Plant GumsJames N BeMiller, Purdue University, West Lafayette, Indiana, USA The term gum is commonly applied to water-soluble, nonstarch polysaccharides of commerce; such gums are present in both higher and lower plants. Plant gums and other gums are used in practical applications primarily to thicken or gel aqueous systems and to control water. They may also function as adhesives, crystallization inhibitors, emulsifying agents, emulsion stabilizers, encapsulating agents, film formers, foam stabilizers, suspending agents, suspension stabilizers, or syneresis inhibitors and/or impart other specific properties. Introductory article Article Contents . Definition, Scope and Occurrence . Pectins . Arabinogalactans . Arabinoxylans . b-Glucan . Psyllium Seed Gum . Algins . Agars, Carrageenans and Furcellaran . Guar, Locust Bean and Tara Gums Definition, Scope and Occurrence To food scientists and many others involved with polymer applications, gums are water-soluble polysaccharides (including modified polysaccharides). Their common characteristic is that they produce viscous aqueous systems, generally at low concentrations (0–5%). When used as food ingredients, they may also, along with protein ingredients, be called hydrocolloids. However, certain hydrophobic substances that in a suitable solvent or swelling agent produce viscous solutions or dispersions are also known as gums. Examples of these substances are high-molecular weight hydrocarbons and other petroleum products, rubbers, certain synthetic polymers, chicle for chewing gum, and the resinous saps that often exude from evergreens and are sometimes commercially tapped to yield, for example, gum balsam and gum resin. Incense gums like myrrh, huataco and frankincense are fragrant plant exudates that are mixtures of resins and carbohydrates but that are gummy because of their content of hydrophobic resin. This article covers naturally occurring, water-soluble, plant polysaccharides used industrially. Other naturally occurring commercial polysaccharidic gums are produced by microorganisms by fermentation; they are not included because they are not of plant origin. These include xanthan/xanthan gum, gellan/gellan gum, curdlan, dextran and others. In the past, some plant gums were called mucilages or were classified as ‘gums and mucilages’. Those called mucilages were those that formed more slimy, that is less pseudoplastic, solutions. However, sliminess is a somewhat controllable physical property, as are other properties, so the category of mucilages has been abandoned. The two starch polymers – amylose and amylopectin – alone or in mixture as they are in most native starches, are gums according to the definition given above. However, because starches are so unique (because they occur in granular form) and their use is so extensive, both in terms of amounts and the variety of applications, they are usually considered separately. Starches and modified starches are not covered in this article; nor are the water-soluble . Tamarind Seed Gum . Inulin . Konjac Mannan . Gum Arabics . Other Exudate Gums cellulose derivatives (because they are not natural) or synthetic water-soluble polymers, both of which have properties that classify them as gums. Occurrence Polysaccharides that are included in the definition of plant gums are cell-wall (structural) materials, energy-storing and carbon-storing substances and exudates. Because higher plants have lignocellulosic cell walls, cellulose is the most abundant polysaccharide and organic compound, but it is not classified as a gum because it is not water soluble. Water-soluble polysaccharides can be extracted from higher plant tissues, some without and some only after delignification. Generally, only those that can be extracted without delignification are also called gums. Those that require delignification and alkaline solutions for extraction are classified as hemicelluloses. A major source of plant gums is marine algae (seaweeds). Seaweeds must be tough but flexible. Therefore, they do not have the strong, but rather brittle, lignocellulosic material in their cell walls, which contain other polysaccharides. Marine algal cell-wall polysaccharides are insoluble in sea water; some can be extracted with hot water or alkaline solutions. Energy-storing and carbon-storing polysaccharides are deposited in seeds, tubers, roots and stems. Other gums exude from wounds in certain trees and shrubs. Gums can be and are classified in several ways. Classification by source is often used: a presentation of plant gums classified in this way is shown in Table 1. (As mentioned earlier, substances classified as gums are also obtained from sources other than plants, and substances other than plant polysaccharides are also called gums.) Also, not all plant gums that impart functionalities to systems (often food 1 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Arabinogalactans 3. Inulin 2. Because of considerable variation in the ratio of arabinosyl to xylosyl units. pectin is a component of a family of polysaccharides known as pectic substances and is a complex. Seeds 1. Exudates A. Agars 3. Gum arabics B. so that commercial pectins are primarily polymers of d -galacturonic acid units of varying methyl ester content (Figure 1). Storage polysaccharides A. Although arabinoxylans constitute only a R O OH O Pectins ‘Pectin’ is a component of the cell walls of all higher plants. Tubers 1. Marine algae (seaweeds) 1. There is limited commercial production and uses are being explored. jellies. The primary use of pectins is for preparation of jams. Locust bean gum 3. Tara gum 4. Members of the family of arabinoxylans are the major nonstarch polysaccharides of wheat endosperm. Guar gum 2. In the native state. Beta-glucan 5. Cell-wall-associated polysaccharides A. i. that is a molecule composed of covalently linked polysaccharides. Psyllium husk gum B. arabinogalactans and arabinogalactan-proteins (supermolecules in which the two biopolymers are covalently linked) are ubiquitous in nature. and frequency of main chain branching. Seeds of legumes. Aloe gel pyranosyluronic acid units (Figure 1). systems) are actually isolated and added as ingredients. The gums are described in the order in which they are listed in Table 1. Gum ghatti C. (DE is a designation of the percentage of carboxyl groups esterified with methanol. Native molecules are altered during extraction. Algins 2. some are solubilized during processing or preparation. Gum tragacanth E. among them its use as a functional food ingredient. Each polysaccharide molecule consists of a linear chain of (1!4)-linked b-d -xylopyranosyl units branched with single a-l -arabinofuranosyl units attached at O2 and/or O3 of backbone units. arrangement of arabinosyl units along the xylan backbone. in the methyl ester form. or CO2 CO2H Figure 1 Primary monomer unit of a pectin molecule. but unlike gum arabic. During extraction. also called pentosans. Commercial pectins are extracted from citrus (primarily lemon and lime) peels and apple pommace. is readily water soluble and does not form highly viscous solutions at low concentration. Arabinoxylans 4. all pectins have some (usually a majority) of the carboxyl groups in the methyl ester form. Arabinogalactans Like pectins. In the native state. −. ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.net . but not exclusively. Pectins 2. Arabinoxylans Arabinoxylans are widespread hemicelluloses of monocots. supermolecule. Tamarind seed gum B. such as soya bean.els. wheat flour arabinoxylans. linear chains of (1!4)-linked a-d -galacto2 OH R= CO2CH3. Arabinogalactans occur in rather high concentrations in the heartwood of species of larch (Larix). Higher land plants 1. it is not a protein-polysaccharide. Konjac mannan III. some of the carboxyl groups are de-esterified and some of the neutral sugar units are removed.. Gum karaya D. all of which are spreadable gels.e. marmalades and preserves. can be fractionated by solubility into several classes. the amount of neutral sugars removed and the degree of neutralization depending on the conditions employed.) All commercial pectins are predominately.Plant Gums Table 1 Classification of plant gums by source I. The extractable gum. contain arabinogalactans. There are two general commercial types: ‘high-methoxyl’ and ‘lowmethoxyl’. like gum arabic. Carrageenans 4. Furcellaran II. the dividing line being a degree of esterification (DE) of 50%. a tree found in the temperate and sub-arctic zones of the northern hemisphere. known as larch arabinogalactan. Some are linear polymers of units linked (1!3). and those composed of mixed units. Different types of gels are formed with alginates from different sources. 3-6anhydro-a-d -galactopyranosyl units. 3 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.and b-d -galactopyranosyl units. the right-hand unit is an a-D-galactopyranosyl unit. The monosaccharide units of k. where they are the principal cell-wall materials. Figure 3 Idealized repeating disaccharide unit structures of k. O − CO2 O OH HO O βManpA − CO2 OH O HO b-Glucan b-Glucans comprise a family of polysaccharides composed of b-d -glucopyranosyl units. Agar contains few or no sulfate half-ester groups. They are linear polymers in which the monosaccharide units are joined by both (1!3) and (1!4) linkages. Curdlan. the monomer units are b-d -galactopyranosyl and. i.e. they have a strong impact on the physical characteristics of doughs and the finished baked product because of their water holding.Plant Gums small proportion (1. CH 2OH R′O O O O CH 2 O O OH − R = R′ = SO3 − R = H. They are being investigated as functional food ingredients. a b-glucan of the unbranched type. iota (i) and lambda (l). elastic.and icarrageenans and furcellaran are b-d -galactopyranosyl and 3. These two types are produced by fungi and yeasts. carrageenans and furcellaran (Danish agar) are extracted from red seaweeds (red algae). with most being obtained from Plantago ovata. all being modified. k( 25%).els. An important and useful property of sodium alginates is their ability to form gels on reaction with calcium ions. Alginates with a higher percentage of poly(guluronic acid) segments form gels more readily and form more rigid. Carrageenans and Furcellaran Agars. a-L-gulopyranosyluronate unit. gelling and interfacial properties. Alginates are the salts (generally sodium salts) of alginic acids. of which there are three idealized types: kappa (k).6-anhydro (internal. Cereal grain b-glucans are known as mixed-linkage bglucans. Alginates with a higher percentage of poly(mannuronic acid) segments form more Agars. those containing only (1!4)-linked a-l-gulopyranosyluronic acid units.( 30%). R′ = 50% SO3 50% H OR ι-Carrageenan κ-Carrageenan Furcellaran Psyllium Seed Gum Psyllium seed (psyllium husk) gum is obtained by extraction with water of the coat (hull) of seed of plants of the genus Plantago. Algins Algins (alginates) are extracted from brown seaweeds (brown algae). which differ structurally in terms of their contents of half-ester sulfate groups and 3. The most extensively produced and used red seaweed polysaccharides are the carrageenans. The primary use of psyllium seed gum (often powdered seed) is as a preventer of constipation. viscosity building. more brittle gels that tend to undergo syneresis. more deformable gels that have a reduced tendency to undergo syneresis. cyclic ether) rings (Figure 3). None of these polymers contains an exact repeating unit structure.6-anhydro-a-d -galactopyranosyl units. i. R′ = SO3 − R = H. others are linear polymers of units linked (1!3) that are branched with other b-d -glucopyranosyl units linked (1!6). linear galactans with alternating monosaccharide units and linkages (Figure 3). In l-type carrageenans. l-Carrageenan contains a. where they are the principal cell-wall (structural) material. furcellaran ( 14%). αLGulpA Figure 2 Monomer units of alginates: bManpA 5 b-Dmannopyranosyluronate unit. Other members of the family are sulfated: in increasing order. Specific properties of algins depend on the percentage of each type of building block. In agar. They are also related structurally. Alginic acids are polymers of d -mannuronic acid and l-guluronic acid units (Figure 2).5%) of wheat flour. Others are used for their immune system-enhancing properties. is an approved food gum. l-carrageenan ( 35%). b-Glucans are also present in bran of cereal grains (especially oat and barley).6-anhydro ring. a unit without the 3.0–1. Alginic acids are composed of three different types of segments: those containing only (1!4)-linked b-D -mannopyranosyluronic acid units.net .and i-type carrageenans and furcellaran. aLGulpA. Commercial guar gum is the powdered endosperm of seeds of the guar plant. locust bean (carob) and tara gum are obtained from seeds of the respective legumes. the average mannosyl to galactosyl ratio being about 3. elastic gels. All three are classified as galactomannans. Locust bean gum polysaccharide molecules have so-called smooth regions that contain no galactosyl units and socalled hairy regions in which most main-chain units are branched. that is polysaccharides consisting of a linear chain of (1!4)-linked b-d -mannopyranosyl units substituted with a-d -galactopyranosyl units attached through O6 of some of the main-chain units (Figure 4). Guaran actually has a slightly higher D-galactosyl side unit content and the branched units are randomly distributed. l-Carrageenan does not gel and is the only member of this family that is nongelling. tamarind gum. the mixture of species (if any) extracted. Tara gum is the ground endosperm of the seeds of the tara (Caesalpinia spinosa) shrub that grows in the northern regions of South America and Africa. its growth conditions. Inulin and inulin oligosaccharides are used as ingredients in functional foods. is the ground endosperm of seeds that are obtained as a byproduct from processing the edible fruit of the tamarind (Tamarindus indica) tree (India. and the treatment used in production. also known as tamarind kernel powder. Guar gum forms high-viscosity. Inulin is obtained commercially primarily from the edible tuber of Jerusalem artichoke (Helianthus tuberosus). furcellaran and xanthan. Bangladesh. Guar. and l-carrageenan the most soluble. strong. 4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.6 : 1 and a flour obtained from diced. The natural polymer contains acetyl ester groups and is not a good gel former. Saponification to remove the ester groups is required to form the heat-stable. a plant cultivated in Asia and used to make noodles and gels that are stable in boiling water. Agar forms the strongest gels. Figure 4 Idealized repeating trisaccharide unit structure of guaran. Tamarind Seed Gum Like the galactomannans (guar. CH2OH HO OH CH2OH HO CH2O O OH O HO O OH O HO O Konjac Mannan Konjac mannan is a glucomannan with a ratio of d mannosyl to d -glucosyl units of 1. lesser amounts are obtained from tap root and stem of chicory (Cichorium intybus). The composition and properties of any preparation are dependent on the species collected. the actual ratio of mannosyl units to galactosyl units being about 1. dried and ground tubers of Amorphophallus konjac. approximately every other main-chain unit on average is substituted with a galactosyl unit. Commercial locust bean gum is the powdered endosperm of seeds of the locust bean (carob) tree. locust bean and tara gums). Burma. Inulin Inulin is a member of a family of small poly(d -fructose) polymers known as fructans. A difference is its gel-forming synergism with kcarrageenan. In guaran.els.Plant Gums Agar is the least soluble member of this family of polysaccharides.net .9 : 1. The main application of tamarind seed gum is in sizing of textiles (cotton and jute). Deesterified konjac mannan exhibits a strong synergism with starch and very strong synergisms with k-carrageenan and xanthan. Only when dispersions of locust bean gum are heated and cooled is high viscosity obtained.8 : 1. Locust Bean and Tara Gums Guar gum. The polysaccharide of locust bean gum has fewer galactosyl units on the main-chain backbone than does guaran. guaranga or Peruvian carob) gum is a galactomannan with a mannosyl to galactosyl ratio and properties that lie between those of guar gum and locust bean gum. guaran is the polysaccharide of guar gum. shear-thinning solutions at low concentration. Tara (also known as huarango. Guar gum is second (to starch) in terms of quantities used as a thickener of aqueous systems. Only small amounts find their way into commerce. although not always in the edible portion of food plants. Locust bean gum has low cold-water solubility and is often used when delayed viscosity development is desired. Fructans occur widely in both food and non-food plants. The general properties of locust bean gum are similar to those of guar gum. Tara gum has fewer branched units and locust bean gum even fewer. Sri Lanka). but it has been almost entirely replaced as a stabilizer for salad dressings. also called gum acacia and acacia gum. New York: Marcel Dekker. 1986) Food Hydrocolloids.net 5 . is still in significant use. The harvest is relatively small and. In: Aspinall GO (ed.Plant Gums Gum Arabics Of the several gums that are dried gummy secretions collected by hand from various trees and shrubs and that have been used for thousands of years. Gum arabic preparations are mixtures of highly branched. structure. Aloe polysaccharides form a mucilaginous gel in Aloe plants. Whistler RL and BeMiller JN (1997) Carbohydrate Chemistry for Food Scientists. Stephen AM (ed. Gum arabic is collected by hand from various species of Acacia. 411–490. This gel is not an exudate gum in the usual sense. 2. Gum arabic is unique among gums because of its high solubility and the low viscosity and Newtonian flow of its solutions. BeMiller JN (2000) Classification. 20% solutions of gum arabic resemble a thin sugar syrup in body and flow properties. 1983. 195–285. 205–236. condiments. Glicksman M (ed. New York: Academic Press. New York: Academic Press. Lewis JG. In: Aspinall GO (ed. pp. hence. DC: American Chemical Society. hair conditioners and other personal care products.) (1982. branch-on-branch. wounds can be from natural causes. ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www. Washington. A major application is in the preparation of dry fixed-flavour powders. Dey PM and Dixon RA (eds) (1985) Biochemistry of Storage Polysaccharides in Green Plants. It produces high solution viscosity. New York: Marcel Dekker. The structure of the polysaccharide component(s) varies with species. For this reason. but trees are also cut deliberately so that they produce gum. In: Lembi CA and Walland JR (eds) Algae and Human Affairs.) (1995) Food Polysaccharides and Their Applications. Painter TJ (1983) Algal polysaccharides. 2. Other Exudate Gums Gum ghatti is obtained from the Anogeissus latifolia tree (India and Sri Lanka). St Paul. Tears of gum exude from wounds in trunks and branches. Whistler RL and BeMiller JN (eds) (1993) Industrial Gums. vol. only gum arabic. 603–611.) The Polysaccharides. Gum arabic also has the unique property of being an effective emulsifier of flavour oils plus an effective stabilizer of the resulting emulsions. Most Aloe species are native to Africa. Cambridge: Cambridge University Press. rather it is obtained by cutting the thick epidermis of leaves and scraping off the gel. 2. CA: Academic Press. London: Academic Press. only small amounts are used commercially. generally in the same applications in which gum arabic is used. The primary use of aloe gel is for the treatment of burns. El-Nokaly MA and Soini HA (eds) (1999) Polysaccharide Applications: Cosmetics and Pharmaceuticals. Caribbean countries and Central America. 3rd edn. Their specific compositions and structures vary with species. certain species are cultivated in Texas. pp. vols 1.) The Polysaccharides. It is also employed as a base for the preparation of hand lotions. season and climate. FL: CRC Press. San Diego. pp. vol. Eliasson A-C (ed. a small tree. shaving lotions. Boca Raton. In: Cho SS and Dreher M (eds) Handbook of Dietary Fiber and Functional Foods. and other acidic foods by xanthan. Most often the principal polysaccharide is a partially acetylated glucomannan. Further Reading BeMiller JN (1999) Structure–property correlations of non-starch food polysaccharides. Macromolecular Symposia 140: 1–15. pp. Gum tragacanth exudes from wounds in the small shrub Astragalus gummifer (Iran and Turkey).els. acidic protein polysaccharides that occur as mixed salts. MN: Eagen Press. it is used as a denture adhesive and in sealing rings for colostomy bags.) (1996) Carbohydrates in Food. New York: Marcel Dekker. Its most important attribute is that it produces pastes with good adhesion. suntan lotions. and chemistry of polysaccharides in foods. 3. Stanley NF and Guist GG (1988) Commercial production and applications of algal hydrocolloids. While most other gums form highly viscous solutions at 1–2% concentration. Gum karaya is obtained from the Sterculia urens tree (India). Its primary attribute is acid stability. Sandford PA and Baird J (1983) Industrial utilization of polysaccharides. shampoos.