Thermodegradation and thermal transitions of an exopolysaccharide produced by Pediococcus damnosus 2.6

Abstract

The thermodegradative behavior of an exopolysaccharide (EPS), produced by Pediococcus damnosus 2.6 from a strain isolated from a ropy Basque Country cider, was studied. Two endothermic processes at 118 and 163°C were detected by differential scanning calorimetry and considered as sub and main glass transitions, respectively, for the EPS. Dynamic thermogravimetry experiments showed: a first weight loss, below 100°C, due to alcohol molecules trapped in the structure of the EPS, a main degradation process involving both dehydration reactions, which yield levoglucosan structures, and scissions of C–O bonds, either in the inner glucan rings or between rings, going on with depolymerization reactions up to 350°C, and char-forming reactions at higher temperatures. From isothermal degradation processes, at short reaction time, an activation energy of 33 kJ/mol was determined. This value changed to 50.4 kJ/mol for conversion ranges of 20–35%, rising to 138 kJ/mol at 50% (determined by dynamic methods). Analyzing the whole process by McCallum's dynamic method, the energy value reaches 293 kJ/mol. Differential thermal analysis indicated at least two simultaneous reactions that are heating rate dependent and influence the whole degradation process.