An experimental and theoretical investigation of chocolate particle size reduction by multi-roll milling

Legarreta Basabe, Xabier (2018). An experimental and theoretical investigation of chocolate particle size reduction by multi-roll milling. University of Birmingham. Eng.D.

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Abstract

Two-stage roll milling of chocolate pastes was studied to investigate the relationships between roll speeds and pressing force with mass processing rate, product particle size and roll torque. A three-roll mill was used, operating in two and three-roll mode for each stage, respectively. A strong correlation (\(R\)2=0.97) was found between the size of the largest particles and the surface coverage of the discharge roll. No evidence was found of shear rate affecting the size of the largest particles (\(p\)=0.13). Negative torque values were measured on the slower rolls under higher forces and/or speed ratios. The minimum specific mechanical energy input (\(S\)\(M\)\(E\)\(I\)) was observed to occur for the lowest pressing force that still resulted in satisfactory material transfer. Theoretical models based on the lubrication approximation (LAT), with and without pressure-dependent viscosity, resulted in lower apparent viscosities found at faster roll speeds, higher speed ratios and/or greater forces. The observed tendency for the material to detach from the slow roll suggests that higher shearing action in the slow boundary causes a greater reduction in paste-roll adhesion, provided that material cohesion exceeds adhesion. A comparison with full 2D FEM solutions revealed differences no greater than 0.02% in the pressure and velocity profiles.

Type of Work:

Thesis (Doctorates > Eng.D.)

Award Type:

Doctorates > Eng.D.

Supervisor(s):