Annual Transactions of the Nordic Rheology

This study investigates the influence of pectin fine structure (i.e. methyl ester content and distribution) on the rheological properties of calcium induced pectin networks. The methyl ester distribution was assessed using the definition of the degree of blockiness (DB) and absolute degree of blockiness (DBabs). The study showed that 1) the ability to form calcium induced pectin networks depends more on the DB and DBabs than the degree of methylation (DM) and 2) that the absolute value of gel strength is increased for pectins with a high degree of blockiness. These results were explained by the effectiveness of the pectins to bind calcium. A pectin with a blocky methyl ester distribution will bind the available calcium in stable junction zones compared to a pectin with random distribution of methyl esters where some calcium will be less effectively bound. It is concluded that the methyl ester distribution is crucial for predictions of the rheological properties of calcium pectin networks and that DBabs seems to be a valid route to assess the methyl ester distribution and predict its rheological properties. INTRODUCTION Pectin is an acidic polysaccharide that is present in the cell wall of plants. It is commercially extracted mainly from lemon peel and apple pomace. The extracted material consists of a 1,4 linked -Dgalacturonate (typically ~90%) to which neutral sugars are attached (< 10%). This part of the pectin is referred to as homogalacturonan. Homogalacturonan is a linear and relatively stiff anionic polymer, which may be esterified by methyl groups on the C-6. Pectins (or homogalacturonan) are typically characterised by their degree of methylation (DM) and the network formation capabilities of pectins are often related to this value. High methoxy pectins (DM>50%) form networks under acidic conditions and at high solid content whereas low methoxy pectins (DM<50%) form ion induced networks. Calcium induced pectin networks are formed according to the egg box model. This model suggests that gelation will proceed in two steps where the first step is a rapid formation of dimeric junction zones where calcium is chelated between the negatively charged carboxyl groups of two pectin chains. It is believed that the Relation between Rheological Properties of Pectin Gels and Pectin Fine