Review Synthesis of the Microbial Polysaccharide Gellan from Dairy and Plant-Based Processing Coproducts Thomas P. West Department of Chemistry, Texas A&M University-Commerce, Commerce, TX 75429, USA; [email protected]; Tel.: +1-903-886-5399 Abstract: This review examines the production of the microbial polysaccharide gellan, synthesized by Sphingomonas elodea, on dairy and plant-based processing coproducts. Gellan is a water-soluble gum that structurally exists as a tetrasaccharide comprised of 20% glucuronic acid, 60% glucose and 20% rhamnose, for which various food, non-food and biomedical applications have been reported. A number of carbon and nitrogen sources have been tested to determine whether they can support bacterial gellan production, with several studies attempting to optimize gellan production by varying the culture conditions. The genetics of the biosynthesis of gellan has been explored in a number of investigations and specific genes have been identified that encode the enzymes responsible for the synthesis of this polysaccharide. Genetic mutants exhibiting overproduction of gellan have also been identified and characterized. Several dairy and plant-based processing coproducts have been screened to learn whether they can support the production of gellan in an attempt to lower the cost of synthesizing the microbial polysaccharide. Of the processing coproducts explored, soluble starch as a carbon source supported the highest gellan production by S. elodea grown at 30 ◦C. The corn processing coproducts corn steep liquor or condensed distillers solubles appear to be effective nitrogen sources for gellan production. It was concluded that further research on producing gellan using a combination of processing coproducts could be an effective solution in lowering its overall Citation: West, T.P. Synthesis of the production costs. Microbial Polysaccharide Gellan from Dairy and Plant-Based Processing Keywords: gellan; polysaccharide; applications; culture conditions; genetics; mutants; processing Coproducts. Polysaccharides 2021, 2, coproducts; Sphingomonas elodea 234–244. https://doi.org/10.3390/ polysaccharides2020016 Academic Editor: Sabina Górska 1. Introduction Received: 13 February 2021 The purpose of this review is to provide both background about the microbial polysac- Accepted: 29 March 2021 charide gellan and, more specifically, to examine bacterial gellan synthesis on dairy and Published: 6 April 2021 plant-based processing coproducts. The anionic heteropolysaccharide gellan is known to be synthesized by Sphingomonas elodea strain ATCC 31461 [1,2]. Although originally clas- Publisher’s Note: MDPI stays neutral sified as Pseudomonas elodea, this strain was reclassified as a species of Sphingomonas [3,4]. with regard to jurisdictional claims in Structurally, the water-soluble gum gellan exists as a tetrasaccharide composed of 20% published maps and institutional affil- glucuronic acid, 60% glucose and 20% rhamnose [5–7]. The native form of gellan has been iations. shown to contain acetyl and L-glyceryl groups. These substituents need to be removed by an alkaline heat treatment to produce a gel. The degree of deacetylation of gellan can be directly correlated to its gel-forming ability [8]. The alkaline treatment of the native biopolymer results in a tetrasaccharide sequence that is anionic (Figure1). As the deacy- Copyright: © 2021 by the author. lated biopolymer cools, a double helix forms from the disordered coils which results in Licensee MDPI, Basel, Switzerland. gelation. Gelation of the polysaccharide is enhanced when the pH of the polysaccharide This article is an open access article becomes more acidic. An acidic pH diminishes the negative charge of the polysaccharide distributed under the terms and molecule that results in greater repulsion within the helix. The addition of monovalent conditions of the Creative Commons cations to a gellan solution increases its rate of gelation [9–14]. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Polysaccharides 2021, 2, 234–244. https://doi.org/10.3390/polysaccharides2020016 https://www.mdpi.com/journal/polysaccharides Polysaccharides 2021Polysaccharides, 2, FOR PEER2021 REVIEW, 2 2 235 Figure 1. TheFigure structure 1. The of structure gellan involves of gellan tetrasaccharide involves tetrasaccharide repeating units repeating consisting units ofconsisting two molecules of two ofmole-D-glucose as well as one moleculecules eachof D-glucose of L-rhamnose as well andas oneD-glucuronic molecule each acid. of L-rhamnose and D-glucuronic acid. When the monovalentWhen the cations monovalent bind to the cations helices, bind they to bind the helices, to interact they with bind the to car- interact with the boxylate groupscarboxylate on the polysaccharide groups on theto diminish polysaccharide repulsion to diminishwithin the repulsion helices. The within bind- the helices. The ing of the monovalentbinding ions of the to monovalent the helices ionsincreases to the with helices ionic increases size. Gellan with ionicbehaves size. like Gellan a behaves like normal polymera normalsolution polymer when low solution monovalent when low cation monovalent concentrations cation are concentrations present. As arethe present. As the monovalent cationmonovalent concentration cation concentrationis increased, a is self-supporting increased, a self-supporting gel is formed. gel The is formed.addi- The addition tion of divalentof cations divalent to cationsa gellan to solution a gellan solutionalso increases also increases its rate itsof rategelation of gelation [9–17]. [ 9The–17 ]. The divalent divalent cationscations bind between bind between the helices the helices to cause to cause gelation. gelation. High Highconcentrations concentrations of diva- of divalent cations lent cations resultsresults in a in reduction a reduction in gel in gelstrength. strength. It has It hasbeen been shown shown that that the theconcentration concentration of divalent of divalent cationscations needs needs to be to equivalent be equivalent to the to thecarboxylate carboxylate group group content content of the of thegellan gellan to achieve to achieve maximummaximum gellan gellan strength. strength. It has It hasbeen been noted noted that that to achieve to achieve maximum maximum gel gel strength, a strength, a higherhigher concentration concentration of monovalent of monovalent cations cations needs needs to tobebe added added to tothe the polymer polymer solution than the divalent cation concentration. The binding properties of gellan allowed a colorimetric solution than the divalent cation concentration. The binding properties of gellan allowed assay to be developed using the dye toluidine blue O where the gellan assay was found to a colorimetric assay to be developed using the dye toluidine blue O where the gellan assay be linear up to a concentration of 0.7 g/L [18]. was found to be linear up to a concentration of 0.7 g/L [18]. In the United States, the source of commercial gellan production is C.P. Kelco (Atlanta, In the United States, the source of commercial gellan production is C.P. Kelco (At- GA, USA). There are a number of commercial applications reported for this polysaccharide lanta, GA, USA). There are a number of commercial applications reported for this poly- gum. With respect to food applications, gellan is used in confectionery jellies, fabricated saccharide gum. With respect to food applications, gellan is used in confectionery jellies, foods, pie fillings and puddings, bakery icings and frostings, dairy products, fruit, milk- fabricated foods, pie fillings and puddings, bakery icings and frostings, dairy products, based and carbonated beverages as well as film or coatings for food adhesion [19–23]. fruit, milk-based and carbonated beverages as well as film or coatings for food adhesion With respect to non-food applications, gellan was originally proposed as a replacement [19–23]. With respect to non-food applications, gellan was originally proposed as a re- for agar in microbial growth media since agar is the most economical polysaccharide gum placement for agar in microbial growth media since agar is the most economical polysac- available to be used as a solidifying agent that produces opaque gels [24–26]. Gellan has charide gum availablefound greater to be used use as a solidifying substitute foragent agar that in produces plant tissue opaque culture. gels The [24–26]. higher clarity of the Gellan has foundgels greater at lower use gellan as a substitute concentrations for agar is a markedin plant advantagetissue culture. over The agar. higher Another advantage clarity of the gelsof at gellan lower is gellan that itconcentrations contains less is impurities a marked comparedadvantage toover agar. agar. The Another use of gellan gels in advantage of gellanplant is tissue that it cultures contains allows less impurities tissue development compared to agar. be observed The usemore of gellan clearly than using gels in plant tissueagar. cultures Gellan allows also has tissue a number development of pharmaceutical to be observed uses more for drug clearly delivery than and enzyme using agar. Gellanimmobilization also has a number [27–34]. of With pharmaceutical its non-toxicity, uses rapid for gelation,drug delivery ability and to retain en- water and its zyme immobilizationbiodegradability, [27–34]. With gellan its non-toxicity, is used in oral rapid formulations gelation, ability for drug to deliveryretain water in capsules, beads and its biodegradability,and