DILUTE-SOLUTION PROPERTIES OF GUAR AND LOCUST BEAN GUM IN SUCROSE SOLUTIONS

The dilute solution properties of guar, native and purified locust bea n gum (LBG) in sucrose solutions (0-40% w/w) have been assessed. The i ntrinsic viscosity of LEG is artificially high due to a contribution f rom polymer/polymer associations. For both galactomannans, the additio n of sucrose was shown to initially decrease the intrinsic viscosity, possibly due to either a reduction in solvent quality or a reduction i n polymer/polymer association, and then increase towards a maximum val ue at 20% sucrose owing to an increase in solvent quality. At 40% w/w sucrose concentrations, the solvent quality decreased owing to competi tion for water, enhancing the extent of polymer contraction. The initi al coil overlap concentration, c, and observation of the onset of she ar thinning, were detected at approximately 0.1% w/v for both galactom annans. This low concentration, along with the relatively low concentr ation dependence above c, C1.8-2.6, indicated a second transition was present at even higher concentrations, c*, marking the onset of the concentrated regime. As opposed to other fully flexible polymer chains , it is proposed that these two transitions were more easily detected for these less compressible, semi-rigid polymers. At dilute concentrat ions, guar exhibited slightly greater specific viscosities at the same degree of space occupancy than LEG. This was attributed to a higher c hain rigidity and possibly a consequence of the higher molecular weigh t resulting in poorer solubility. Furthermore, large Huggins coefficie nts were determined for guar in sucrose solutions reflecting guar's po orer solubility than in water. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.