Role of galactomannan composition on the binary gel formation with xanthan

Abstract

The influence of the galactomannan characteristic ratios ( M/ G) on the temperature of gelation ( T g) and the gel strength of mixtures of galactomannan with xanthan is reported. Two galactomannans were investigated: one highly substituted from the seeds of Mimosa scabrella ( M/ G=11), and the other, less substituted, from the endosperm of Schizolobium parahybae, with ( M/ G=30) [Ganter JLMS, Zawadzki-Baggio SF, Leitner SC, Sierakowski MR, Reicher F. J Carbohydr Chem 1993;12:753]. The xanthan:galactomannan systems (4:2 g l −1, in 5 mM NaCl) showed a temperature of gel formation ( T g) of 24°C for that of S. parahybae [Bresolin TMB, Milas M, Rinaudo M and Ganter JLMS. Int J Biol Macromol 1998;23:263] and 20°C for the galactomannan of M. scabrella, determined by viscoelastic measurements and microcalorimetry. A T g of 40–50°C was found by Shatwell et al. [Shatwell KP, Sutherland IW, Ross-Murphy SB, Dea ICM. Carbohydr Polym 1991;14:29] for locust bean gum-LBG ( M/ G=43). Lundin and Hermansson [Lundin L, Hermansson AM. Carbohydr Polym 1995;26:129] reported a difference of 13°C for T g of two LBG samples with M/ G=3 (40°C) and 5 (53°C), in mixtures with xanthan. It appears that the more substituted galactomannans have lower temperatures of gelation in the presence of xanthan. The mechanism of gelation depends also on the M/ G ratio. For the lower values it involves only disordered xanthan chains in contrast to M/ G ratios higher than 3. In addition, the presence of the galactomannan from M. scabrella increased slightly the temperature of the conformational change ( T m) of xanthan probably due to the ionic strength contribution of proteins (3.9%) present in the galactomannan. On the other hand, the galactomannans from S. parahybae, with 1.5% of proteins and M. scabrella, with 2.4% of protein, did not show this effect, the T m of xanthan alone or in a mixture being practically unchanged.