International Journal of Biological Macromolecules 66 (2014) 74–80
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Review
Locust bean gum: Processing, properties and food applications—A review Sheweta Barak, Deepak Mudgil ∗ Department of Dairy and Food Technology, Mansinhbhai Institute of Dairy and Food Technology, Mehsana 384002, Gujarat, India
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Article history: Received 28 December 2013 Received in revised form 4 February 2014 Accepted 9 February 2014 Available online 16 February 2014 Keywords: Locust bean gum Carob bean gum Dietary fiber Properties Food application
a b s t r a c t Locust bean gum or carob gum is a galactomannan obtained from seed endosperm of carob tree i.e. Ceratonia siliqua. It is widely utilized as an additive in various industries such as food, pharmaceuticals, paper, textile, oil well drilling and cosmetics. Industrial applications of locust bean gum are due to its ability to form hydrogen bonding with water molecule. It is also beneficial in the control of many health problems like diabetes, bowel movements, heart disease and colon cancer due to its dietary fiber action. This article focuses on production, processing, composition, properties, food applications and health benefits of locust bean gum. © 2014 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4. 5. 6.
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-food applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composition and chemical structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Hydration rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Synergistic gel formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6. Water adsorption isotherm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Edible films/coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Beverages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Bakery products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4. Noodles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5. Ice cream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6. Low-fat yoghurt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Health benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∗ Corresponding author. Tel.: +91 9896672001. E-mail address: [email protected] (D. Mudgil). http://dx.doi.org/10.1016/j.ijbiomac.2014.02.017 0141-8130/© 2014 Elsevier B.V. All rights reserved.
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1. Introduction Locust bean gum is a white to creamy white powder obtained after milling of seed endosperm of fruit pod of the carob tree, a member of legume family, botanically known as Ceratonia siliqua L. which is found in Mediterranean regions. Hence, locust bean gum is also known as carob gum. The carob seed is consists of three parts i.e. husk, endosperm and germ. The processing of carob gum first involves the removal of hull from the seed which can be attained via thermo-mechanical or by chemical treatment [1]. After removal of the outer layer i.e. the hull, the seeds are split lengthwise and the germ portion is separated from the endosperm of carob seed. Further, the isolated endosperms are subjected to grinding, sifting, grading, packaging and finally marketed as locust bean gum or LBG. All the operations in LBG processing are designed such that any impurity from husk or germ portion can be avoided which can ultimately alter the properties of the carob gum. Galactomannans are linear polysaccharides consists of -(l–4)-mannose backbone with single d-galactopyranosyl units attached via ␣-(l–6) linkages as side branch. These side branches are not distributed uniformly in the main backbone chain [2,3]. There are also present some unsubstituted -d-mannopyranosyl chain segments, alternating with -d-mannopyranosyl units substituted with ␣-d-galactopyranosyl side branches [4–7]. Carob galactomannan is one of the commercial galactomannans guar gum and tara gum; and among these galactomannans LBG has the lowest galactose content about twenty percent [8]. Locust bean gum generally has an average mannose to galactose ratio of about 3.5 which is highest among the commercially available galactomannan such as guar gum (1.8) and tara gum (3.0). The degree of galactose substitution on mannose chain affects water solubility of the galactomannan. This is reason because guar gum is cold water soluble whereas LBG shows low solubility at ambient temperature and there is a requirement of heat treatment for maximum solubility to achieve the best water binding capacity [9]. The molecular size, mannose to galactose ratio, galactose distribution in the mannose backbone chain influences solubility and also controls the rheological properties of LBG. Different chromatography techniques can be used to characterize carob galactomannan structure. The molecular weight distributions can be determined using size exclusion chromatography and mannose to galactose ratio can be generally determined using gas chromatography. LBG is considered as the first galactomannan used as additive in industries such as paper industry, textile industry, pharmaceutical industry, cosmetic industry and food industries. The important characteristic of this gum that make it a useful industrial gum is its ability to form very viscous aqueous solution at relatively low concentration, to stabilize emulsion and to replace fat in many food products. Being non-ionic in nature, locust bean gum solutions are not influenced by pH, salts and heat treatment. Locust bean gum can be used in combination with other hydrocolloids such as carrageenan and xanthan gum as it shows synergistic action with them and forms a gel with more elasticity and strength [10]. Locust bean gum is suitable for many food applications as it provides a creamy mouthfeel. It is typically added in cream-cheese spreads to impart richness and spreadability. It is especially useful in preventing syneresis in various food products. LBG also create a smooth mouthfeel in sauces. LBG’s ability to bind water makes it an excellent choice for frozen applications, such as ice cream as it will slow down and reduces the size of ice-crystal formation as the moisture is retained within the ice cream. LBG also finds application in yoghurt manufacture. In yoghurt, it reduces the syneresis and enhances water holding capacity of yoghurt when used at certain levels [11]. Recently, LBG is used in edible films and improved the properties of films such as water vapor permeability, oxygen permeability, tensile strength and elongation-at-break [12]. In edible coatings, when locust bean gum is used in combination with
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other hydrocolloid such as -carrageenan; it improves the properties of the film due to its synergistic action. LBG is also used as thickener, stabilizer and gelling agent in various food products such as baked foods, beverages, dairy products and processed fruit products. Locust bean gum is classified as GRAS (Generally recognized as safe) by FDA and is used for its stabilizing, thickening and fat-replacing properties [13]. 2. Production Locust bean gum is extracted from the seed endosperm of the carob tree plant botanically known as C. siliqua. It belongs to the subfamily Caesalpinioideae of the Leguminosae family [14]. Carob plant is typical tree of semiarid environments. It is very abundant in the Mediterranean region since ancient times and is currently produced in Spain, Italy, Cyprus and other Mediterranean countries. Its localization also extends to different regions of North Africa, South America, and Asia. Other known carob producing countries are Morocco, Greece, Algeria, Turkey, Israel, India and Pakistan. Carob plant is long-lived evergreen tree and after germination, it grows to about 10 m height in 10–15 years. The carob tree may not be fully grown until it is 50 years old and it starts to bear good quantities of pods at the age of 15 years. The pods reach to full size in July but are ripened in October. Hot and dry climatic conditions are required for good yields. Large trees can yield up to half ton of pods per annum. Carob tree yields large brown fruits known as carob pods. These carob pods are sickle shaped and are 10–20 cm in length and 2–4 cm in width. These sickle shaped pods contain 10–15 oval shaped carob seeds or kernels. The polysaccharide from seed endosperm of carob tree is also referred in the literature as carob bean gum, carob seed gum, carob flour, or even ceratonia [15]. World production of carob seeds is estimated at about 315,000 tons per year, produced from two lakhs hectare. The main producers include Morocco (38%), Spain (28%), Italy (8%), Portugal (8%), Greece (6%), Turkey (6%) and Cyprus (2%) [14]. Recently, growing interest for locust bean gum has been observed due to its various industrial applications. Locust bean gum is used as a thickening and stabilizing agent in food, cosmetic and pharmaceuticals industries [16]. In food industry it is a food additive with E-number E-410 in the European Union [17]. Pharmaceutical applications of locust bean gum are mainly due to its ability as controlled release excipient in tablets. Biodegradability, low toxicity and low cost of locust bean gum contribute for its increasing utilization in various fields. 3. Processing The carob pods are kibbled to separate the seeds from the pulp portion. The processing of carob seeds includes dehusking (acid or thermo-mechanical), splitting, milling, sifting, clarification and drying. The processing of carob seeds is very difficult due to its very tough and hard seed coat. The dehusking of carob kernels is achieved by treatment of carob seeds with dilute sulphuric acid or with thermo-mechanical treatment known as acid peeling and thermal peeling, respectively. In acid peeling process, carob seeds are treated with sulphuric acid at elevated temperature to carbonize the seed coat. The remaining portions of the seed coat are separated and removed from the endosperm portion by efficient washing and brushing process. The peeled kernels are dried and cracked and the more friable germ gets crushed. The germ parts are sifted off from the unbroken endosperm halves known as splits. The carob bean gum produced from the carob splits by this process is whitish in color and has higher viscosity. In thermo-mechanical peeling process, roasting of carob kernels are carried out in rotating furnace where the seed coat gets pop off from the internal portions of carob kernels. The endosperm halves or carob splits are obtained
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S. Barak, D. Mudgil / International Journal of Biological Macromolecules 66 (2014) 74–80 Table 1 Composition of carob seed. Part
Proportion (%)
Husk Endosperm Germ
30–33 42–46 23–25
Locust bean gum also reduces or controls diabetes due to its high gelling ability which on ingestion causes satiety sensation [20]. In textile industry, locust bean gum is used in combination with starch as sizing agent. Locust bean gum is utilized as a thickener in paints. Locust bean gum is also used in paper making, pet foods and cosmetics products. 5. Composition and chemical structure
Fig. 1. Flow diagram for manufacturing of locust bean gum.
after mechanical processing and are recovered from the roasted seed coat or husk and the crashed germs. The isolated endosperm halves are milled and sieved to obtain fine particle size powder of native carob bean gum or locust bean gum (Fig. 1). The product obtained from this process is somewhat darker in color due to the heating or roasting operation. Thermo-mechanical process leads to no effluent as it do not involve sulphuric acid as processing aid. Clarification of native locust bean gum may be achieved by dispersing it in water and dissolved by heating. This solution is subjected to filtration to remove insoluble substances and to obtain clear solution. Locust bean gum is recovered by precipitation with isopropanol or ethanol, followed by filtering, drying and grinding or milling, to obtain fine particle size powder of clarified or purified carob bean gum. 4. Non-food applications Locust bean pods were utilized as cattle feed for a very long time but now its seed endosperm powder is utilized as locust bean gum in various industries such as food, cosmetic, pharmaceutical, textile, paint, mining, oil drilling and construction industries for its thickening and stabilizing properties. In pharmaceutical industries, locust bean gum is used in the production of solid monolithic matrix systems, films, beads, micro-particles, nano-particles, inhalable and injectable systems, as well as in viscous liquid and gel formulations. In these dosage forms, locust bean gum performs different functions such as binders, viscosity enhancers, stabilizers, matrix formers, drug release modifiers, coatings, disintegrators, solubilizers, emulsifiers, suspending agents, gelling agents, and bioadhesives [18]. Biopharmaceutical applications of locust bean gum are mainly attributed to its gelling capacity and synergies with other polysaccharides. The most common application of locust bean gum is the formulation of oral delivery systems based on tablets, hydrogels and multiparticulate systems. Apart from the binding and stabilizing ability, locust bean gum also act as a bioactive substance which has hypolipidemic effect, decreasing low density lipoprotein (LDL) cholesterol due to high dietary fiber content [19].
Locust bean kernel is composed three major components which are outer husk (30–33%), germ portion (23–25%) and endosperm portion (42–46%) by seed weight (Table 1). The major portion of locust bean endosperm is galactomannan, which comprises approximately 80% weight of the endosperm and the rest corresponding to proteins and impurities. The protein content of locust bean gum is reported to include albumin and globulin (32%), while the remaining 68% correspond to glutelin [21]. Impurities mainly refer to ash and acid-insoluble matter [22]. After seed processing, crude galactomannan can be further submitted to several processes to eliminate both the protein content and impurities. Impurities usually remain insoluble even when heating at temperatures up to 70 ◦ C [23]. Treatment with isopropanol is considered very efficient in the elimination of proteins as insoluble impurities. The composition of commercial locust bean gum is presented in Table 2. The commercial locust bean gum powder contains approximately 10–12% moisture, 5% protein, 1.0% ash, 1.0% crude fiber, 0.5% fat and 80–85% galactomannan [24,25]. Possible impurities in commercial locust bean gum powder are husk and germ which are reflected by acid-insoluble matter and protein content, respectively. Residual amount of ethanol or isopropanol used during washing or extraction process also act as impurities in locust bean gum powder. Locust bean gum is comprised of a high molecular weight polysaccharide composed of galactomannan. Locust bean galactomannan is composed of two units i.e. galactose and mannose. Locust galactomannan consists of a linear chain of (1→4)-linked d-mannopyranosyl units with (1→6)-linked ␣-d-galactopyranosyl residues as side chains (Fig. 2). Locust galactomannans are highly polydispersed like other polysaccharides found in nature. In literature molecular weight of locust galactomannan is reported as approximately 310,000. However, recent molecular weight estimation techniques such as gel permeation chromatography suggest that average molecular weight of locust galactomannan varies significantly, typically ranging from 0.3 to 2.0 million, depending on the seed source, plant growing conditions and manufacturing processes. The galactomannan molecule is considered to exhibit an extended ribbon-like structure at the solid state and a semi flexible coil-like conformation in solution. The galactose to mannose ratio of locust bean gum is approximately 1:3.1–1:3.9 [25–27]. The Table 2 Composition of locust bean gum. Constituent Galactomannan Moisture Protein content Fat content Crude fiber Ash
Proportion (%) 80.0–85.0 10.0–12.0 5.0–6.0 0.5–0.9 0.8–1.0 0.5–1.0
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Fig. 2. Structure of locust bean gum.
mannose and galactose content has been reported as 77–78% and 21–23%, respectively [25]. The distribution of d-galactosyl residues or side chains along the mannose backbone chain can be random, blockwise and ordered. 6. Properties The functional properties of locust galactomannan like other polysaccharides are dependent on their behavior in an aqueous medium. Locust bean gum is partially soluble in cold water. Locust galactomannan requires heating for complete solubilization in water. If galactomannan sol is heated above 80 ◦ C to achieve complete solubilization, care should be taken, as heating to such extent may cause oxidative–reductive depolymerization of galactomannan chain and reduction in viscosity of final solution may observed. In pH range 4–9, The Viscosity of locust bean gum solution decreases with pH increasing above 9 and decreasing below 4. Locust bean gum is relatively stable against mechanical distortion. The viscosity property of locust bean gum depends on various factors such as molecular weight, concentration, shear rate and solubilization method. Locust bean gum is often regarded as less viscous than guar gum and tara gum. 6.1. Solubility Locust bean gum is partially soluble in cold water and needs to be heated to reach maximum solubility. Locust bean gum shows solubility in water of approximately 70–85% when heated to 80 ◦ C/30 min [28]. This difference in solubility may be due to the high molecular weight galactomannan component and galactomannan with lower galactose residues as side chain which can solubilized at higher temperature. It shows that it is hot water soluble and has lesser solubility in water than other galactomannans such as guar gum which is cold water soluble gum [5]. Generally, the solubility of locust bean gum powder does not exceed 90%, which is further dependent on some factors or variable such as particle size or granulation and impurities such as husk or germ contamination [16].
molecules aligns in the direction of flow which leads to decrease in viscosity and thus explain the non-Newtonian behavior of locust bean gum solution at higher shear rates [29]. Aqueous solutions of locust bean gum at 0.5–2.0% concentration shows a typical behavior of macromolecular biopolymer with dominating loss modulus (G ) over storage modulus (G ) in lower frequency range whereas storage modulus dominates the loss modulus in high frequency range. It means that locust bean gum solutions with concentrations ranging from 0.5% to 2.0%, shows a liquid behavior at lower frequency (G > G ) and a solid-like behavior at higher frequencies (G > G ). G value of locust bean gum aqueous solution at same concentration is lower than fenugreek galactomannan, guar galactomannan and tara galactomannan. In frequency sweep tests, crossover frequency (where G = G ) of locust bean gum solution shifts to lower frequency values with increase in gum concentration [30]. 6.3. Viscosity The most significant property of locust bean gum is its ability to hydrate in hot water to give viscous solution. The thickening capacity of locust bean gum depends on certain factors such as particle size, polymer concentration, molecular weight distribution, shear rate, solubilization methods and impurities. Locust bean gum is generally less viscous galactomannan than guar and tara galactomannan. Viscosity values of locust bean gum solutions in the Newtonian plateau can be used to analyze molecular characteristics. Intrinsic viscosity of locust bean gum solution is determined based on concentration dependence of Newtonian viscosities of dilute solutions. Intrinsic viscosity is related molecular weight through a power-law equation known as Mark–Houwink–Sakurada equation. Power-law exponent ‘˛’ for locust bean gum has been reported as 0.77 when water is used as solvent. Intrinsic viscosity and molecular weight of locust bean gum is lower among galactomannans. Viscosity of locust bean gum solutions is concentration dependent and generally shows increase with increase in gum concentration. 6.4. Hydration rate
6.2. Rheology Rheology is the study of flow (in liquids) and deformation (in solid or semisolids) of material under the effect of applied force. Locust bean gum in aqueous solutions shows a non-Newtonian behavior (pseudoplastic steady-flow behavior or shear-thinning behavior) at high shear rates, but exhibits Newtonian flow behavior at low shear rates. At lower shear rates, the disruption of molecular entanglements by the shear may be balanced by the reformation of new entanglements, so that the viscosity kept constant and the locust bean gum solutions show Newtonian flow behavior whereas at higher shear rates, disruption of the entanglements predominated over the reformation of new entanglements,
Rate of hydration of commercial locust bean gum varies depending on the grades. Locust bean gum is generally graded based on the particle size, gum content and insoluble. Locust bean gum requires heating at 80 ◦ C for 30 min for its full hydration. Hydration of about 2 h is required in order to reach maximum viscosity for practical applications [31]. Major controlling factor of hydration rate of locust bean gum is particle size of locust bean gum powder. Fine particle size gum powder hydrates easily and more quickly than relatively coarser particle size gum powder. Commercially, fine particle size locust bean gum powder is available for quick hydration. However, a considerable time interval for complete or maximum hydration of locust bean gum powder is still desired.
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6.5. Synergistic gel formation Locust bean gum do not form gel under normal conditions but upon freeze–thaw treatment a weak gel can be obtained. Locust bean gum forms gel in the presence of large amount of sucrose. Gel strength of locust bean solution in combination with sugar shows an initial increase and subsequent decrease with increasing concentration of sugar. Initial increase in gel strength may be attributed to reduction in water content due to increased concentration of sugar. The subsequent decrease in gel strength attributed to inhibition of polymer–polymer association by binding of sugar molecules to polymer chains which leads to decrease in gel strength arising from decrease in strength of binding. Maximum gel strength of locust bean gum solutions can be attained at 45% (by weight) fructose, 50% (by weight) sucrose or sorbitol, and 55% (by weight) glucose [32]. Locust bean gum also shows a useful synergistic increase in gel strength on blending with other gums such as xanthan gum, -carrageenan etc. X-ray diffraction analysis of xanthan-locust bean gum blends shows new diffraction patterns that are absent in xanthan and locust bean when studied separately which suggest intermolecular binding between xanthan and locust bean gum. X-ray diffraction studies also indicate random aggregation between galactomannan and -carrageenan which reveals small degree of intermolecular binding. It is well accepted fact that by replacement part of carrageenan with locust bean gum, increases the gel strength indexed by dynamic rigidity modulus and compressive Young’s modulus. This increase in gel strength increases with increase in concentration of locust bean gum and then decreases after a peak point in mixed gel and is generally known as synergistic effect [33]. 6.6. Water adsorption isotherm Water adsorption isotherm represents the relationship between equilibrium moisture content vs. water activity. Torres et al. (2012) studied the water adsorption isotherm of locust bean gum at different temperatures (20–65 ◦ C) and reported increase in equilibrium moisture content with increasing water activity following the shape corresponding to type II isotherms according to Brunauer’s classification [34,35]. Torres et al. (2012) also reported that the equilibrium moisture content of locust bean gum decreased with increasing temperature at each water activity level [35]. This can be explained as at high temperatures, the activation of water molecules changes to higher energy levels and the links become less stable and break away from the water-binding sites of galactomannan, hence the equilibrium moisture content decreases. Isosteric sorption heat of locust bean gum decreased with increasing equilibrium moisture content. Locust bean gum shows high sorption heat of about 27 kJ mol−1 which exhibits less hygroscopicity of the gum [35].
or without the addition of plasticizers and surfactants [12]. Locust bean gum has been used to form edible films/coatings due to its edibility and biodegradability and can be used as an alternative to reduce negative effects of minimal processing on fresh-cut fruits [36]. Hydrophilic properties of locust bean gum provide a good barrier due to its carbon dioxide permeability, oxygen permeability, water vapor permeability, tensile strength and elongation-at-break under certain conditions. Locust bean gum in edible film and coating may also serve as carrier of additives and bioactive components [37]. Edible films formed by mixed systems of locust bean gum and -carrageenan showed improved properties. The addition of -carrageenan to locust bean gum improved the barrier properties of the film which leads to decrease in water vapor permeability. Improved values of elongation-at-break of edible films were also reported when the ratio of -carrageenan and locust bean gum was mixed in the ratio of 80:20% (w/w). -carrageenan and locust bean gum blend films enhance the tensile strength compared to tensile strength of films prepared individually by carrageenan and locust bean gum. Improvement in -carrageenan and locust bean gum film properties are reported due to hydrogen bonds interactions between -carrageenan and locust bean gum observed via Fourier Transform Infrared (FTIR) spectroscopy analysis [38]. 7.2. Beverages Locust bean gum is very popular as thickening and stabilizing agent in used in various beverages. Locust bean gum solutions are stable at wide range of pH which makes it a unique stabilizer and thickener in most of the beverages. Guar gum is soluble in hot water and most of the beverages require heat processing which enable the locust bean gum to use in beverages. It improves the keeping qualities of beverages via resistance to phase separation and thickening. 7.3. Bakery products Its application for bakery purposes results in higher baked product yields; it improves the final texture and adds viscosity in dough. Addition of guar gum in cookies dough improves the machinability of the dough which helps in the better handling of dough with minimum requirement of energy and time. Locust bean gum is also used to increase volume and to retard the aging of bakery products [39]. The addition of locust bean gum to wheat flour suspension decreases the pasting temperature and increases the peak viscosity, trough viscosity, breakdown, final viscosity and setback values. Water absorption capacity and dough development time of wheat flour dough also increases on addition of locust bean gum [40]. Locust bean gum can also be used as binding agent as a substitute for gluten in gluten-free bread formulations based on corn starch with improved loaf volume and crumb structure [41].
7. Food applications 7.4. Noodles Locust bean gum is used as an additive in food industry due to its thickening and stabilizing property. Its utilization as stabilizer and thickener in food products is popular as it is obtained from a natural source. Its functionality is due to its water phase management in food products. 7.1. Edible films/coating The use of edible films or coatings composed of natural polymers and food additives have been constantly increasing in the food industry to enhance the shelf life of fresh fruits, vegetables and meat products. These films/coatings can be produced from biopolymers such as polysaccharides, proteins, lipids, resins, with
Incorporation of locust bean gum in noodles dough improves the dough rheology as well as the textural characteristics of the cooked noodles. Improvement in textural properties of noodles is attributed to the strengthening effect of gum on the gluten network which results in better textural properties of noodles. Addition of locust bean gum reduces the cooking loss and swelling index of noodles [42]. 7.5. Ice cream Locust bean gum alone or in combination with guar gum is used in frozen dairy products for desired textural properties such
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as viscosity enhancement and ice recrystallization inhibition. But, milk proteins and locust bean gum systems are incompatible in solution systems and leads to separation of phases [43]. These two separated phase consists of water soluble and water insoluble components. Water insoluble components include colloidal protein and fat components (opaque); whereas other phase include dissolved solutes (transparent). The difference in optical properties of the two phases makes the product undesirable in appearance and ultimately for consumption. To solve the problem of phase separation, carrageenan is used in combination with locust bean gum which leads to phase stabilization dairy systems via casein and carrageenan electrostatic interaction [44–46]. The phase separation in soft-serve ice cream mixes is undesirable from consumer point of view. The effect of -carrageenan, locust bean gum and casein–whey protein ratio on the phase stability of soft-serve ice cream mixes during storage of 3 weeks at 5 ◦ C is also studied and reported that -carrageenan 0.015–0.02% (w/w) and locust bean gum 0.06–0.2% (w/w) is required to stabilize the ice cream mix against serum or phase separation [47]. 7.6. Low-fat yoghurt Low-fat yoghurt has a texture different from that of full-fat yoghurt. Fat component in food is mainly responsible for the texture of that product. There have been many studies reported in the literature related to improvement of texture and body of non-fat or low-fat yoghurt. The main examples are use of exopolysaccharides producing starter cultures [48] and addition of food hydrocolloids which provide good stability and desirable texture in low-fat yoghurt [49,50]. Hydrocolloids and protein concentrations in lowfat yoghurt need to be optimized to allow for maximum interaction between the hydrocolloid and protein. If it is not optimized, then hydrocolloid–hydrocolloid or protein–protein interactions may predominate and ultimately affects the milk reactivity. The milk reactivity is highly dependent on gum concentration [51]. Locust bean gum when added at 0.02% (w/w) to low-fat yoghurt, increases firmness, viscosity and water-holding capacity and reduces syneresis [11]. 8. Health benefits Locust bean gum comes under the category of viscous soluble fiber. Due to its effect on viscosity and food structure, it can alter the rate of carbohydrate degradation during digestion. This is likely to have beneficial effects for the regulation of postprandial blood sugar and insulin levels, key events in the prevention and treatment of obesity and diabetes [52]. Viscous soluble fiber such as locust bean flattens postprandial glycaemia more consistently than wheat bran and other insoluble fibers [53]. Recent studies on gums as dietary fiber reported their protective effect on cardiovascular disease. Incorporation of gums such as guar gum & locust bean gum as dietary fiber in diet can be beneficial in reducing inflammation and inflammatory bowel diseases, Crohn’s disease and ulcerative colitis [54]. The physiological effects and viscosity property of locust bean gum is comparable to guar gum and -glucan [55]. 9. Conclusions Locust bean gum is derived from the seed endosperm of C. siliqua. It is a hydrocolloid with unique functional properties which makes it a novel thickener and stabilizer. It gives viscosity when added to different food products and ultimately improves the textural and other functional properties through water phase management. It has wide applications in the sectors such as food, pharmaceutical, textile, oil, paint, paper and cosmetics. In food sector,
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it is utilized as thickener and stabilizer in ice cream, sauce, beverages, bakery and meat industry. It is also a source of soluble fiber and can be utilized for the development of dietary fiber enriched food products. Its consumption in routine diet reduces the risk of heart diseases, diabetes and digestive disorders in human beings.
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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac
Review
Locust bean gum: Processing, properties and food applications—A review Sheweta Barak, Deepak Mudgil ∗ Department of Dairy and Food Technology, Mansinhbhai Institute of Dairy and Food Technology, Mehsana 384002, Gujarat, India
a r t i c l e
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Article history: Received 28 December 2013 Received in revised form 4 February 2014 Accepted 9 February 2014 Available online 16 February 2014 Keywords: Locust bean gum Carob bean gum Dietary fiber Properties Food application
a b s t r a c t Locust bean gum or carob gum is a galactomannan obtained from seed endosperm of carob tree i.e. Ceratonia siliqua. It is widely utilized as an additive in various industries such as food, pharmaceuticals, paper, textile, oil well drilling and cosmetics. Industrial applications of locust bean gum are due to its ability to form hydrogen bonding with water molecule. It is also beneficial in the control of many health problems like diabetes, bowel movements, heart disease and colon cancer due to its dietary fiber action. This article focuses on production, processing, composition, properties, food applications and health benefits of locust bean gum. © 2014 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4. 5. 6.
7.
8. 9.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-food applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composition and chemical structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Hydration rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Synergistic gel formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6. Water adsorption isotherm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Edible films/coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Beverages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Bakery products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4. Noodles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5. Ice cream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6. Low-fat yoghurt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Health benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∗ Corresponding author. Tel.: +91 9896672001. E-mail address: [email protected] (D. Mudgil). http://dx.doi.org/10.1016/j.ijbiomac.2014.02.017 0141-8130/© 2014 Elsevier B.V. All rights reserved.
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1. Introduction Locust bean gum is a white to creamy white powder obtained after milling of seed endosperm of fruit pod of the carob tree, a member of legume family, botanically known as Ceratonia siliqua L. which is found in Mediterranean regions. Hence, locust bean gum is also known as carob gum. The carob seed is consists of three parts i.e. husk, endosperm and germ. The processing of carob gum first involves the removal of hull from the seed which can be attained via thermo-mechanical or by chemical treatment [1]. After removal of the outer layer i.e. the hull, the seeds are split lengthwise and the germ portion is separated from the endosperm of carob seed. Further, the isolated endosperms are subjected to grinding, sifting, grading, packaging and finally marketed as locust bean gum or LBG. All the operations in LBG processing are designed such that any impurity from husk or germ portion can be avoided which can ultimately alter the properties of the carob gum. Galactomannans are linear polysaccharides consists of -(l–4)-mannose backbone with single d-galactopyranosyl units attached via ␣-(l–6) linkages as side branch. These side branches are not distributed uniformly in the main backbone chain [2,3]. There are also present some unsubstituted -d-mannopyranosyl chain segments, alternating with -d-mannopyranosyl units substituted with ␣-d-galactopyranosyl side branches [4–7]. Carob galactomannan is one of the commercial galactomannans guar gum and tara gum; and among these galactomannans LBG has the lowest galactose content about twenty percent [8]. Locust bean gum generally has an average mannose to galactose ratio of about 3.5 which is highest among the commercially available galactomannan such as guar gum (1.8) and tara gum (3.0). The degree of galactose substitution on mannose chain affects water solubility of the galactomannan. This is reason because guar gum is cold water soluble whereas LBG shows low solubility at ambient temperature and there is a requirement of heat treatment for maximum solubility to achieve the best water binding capacity [9]. The molecular size, mannose to galactose ratio, galactose distribution in the mannose backbone chain influences solubility and also controls the rheological properties of LBG. Different chromatography techniques can be used to characterize carob galactomannan structure. The molecular weight distributions can be determined using size exclusion chromatography and mannose to galactose ratio can be generally determined using gas chromatography. LBG is considered as the first galactomannan used as additive in industries such as paper industry, textile industry, pharmaceutical industry, cosmetic industry and food industries. The important characteristic of this gum that make it a useful industrial gum is its ability to form very viscous aqueous solution at relatively low concentration, to stabilize emulsion and to replace fat in many food products. Being non-ionic in nature, locust bean gum solutions are not influenced by pH, salts and heat treatment. Locust bean gum can be used in combination with other hydrocolloids such as carrageenan and xanthan gum as it shows synergistic action with them and forms a gel with more elasticity and strength [10]. Locust bean gum is suitable for many food applications as it provides a creamy mouthfeel. It is typically added in cream-cheese spreads to impart richness and spreadability. It is especially useful in preventing syneresis in various food products. LBG also create a smooth mouthfeel in sauces. LBG’s ability to bind water makes it an excellent choice for frozen applications, such as ice cream as it will slow down and reduces the size of ice-crystal formation as the moisture is retained within the ice cream. LBG also finds application in yoghurt manufacture. In yoghurt, it reduces the syneresis and enhances water holding capacity of yoghurt when used at certain levels [11]. Recently, LBG is used in edible films and improved the properties of films such as water vapor permeability, oxygen permeability, tensile strength and elongation-at-break [12]. In edible coatings, when locust bean gum is used in combination with
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other hydrocolloid such as -carrageenan; it improves the properties of the film due to its synergistic action. LBG is also used as thickener, stabilizer and gelling agent in various food products such as baked foods, beverages, dairy products and processed fruit products. Locust bean gum is classified as GRAS (Generally recognized as safe) by FDA and is used for its stabilizing, thickening and fat-replacing properties [13]. 2. Production Locust bean gum is extracted from the seed endosperm of the carob tree plant botanically known as C. siliqua. It belongs to the subfamily Caesalpinioideae of the Leguminosae family [14]. Carob plant is typical tree of semiarid environments. It is very abundant in the Mediterranean region since ancient times and is currently produced in Spain, Italy, Cyprus and other Mediterranean countries. Its localization also extends to different regions of North Africa, South America, and Asia. Other known carob producing countries are Morocco, Greece, Algeria, Turkey, Israel, India and Pakistan. Carob plant is long-lived evergreen tree and after germination, it grows to about 10 m height in 10–15 years. The carob tree may not be fully grown until it is 50 years old and it starts to bear good quantities of pods at the age of 15 years. The pods reach to full size in July but are ripened in October. Hot and dry climatic conditions are required for good yields. Large trees can yield up to half ton of pods per annum. Carob tree yields large brown fruits known as carob pods. These carob pods are sickle shaped and are 10–20 cm in length and 2–4 cm in width. These sickle shaped pods contain 10–15 oval shaped carob seeds or kernels. The polysaccharide from seed endosperm of carob tree is also referred in the literature as carob bean gum, carob seed gum, carob flour, or even ceratonia [15]. World production of carob seeds is estimated at about 315,000 tons per year, produced from two lakhs hectare. The main producers include Morocco (38%), Spain (28%), Italy (8%), Portugal (8%), Greece (6%), Turkey (6%) and Cyprus (2%) [14]. Recently, growing interest for locust bean gum has been observed due to its various industrial applications. Locust bean gum is used as a thickening and stabilizing agent in food, cosmetic and pharmaceuticals industries [16]. In food industry it is a food additive with E-number E-410 in the European Union [17]. Pharmaceutical applications of locust bean gum are mainly due to its ability as controlled release excipient in tablets. Biodegradability, low toxicity and low cost of locust bean gum contribute for its increasing utilization in various fields. 3. Processing The carob pods are kibbled to separate the seeds from the pulp portion. The processing of carob seeds includes dehusking (acid or thermo-mechanical), splitting, milling, sifting, clarification and drying. The processing of carob seeds is very difficult due to its very tough and hard seed coat. The dehusking of carob kernels is achieved by treatment of carob seeds with dilute sulphuric acid or with thermo-mechanical treatment known as acid peeling and thermal peeling, respectively. In acid peeling process, carob seeds are treated with sulphuric acid at elevated temperature to carbonize the seed coat. The remaining portions of the seed coat are separated and removed from the endosperm portion by efficient washing and brushing process. The peeled kernels are dried and cracked and the more friable germ gets crushed. The germ parts are sifted off from the unbroken endosperm halves known as splits. The carob bean gum produced from the carob splits by this process is whitish in color and has higher viscosity. In thermo-mechanical peeling process, roasting of carob kernels are carried out in rotating furnace where the seed coat gets pop off from the internal portions of carob kernels. The endosperm halves or carob splits are obtained
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S. Barak, D. Mudgil / International Journal of Biological Macromolecules 66 (2014) 74–80 Table 1 Composition of carob seed. Part
Proportion (%)
Husk Endosperm Germ
30–33 42–46 23–25
Locust bean gum also reduces or controls diabetes due to its high gelling ability which on ingestion causes satiety sensation [20]. In textile industry, locust bean gum is used in combination with starch as sizing agent. Locust bean gum is utilized as a thickener in paints. Locust bean gum is also used in paper making, pet foods and cosmetics products. 5. Composition and chemical structure
Fig. 1. Flow diagram for manufacturing of locust bean gum.
after mechanical processing and are recovered from the roasted seed coat or husk and the crashed germs. The isolated endosperm halves are milled and sieved to obtain fine particle size powder of native carob bean gum or locust bean gum (Fig. 1). The product obtained from this process is somewhat darker in color due to the heating or roasting operation. Thermo-mechanical process leads to no effluent as it do not involve sulphuric acid as processing aid. Clarification of native locust bean gum may be achieved by dispersing it in water and dissolved by heating. This solution is subjected to filtration to remove insoluble substances and to obtain clear solution. Locust bean gum is recovered by precipitation with isopropanol or ethanol, followed by filtering, drying and grinding or milling, to obtain fine particle size powder of clarified or purified carob bean gum. 4. Non-food applications Locust bean pods were utilized as cattle feed for a very long time but now its seed endosperm powder is utilized as locust bean gum in various industries such as food, cosmetic, pharmaceutical, textile, paint, mining, oil drilling and construction industries for its thickening and stabilizing properties. In pharmaceutical industries, locust bean gum is used in the production of solid monolithic matrix systems, films, beads, micro-particles, nano-particles, inhalable and injectable systems, as well as in viscous liquid and gel formulations. In these dosage forms, locust bean gum performs different functions such as binders, viscosity enhancers, stabilizers, matrix formers, drug release modifiers, coatings, disintegrators, solubilizers, emulsifiers, suspending agents, gelling agents, and bioadhesives [18]. Biopharmaceutical applications of locust bean gum are mainly attributed to its gelling capacity and synergies with other polysaccharides. The most common application of locust bean gum is the formulation of oral delivery systems based on tablets, hydrogels and multiparticulate systems. Apart from the binding and stabilizing ability, locust bean gum also act as a bioactive substance which has hypolipidemic effect, decreasing low density lipoprotein (LDL) cholesterol due to high dietary fiber content [19].
Locust bean kernel is composed three major components which are outer husk (30–33%), germ portion (23–25%) and endosperm portion (42–46%) by seed weight (Table 1). The major portion of locust bean endosperm is galactomannan, which comprises approximately 80% weight of the endosperm and the rest corresponding to proteins and impurities. The protein content of locust bean gum is reported to include albumin and globulin (32%), while the remaining 68% correspond to glutelin [21]. Impurities mainly refer to ash and acid-insoluble matter [22]. After seed processing, crude galactomannan can be further submitted to several processes to eliminate both the protein content and impurities. Impurities usually remain insoluble even when heating at temperatures up to 70 ◦ C [23]. Treatment with isopropanol is considered very efficient in the elimination of proteins as insoluble impurities. The composition of commercial locust bean gum is presented in Table 2. The commercial locust bean gum powder contains approximately 10–12% moisture, 5% protein, 1.0% ash, 1.0% crude fiber, 0.5% fat and 80–85% galactomannan [24,25]. Possible impurities in commercial locust bean gum powder are husk and germ which are reflected by acid-insoluble matter and protein content, respectively. Residual amount of ethanol or isopropanol used during washing or extraction process also act as impurities in locust bean gum powder. Locust bean gum is comprised of a high molecular weight polysaccharide composed of galactomannan. Locust bean galactomannan is composed of two units i.e. galactose and mannose. Locust galactomannan consists of a linear chain of (1→4)-linked d-mannopyranosyl units with (1→6)-linked ␣-d-galactopyranosyl residues as side chains (Fig. 2). Locust galactomannans are highly polydispersed like other polysaccharides found in nature. In literature molecular weight of locust galactomannan is reported as approximately 310,000. However, recent molecular weight estimation techniques such as gel permeation chromatography suggest that average molecular weight of locust galactomannan varies significantly, typically ranging from 0.3 to 2.0 million, depending on the seed source, plant growing conditions and manufacturing processes. The galactomannan molecule is considered to exhibit an extended ribbon-like structure at the solid state and a semi flexible coil-like conformation in solution. The galactose to mannose ratio of locust bean gum is approximately 1:3.1–1:3.9 [25–27]. The Table 2 Composition of locust bean gum. Constituent Galactomannan Moisture Protein content Fat content Crude fiber Ash
Proportion (%) 80.0–85.0 10.0–12.0 5.0–6.0 0.5–0.9 0.8–1.0 0.5–1.0
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Fig. 2. Structure of locust bean gum.
mannose and galactose content has been reported as 77–78% and 21–23%, respectively [25]. The distribution of d-galactosyl residues or side chains along the mannose backbone chain can be random, blockwise and ordered. 6. Properties The functional properties of locust galactomannan like other polysaccharides are dependent on their behavior in an aqueous medium. Locust bean gum is partially soluble in cold water. Locust galactomannan requires heating for complete solubilization in water. If galactomannan sol is heated above 80 ◦ C to achieve complete solubilization, care should be taken, as heating to such extent may cause oxidative–reductive depolymerization of galactomannan chain and reduction in viscosity of final solution may observed. In pH range 4–9, The Viscosity of locust bean gum solution decreases with pH increasing above 9 and decreasing below 4. Locust bean gum is relatively stable against mechanical distortion. The viscosity property of locust bean gum depends on various factors such as molecular weight, concentration, shear rate and solubilization method. Locust bean gum is often regarded as less viscous than guar gum and tara gum. 6.1. Solubility Locust bean gum is partially soluble in cold water and needs to be heated to reach maximum solubility. Locust bean gum shows solubility in water of approximately 70–85% when heated to 80 ◦ C/30 min [28]. This difference in solubility may be due to the high molecular weight galactomannan component and galactomannan with lower galactose residues as side chain which can solubilized at higher temperature. It shows that it is hot water soluble and has lesser solubility in water than other galactomannans such as guar gum which is cold water soluble gum [5]. Generally, the solubility of locust bean gum powder does not exceed 90%, which is further dependent on some factors or variable such as particle size or granulation and impurities such as husk or germ contamination [16].
molecules aligns in the direction of flow which leads to decrease in viscosity and thus explain the non-Newtonian behavior of locust bean gum solution at higher shear rates [29]. Aqueous solutions of locust bean gum at 0.5–2.0% concentration shows a typical behavior of macromolecular biopolymer with dominating loss modulus (G ) over storage modulus (G ) in lower frequency range whereas storage modulus dominates the loss modulus in high frequency range. It means that locust bean gum solutions with concentrations ranging from 0.5% to 2.0%, shows a liquid behavior at lower frequency (G > G ) and a solid-like behavior at higher frequencies (G > G ). G value of locust bean gum aqueous solution at same concentration is lower than fenugreek galactomannan, guar galactomannan and tara galactomannan. In frequency sweep tests, crossover frequency (where G = G ) of locust bean gum solution shifts to lower frequency values with increase in gum concentration [30]. 6.3. Viscosity The most significant property of locust bean gum is its ability to hydrate in hot water to give viscous solution. The thickening capacity of locust bean gum depends on certain factors such as particle size, polymer concentration, molecular weight distribution, shear rate, solubilization methods and impurities. Locust bean gum is generally less viscous galactomannan than guar and tara galactomannan. Viscosity values of locust bean gum solutions in the Newtonian plateau can be used to analyze molecular characteristics. Intrinsic viscosity of locust bean gum solution is determined based on concentration dependence of Newtonian viscosities of dilute solutions. Intrinsic viscosity is related molecular weight through a power-law equation known as Mark–Houwink–Sakurada equation. Power-law exponent ‘˛’ for locust bean gum has been reported as 0.77 when water is used as solvent. Intrinsic viscosity and molecular weight of locust bean gum is lower among galactomannans. Viscosity of locust bean gum solutions is concentration dependent and generally shows increase with increase in gum concentration. 6.4. Hydration rate
6.2. Rheology Rheology is the study of flow (in liquids) and deformation (in solid or semisolids) of material under the effect of applied force. Locust bean gum in aqueous solutions shows a non-Newtonian behavior (pseudoplastic steady-flow behavior or shear-thinning behavior) at high shear rates, but exhibits Newtonian flow behavior at low shear rates. At lower shear rates, the disruption of molecular entanglements by the shear may be balanced by the reformation of new entanglements, so that the viscosity kept constant and the locust bean gum solutions show Newtonian flow behavior whereas at higher shear rates, disruption of the entanglements predominated over the reformation of new entanglements,
Rate of hydration of commercial locust bean gum varies depending on the grades. Locust bean gum is generally graded based on the particle size, gum content and insoluble. Locust bean gum requires heating at 80 ◦ C for 30 min for its full hydration. Hydration of about 2 h is required in order to reach maximum viscosity for practical applications [31]. Major controlling factor of hydration rate of locust bean gum is particle size of locust bean gum powder. Fine particle size gum powder hydrates easily and more quickly than relatively coarser particle size gum powder. Commercially, fine particle size locust bean gum powder is available for quick hydration. However, a considerable time interval for complete or maximum hydration of locust bean gum powder is still desired.
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6.5. Synergistic gel formation Locust bean gum do not form gel under normal conditions but upon freeze–thaw treatment a weak gel can be obtained. Locust bean gum forms gel in the presence of large amount of sucrose. Gel strength of locust bean solution in combination with sugar shows an initial increase and subsequent decrease with increasing concentration of sugar. Initial increase in gel strength may be attributed to reduction in water content due to increased concentration of sugar. The subsequent decrease in gel strength attributed to inhibition of polymer–polymer association by binding of sugar molecules to polymer chains which leads to decrease in gel strength arising from decrease in strength of binding. Maximum gel strength of locust bean gum solutions can be attained at 45% (by weight) fructose, 50% (by weight) sucrose or sorbitol, and 55% (by weight) glucose [32]. Locust bean gum also shows a useful synergistic increase in gel strength on blending with other gums such as xanthan gum, -carrageenan etc. X-ray diffraction analysis of xanthan-locust bean gum blends shows new diffraction patterns that are absent in xanthan and locust bean when studied separately which suggest intermolecular binding between xanthan and locust bean gum. X-ray diffraction studies also indicate random aggregation between galactomannan and -carrageenan which reveals small degree of intermolecular binding. It is well accepted fact that by replacement part of carrageenan with locust bean gum, increases the gel strength indexed by dynamic rigidity modulus and compressive Young’s modulus. This increase in gel strength increases with increase in concentration of locust bean gum and then decreases after a peak point in mixed gel and is generally known as synergistic effect [33]. 6.6. Water adsorption isotherm Water adsorption isotherm represents the relationship between equilibrium moisture content vs. water activity. Torres et al. (2012) studied the water adsorption isotherm of locust bean gum at different temperatures (20–65 ◦ C) and reported increase in equilibrium moisture content with increasing water activity following the shape corresponding to type II isotherms according to Brunauer’s classification [34,35]. Torres et al. (2012) also reported that the equilibrium moisture content of locust bean gum decreased with increasing temperature at each water activity level [35]. This can be explained as at high temperatures, the activation of water molecules changes to higher energy levels and the links become less stable and break away from the water-binding sites of galactomannan, hence the equilibrium moisture content decreases. Isosteric sorption heat of locust bean gum decreased with increasing equilibrium moisture content. Locust bean gum shows high sorption heat of about 27 kJ mol−1 which exhibits less hygroscopicity of the gum [35].
or without the addition of plasticizers and surfactants [12]. Locust bean gum has been used to form edible films/coatings due to its edibility and biodegradability and can be used as an alternative to reduce negative effects of minimal processing on fresh-cut fruits [36]. Hydrophilic properties of locust bean gum provide a good barrier due to its carbon dioxide permeability, oxygen permeability, water vapor permeability, tensile strength and elongation-at-break under certain conditions. Locust bean gum in edible film and coating may also serve as carrier of additives and bioactive components [37]. Edible films formed by mixed systems of locust bean gum and -carrageenan showed improved properties. The addition of -carrageenan to locust bean gum improved the barrier properties of the film which leads to decrease in water vapor permeability. Improved values of elongation-at-break of edible films were also reported when the ratio of -carrageenan and locust bean gum was mixed in the ratio of 80:20% (w/w). -carrageenan and locust bean gum blend films enhance the tensile strength compared to tensile strength of films prepared individually by carrageenan and locust bean gum. Improvement in -carrageenan and locust bean gum film properties are reported due to hydrogen bonds interactions between -carrageenan and locust bean gum observed via Fourier Transform Infrared (FTIR) spectroscopy analysis [38]. 7.2. Beverages Locust bean gum is very popular as thickening and stabilizing agent in used in various beverages. Locust bean gum solutions are stable at wide range of pH which makes it a unique stabilizer and thickener in most of the beverages. Guar gum is soluble in hot water and most of the beverages require heat processing which enable the locust bean gum to use in beverages. It improves the keeping qualities of beverages via resistance to phase separation and thickening. 7.3. Bakery products Its application for bakery purposes results in higher baked product yields; it improves the final texture and adds viscosity in dough. Addition of guar gum in cookies dough improves the machinability of the dough which helps in the better handling of dough with minimum requirement of energy and time. Locust bean gum is also used to increase volume and to retard the aging of bakery products [39]. The addition of locust bean gum to wheat flour suspension decreases the pasting temperature and increases the peak viscosity, trough viscosity, breakdown, final viscosity and setback values. Water absorption capacity and dough development time of wheat flour dough also increases on addition of locust bean gum [40]. Locust bean gum can also be used as binding agent as a substitute for gluten in gluten-free bread formulations based on corn starch with improved loaf volume and crumb structure [41].
7. Food applications 7.4. Noodles Locust bean gum is used as an additive in food industry due to its thickening and stabilizing property. Its utilization as stabilizer and thickener in food products is popular as it is obtained from a natural source. Its functionality is due to its water phase management in food products. 7.1. Edible films/coating The use of edible films or coatings composed of natural polymers and food additives have been constantly increasing in the food industry to enhance the shelf life of fresh fruits, vegetables and meat products. These films/coatings can be produced from biopolymers such as polysaccharides, proteins, lipids, resins, with
Incorporation of locust bean gum in noodles dough improves the dough rheology as well as the textural characteristics of the cooked noodles. Improvement in textural properties of noodles is attributed to the strengthening effect of gum on the gluten network which results in better textural properties of noodles. Addition of locust bean gum reduces the cooking loss and swelling index of noodles [42]. 7.5. Ice cream Locust bean gum alone or in combination with guar gum is used in frozen dairy products for desired textural properties such
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as viscosity enhancement and ice recrystallization inhibition. But, milk proteins and locust bean gum systems are incompatible in solution systems and leads to separation of phases [43]. These two separated phase consists of water soluble and water insoluble components. Water insoluble components include colloidal protein and fat components (opaque); whereas other phase include dissolved solutes (transparent). The difference in optical properties of the two phases makes the product undesirable in appearance and ultimately for consumption. To solve the problem of phase separation, carrageenan is used in combination with locust bean gum which leads to phase stabilization dairy systems via casein and carrageenan electrostatic interaction [44–46]. The phase separation in soft-serve ice cream mixes is undesirable from consumer point of view. The effect of -carrageenan, locust bean gum and casein–whey protein ratio on the phase stability of soft-serve ice cream mixes during storage of 3 weeks at 5 ◦ C is also studied and reported that -carrageenan 0.015–0.02% (w/w) and locust bean gum 0.06–0.2% (w/w) is required to stabilize the ice cream mix against serum or phase separation [47]. 7.6. Low-fat yoghurt Low-fat yoghurt has a texture different from that of full-fat yoghurt. Fat component in food is mainly responsible for the texture of that product. There have been many studies reported in the literature related to improvement of texture and body of non-fat or low-fat yoghurt. The main examples are use of exopolysaccharides producing starter cultures [48] and addition of food hydrocolloids which provide good stability and desirable texture in low-fat yoghurt [49,50]. Hydrocolloids and protein concentrations in lowfat yoghurt need to be optimized to allow for maximum interaction between the hydrocolloid and protein. If it is not optimized, then hydrocolloid–hydrocolloid or protein–protein interactions may predominate and ultimately affects the milk reactivity. The milk reactivity is highly dependent on gum concentration [51]. Locust bean gum when added at 0.02% (w/w) to low-fat yoghurt, increases firmness, viscosity and water-holding capacity and reduces syneresis [11]. 8. Health benefits Locust bean gum comes under the category of viscous soluble fiber. Due to its effect on viscosity and food structure, it can alter the rate of carbohydrate degradation during digestion. This is likely to have beneficial effects for the regulation of postprandial blood sugar and insulin levels, key events in the prevention and treatment of obesity and diabetes [52]. Viscous soluble fiber such as locust bean flattens postprandial glycaemia more consistently than wheat bran and other insoluble fibers [53]. Recent studies on gums as dietary fiber reported their protective effect on cardiovascular disease. Incorporation of gums such as guar gum & locust bean gum as dietary fiber in diet can be beneficial in reducing inflammation and inflammatory bowel diseases, Crohn’s disease and ulcerative colitis [54]. The physiological effects and viscosity property of locust bean gum is comparable to guar gum and -glucan [55]. 9. Conclusions Locust bean gum is derived from the seed endosperm of C. siliqua. It is a hydrocolloid with unique functional properties which makes it a novel thickener and stabilizer. It gives viscosity when added to different food products and ultimately improves the textural and other functional properties through water phase management. It has wide applications in the sectors such as food, pharmaceutical, textile, oil, paint, paper and cosmetics. In food sector,
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it is utilized as thickener and stabilizer in ice cream, sauce, beverages, bakery and meat industry. It is also a source of soluble fiber and can be utilized for the development of dietary fiber enriched food products. Its consumption in routine diet reduces the risk of heart diseases, diabetes and digestive disorders in human beings.
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