Role of galactomannan composition on the binary gel formation with xanthan

The influence of the galactomannan characteristic ratios (M/G) on the tempe rature 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, les s 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 degrees C for that of S. parahybae [Bresolin TMB, Milas M, Rinaudo M and Ganter JL MS. Int J Biol Macromol 1998;23:263] and 20 degrees C for the galactomannan of hi. scabrella, determined by viscoelastic measurements and microcalorim etry. A T-g of 40-50 degrees C was found by Shatwell et al. [Shatwell KP, S utherland IW, Ross-Murphy SE, Dea ICM. Carbohydr Polym 1991;14:29] for locu st bean gum-LEG (M/G=43). Lundin and Hermansson [Lundin L, Hermansson AM. C arbohydr Polym 1995;26:129] reported a difference of 13 degrees C for T-g o f two LEG samples with M/G = 3 (40 degrees C) and 5 (53 degrees C), in mixt ures with xanthan. It appears that the more substituted galactomannans have lower temperatures of gelation in the presence of xanthan. The mechanism o f 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 sli ghtly the temperature of the conformational change (T-m) of xanthan probabl y due to the ionic strength contribution of proteins (3.9%) present in the galactomannan. On the other hand, the galactomannans from S. parahybae, wit h 1.5% of proteins and M. scabrella, with 2.4% of protein, did not show thi s effect, the T-m of xanthan alone or in a mixture being practically unchan ged. (C) 1999 Elsevier Science B.V. All rights reserved.