A study of the isomerisation of aconitic acid with reference to sugarcane processing.

Abstract

Chromatographic methods were used to determine the absolute values of cis-aconitic acid and trans-aconitic acid present in sugar cane factory processing streams and to determine the rates of isomerisation from the trans-aconitic acid to the cis-aconitic acid isomer. A reproducible, solid phase, ion exchange extraction method was developed to isolate the organic acids found in sugar factory process streams. The isolated acids were quantified using a dual ion-exclusion column, high performance liquid chromatographic method. To improve resolution of the acids, columns were maintained at different temperatures, whilst the combined use of both ultra-violet (UV) and refractive index detection proved useful in peak identification. Concentrations of the aconitic acid isomers were used to calculate the cis/trans aconitic acid isomer ratio occurring across the different processes found in a sugar cane factory. trans-Aconitic acid was found to be the predominant form present in the cane entering the factory. Analysis showed that isomerisation of the trans-aconitic acid to the cis-aconitic acid isomer occurred during processing. To understand and model this reaction, a reproducible experimental isomerisation method was developed making use of buffers to maintain pH conditions during experiments. A chromatographic analysis method, using ion-exclusion chromatography and UV detection, was developed to analyse the isomerisation reaction mixture. Chromatography was used in both an on-line and off-line mode for quantitation of the isomers. The method was used to study the isomerisation under conditions similar to those found in the factory. These included pH, temperature, ionic strength and the presence of monovalent and divalent cations found in sugar cane juices. It was shown that the isomerisation is a first order reversible reaction under the conditions studied. Temperature and pH were shown to be the important isomerisation variables. Temperature enhances the rate of isomerisation of the trans-aconitic isomer to the cis-aconitic acid isomer whilst pH affects the ultimate cis/trans aconitic acid ratio attained. Ionic strength was found to be a relatively unimportant factor. The presence of divalent and monovalent cations, at concentrations usually found in cane juice, was shown to have little effect on the rate of isomerisation. Activation parameters, including the activation energy (Ea), pre-exponential factor (log A), enthalpy (?H‡) and entropy of activation (?S‡), were calculated at each combination of buffer concentration and pH used in the experimental procedures. The values recorded are of a similar value to those reported for structurally similar compounds. cis-Aconitic acid was shown to undergo decarboxylation to itaconic acid. This occurred at low pH values and high temperatures. A detailed study was not undertaken since the conditions under which it occurs are considered extreme from the viewpoint of a sugar technologist. A model describing the equilibrium cis/trans aconitic acid isomer ratio was developed as a function of pH, temperature and time from the kinetic results. This was used to predict the equilibrium ratio for the aconitic acid isomers at the output of various processes in the sugar factory. Given the time, average pH and temperature the model can successfully predict the equilibrium ratio for the relevant process stream.