Degradation kinetics of beetroot pigment encapsulated in polymeric matrices

Kinetic studies on the degradation of water‐soluble beetroot pigment, mainly consisting of the betalain betanin, encapsulated in three different matrices (pullulan and two maltodextrin samples differing in their molecular weight) were carried out under various water activity (aw = 0.23, 0.43, 0.64, 0.75 and 0.84) and temperature (30, 40 and 50 °C) conditions. The water sorption behaviour of these materials was also examined. Degradation of the pigment was monitored by absorbance measurements at 537 nm (λmax of betanin). The highest values of the rate constants for degradation were observed at an intermediate water activity level (aw = 0.64) for all matrices and all three storage temperatures examined. An attempt to relate the degradation kinetics to the molecular mobility of the wall material was not successful. Pigment losses were observed even at temperatures below the glass transition temperature (Tg) of the polymeric matrices, although degradation was largely slowed down in the glassy state. In the vicinity of the Tg zone, where all polymers go through a glass → rubber transition, there was not a distinct change in the reaction rate, which could reflect the pronounced changes in molecular mobility of the wall material. In fact, some of the lower degradation rates were observed mostly under conditions where the matrices were fully plasticised (ie rubbery) and ‘collapsed’, implying that the degradation kinetics is not governed by factors related only to the physical state of the polymeric wall material.