Rheological properties of Ca2+-gels of partially methylesterified polygalacturonic acid : effect of "mixed" patterns of methylesterification

Subjecting homogalacturonan to a combination of processes which modify the extent of pectin's methylesterification results in the generation of distinct patterns of methylesterification on the polymer. In this study, the influence of these "mixed" patterns of methylesterification on the rheological characteristics of Ca2+-gels is extensively examined, using partially methylesterified poly-d-galacturonic acid (mPGA) as a model for homogalacturonan. The uncontrolled pattern of methylesterification induced on mPGA was combined with that resulting from subsequent controlled chemical or enzymatic de-esterification. Afterwards, the produced mPGAs were characterised in terms of degree and pattern of methylesterification. The latter structural feature was quantified as "absolute degree of blockiness" through enzymatic fingerprinting. Characterised mPGAs were used for the preparation of gels with various calcium ion (Ca2+) concentrations. All Ca2+-pectin systems formed "true" gels with a strong elastic behaviour, but vary in gel stiffness. The gels’ rheological characteristics not only depend on the extent of mPGA de-esterification, but also on the "mixed" patterns of methylesterification. The experimental results also indicate that extremely short non-cooperative junction zones contribute to the stiffness of Ca2+-mPGA gels.