Bioprospecting of Microorganism-Based Industrial Molecules. Группа авторов
different sphingans.
Source: Based on [3].
3.5.2.2 Welan Gum
Welan gum produced by Sphingomonas spp ATCC 31555 (formerly known as Alcaligenes sp. ATCC 31555). Welan gum is also an anionic HePS, which is composed of l‐mannose, l‐rhamnose, d‐glucose, and d‐glucuronic acid in the molar ratios 1.0:4.5:3.1:2.3. The side chain present could be either l‐rhamnose or l‐mannose [3]. Welan gum was previously known as EPS S‐130 and commercially produced by CP Kelco, division of J.M. Huber Corporation. The molecular weight of welan gum is about 1000 kDa, and it is non‐gelling polysaccharide, unlike gellan explained above [32]. Even so, welan possesses high viscosifying properties due to which it has wide‐spread applications in cement technology and oil drilling industries. It has the potential to be used for food application as it can act as a thickening, suspending, binding, and emulsifying agent [3].
3.5.2.3 Rhamsan Gum
Rhamsan gum produced by Sphingomonas spp. RH‐1 under aerobic conditions using glucose‐containing medium. Rhamsan gum has a similar backbone like all sphingans consisting of glucose, glucuronic acid, and rhamnose; and the difference lies with the side chain present, which in the case of rhamsan are a O,6‐linked glucosyl disaccharide linked by β(1,6) linkages. Rhamsan under temperature higher than 100 °C can maintain high viscosity and thermostability. Due to its ability to tolerate high phosphate and sodium chloride concentration, it has a broader application in the food industry. Rhamsan has been approved as a food additive by the Japanese Ministry of Health and Welfare in 1996 [21].
3.5.2.4 Diutan Gum
Diutan gum produced by Sphingomonas sp. ATCC 53159 under aerobic conditions. Diutan differs from other sphingans by possessing a disaccharide of l‐rhamnosyl units as a side chain. Diutan gum can act as thickening, binding, suspending, stabilizing, and emulsifying agent. Diutan gum is used in concrete and cement due to its ability to improve the viscosity of cement paste [22].
3.5.3 Pullulan
Pullulan is a HoPS composed of glucose monomers as organized as maltotriose subunits linked by α‐(1,6) linkages. It is produced by fungus Aureobasidium pullulans intracellularly using UDP‐glucose as a precursor followed by export outside the cell [5]. Other fungi have also been reported to produce pulluan, e.g. Eurotium chevalieri, Cryphonectria parasitica, Cyttaria darwinii, and yeast Rhodotorula baracum [23]. Pullulan is a neutral polysaccharide, and its molecular weight can range between 10 and 3000 kDa, and this variability is dependent on its production conditions. Pullulan shares a similar composition with maltodextrin, but it possesses more α‐(1,6) linkages (30%) compared to the later (20%). The higher occurrence of α‐(1,6) linkages results in show digestibility of pullulan compared to starch and hence is used for low‐calorie food products [12].
Pullulan is a non‐gel forming polysaccharide, but it increases the viscosity of a solution, when in the aqueous state even in relatively low concentrations. Pullulan can maintain its viscosity even in high heat, changes in pH, and metal ions [33]. To improve the properties of pullulan, different chemical modifications have been made in pullulan such as small functional groups like carboxymethyl, sulfate, phosphate, acetate, and alkyl esters [34]. Pullulan is used to make capsule shells, tablet coatings, and edible flavored films due to its film‐forming properties. Pullulan has been used as a nontoxic food additive in Japan since 1976 and has been regarded as GRAS by US FDA [12]. When consumed, pullulan can alter the composition of intestinal microbiota, specifically stimulating the growth of bifidobacteria and hence acting as a prebiotic [35]. Recently, when phthalyl pullulan nanoparticle was treated with probiotic Lactobacillus plantarum, the production of antimicrobial peptide plantaricin increased and showed more significant inhibition of Escherichia coli and Lis. monocytogenes growth [36].
3.5.4 Other Microbial Gums
Besides the above mentioned microbial gums, there are several other gums produced from microbial sources; a few examples are as follows.
Scleroglucan is an EPS produced by fungus Sclerotium glucanicum. It is HoPS of glucose linked by β‐(1,3) and β‐(1,6) linkages where every third glucose is connected by β‐(1,6) linkages. The molecular mass of scleroglucan can vary from strain to strain. The average molecular weight ranges between 320 and 6000 kDa [24]. It is used as thickeners, stabilizers, and gelling agents in food products due to its resistance to temperature, hydrolysis, and electrolytes. It can inhibit syneresis in starch‐based foods that occur during refrigeration. It is also used in oil recovery operations in the sea [12].
Curdlan is a β‐glucan composed of glucose monomer linked by β‐(1,3) linkages. Curdlan is produced by bacterium Alcaligenes faecalis. Curdlan is not soluble in water or acidic solution but is only soluble in alkaline pH. Curdlan forms two types of gels at different temperatures: (i) high‐set gel (at above 80 °C) and (ii) low‐set gel (between 60 and 80 °C) [2]. The high‐set gel of curdlan is used in food products, as it is a thermo‐irreversible gel and stable during deep‐fat frying and freeze‐thawing. Curdlan is used in low‐calorie foods, as a fat replacer and in food products like freezable tofu noodles [6].
Levan is a HoPS composed of fructose units linked by β‐(2,6) linkages in the main chain with β‐(2,1) linked branches. Levan has been reported to be produced by a wide range of microbes such as Bacillus subtilis, Bacillus polymyxa, Aerobacter levanicum, Leuconostoc mesenteroides, Streptococcus sp., Pseudomonas sp., and Corynebacterium laevaniformans. Levan has a molecular weight of 2–100 MDa that varies between producer organisms. Levan is used in food, pharmaceutical, and cosmetic industries. It is used as stabilizing, emulsifying, and sweetening agents in food products. In the cosmetic industry, levan is utilized for its moisturizing effect, cell proliferation effect, and anti‐inflammatory effects [12].
Dextran is a type of α‐glucan, which consists of a linear chain of α‐(1,6) linked D‐glucose units with various branched linkages at α‐(1,2), α‐(1‐3), and α‐(1‐4) positions of D‐glucose [25]. The degree of branching and molecular weight of dextran varies with dextransucrase enzyme and producers strain. Dextran is commercially available in different molecular weights and is commonly used as molecular weight standards in GPC analysis. Dextran is used in the food industry as a food hydrocolloid and is prebiotic in nature [26].
Alginate, an important gum, is a copolymer of D‐mannuronic acid and L‐guluronic acid. It is usually produced by brown algae. However, alginate is also produced by bacterial sources like Azotobacter vinelandii, Azotobacter chroococcum, and several species of Pseudomonas. These are called bacterial alginates. Bacterial alginates are used in pharmaceutical industries due to their high purity, and they possess immunomodulatory activity [37].
Carrageenan is a gel‐forming biopolymer produced by edible red seaweed (algae). It is composed of alternating units of D‐galactose and 3,6‐anhydro‐galactose joined by α‐1,3 and β‐1,4‐glycosidic linkage. On the basis of their solubility in potassium chloride, carrageenans are classified into λ, κ, ι, ε, μ–carrageenans. They have average molecular weight of above 100 kDa with 15–40% ester sulfate groups. Carrageenan has been reported to possess immunomodulatory, anticoagulant, antithrombotic, antiviral, and antitumor effects [38].
3.6 Applications of Microbial Gums
By virtue of their various properties, microbial gums are utilized in several types of applications in research and industry. In many cases, one microbial gum is used with another gum of either microbial or nonmicrobial origins that results in the enhancement of the final desire property. In this section, we will describe the use of different microbial gums in various sectors. The different types of applications of microbial gums have been mention in Table 3.2.