Thermally reversible acid-induced gelation of low-methoxy pectin

摘要

Gelation of low-methoxy pectin (DE 31.1) on cooling under acidic conditions in the absence of Ca~(2+) has been investigated by rheologican measurements under low-amplitude oscillatory oscillatory shear.The mechanical spectra obtained after 60 min at 5degC showed a progressive increase in solid-like response (increasing G;decreasing tan #delta#;increasing frequency-dependence of #eta#~*) as the pH was reduced from 4.0 to 1.6, with formation of a critically crosslinked network at -pH 3.0 (for a polymer concentration of 3.0 wt%).By extrapolation from X-ray fibre diffraction analysis of pectic acid,it is suggested that crosslinking occurs by association of three-fold helices.At pH values between -3.5 and -2.5 there is no detectable thermal hysteresis between the sol-gel transition on cooling and gel-sol transition on heating,and both are accompanied by a sigmoidal change in optical rotation (attributed to formation and melting of three-fold order).Substantial hysteresis is,however,observed at lower and higher,pH,and is attributed to extensive aggregation as electrostatic repulsion is suppressed (below-pH 2.5) and slow formation of intermolecular hydrogen bonds by protoated carboxyl groups (above-pH 3.5), respectively.The transition enthalpy from DSC heating scans has a maximum value of #DELTA#H=11 J/g at -pH 3.0 but decreases sharply at lower and higher pH, with accompanying loss of a detectable transition in optical rotation.It is suggested that the chain conformation in solution at low pH is predominantly three-fold with therefore,little conformational change on adoption of the ordered,intermolecular structure,whereas at high pH the solution conforma-tion is predominantly two-fold,with only limited conversion to the three-fold (acid) from on cooling.2000 Elsevier Science.Ltd.All rights reserved.