Rheological characteristics of pectin gelation in sugar-acid systems

The plant cell wall polysaccharide pectin is a food ingredient with high functionality and good consumer acceptance. It is used in a wide range of food systems and traditionally applied as gelling agent, but it has also relevant properties beside gelation. Pectin forms gels in a sugar- acid environment, like classical jam, by cold-set and ionotropic gelation, acting separately as well as in combination. Structure formation of pectin is strongly determined by several internal and external factors, such as pectin type, molecular characteristics like degree of methoxylation and pattern of free carboxyl groups, or environmental conditions, like pH and calcium ions in the gel system as well as cooling conditions during gelation. Despite of many decades of research, their single effects and interactions are not completely understood. Therefore, this thesis was focused on a systematic examination of some of these factors, in order to clarify their relevance for the pectin gelling process and the final gel properties. A new method for the investigation of pectin gelation was introduced. Characteristic temperatures of the structuring process were calculated from the structuring velocity curve, derived from rheological oscillation measurements. They reflected the typical kinetic of the gelation process. This method allowed the comparison of pectin gelation, independent on pectin type and molecular parameters, and it was a prerequisite for the broad examination of the impact of internal and external factors. The gelation of 15 commercial citrus pectin samples, both high- and low-methoxylated, partly amidated, was examined in several single studies. An investigation of the structuring process of high-methoxylated pectin revealed, that pH as well as cooling conditions affected the gelation kinetics. The pH determined the degree of dissociation and, thus, the number of the formed hydrogen bond, and the cooling rate was crucial for the dominating type of junction zones in the course of the gelling process. The threshold of the change from dominating hydrophobic to hydrogen bond was reflected in the structuring velocity curves. The gelation of low-methoxylated pectin was affected by additional factors and to a different extent, since beside a certain cold-set gelation, the ionotropic gelation is the dominant mechanism. It is less temperature-dependent than cold-set gelation, because of additional formed calcium bridges, which are independent on temperature. The structuring process was affected by added acid (pH) and calcium ions. The structure formation was divided into different phases with varying dominating mechanisms, which were reflected in the structuring velocity curves. Comparing the structure formation kinetics of the low-methoxylated pectin with and without amid groups, amidation supported the gelation due to additional hydrogen bonds formed between the amide groups it was found that additional hydrogen bonds were formed between the amide groups. Thus, structure formation of amidated pectin was less dependent on pH and added calcium ions than that of non-amidated, since the amide group supported the dominating ionotropic gelation. Beside the studies of the commercial pectins, 12 special samples have been prepared in laboratory-scale for a study, that investigated the effect of the pattern of free carboxyl groups on the gelation kinetic of high- and low-methoxylated pectins. The molecular characteristics of one commercial high-methoxylated pectin were modified by three different methods, using acid, microbial or plant pectinmethylesterases. The modified pectin samples of the obtained 3 groups had a comparable degree of methoxylation but differed in their pattern of free carboxyl groups (block-wise or random). Their gelling kinetics as well as gel properties varied during cold-set as well as ionotropic gelation. Independent of the modification method and degree of methoxylation, pectin samples with a random distribution showed similar structure formation kinetics and properties of the resulting gels, that differed from those of pectin samples with a blockwise distribution. The newly developed method for the first time allowed the direct comparison of the gelation of pectins with any degree of methoxylation and with varying gelling mechanisms. The presented work demonstrates the high impact of selected factors on the gelation process. Even moderate deviations from the standard gel composition or gelation procedure significantly affect the structure formation and the properties of the final gels after cooling. The results, obtained with the newly developed method, provide deeper insights into the interactions between molecular properties, gel composition and procedures of pectin gelation.