Analysis of the cocoa roasting process (Theobroma cacao L)

Abstract

En esta tesis doctoral se analizó el proceso de torrefacción de partículas de cacao, se estudió la cinética de formación y volatilización de compuestos de bajo peso molecular, el efecto del tamaño de partícula, velocidad de calentamiento y tipo de gas portador (inerte u oxidante), así como el modelamiento del proceso en condiciones no isotérmicas. La investigación se llevó a cabo en la Universidad Nacional de Colombia y en la "Rijksuniversiteit Groningen" en Los Paises bajos. La experimentación se realizó en TGA y un microrreactor (PTV-GC-MS) con pequeñas partículas de cacao, identificando la composición química de los gases liberados durante el calentamiento. De esta forma, fue posible explicar el proceso de degradación térmica mediante un esquema cinético de tres etapas claramente definido.

In this research, the effect of infrared roasting at 100, 150 and 200 °C on the main physical and chemical changes of cocoa nibs were identified. By MDSC, the fusion of fat at around 35 °C, desolvation of water and volatile compounds were found, and obstruction of porosity was determined by BET approach. Additionally, a decrease in reducing sugars with increasing pyrazines content due to Maillard reaction was identified. Through experiments with small cocoa powder samples under non-isothermal conditions, high differences were found in the volatilization in the air compared with nitrogen. Smaller particles have a greater slope of weight loss due to their Bigger specific surface area available for oxygen interaction compared to the area of the large ones, facilitating reactions such as lipid oxidation during heating. For a specific kinetic analysis, the DTG curves were filtered by Fourier deconvolution (6 peaks for N2 and 7 peaks for the air), and the kinetic parameters were determined using the global Friedman model (the entire curve) and for each peak resulting from the deconvolution. In addition, the chemical composition of these peaks was identified by PTV-GC-MS and similarly, the aromatic components remaining in the solid after the heating by SPME-GC-MS. It was found that at temperatures starting from 150 °C, a high number of compounds are generated which are volatilized to the gas phase when higher temperatures are reached. Consequently, a higher number of species was identified in the gas phase. The types of components identified in both the gas and solid phases were acids, alcohols, aldehydes, pyrazines, esters, ketones, alkanes, alkaloids, phenol, pyrroles, and pyridines. All these components are important for the quality of aroma and flavour of cocoa, especially pyrazines, aldehydes, and ketones.

Finally, a phenomenological model able to predict the behaviour of a small cocoa particle using the PTV injector as a u-reactor was obtained. This methodology and model were able to explain the effect of the heating rate on the presence of thermal differences, pressures, and the gas outlet rate, as well as the identification of aromatic compounds volatilized at different temperatures, making it an effective tool to analyze with more depth the production of volatiles in thermochemical and kinetic processes of complex biomass that are generally carried out in TGA.