Rheological Investigation of Pectin Deesterified using Salt-independent Pectin Methylesterase from Citrus

During deesterification, the ester groups on the pectin from citrus peel by-products can be removed in a random or block-wise (sequential) manner. Pectins containing these blocks of unesterified groups on the chain are highly reactive to divalent cations such as calcium ion. In solution, in the presence of calcium ions, pectins demethylated by plant pectin methylesterases (PME) can exhibit unique rheological properties. One such property is that they can produce pseudoplastic solutions that are needed for suspension aids for food and non-food applications.A non-calcium sensitive pectin with a 94% degree of methylation was demethylated to a value of either 55% degree of methylation (DM) at pH 4.5 or 62% DM at pH 7.5 using monocomponent citrus salt-independent PME. Rheology measurements on these pectins indicated that to reach a maximum value storage modulus G', a cure time of 20 hours was needed. At an angular frequency of 0.01 rad.s(-1) the G', G '' (loss modulus), and tan (delta) (ratio of loss to storage modulus) values were 9.69 Pa, 0.247 Pa, and 0.0255 Pa, respectively, for the pH 4.5 sample. This indicates that the modulus is largely comprised of an elastic component that is important for suspension applications. A maximum value of storage modulus was observed at mole ratios (calcium ion to free unesterified carboxyl groups) of 1.0 and 1.2, where the G' values were 34.5 and 52.1 Pa, respectively. The decrease in G' to 0.14 Pa observed after shearing for 15 seconds signifies the need for improvement of modulus recovery after shear.