Kinetic Model of Pectin Demethylation

−− Pectins are polysaccharides that act as a cellular binder in the peel of many different fruits and vegetables. An important feature of pectins is the esterification of the galacturonic acid residues with methanol. The degree of methylation is defined as the number of moles of methanol per 100 moles of galacturonic acid. The objective of the present work was to study the acid demethylation of apple pectin at different temperatures, by following changes in the degree of methylation. A solution of high methoxyl apple pectin was demethylated with concentrated HCl (pH = 0.5). The processes were carried out for 120 min at T = 80°C and 400 min at T = 65°C and T = 50°C. Anhydrogalacturonic acid content, degree of methylation and intrinsic viscosity as a function of reaction time were determined. Results showed pectin demethylation followed a second order kinetics and demethylation rate increased exponentially with temperature. Results also showed pectin “purity”, as anhydrogalacturonic acid content, increased with demethylation. However, intrinsic viscosity reduced with reaction time. This behavior was associated with the hydrolysis of non-polyuronic material during the acid treatment, lowering in that way the average molecular weight. Keywords−− Pectin, Demethylation, Kinetic model, Bio-polymer, Low methoxyl.