Microstructure and Gelation Behaviour of High Methoxyl and Low Methoxyl Pectin Gels and their Mixtures

Doctoral thesis, 2005

The objective of this thesis was to explore the functional behaviour of high methoxyl (HM), low methoxyl (LM) and mixed HM/LM pectin gels of different chemical characteristics with emphasis on microstructure and kinetic rheological behaviour. The rheological properties and the structure of pectin gel networks were investigated for pectins under different sucrose, calcium and pH conditions by the means of oscillatory measurements and transmission electron microscopy (TEM). The gel microstructure of HM and LM pectin showed similar appearance at pH 3. Both HM and LM pectins formed open and aggregated network structures with many pores about ~ 500 nm in diameter. Mixed HM/LM pectin gels revealed different gel structures than the HM and LM pectin gels. The microstructure of the mixed HM/LM pectin gels was strongly affected by the presence of calcium. The kinetic behaviour during gel formation differed for HM and LM pectins. HM pectins does not require calcium ions for gel formation but this work showed that calcium has a profound effect on the kinetic behaviour and in some cases also the microstructure. Rapid gel formation was obtained for LM pectin under all investigated conditions. The methyl esters and amide groups play a decisive role for the functionality of pectins. This work also showed that the distribution of methyl esters has a bearing on the functionality of the gels, both in the absence and the presence of calcium. LM pectin gelation occurs at a wide range of pH values, at reduced sugar concentrations or no sugar at all. The gelling conditions are different at acidic and neutral pH values, and it was shown that the kinetic behaviour and the microstructure of LM pectin were dissimilar at pH 3 and pH 7. The gel microstructure of LM pectin was much denser at pH 7 than at pH 3. The structural variations could be a direct result of the differing kinetic behaviour under acidic and neutral conditions or a result of a conformational transition that is associated with a pH change. There is an increasing demand for low-sugar food products with similar texture properties as high-sugar products. Pectin mixtures are interesting candidates for replacing the consistency given by the presence of sucrose. A strong synergistic effect in rheological behaviour occurred for mixed HM/LM pectin gels in the presence of sucrose and calcium. A HM pectin gel could be replaced with a mixed HM/LM pectin gel of lower sugar content with maintained rheological behaviour. The kinetic rheological behaviour of the mixed gels was influenced by both HM and LM pectin gelation. Different gel formation behaviour of the mixed HM/LM pectin gels could be achieved depending on the degree of blockiness of the HM pectins.