The influence of the storage conditions heat and humidity on conformation, state transitions and degradation behaviour of dried pectins

For testing the influence of long-term storage on pectins, differently prepared samples were incubated under defined temperature and relative humidity in a climate chamber. This caused pectin demethoxylation, depolymerisation and non-enzymatic browning and influenced the dissolution behaviour and pH. The extent of chemical changes and the reaction mechanisms depended on incubation conditions, type and "history" of the pectins and on their conformation (C-4(1) or C-1(4) chair). Higher temperature and higher humidity increased demethoxylation and browning. Depolymerisation was high even at low degree of methoxylation. Therefore, an additional mechanism besides beta-elimination and glycosidic linkage cleavage was suggested. The pectin conformation was found to be crucial for all results: demethoxylation was favoured by an equatorial position of the methoxycarbonyl group of the C-4(1) conformation. Transitions to the C-1(4) conformation happened during pectin incubation in the climate chamber and were detected as shrinkage of endothermic pre-peaks in DSC signals of thermal analysis. The conformation transition and the C-1(4) conformation were favourable for pectin degradation during thermal treatment because they fulfil the stereo-electronic requirements for certain degradation reactions. The thermal analysis peaks of the most incubated materials were shifted to the left because of pectin demethoxylation. The pectin conformation influenced also the extent and velocity of state transitions to a more rubbery state by water adsorption during incubation and to a more fluid state after melting of the crystalline form of the C-1(4) conformation. In the rubbery or fluid state all pectin changes were accelerated. (C) 2008 Elsevier Ltd. All rights reserved.