Purified galactomannans for industrial usage are obtained from four main plant sources in the sub family Ceasalpinioideae which are well characterized. There is Guar gum from the seeds of Cyamopsis tetragonoloba, Locust bean gum from seeds of Ceratonia siliqua (Carob tree), Tara gum from seeds of Cesal pinia spinosa, and Cassia gum from seeds of Cassia obtusifolia. Extraction of galactomannans involves de-hulling of seeds, crushing to remove the embryo, followed by milling of the endosperm to produce crude flour. The flour can be purified by dissolving in hot water followed by filtration and precipitation with isopropanol to remove impurities. The properties of galactomannans from different sources varies depending on the structure of the galactomannan which is characteristic of the source species.
The use of Locust bean galactomannan (LBG) in the Mediterranean and the Middle East has been part of traditional food preparation for hundreds of years. LBG has a galactose to mannose ratio of 1 : 4 and molecular weights ~300 000. The cold-water solubility of LBG is low and dispersions need to be heated to 85 °C to achieve good dissolution, concentrations of up to 5% w/v being possible. Low solubility is due to the tendency for the linear mannan chains to strongly hydrogen bond to each other in unsubstituted regions of the chain, limiting opportunities for interaction with water molecules.
Though LBG does not itself form gels it can be used together with other hydrocolloids to provide gel formation. This synergestic gel formation can be
observed with other nongelling polysaccharides such as xanthan gum. This property is attributed to the ability of the nonsubstituted regions of the linear
mannan backbone being able to hydrogen bond to helical regions of the other hydrocolloids and provide cross-linking. LBG is widely used as a thickener and stabilizer in many foods such as ice cream, cheese spreads, salad creams, processed meat products, and pie fillings. A major use of LBG, and Guar, is in ice cream where it can act as an effective stabilizer to prevent ice crystal growth at low temperatures by restricting accretion of liquid water to existing ice crystals. The presence of large ice-crystals leads to a significant deterioration in ice-cream texture. Guar galactomannan has long been used as a food ingredient in East. Guar is a linear ?-(1-4) mannan with a higher proportion of galactose substituents
than LBG, having a glactose:mannose ratio of 1 : 2 and this is reflected in the easier dispersion of Guar when compared with LBG. Guar can be dissolved at lower temperatures (20 °C) than LBG as the extent of unsubstituted regions of mannan chain is smaller, reducing opportunities for interchain hydrogen bonding that would lead to aggregate formation and prevent hydration. The rheology of Guar is similar to that of LBG - pseudoplastic and decreasing with temperature, with good pH stability. Gel formation is again only observed when other polysaccharides are added, but the ability of Guar to participate in such synergistic actions is weaker than that of LBG, and this is also attributed to the reduced extent
of galactose free regions of the mannan chain, which could form close hydrogen bonding with another polysaccharide.
Galactomannan from Tara seeds has a galactose mannose ratio of 1 : 3, intermediate between those of LBG and Guar. The rheological properties are similar to those of LBG. Cassia galactomannan has an average galactose/mannose ratio of 1 : 5 and can only be solubilized after boiling when a high viscosity solution can be obtained. The structure of the gum is rather variable and fractions of different solubility are obtained due to varying degrees of galactose substitution.
Like the above tree seeds of the legume subfamily Ceasalpinioideae the major grain legumes in the subfamily Papilionoideae can also contain galacto mannans. Fenugreek is a legume from the Mediterranean region, which contains high levels of a seed endosperm galactomannan with a galactose mannose ratio approaching 1 : 1. Fenugreek galactomannan is cold-water soluble, forming solutions with a lower viscosity than the other galactomannans. It has potential to act as an emulsifier and can show a good ability to stabilize oil/water interfaces. Soybeans contain a potentially useful seed galactomannan with a galactose/mannose ratio approaching 1 : 2. Lupin seeds are unusual in containing a reserve polysaccharide which is structurally unrelated to other legume reserve galactomannans, this is a galactan (or arabinogalactan) with a main chain of b-(1À4) linked galactose and low levels of arabinose substitution.