration to remove the catalyst, evaporation to concentrate the xylitol, and crystallization to obtain pure xylitol.

Xylitol via Fermentation of Xylose

An alternative to the chemical approach for xylitol production is to use microorganisms, such as yeast or bacteria, that have the ability to convert xylose into xylitol. The process is essentially a specialized form of fermentation.

Several species of yeasts and bacteria have been identified as potential candidates for this fermentation, including species of Candida, Pichia, and Debaryomyces. Some microorganisms naturally produce xylitol, while others have been genetically modified to enhance their xylitol production capabilities. Some yeast strains have been shown to tolerate high sugar concentrations and produce high xylitol yields.

During the fermentation process the microorganisms metabolize xylose primarily through the pentose phosphate pathway. In this pathway, xylose is first reduced to xylitol by xylose reductase, an enzyme that the microorganisms produce. This step requires the co-factor NADPH. Xylitol is then oxidized to xylulose by xylitol dehydrogenase using NAD+. However, the latter step is typically slower and less efficient, leading to an accumulation of xylitol, which is then excreted by the cells.

Some important parameters for the fermentation are described below:
  • Fermentation Time - Fermentation time can affect xylitol yield. In batch fermentation, the process might take several days. In continuous or fed-batch fermentation, it is controlled by feeding rates and other parameters.
  • Fermentation Mode - The mode of fermentation can influence xylitol production. In batch fermentation, all nutrients are added at the beginning, while in fed-batch or continuous fermentation, nutrients are added over time. Fed-batch or continuous fermentation can potentially give higher xylitol yields, as they allow better control over the fermentation conditions.
  • Presence of Fermentation Inhibitors - Depending on the source of xylose (e.g the hydrolysis or pretreatment method employed), the feed may contain toxic compounds that inhibit the growth of the microorganisms or the fermentation process. These toxins need to be removed or neutralized before or during fermentation. Alternatively, microorganisms that are less sensitive to these inhibitors should be used.

Chemical Vs Biological Approach

Advantages of the chemical method over the biological method:
  • High yield - The chemical method generally provides a high yield, often greater than 90%.
  • Robust and Predictable - The process is less susceptible to contamination compared to biological methods and it has less variability, leading to more predictable outputs.
  • Scalability - This method is often used in commercial production as it can be effectively scaled up for industrial application.
Disadvantages of the chemical method over the biological method:
  • High Energy Consumption - The process requires high temperatures and pressures, leading to substantial energy consumption.
  • Use of Hazardous Materials - The use of potentially dangerous substances like hydrogen gas and catalysts like nickel, ruthenium, or palladium can pose safety and environmental risks.
  • Catalyst Deactivation - he catalyst may need to be frequently replaced due to deactivation over time, adding to the process costs.
Advantages of the biological method over the chemical/catalytic method:
  • Lower Energy Requirements - The process operates at ambient or near-ambient conditions, resulting in less energy consumption.
  • Environmentally Friendly - It does not require the use of harsh chemicals, and does not generate hazardous waste.
  • Flexibility - There's potential for further transformation of xylose into other value-added products due to the metabolic versatility of microorganisms.
Disadvantages of the biological method over the chemical method:
  • Lower Yield - The yield of xylitol is typically lower than the chemical method, though research is ongoing to improve this.
  • Contamination Risks - The fermentation process is susceptible to contamination, which could disrupt xylitol production.
  • Feedstock Issues - The hydrolysates used as feedstock can contain inhibitors that negatively impact microbial growth and fermentation, necessitating additional detoxification steps.
  • Separation and Purification - The fermentation broth contains various by-products, necessitating extensive purification steps to obtain pure xylitol.

Xylitol from Hexose Sugars

While xylitol is typically derived from the pentose sugar xylose, it is also possible to produce xylitol from hexose sugars like glucose, derived from the cellulose fraction of lignocellulosic biomass. This involves the use of certain microorganisms that can convert hexose sugars into xylitol. It is worth noting, however, that this process is less straightforward and efficient than the direct reduction of xylose to xylitol, and is still a subject of ongoing research.

One approach for converting hexose sugars into xylitol involves genetically engineered microorganisms. Through metabolic engineering, the natural metabolic pathways in these microorganisms can be modified to divert glucose metabolism towards the production of xylitol. A commonly used microorganism for this purpose is Saccharomyces cerevisiae (yeast), which naturally metabolizes glucose through glycolysis. By introducing the genes for xylose reductase and xylitol dehydrogenase (enzymes that convert xylose into xylitol), and modifying other metabolic pathways, the yeast can be engineered to convert glucose into xylitol.

However, it is important to highlight that this is a challenging process. Glucose naturally tends to be metabolized to ethanol in yeast, and redirecting this metabolism towards xylitol production requires significant metabolic engineering. Furthermore, the yield of xylitol from glucose is typically lower than that from xylose, as the conversion involves more steps and is less direct. Therefore, while it is technically possible to produce xylitol from hexose sugars, it is typically more practical and efficient to use pentose sugars like xylose as the starting material and then to use the biomass-derived hexose sugars as substrates for the production of other biobased chemicals (e.g. bioethanol, biobutanol etc.).

Higher-Value Chemicals from Xylitol

Xylitol can be also be upgraded to other higher value chemicals. Below are listed a few examples of chemicals that can be derived from xylitol:
  • Furans - Xylitol can be converted into furanic compounds, such as furfural and hydroxymethylfurfural (HMF), through dehydration reactions. Furans are versatile platform chemicals that can be further transformed into a variety of high-value chemicals and materials.
  • Xylaric acid - Xylitol can be oxidized to form xylaric acid, a compound with potential applications in biodegradable polymers and as a building block for chemical synthesis.
  • Xylonates - Oxidation of xylitol can also lead to the production of xylonates, such as xylonic acid, which has potential uses in food, pharmaceuticals, and chemical industries.
  • Polyols - Xylitol is itself a polyol, but it can also be chemically modified to produce other polyols with different properties, which can be used in various applications like pharmaceuticals, food products, and polymer synthesis.
  • Biofuels - Xylitol can be used as a starting material for the production of biofuels. For example, it can be converted into butanol through a series of fermentation and catalytic steps.

Bioprocess Development for Xylitol Production - How Celignis Works With Our Clients

At Celignis, we have the expertise and infrastructure to be able to produce xylitol from biomass-derived sugars using both the chemical/catalytic and biological approaches. Developing a fully-integrated process for xylitol production from a feedstock involves a number of different steps, outlined below. We can work on projects where all these steps are undertaken or we can focus on the optimisation of a particular stage in the process. The latter is particularly relevant where our clients already have a technology for xylitol production and are looking for optimising the process at particular nodes. Additionally, where an exisiting bioproces, for example one focused on cellulose valorisation, already has a side-stream containing xylose (for example, as the liquid output of its pretreatment) then our work can focus on tailoring a bioprocess for the production of xylitol from that side-stream.

1. Understanding Your Requirements

Prior to undertaking bioprocess projects we learn from our clients what their targets are from the process as well as whether there are any restrictions or requirements that may need to form the boundaries of the work that we undertake. These help to guide us to then prepare a potential bioprocess development project.

For example, in the context of xylitol production, the primary focus of one client may be on the production of xylitol in the highest possible yields from a particular feedstock whereas, for another client, the primary focus may be on cellulose valorisation, with the production of xylitol from the xylose fraction being a lower priority. These different preferences are likely to influence our choices of pretreatment technology, hydrolysis and fermentation approaches, and process conditions.

Additionally, based on their existing infrastructure or other factors, the client may express a preference for a certain route (chemical or biological) for the production of xylitol which will affect our choices in subsequent stages of the project.

2. Detailed Feedstock Analysis

In cases where you have already selected a feedstock for the bioprocess, we would then undertake a detailed compositional analysis (P10 or, ideally, P19) of representative samples of that feedstock.

In cases where the feedstock has not yet been selected we can review your list of candidate feedstocks, selecting top candidates based on our prior experience in their analysis and bioprocessing. If you do not have a list of candidate feedstocks then we can provide one, based on your location and the requirements outlined in Stage 1. We would then analyse in detail these priority feedstocks and come to a decision, based on the compositional data and other relevant factors (e.g. price, supply, consistency etc.) on a selected feedstock for the project.

At this point of the project, the Celignis Bioprocess team typically meet to discuss and prepare a project proposal for the development of a bioprocess for xylitol production from this feedstock. After this proposal is reviewed by the client, and revised if needed, we are then ready to start work on the next stages.

3. Hydrolysis to Xylose (Lab-Scale)

We will undertake a number of hydrolysis experiments, focused on the extraction of xylose from the feedstock. These are often termed pretreatment experiments, when considering lignocellulosic biomass as a whole since methods for hemicellulose hydrolysis may not hydrolyse cellulose. Alternatively, depending on the we workplan established in Stage 2, we can work on the more extensive hydrolysis of biomass, releasing sugars from both hemicellulose and cellulose. However this approach would itself require some form of pretreatment in order to allow for the enzymes to effectively hydrolyse the biomass.

Our hydrolysis experiments will follow a scientifically-based Design of Experiments (DoE) protocol where the criteria and boundaries for this DoE are formulated in close collaboration with our clients, considering the chemistry of the feedstock(s) and our understandings of the mechanisms of biomass hydrolysis conversion.

We usually recommend that these initial optimisation experiments are undertaken at the lab-scale (around TRL3) in order to reduce costs and the length of the project. For each experiment we analyse the solid and liquid outputs of the process, leading to a detailed data-set where effects of process conditions on the yield and composition of the various streams can be explored and mapped.

We can also undertake a second iteration of lab-scale experiments in order to fine-tune the conditions based on the knowledge gained in the initial experiments.

4. Xylitol Production

At the project proposal stage we will have decided with regards to whether the chemical or biological route would be applied for the production of xylitol from xylose.

In either case, our experimental work would follow a Design of Experiments (DoE) protocol, as outlined in Stage 3, in order to optimise the various process parameters for xylitol production. As before, these optimisation experiments are recommended to be undertaken at the lab-scale in order to accelerate the outputs and reduce project costs.

This stage can follow the hydrolysis optimisation activities, however in some cases it is recommended that these two stages overlap due to the interdependencies between them with regards to the ultimate yields and efficiencies of xylitol production.

5. Xylitol Recovery

Based on the outputs of the prior lab-scale Stages we can optimise the methods employed for separating and purifying the xylitol from the process. We can also potentially look at the recovery of other compounds from the liquid.

It is possible for this Stage to run alongside Stage 4.

6. Valorisation of Remaining Biomass

Depending on the pretreatment, hydrolysis, and fermentation processess employed in the project, we can look at valorising those biomass components that are not used for xylitol production. For example, we can assess the cellulose residue as a feedstock for enzymatic hydrolysis and for the subsequent fermentation of the liberated glucose to bioethanol.

7. Validation at Higher TRLs

Once we have concluded our optimisation of the xylitol bioprocess conditions at the lab-scale we can then test those conditions at higher technology readiness levels (TRLs). The scales at which we can operate are dependent on the type of technology employed, but can reach up to 100 litres.

We have all of the necessary downstream equipment to efficiently handle the solid and liquid streams arising from these scaled-up activities.

If we find that there are differences between the yield and compositions of the different streams, compared with our lab-scale experiments, then we can explore the potential reasons for these and work on final tweaks to optimise the bioprocess for higher TRLs.

8. Technoeconomic Analysis (TEA)

The Celignis team, including Oscar our chief TEA expert, can undertake a detailed technoeconomic analysis of the developed process. We apply accurate and realistic costing models to determine the CAPEX and OPEX of simulated and pilot scale processes which are then used to determine key economic indicators such as IRR, NPV and payback periods.

Within these TEAs we can undertake sensitivity analyses to assess the effect of variable costs and revenues on the commercial viability of the xylitol production process.

Our preferred approach is to include TEA studies at each stage of the development of the bioprocess, so that the process can be optimised in a commercially-relevant way, followed by a more detailed TEA after the process has been optimised and tested at higher TRL levels.

Click here to read more about the technoeconomic analysis (TEA) services offered by Celignis.

Contact Celignis Bioprocess

With regards to the development and optimisation of bioprocesses for the production of xylitol from biomass, the Celignis Bioprocess team members with the most experience in undertaking such projects are listed below. Feel free to contact them to discuss potential projects.

Lalitha Gottumukkala

Founder of Celignis Bioprocess, CIO of Celignis


Has a deep understanding of all biological and chemical aspects of bioproceses. Has developed Celignis into a renowned provider of bioprocess development services to a global network of clients.

Oscar Bedzo

Bioprocess Project Manager & Technoeconomic Analysis Lead


A dynamic, purpose-driven chemical engineer with expertise in bioprocess development, process design, simulation and techno-economic analysis over several years in the bioeconomy sector.

Dan Hayes

Celignis CEO And Founder

PhD (Analytical Chemistry)

Dreamer and achiever. Took Celignis from a concept in a research project to being the bioeconomy's premier provider of analytical and bioprocessing expertise.

Other Biobased Chemicals


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