L OCUST B EAN G UM

4.2.1 L OCUST B EAN G UM

4.2.1.1 Source

Locust bean gum is also called carob bean gum since it is separated and refined from the endosperm of the seed of the carob tree. Carob is a long-lived evergreen

tree that grows to about 10 m in height in 10 to 15 years after germination and yields large brown fruits called carob pods. The sickle-shaped carob pods are 10 to 20 cm long and 2 to 4 cm wide and contain 10 to 15 oval-shaped seeds or kernels. Locust bean has been used as a food ingredient in the eastern Mediterranean region since ancient times and is currently produced mostly in Spain, Italy, Cyprus, and other Mediterranean countries.

4.2.1.2 Method of Production

The production of commercial locust bean, guar, and tara gums is similar, involving separation of endosperms from the seed hull and germ, followed by grinding and sifting of the endosperm into fine particle-sized flour. Further purification is made by repeated alcohol washings. Most commercial gums contain more than 80% galactomannan. The quality or purity of the final gum product depends on the extent of endosperm separation and de-hulling. Remaining fragments of the hull may appear as dark specks and deteriorate product quality.

The carob pod contains approximately 10% of seeds by weight. The endosperm is commonly called splits because two spherical halves constitute the endosperm surrounding the germ. The splits are milled to obtain flours that give a cloudy solution when dissolved in water. The flour is then dispersed into hot water and insoluble particles are removed by diatomaceous earth filtration. The clarified solution is then precipitated using isopropyl alcohol, washed with alcohol, pressed, dried, ground, and sieved. The final product is a white to cream-colored powder that should give

a clear solution when dissolved in water.

4.2.1.3 Chemistry and Structural Features

The molar ratio of galactose to mannose of locust bean gum is approximately 1:4. The distribution of D -galactosyl residues along the backbone chain can be random, blockwise, and ordered, where there are long runs of unsubstituted mannosyl units and block condensation of galactosyl units. 5–8

Like many polysaccharides found in nature, galactomannans are highly polydis- persed. The average molecular weight varies significantly, typically ranging from

0.3 to 2 million, depending on the source of seed, growing and harvesting conditions, and manufacturing processes. The galactomannan molecule is considered to adopt an extended ribbon-like structure at the solid state and a semiflexible coil-like

conformation in solution. 9 The flexibility of the mannan backbone in solution seems to be limited to some extent since galactomannan oligosaccharides tend to precipitate in an aqueous solution, which is not the case with an ideally flexible polysaccharide

chain such as pullulan. 10 Nevertheless, the persistence length, a measure of the flexibility of a polymer chain, has been evaluated to be ca. 3–5 nm for a molecularly

Seed Polysaccharide Gums

solubilized locust bean gum using the pressure cell solubilization method. 11 This value corresponds to the end-to-end length of only several linearly connected glu- copyranoses and is much smaller than the value normally found for a food polysac- charide; in the latter case, aggregation was frequently not considered.

4.2.1.4 Functional Properties and Applications

Locust bean gum is an effective thickener or stabilizer since it provides a relatively high viscosity at low concentrations. Locust bean gum is only partially soluble in cold water. Care should be taken, however, if a galactomannan sol is heated above 80°C to achieve complete solubilization, since heating to such an extent may cause oxidative–reductive depolymerization of the main chain and reduction of viscosity of the final solution. 12,13 In the pH range from 4 to 9, the viscosity of a locust bean gum solution is fairly stable, while the viscosity decreases with pH increasing above

9 or decreasing below 4. 14,15 In an acidic condition, acid-catalyzed hydrolysis may occur especially on heating. Locust bean gum is relatively stable against mechanical distortion. It is therefore recommended to moderately apply heat with stirring to prepare a homogeneous gum solution.

The thickening ability of locust bean gum depends on various factors, such as molecular weight distribution, polymer concentration, shear rate, solubilization methods, etc.; it is often regarded as a less viscous galactomannan than guar and tara gums. Locust bean gum solutions ordinarily exhibit pseudoplastic steady-flow behavior or shear thinning at high shear rates, but Newtonian flow behavior may be

observed at low shear rates. 16 Viscosity values in the Newtonian plateau can be used to evaluate molecular characteristics. The intrinsic viscosity is determined based on the concentration dependence of Newtonian viscosities of dilute solutions and is related to the molecular weight through a power-law equation called the Mark–Hou- wink–Sakurada equation. The value of the power-law exponent α is known to represent the stiffness of the polymer chain and the nature of polymer–solvent interactions. In the case of locust bean gum, the α value has been reported to be

0.77, suggesting that water is a relatively good solvent for the gum and that the molecule behaves essentially as a flexible coil. 11 A higher α value of 0.98 was reported for highly polydispersed samples. 16

Locust bean gum does not normally form a gel, while a weak gel can be obtained upon freeze–thaw treatment 17 or in the presence of a large amount of sucrose. 18 There is also a useful synergistic increase in viscosity or gel strength by blending locust bean gum with certain helix-forming or rigid polysaccharides, including xanthan, κ-carrageenan, and yellow mustard gum. 1–4 X-ray fiber diffrac- tion studies on xanthan–locust bean gum mixtures have revealed new diffraction patterns that are absent in individual polysaccharides, suggesting intermolecular binding between these two types of polysaccharides. 19,20 The formation of a three- dimensional polymer network is then possible if the intermolecularly bound sec- tions play a role as junction zones that are connected by unbound sections of the molecular chains. However, no such preferential intermolecular alignment has been evidenced between κ-carrageenan and locust bean gum in the x-ray fiber diffraction studies, indicating random aggregation between galactomannan and the

Functional Food Carbohydrates

surface of κ-carrageenan crystallites or only a small degree of intermolecular binding. 19,20

Locust bean gum is one of the most extensively utilized gums in the world. In the food and pet food industries, the gum is widely used as texturizing, thickening, and stabilizing agents, usually in the amount of <1% of the product weight. One of the important applications of locust bean gum is in ice cream products. It improves the smoothness of the body and handling properties, and gives uniformity of the

product and desired resistance to melting. 21 Locust bean gum forms a structured gel network after freezing and temperature cycling between –18 and –10°C; this would explain why locust bean gum-stabilized products are more stable against temperature fluctuations. The mechanism of preventing crystallization has been attributed to its ability to limit the rate of growth of the ice crystals during recrystallization without affecting the initial ice crystal formation processes. 21,22 It is recommended to add a small amount of κ-carrageenan together with locust bean gum in ice cream and other dairy products since syneresis or whey separation caused by the incompatibility

between locust bean gum and milk proteins can be prevented. 23 The synergy between locust bean gum and κ-carrageenan produces a cohesive gel and is used in meat and dessert products.

Other food applications include bakery products, pie fillings, sauces, dressings, and creams. Locust bean gum has been used for improving the texture and stability

of starch-based products. The viscosity of a mixture of gelatinized starch and locust bean gum is normally much higher than that of starch or galactomannan alone,

demonstrating a strong synergy. 24 Gelatinized starch paste can be regarded as a suspension of swollen starch granules in a continuous aqueous phase containing amylose leached out from the granules during gelatinization. Since locust bean gum is likely to be confined in the aqueous phase, its local concentration or effective concentration is anticipated to be higher than the bulk concentration. Thus, it is not surprising that the overall viscosity is higher than what is expected based on the bulk concentration. Other factors influencing properties of starch–galactomannan mixtures are possible reduction in the amount of amylose leaching, suppression of granule swelling, and acceleration of retrogradation since galactomannans reduces the amount of water available for starch. The textile industry uses locust bean gum, sometimes in combination with starch, as a sizing agent and a thickener for paints. Other users of locust bean gum are the pharmaceutical, cosmetics, mining, oil drilling, and construction industries.

4.2.1.5 Physiological Properties and Health Benefits

Locust bean gum is regarded as a dietary fiber and a potential dietary supplement in weight control and treatment of diabetes and hyperlipidemia. Generally observed

physiological effects of dietary fibers are often considered as a result of absorption or binding of nutrient compounds by fibers at a high level of viscosity that slows down the mechanical disruption of foods and flow of nutrients in the gut, eventually

leading to a lower degree of absorption of nutrients. 25 Locust bean gum has been shown to decrease postprandial plasma glucose and insulin levels in diabetic humans. 26 Hypocholesterolemic effects have also been confirmed for LBG, but

Seed Polysaccharide Gums

underlying mechanisms remain unclear; influence of the gum on the secretion of the gastrointestinal hormones has been indicated. 27