Galactomannan hydration

Guar gum is a galactomannan used like locust bean gum and at approximately the same levels. However, unlike locust bean gum, it swells in cold solution and requires no heat for complete hydration. It also induces mix separation which may be controlled with carrageenin. It is well suited for use in HTST systems. 

Delayed Crosslinking Additives. Glyoxal [458,460,461] is effective as a delay additive within a certain pH range. It bonds chemically with both boric acid and the borate ions to limit the number of borate ions initially available in solution for subsequent crosslinking of a hydratable polysaccharide (e.g., galactomannan). The subsequent rate of crosslinking of the polysaccharide can be controlled by adjusting the pH of the solution. 

A number of problems arise in connection with the measurement of molecular weights of galactomannans. Firstly, it is very difficult to obtain true solutions of many of the gums, especially of locust-bean gum and guar gum consequently, the solutions must be filtered or centrifuged to remove undissolved or partially hydrated gum. This immediately raises the problem of fractionation of the sample, as the... 

The linear, but highly branched, molecular structure of the galactomannans is the reason for some specific properties which are quite different from those of the unbranched cellulose-like and water-insoluble mannans and glucomannans. Galactomannans are hydrated in cold water and give stable solutions even in acidic formulas. The interactions of galactomannans with other polysaccharides are the base of a variety of industrial applications. 

With the exception of cellulose and chitin, plant polysaccharides are usually hydrated. Hydration often occurs in the crystalline regions as well as in the amorphous areas. When water of hydration is found in the crystallites, it may or may not affect the conformation of the polysaccharide backbone and in most cases, it affects the unit-cell dimensions, while in a few cases, the water appears to have no effect on unit-cell dimensions. The structures of six hydrated neutral polysaccharides will be examined with regards to the state of water of hydration in the structure. It wi 11 be seen that water may occur as columns or as sheets in these structures. The structures that will be discussed are (1 4)-3-p-xylan, nigeran, amylose, galactomannan, (1 3)-3-p-gTucan and (1 3)-s-P-xy1 an. The chemical structures of these polysaccharides are shown in Figure 1. 

Since crystalline mannan I is not hydrated, it seems clear that the role of the (l- -6)-a-D-gal actose substituent is to encourage hydration and plasticity. In wood cells where the galactoglucomannan (glucose present in the backbone) is a matrix substance, plasticization is probably the desired property. In seeds where galactomannan is a constitutent of an endosperm, controlled hydration to facilitate attack by some enzyme is probably the important feature. The facility of hydration can control the specific time of germination of seeds especially in a desert enV i ronment. 

Seed germination is a phenomenon which requires moisture, hence hydration of the polysaccharide in the endosperm. Galactomannans seem tailored to adapt to the environmental requirements of plants located in tropical areas where moisture is seasonal. By comparison, starch hydration is probably more gradual and reversible, a situation more in keeping with a temperate climate. 

Tara gum is the galactomannan obtained from the seed of Tara shrub, Caesalpinia spinosa, mostly found in northern Africa and Peru of South America. The main polysaccharide chain is formed of (1 4)-llnked 3-D-mannopyranosyl units, with single a-D-galactopyranosyl units attached with average three main chain units via (1 6]-linkages. Molecular weight of tara gum may fall between 300,000 and 1,000,000. It is 70% soluble in cold water and hydrated 100% at temperature above 80°C showing pseudoplastic behavior in solution. 

Other hemicelluloses, composed mainly of hydrocolloids, are used primarily as an extracellular energy, raw materials storage system, and as a water retention mechanism in seeds. Galactoglucomannans, glucomannans, galactomannans, and h-glucans tend to be heavily hydrated and have fewer, if any, ester-linked side chains [57]. 

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