The unique sensory characteristics perceived during chocolate consumption are largely determined by the cocoa variety, growing region, and harvesting season but are predominantly developed during cocoa post-harvest processes, such as fermentation, drying, roasting, and conching. During chocolate production, numerous hours are invested, with conching representing a particularly time-consuming, energy-intensive and therefore costly step. Consequently, the conching process has been subject of previous research activities, yet it remains incompletely understood as contradictory results regarding aroma changes during conching have been reported. This thesis aimed to deepen the understanding of crucial mechanisms occurring during conching by investigating aroma and texture changes throughout dark chocolate production, with a specific focus on conching. In the initial approach, aroma-relevant changes during chocolate production were investigated by analyzing aroma-active volatiles and volatile organic compounds (VOCs) in cocoa liquor as starting material, in flakes after refining, and in chocolate after conching. A total of 63 aroma-active regions were detected using aroma extract dilution analysis and gas chromatography-mass spectrometry/ olfactometry, with their number found to decrease throughout production. VOC intensities determined by gas chromatography-ion mobility spectrometry also generally decreased during processing, indicating reduced VOC concentrations, although individual VOCs exhibited increased intensities at specific stages. Additionally, numerous highly volatile organic compounds – even if mostly not being aroma-active – were detected, offering potential as novel markers for monitoring the chocolate production. Quantitative analysis of selected aroma-active compounds during lab-scale conching revealed decreasing concentrations within the fat phase. For compounds with lower n-octanol/ water-partition coefficients and lower boiling points, more pronounced declines were determined. Simultaneously, a decreasing complex viscosity was observed with increasing conching time, correlating with an increasing proportion of free cocoa butter during conching. Conching is assigned to be a complex interplay of compound formation, volatilization, and re-distribution, which collectively shape the aroma characteristics of chocolate. Gaining insight into the mechanisms inside a conche is therefore inevitable for targeted process control. The distribution and migration of selected aroma-active compounds between chocolate constituents (cocoa butter, cocoa particles, sugar particles) were therefore examined under conching like conditions using model systems of varying composition. These included amorphous and crystalline sucrose as well as systems with varying fat contents. Compound concentrations were quantified in fat and particle phases. The diffusion activity was found to depend on both, the physico-chemical properties of the compounds and the system composition. Less polar compounds increasingly accumulated in the fat phase. Further, cocoa particles were found to be potential aroma reservoirs, releasing aroma-active compounds into the fat phase and thereby contributing to the chocolate’s sensory development. Notably, acetic acid concentrations raised in all model systems containing cocoa or crystalline sugar particles, indicating the formation of free acetic acid during conching. Finally, combining aroma and rheological characterization of plastic masses during pilot-scale conching, conducted under varying processing conditions, highlighted the need for compromises to achieve optimal flow properties and flavor of dark chocolate. It was shown that some positive outcomes, such as retention of desired aroma-active compounds and the viscosity reduction of plastic masses, were inversely related during conching at elevated temperature or in the absence of residue from previous trials (pre-charge) on the conche vessel wall. Desired compounds were found to particularly decrease during conching at elevated temperature, whereas concentrations of selected compounds mostly remained highest after conching in a clean conche without pre-charge. Likewise, conching temperature and the absence of pre-charge had the strongest influence on the rheological properties of plastic masses, resulting in lowest and highest complex viscosity, respectively. The findings presented in this thesis provide fundamental insights into different mechanisms occurring during conching, giving a basis for a deepened process understanding and potential future research directions with respect to process automation in chocolate manufacturing.
Yvonne Guckenbiehl (Thu,) studied this question.