Xanthan gum sounds like a planet straight out of science fiction but it is one of the most useful and surprisingly common materials to be found in food, the oil industry, pharmaceutical and personal care products. Before xanthan, the only gums used in food were plant-based such as pectin and gum acacia. Xanthan has particular thickening properties and characteristics which make it stand out compared to most other gums.
From a regulatory perspective in the use of foods, xanthan gum has been widely accepted and its specifications set out by the Food Chemicals Codex and National Formulary. It is E415 in the EC ingredient list. In 1988 the ADI (acceptable daily intake) of xanthan gum was changed to ”not specified” confirming its status as an extremely safe food additive.
The ingredient is often categorized by many terms: a polymer, a hydrocolloid, gum or polysaccharide. It is used to thicken fluids like beverages, creams, sauces etc. Increasing viscosity to improve or change mouth feel is a useful property of most gum ingredients and xanthan gum has regularly been chosen for this purpose.
Xanthan is also a naturally produced ingredient and works well when combined with other polysaccharides to produce products with unusual flow behaviour. The unusual properties mean that it has very wide stability over a range of temperatures and can be used in both acid and alkaline conditions (Garcia-Ochoa et al., 2000). There is good chemical stability too and appears not to breakdown or hydrolyse easily.
The gum is produced by bacteria from the Xanthomonas genus. They are gram-negative and notable pathogens of a range of plants. Some are highly specific to particular plant species whilst others have a general range of attack. They all produce black rot which usually leads to death of the plant.
Xanthan is produced commercially by the gram-negative bacterium Xanthomonas campestris. Other Xanthomonas species producing xanthan gums include: X. arboricola, X. axonopodis, X. campestris, X. citri, X. fragaria, X. gummisudans, X. juglandis, X. phaseoli, X. vasculorium.
History
The gum was first examined in the 1950s by Allene Rosalind Jeanes of the USDA. The USDA began developing xanthan gum for the food industry in the 60s but its unusual flow properties also attracted the attention of the oil industry who have been using it since to recover oil from drilling sites. It was approved for food use in 1969 (Federal Register 345376).
Ice cream especially is a popular food that makes use of its pseudoplastic viscosity behaviour.
The gum has been widely reviewed: see Urlacher & Noble, 1997. Its fermentation has been reviewed as has xanthan production and extraction: Kennedy & Bradshaw, 1984; Vincent, 1985; Sutherland, 1990; Galindo, 1994 Habibi et al., 2017; Bhat et al., 2022.
Chemistry Of Xanthan Gum
Xanthan has a main chain (backbone) which is a linear backbone of 1,4-linked β-D-glucose with links at positions 1 and 4. The polymer skeleton is similar to cellulose (Jansson et al., 1975). It can also be described as repeating units of pentasaccharides with glucose, mannose and glucuronic acid in a ratio of 2:2:1. There are pyruvate and acetyl substituent substituent groups. The side chain is composed of glucuronic β‐d‐mannose‐(1,4)‐β‐d‐glucuronic acid and (1,2)‐α‐d‐mannose (Fitzpatrick et al., 2013).
It is known from many studies that the amounts of D-glucose, D-mannose, D-glucoronic acid and residues of acetic and pyruvic acid will vary to some extent in xanthan from the same species and from different Xanthomonas species. Culture conditions also have a part to play too on the structure (Sutherland, 1983).
Substitutions in xanthan come from pyruvic acid and acetal. Acetate groups are present as substituents at the O(6) position of the non-terminal mannose. Typically, acetate groups can be found on 60–70% of the internal mannose residues whereas 30–40% of the terminal mannose residues contain pyruvate residues (Abbaszadeh et al., 2015).
Approximately one-half of the terminal d-mannose contains a pyruvic acid residue linked via keto group to the four and six positions. The d-mannose unit linked to the main chain contains an acetyl group at position O-6. The presence of acetic and pyruvic acids produces an anionic polysaccharide type.
The level of substitution affects the molecular weight. The degree of substitution depends on the bacterial strain used as well as fermentation medium composition and operating conditions (Cadmus et al., 1978; Kennedy & Bradshaw, 1984). The pyruvate and acetate content influences the interaction between molecules of xanthan, and its interactions with other polymers such as pectin and galactomannans (Kang & Pettit, 1993; Peters et al., 1993).
The Molecular Size of Xanthan Gum
The average molecular weight (MMW) of xanthan chains might range from 1 x 10E6 to 20 x 10E6 g/mol, depending on the biosynthesis conditions and interchain association.
During fermentation, the mean molecular weight (MMW) was reported to increase from 7.2 x 106 to 9.3 X 106 kg/kgmol during the exponential growth phase. The molecular weight remains constant thereafter in the plateau phase or certainly in the 20 hours following the exponential growth phase which is usually the lifetime of a fermentation (Herbst et al., 1988). In the declining or lytic phase, the molecular weight starts to decrease but slowly and finally reaches close to approx. 8.5 × 106 kg/kgmol. It’s just worth noting that in Herbst’s studies the oxygen tension was kept above 20% – more on that later.
Xanthan Gum Properties And pH
Xanthan gum behaves as a polyanion at pH > 4.5 due to the deprotonation of O-acetyl and pyruvyl residues. Following extraction from a fermentation broth and without any heating, xanthan chains are arranged as single helices stabilized by Ca2+ ions, which can be irreversibly denatured, changing to coils (Baumgartner et al., 2008).
The solubility of xanthan is manipulated to help with extraction after fermentation as well as demonstrating aspects of its chemistry. Polyvalent cations such as calcium, aluminum, and quaternary ammonium salts are especially effective in the precipitation of xanthan gum due to ion binding of the cations to the ionized groups on the polyanionic polysaccharide (Garcia-Ochoa et al., 2000).
Analysis
Xanthan gums are commercially sized using gel chromatography (Herbst et al., 1988) and size-exclusion chromatography (Suh et al., 1990). The pyruvate content is measured using HPLC following hydrolysis with hydrochloric acid (HCl) at 80 ºC and extraction with ethyl acetate as previously reported (Papagianni et al., 2001).
The viscosity of solutions is monitored using rheometers. It can be a measure too of molecular weight.
Commercial Production Of Xanthan Gum
Xanthan gum is industrially produced by a bacteria called Xanthomonas campestris which is aerobically fermented. It is a typical secondary metabolite which means it is not directly associated with growth and is only produced when carbohydrate in the fermentation medium is in excess.
The gum is extracted, impurities removed and then dried to a powder. The outline process is to create an inoculum which is added to a bioreactor containing the production medium. This forms a fermentation broth where biomass is grown and xanthan gum excreted. Being extracellular makes recovery especially easy. The whole broth is heat treated in a pasteurization step to kill the bacterial cells. The cells are removed leaving a clarified broth which is treated with alcohol or some other agent that precipitates the xanthan. This xanthan is separated and purified, then washed, dewatered and dried for presentation to the customer.
Preparing The Inoculum
As in keeping with best practice in fermentation, strains of X. campestris which are proven to be high yielding for xanthan are preserved in cold-storage for long-term exploitation and development. A small amount of inoculum is routinely grown on a solid surface and then into liquid media to produce enough inoculum for large-scale fermentation. A typical YM agar plate medium for this would be d-glucose (10g/l), bacteriological peptone (3g/L), yeast extract (3g/L), malt extract (3g/L) and agar (24g/L).
During inoculum buildup, the cell concentration is increased but production of xanthan is minimized because xanthan around the cells impedes mass transport of nutrients and extends the lag phase of growth. Suppressing xanthan production whilst building up the cell mass, requires multiple stages of inoculum development (Sharma et al., 2014).
A typical inoculum volume would be between 5% and 10% (v/v) of the final batch volume. Increasingly larger batches are developed in stages through the process. The medium used for preparation of the inoculum in g/L: 20.0 sucrose, 3.0 yeast extract, 0.86 NH4NO3, 2.5 Na2HPO4 and 2.5 KH2PO4 (Alves, 1991, Lima, 1999, Serrano-Carreón et al., 1998, Faria et al., 2011). The average cellular concentration of inoculum was 0.29 ± 0.05 g/L. The growth of the microorganism in its inoculum requires a medium grown under agitation (150 rpm) in a shaker at a temperature of 28 ± 1°C for 16 hours.
The growth of X. campestris on a large scale is affected by all the traditional factors that influences any fermentation: type of fermenter vessel or bioreactor, type of operation, medium composition and quantity, culturing conditions such as temperature, dissolved oxygen and pH (Nasr et al., 2007).
Most manufacturers prefer batch production to continuous and so use a conventional stirred tank fermenter (Rosalam & England, 2006). In the early years of fermentation studies, both Leach et al., and Lilly et al., reported yields around 6g xanthan per litre.
The temperature for fermentation is optimal at 28ºC but this depends in part on the growth media. Most producers choose between 25 and 30ºC (Rogovin et al., 1965; Moraine and Rogovin, 1971; Silman and Rogivin, 1972; Kennedy et al., 1982; De Vuyst et al., 1987). There is a direct correlation between the specific growth rate and temperature.
The fermentation is finished very often from 20 hours onwards. A variety of commercial producers will finish between 72 and 96 hours before starting the extraction.
Fermentation Media
Generally, a medium for any fermentation needs a carbon and nitrogen source, various micronutrients such as potassium, iron, and calcium salts and may be some other specific minerals and vitamins. The optimal fermentation is designed using various methods based on response surface and central box design methods. The media is generally described as synthetic rather than using plant-derived sources.
A typical production medium would be in g/L; 27.0 sucrose, 2.0 yeast extract, 0.8 NH4NO3, 2.5 Na2HPO4, 2.5 KH2PO4 and 0.5 antifoam (Faria et al., 2009). In their study, temperature was set at 28ºC. The pH was maintained at 7.5.
Garcia-Ochoa et al., in 1992 proposed the following: sucrose (40 g/L), citric acid (2.1 g/L), NH4NO3 (1.144 g/L), KH2PO4 (2.866 g/L), MgCl2 (0.507 g/L), Na2SO4 (0.089 g/L), H3BO3 (0.006 g/L), ZnO (0.006 g/L), FeCl3.6H2O (0.020 g/L), CaCO3 (0.020 g/L), and concentrated HCl (0.13 ml) and the pH was adjusted to 7.0 by adding NaOH.
The pH depends on the media used but is usually made neutral because this is the optimum for bacterial growth. A few researchers do not think pH control is relevant. When pH is controlled, xanthan production ceases once the stationary growth phase is reached. This effect is independent of the alkali used to control the pH. When pH is not controlled, the gum production continues during the stationary phase of growth (Garcia-Ochoa et al., 1996).
Carbon Sources
Glucose and sucrose are the most commonly used carbon sources in most fermentations and xanthan production is no different (Dey & Chatterji, 2024). Glucose is the better of the two as a sole carbon source. The concentration of carbon source affects the xanthan yield; a concentration of 2 to 4% of either sucrose or glucose is preferred (Souw & Demain, 1979,1980). Glucose is still the best source for product yield, its supply and xanthan quality (Salah et al., 2011).
If the fermentation proceeds to a point where the glucose concentration begins to approach zero, xanthan production stops. It is known that keeping the glucose concentration between 30 and 40g/kg is ideal. It prevents inhibition of cell growth along with the ending of xanthan production. That also occurs if the glucose concentration is higher than 40g/kg (Funahashi et al., 1987). This group achieved a concentration of 43 g/kg broth after 96 hours cultivation. Some researchers use a fed-batch approach to obtain a good yield of product.
One study by Leela & Sharma (2000) evaluated the effect of several types of sugars as carbon sources for the fermentation of a wild strain of X. campestris GK6. The obtained xanthan gum yields in decreasing order were obtained in media containing glucose (14.7 g/L), sucrose (13.2 g/L), maltose (12.3 g/L), and soluble starch (12.1 g/L).
– Alternative Carbon Sources
A number of alternative sources of carbon are available because they are cheaper to use, might prove more economically effective as disposal costs become more prohibitive. A number of these also double up as it were as nitrogen sources.
Sugar beet molasses as a carbon source might prove an alternative to made-up media. It also provides a substantial portion of nitrogen as well as key essential vitamins and minerals. Production of xanthan up to 22.8 g/kg of fermentation medium using molasses is known (De Vuyst & Vermiere, 1994).
Other carbohydrate/carbon sources include various forms of starch especially hydrolyzed starch, apple pomace, cassava bagasse, green coconut shells, corn syrups, hydrolysed rice, barley & corn flour, acid whey, citrus waste, coconut juice, olive mill wastewaters, sugar cane molasses (El-Salam et al., 1994), date juice palm (Salah et al., 2011), corn steep liquor, glycerol (glycerine) and vegetable leftovers (Santo et al., 2011).
– The Need for Dipotassium monohydrogen phosphate
The addition of dipotassium monohydrogen phosphate (K2HPO4) has a significant positive effect on both gum and biomass production. The maximum xanthan gum production was 53g/l after 24 hours using 175g/l molasses, 4 g/l K2HPO4 and starting neutral pH. The hydrogen phosphate salt serves as a buffering agent (Kalogiannis et al., 2003).
A base such as sodium hydroxide is added to neutralise the anionic groups present in xanthan and bring the pH beck to around 7.
The Nitrogen Source
The nitrogen source is also critical. The nitrogen concentration in the medium alongside the glucose concentration affects production amounts. Cell growth stops when the nitrogen concentration approaches zero compared to xanthan production which stops when the glucose concentration approaches zero (Rogovin et al., Flores et al.,).
Ammonium as its chloride is a better substrate for biomass accumulation, while xanthan gum yields are higher with nitrate used as the nitrogen source. Studies by Letisse showed this to be the case when sucrose was the source. The two nitrogen sources ammonium chloride or sodium nitrate at an initial starting amount of 0.055% nitrogen equivalent was used.
Yeast extract, often serves as an additional nitrogen source, but has a negative effect on both xanthan and biomass production (Kalogiannis et al. 2003). It is likely that the organic nitrogen content in molasses is sufficient to support the growth of X. campestris whilst any further increase by the addition of yeast extract actually has a detrimental effect.
Other nitrogen sources that can be used are distillers’ dried solubles (DDS) in conjunction with glucose and minerals (Moraine & Rogovin, 1973). In the study of Moraine & Rogovin, formation of xanthan does not ultimately require active growth but the rate of production rises with cell concentration. It seems in that fermentation that the specific product formation rate dropped as the viscosity rose. The level of glucose concentration and dissolved oxygen which was up to 90% in some cases had no impact on the rate of xanthan production. They also made sure the pH stayed around 7 with a temperature close to 28°C. It could be the case that xanthan is produced when carbohydrate is directed to both cell mass and gum production.
Urea has been a supplement used in conjunction with DDS. That was a small-scale single-stage continuous fermentation (Silman & Rogovin, 1970). In that study the highest dilution rate was D = 0.0285/hr. the steady-state rate of xanthan production was 0.36 g/kg/hr. In that study the steady state yield was 68%. In continuous fermentation, xanthan production rate was dependent on the dilution rate and pH.
The addition of organic acids including citric acid, pyruvic acid, alpha-ketoglutarate and succinic acid can stimulate xanthan production but too much produces inhibition (Souw & Demain, 1979). Thus a careful balance is needed.
The balance of nutrients such as nitrogen, phosphorus, and magnesium on the amount of biomass, and the influence of nitrogen, phosphorus, and sulfur on xanthan production, are significant. All these factors influence sucrose consumption (
Tap water can contribute some minerals such as magnesium – it is one of the cheapest medium materials available. Some waters contain up to 34 mg/l of magnesium.
Triton 80 has been reported to improve xanthan yield and polymer quality by X. campestris grown on a chemically defined medium (Galindo & Salcedo, 1996). Some raw media materials such as molasses may contain surfactants as addition of Triton 80 has no improvement on xanthan yield (Kalogiannis et al., 2003).
Cheese whey is a waste byproduct of cheese production. The cheese whey:sucrose ratio and the supplementation of the medium was investigated using a Central Composite Design (CCD) in order to improve the xanthan gum production (Silva et al., 2009). Lactose in the whey serves as the carbon source. The producers claim to make 30g/l of xanthan for 40g/l of whey powder. In Silva’s study, maximum gum production was observed after 72 hours using cheese whey as the sole carbon source, 0.1% (w/v) MgSO4 7H2O and 2.0% (w/v) of K2HPO4, yielding approximately 25 g/L.
Green production of xanthan is now a priority given that fermentation media is expensive. Substrates that are often used in pathogenic sense might prove useful. Orange peels from the citrus processing industry, grape peels and vineyard/winery wastewater all appear to have promise (Roncevic et al., 2019). One Chinese research group found that Melaleuca alternifolia residue hydrolysate to be a potential substrate. This medium comes from waste material used in processing (Li et al., 2022).
An example of response surface methodology in the design and optimisation of media and fermentation conditions was demonstrated by Psomas et al., (2007). They used synthetic broths around Luria-Bertani plus glucose, (LBG) without pH control. The individual and interactive effects of three independent variables of agitation rate (100–600 rpm), temperature (25–35 C), time of cultivation (24–72 h) on xanthan gum and biomass production were studied, using a face-centered composite design of experiments. Optimal xanthan gum production was found at 600 rpm 30 °C at 72 h and biomass at 600 rpm, 25 °C at 72 hours.
Another suitable production medium using corn starch already mentioned would be (g/l): corn starch 30, corn steep liquor 2.8, sodium glutamate 2.4, CaCO3 4, NaCl 1.
Production Kinetics
The design and scale-up of production reactors for xanthan production has a long history. A variety of kinetic models of varying complexity have been developed (Moraine and Rogovin, 1966; Weiss and Ollis, 1980; Pinches and Pallent, 1986; Quinlan, 1986; Schweickart and Quinlan, 1989; Pons et al., 1989; et al., 1995a, 1998).
Two phases are known. The first is a tropophase where there is rapid cell growth with little or no xanthan production and then the idiophase where optimal biomass is reached and the bacteria start producing xanthan.
Being a secondary metabolite, the growth rate of the bacteria has a bearing on xanthan production. The specific growth rate is a major determinant of the rate of production of xanthan.
A conventional batch fermentation in a stirred-tank reactor (STR) with free cells usually has a productivity of only 0.5 g/L/h or lower. In continuous culture, the overall rate of xanthan production is constant in the dilution range of 0.05 to 0.2 per hour. the amount of xanthan gum produced per unit cell mass in continuous culture increases with a decreasing growth rate (Kuppuswami, 2014).
Viscosity of fermentation broths
As in applications, the viscosity of xanthan solutions increases strongly with increasing concentration. This behaviour is attributed to intermolecular interactions and molecular entanglement. This increases the effective macromolecule dimensions and molecular weight of the polymer.
In a fermentation, production stops in the late log phase when the viscosity is too high. This may be after 36 to 48 hours. In this case a level of between 2.5 and 3% with a viscosity of 20,000 cps is usually the case.
Adding salts affects xanthan viscosity. At low polymer concentration the viscosity declines slightly when a small amount of salt is added to solution. This is attributed to a reduction in molecular dimensions resulting from diminished intermolecular electrostatic forces (Smith and Pace, 1982). The viscosity of solution increases when larger amounts of salt are added. This is attributed to increased interactions between polymers (Smith & Pace, 1982; Milas et al., 1985). The viscosity is independent of salt concentration when the salt concentration exceeds 0.1% w/v (Kang & Pettit, 1993).
Some producers have tried precipitating the xanthan gum as it forms to reduce viscoity. Such a method has poisoned the Xanthomonas by also causing them to precipitate too and also requires removal of the precipitant too adding to further cost.
Oxygen Transfer In Fermentation Broths
A typical fermentation is aerobic where the typical aeration rates are usually in the range of 0.5 to 1.0 volumes of air per volume of liquid per minute (VVM).
Given that X. campestris is a strictly aerobic bacterium, the rate of oxygen transmission is a limiting factor (Becker et al., 1998). It increasingly becomes important as xanthan is excreted because of the increasing viscosity of the fermentation. If the oxygen transmission rate drops, the molecular weight of the gum also drops (Suh et al., 1990). Low molecular weight xanthan is not as viscous although this could be an advantage in some applications.
The optimum dissolved oxygen concentration anywhere in the fermenter is said to be somewhere between 10 and 30%. No commercial or research institute can give a precise figure because each fermenter model is often different to another. Clearly, aeration rate, impellar design and agitation speed are the key parameters here ((Carreón et al., 1998; Ochoa et al., 2000).
To avoid oxygen limitation means using high speed shear mixers, raising the oxygen partial pressure and minimising stagnant zones in the fermenter (Herbst et al., 1989).
A yield of 62% is usually expected if oxygen is not the limiting factor where the concentration achieved is 28 g/l, and where glucose is the carbon source. The specific growth rate is about 0.22 per hour. Whilst oxygen is a limiting factor other nutrients run out such as the carbon or nitrogen source (Moraine & Pogovin, 1979; Peters et al., 1989; Shu & Yang, 1990).
Another approach is to add a immiscible oil such as soybean, sunflower or other food-grade oil during fermentation. Supposedly significantly higher concentrations of gum are generated because of the emulsive nature of the fermentation. During fermentation, enough oil is added where the culture medium contains between 40 and 70% of its weight in a water insoluble oil in which the xanthan gum is also insoluble. It is claimed the oxygen transfer efficiency rises which improves biomass production as well as gum production. An emulsifier is needed. After 36 to 48 hours, the fermentation is stopped and 1.2 to 1.5 volumes of water insoluble oil is added along with further nutrition. the growth continues on for beyond 48 hours until one of the components is exhausted or when the xanthan yield reaches peak production and then stops (US Patent4352882A: Kelco). What is not clear from this patent is how the xanthan gum is then extracted and treated.
Fermenter design has been modified to improve oxygen transfer and claims made this increased xanthan production. Examples include the Static-mixing Loop Fermentor (PS-Loop Fermentor) (Ping et al., 1994).
To reduce agitation and aeration costs, the bacteria has been immobilized on a novel centrifugal fibrous-bed reactor using natural attachment to the fibres (Yang et al., 1996). Mixing and aeration were claimed to be overcome by continuous pumping and circulating the medium broth through the rotating matrix. This ensured intimate contact between substrates and gas with the cells whilst the xanthan gum continued to be separated from the cell mass. There were very few suspended cells suggesting all the cells were immobilised. The reactor could be operated in repeated batch mode. In their example this was 8 times. The specific productivity of xanthan was lower in this type of reactor compared to a stirred-tank reactor. This was due to lower cell viability (about 60%) and limitations in oxygen transfer.
Few studies exists on immobilization of Xanthomonas generally but cells have also been encapsulated onto calcium alginate beads and calcium alginate–polyvinyl alcohol-boric acid (CA–PVA) beads (Niknezhad et al., 2016). The authors claimed xanthan gum levels of 8.2 and 9.2 g/L respectively. This is not especially high from a commercial perspective. The growth medium was glucose and hydrolysed starch. The modified alginate beads were more robust and said to be more efficient than those cells immobilised on just calcium alginate. It’s not clear if economically there would be a better performance than a conventional STR.
Impellar Design
A variety of impellars are available for this type of highly viscous fermentation. It is advisable to try a low-shear impellar in an effort to maintain a uniform viscosity throughout the fermenter.
Impellars range from the Rushton turbine through to helical and ribbon impellars. A close clearance impellar with a spiral, serrated or anchor-style works well where the broth is continually swept upwards. Double helical and INTERMIG impellars also produce notable results.
Herbst et al., (1992) updated the volumetric mass transfer coefficient data because they were not satisfactory. A constant specific oxygen transfer rate was found to be the appropriate scale-up criterion for obtaining optimal; yields and constant productivity. The INTERMIG impeller for mixing was preferred over the Rushton impeller (Herbst et al., 1992).
Baffles are always needed. Placing these at a CB/D of 0.05 is ideal compared to a standard baffle placing of CB/D = 0.15 which is the standard baffle condition. This is because the former placing prevents secondary circulation form forming vortices like a ring. Incidentally, C is the distance of the centre of an impellar from the floor relative to the diameter of the impellar. A value of C=0.75D is best for flow with shear-thinning solutions.
Good examples of impellars used in fermentation are the LR500 (Satake Chem. Equip.), Lightnin A-315 (Galindo(t?) & Nienow, 1992). The latter type is not ideal for highly viscous solutions because of severe power drops and torque instability. Some designers have combined a helical impellar at the lower end with a Rushton style in the middle to achieve suitable draw of fermentation broth.
Extraction Of Xanthan
The final fermentation broth contains 20 to 30 g/L xanthan (2-3% by weight), 1 – 10 g/L cells, and 3- 10 g/L residual nutrients, and other metabolites (Garcia-Ochoa et al., 1993). Xanthan broths are extremely viscous which makes extraction much more difficult than others. Most recovery yields are at worst 65% of the total content in the broth whilst the best is still only 80%. There are no papers save for some claims in Chinese patents that anybody has achieved higher than this figure and even then it is only by a couple of percentage points if the figures are to be trusted.
The removal of cells is the most tricky because the density difference between cell and broth is often minimal. It requires additional energy to mix in recovery agents. To facilitate extraction, dilution 50% with tap water is possible but not recommended. It is preferred that broth be diluted with alcohol because this helps minimise costs with the final precipitation.
The end use for the gum dictates extraction and final polishing. Xanthan for food use requires stringent purification so all recovery materials need to be removed.
The bacteria cells are destroyed usually by pasteurisation to stop further damage to the gum and then separated. Whilst heat treatment helps in separation of xanthan it can also damage and extracellular materials, especially polymers which reduces yield. Care is exercised here. A typical operation is heat treatment between 80 and 130°C for between 10 and 20 minutes although a HTST type process is preferred. The pH of the broth is usually between 6.3 and 6.9 (Smith & Pace, 1982) towards the end of fermentation. Heating also improves solubility of the gum, appears to improve separation from the cells and reduces the viscosity in any subsequent processing. A typical tube-in-tube pasteuriser should be used rather than plate and frame.
The bacterial cells are separated from the feedstock usually using centrifugation (12,000g for a minute) or microfiltration/ultrafiltration. Centrifugation is preferred over filtration. A certain amount of xanthan gum is retained with the cells. The solids recovered are resuspended and washed and then recentrifuged to recover xanthan that passes into solution.
A hot solution from the pasteuriser helps with filtration in particular because any reduction in viscosity is a necessity. In the discussion on precipitation of the xanthan, some precipitating solvents are added just below precipitation concentration simply to reduce the viscosity by dilution.
Precipitation
The xanthan is precipitated using solvents such as acetone and methanol although on a large-scale pure isopropanol precipitation in the presence of a salt such as KCl is most commonly used. The salt concentration level is around 1g/l (Smith, 1983) whilst the IPA concentration is 42%w/w. The FDA also demands that isopropanol be used for food-grade xanthan. No reagent used here must reduce the molecular weight of the gum and this really goes for the whole of the extraction and processing. Isopropanol is also needed to wash any precipitated product remaining in vessels and filter systems.
Monovalent salts such as NaCl and KCl in their own right cannot produce precipitation. The polyvalent cations such as calcium, aluminium and quarternary ammonium salts are however effective in precipitation (Pace & Righelato, 1981; Kennedy & Bradshaw, 1984).
IPA precipitation is so expensive that its recovery and reuse is critical.
In solution, xanthan is like a hydrophilic colloid. Precipitation occurs because the solubility of macromolecules disrupts the colloid. Salts, water-miscible non-solvents and even evaporation can all be exploited. As well as alcohol, salts especially tri- and tetravalent salts are good. Alcohol precipitation is a standard practice for gums – the pectin industry also relies on alcohol precipitation.
The amount of IPA needed for total precipitation in volume terms is always the same ratio of 3.2 and as mentioned earlier a final concetration in broth of 42-43%w/w. The ratio when ethanol is used is 6 (Garcia-Ochoa et al., 1993). It also means at least 3 volumes of alcohol is needed for 1 volume of broth for precepitation to start occurring. Apparently, 18MT/hr of IPA is required to produce about 620 kg/hr of xanthan gum which is a ratio of 29:1.
If the precipitate is centrifuged away, then about 20% of the filtrate is sent to an IPA recovery unit. Here a distillation column is used to recover an 85%/w IPA fraction. As in keeping with the economical use of a distillation column, the feed stream is usually preheated before entering the distillation column using residual heat from the distillate stream and from the bottoms stream. It is possible for the feed stream to enter the distillation column at about 80C. The bottoms stream when it has exchanged heat with the feedstream is further cooled to 40C and then treated as for wastewater.
The volumetric xanthan productivity achieved in the reactor was ∼1 g/L/h based on the total liquid volume and ∼3 g/L/h based on the fibrous-bed volume. By comparison, conventional batch fermentation in a stirred-tank reactor (STR) with free cells usually has a productivity of only 0.5 g/L/h or lower. The high productivity in CPBR was attributed to the relatively high cell density, ∼7 g/L, in the reactor.
Alcohols when added also helps in the removal of impurities and coloured molecules.
If chemical treatments are used, the pH must not be raised too much or it removes pyruvate substitution in the xanthan. When salts are added for precipitation, cation binding of the added salt to ionic groups on the polymer produces charge neutralisation. calcium, aluminium and quarternary ammonium salts are very effective precipitants. Sodium chloride is not effective although KCl is sometimes used.
In some purifications, the addition of divalent salts helps reduce the amount of alcohol required for precipitation compared to monovalent salts (Garcia-Ochoa et al., 1993). In all cases, chloride is the preferred anion. the total precipitation of xanthan is reached with a VIPA/Vo ratio smaller than 0.9 using calcium or magnesium. calcium is slightly better. The only major issue is that the xanthan is almost insoluble. The salt concentration is around 1g/L.
The xanthan is then decanted as a precipitate where it is further washed in alcohol. It is dried and milled to produce a white -cream-coloured free-flowing powder.
Some researchers have used dialysis following alcohol-, acid- and alkali-precipitation as a way to clean up xanthan (Li et al., 2022). The quality of the xanthan gum was better with dialysis using alcohol precipitation although it is generally acknowledged that whatever approach is taken, an alcohol method is better than using pH based approaches.
Any light phase from the decanter can be treated to recover xanthan gum which is recycled in the precipitation step. The methanol or isopropanol will be 50% by volume and is ultrafiltered before being recovered by distillation (Lipnizki, 2010).
Application of Ultrafiltration
Ultrafiltration is one of the most effective methods of reducing solvent use to precipitate xanthan by concentrating it further even though it may be a viscous solution. Whilst membrane processes are generally high-shear it does not seem to affect the molecular weight or the rheological properties of the gum solution.
A typical membrane system would be a polysulphone membrane hollow fibre of 500,000 MWCO cut-off in a tubular arrangement.
Given that on average the starting solution is 2-3% by weight xanthan, ultrafiltration has been successfully employed to concentrate it further. The concentration factor is 5 or higher and can reduce the amount of alcohol needed by at least 80%.
An ultrafiltration retentate can be generated of 5 to 15% by weight of high molecular-weight xanthan. The recovery yield is 95%. The permeate flux was constant up to around 8% before it dropped away. The loss of flux occurred because the pumping shear rate rose as did the viscosity (Lo et al., 1996). It appears that fouling is not an issue because the membrane gel layer probably does not form as it is still shear-thinning at the membrane surface. Lo et al (1996) reported that “the filtrate flux decreased with increasing xanthan concentration and increased with increasing pumping (shear) rate and trans-membrane pressure difference”. There is no benefit in altering pH of the broth.
The permeate is mostly the watery broth with some fragments of xanthan, sugars and salts. This permeate stream can be passed through reverse osmosis membranes to remove the water which is reused in fermentation. It is reckoned that 80% of the energy used in purifying xanthan gum has been saved using UF (Lo et al., 1997).
Fermentation Byproducts
The biomass recovered from fermentation has a certain valuable as a fertilizer. This material is dried to below 10% moisture content using a rotary or drum dryer. A small amount of cost of operation is obtained if this biomass is sold for about $50-60/MT.
During the production of xanthan gum, Xanthomonas produces xanthomonadins, which are a class of water-insoluble, brominated, aryl polyene yellow pigments (Amanullah et al., 1998). These pigments are membrane bound. Their role is to protect the bacteria from UV damage because they have potent antioxidant activity and allows them to infect their hosts.
Economics
The cost of fermentation medium can be very high – it is a critical element of commercial production. The use of cheaper substrates, instead of the commonly used ones such as glucose or sucrose, might result in a lower cost of the final product. We have mentioned waste resources including cheese whey for example.
In terms of running the fermentation, agitation and aeration remain the most costly maintenance item because of the mixing needed to overcome the high broth viscosity .
The production cost of food grade xanthan gum in the downstream purification steps can be as high as 50%, which would not be required for nonfood applications (Palaniraj & Jayaraman, 2011). The costs of aeration and agitation (electricity costs) are 25% and 20% respectively of the total cost of a batch fermentation (FoodWrite Ltd, unpubl. results).
Major suppliers nationally include the USA, China, India and various European Community countries. CP Kelco, Meerck and Pfizer are established suppliers in the USA. KELTROL T is a well known brand name from Kelco. Satiaxane CX190 is produced by Degusa Texturant Systems (France) as well as Rhone Poulence, Mero-Pousselot-Santia and Sanofi-Elf. The major producer in China is Saidy Chemical and in Austria it is Jungbunzlauer.
The price of xanthan in 2022 was around £200 to £360/kg for a 25kg sack depending on quality and batch size. A 1000kg (1 metric tonne) is economically cheaper but not that much more.
Uses
Extremely small amounts are used. The ingredient is often dissolved in water using a high shear mixer before added to the main formulation. It forms biodegradeable films when combined with other gums and polysaccharides. Chitosan is a common material to be combined with xanthan as it produces some unusual films as a result (Horn et al., 2015).
Fermentation and downstream extraction performance is also evaluated on the quality of the gum produced. Qualities such as pseudoplasticity and its behaviour in solid gel systems are good approaches. Likewise, flow curves and linear viscoelastic behaviour are suitable. Some researchers have used rheology as their main measure – eg. the storage modulus (G′) and loss modulus (G″) in frequency sweep tests (Li et al., 2022).
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I bought this gum to help me with my keto diet and it’s fab, also arrived very quickly when I ordered it from Holland & Barrett. Sounds like another weird ingredient from the nuclear war. Have to say though that it probably is a useful one in replacing some of the trans fat I was feeding myself.