Locust bean gum is a polysacharide (a long chain made of sugars) made of the sugars galactose and mannose.
Some other familiar polysacharides are starch and cellulose, which are made of long chains of the sugar glucose.
In locust bean gum, the ratio of mannose to galactose is higher than in guar gum, giving it slightly different properties, and allowing the two gums to interact synergistically so that together they make a thicker gel than either one alone.
Locust bean gum is extracted from the endosperm of the seeds of the carob tree Ceretonia siliqua, which grows in Mediterranean countries.
The ancient Egyptians used locust bean gum to bind the wrapping of mummies.
In more recent times is is used as a thickener in salad dressings, cosmetics, sauces, as an agent in ice cream that prevents ice crystals from forming, and as a fat substitute.
In pastry fillings, it prevents "weeping" (syneresis) of the water in the filling, keeping the pastry crust crisp.
It has a very high viscosity (thickness) even when very little is used.