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.