Siegfried Bolenz

Impact of fat content during grinding on particle size distribution and flow properties of milk chocolate

Consumer acceptance depends on taste and mouth feeling. Processing can generate varying particle size distributions (PSD), which again influence flow parameters. Chocolate can be produced by roller refining and conching or alternatively by ball milling. Fat content during milling is an important parameter of both processes. The aim of this project was to elucidate relationship between this processing parameter, PSD and flow properties, so they can be controlled. Milk chocolate samples were produced in pilot scale and measured. The results showed that varying fat contents influences PSD for both processing types. Higher fat contents resulted in bimodal distributions for roller refining. Small particles fill voids between big ones, improve packing density and liberate fat for flowing. Nevertheless, fat content during refining should not be too high either, as downstream conching requires low initial content. Pre-dried raw materials were used for ball milling, since this process offers few possibilities to evaporate water and undesired volatiles, unlike conventional conching. Samples showed narrower, multi-modal PSDs. The lack of fine particles decreases packing density and results in higher viscosity at medium and high shear rates. Viscosity at low shear can be less than for roller refined products, which is a consequence of reduced specific surface. Unlike roller refiners, the PSDs became broader at lower fat contents. Almost bimodal PSDs were achieved in lab scale at very low fat contents. Under industrial conditions, the mass must be pumpable for milling. This is a restriction for influencing properties via PSD and needs further optimization.

Improvement of process parameters and evaluation of milk chocolates made by the new coarse conching process

Chocolate mass is produced by roller refining and downstream conching or alternatively by ball milling. The latter requires high fat contents; water and off-flavours cannot be volatilized as in classical dry conching. To overcome this, coarse conching is performed first, applying the high shear vortex chamber built into the kneading zone. It does not only intensifies energy and mass transfer, but also reduces particle size of crystal sugar. Finally, grinding is done downstream. Only some examples from the extensive initial development phase are shown, e.g. on the behaviour of skim and whole milk powder. During process optimization, three variables were combined with two full factorial experimental designs carried out using differently scaled ball mills. The resulting flow properties of the samples were combined in model equations. All results showed evidence that during grinding, the lecithin must be added as late as possible in order to achieve low viscosities over the entire flow curve. Using smaller grinding balls resulted in higher yield values, but lower viscosities at higher shear. Intensive extra shear after industrial-scale milling had little impact, but did resulted in lower viscosities at high shear after using the pilot-scale mill. After finding optimized settings, confirmation trials were run allowing reduced energy consumption by the vortex chamber. Evaluation of sensory perception showed cocoa taste to be more intense in samples from the coarse conching process when compared to the conventional process. Finally, the results allowed line set-ups for small scale as well as for continuous large-scale production.