Rheology of chocolate. Rheological studies of chocolate in relation to their flow and mixing properties during manufacture.

摘要

An investigation has been carried out into the rheology ofudchocolate in relation to its flow and mixing features in a realudindustrial environment. The chocolate manufacturing plant ofudRowntree at York provided a base for this study.udThe project aims were:uda) to measure the viscous and time dependent properties ofudchocolate.udb) to explain the observed flow properties in relation to theudconstituents of chocolate.udc) to determine the shear rate which, for a given recipe,udyields a minimum stable viscosity (of particular commercialudvalue).udd) to assess the type of mixer able to provide this duty.udThe experimental work involved rheological studies with concentricudcylinder and tubular viscometers, operated to measure viscosity asuda function of shear rate and shearing time.udThe chocolate samples studied were taken from various points in theudmanufacture process at Rowntree, York. Model chocolate systems wereudmade from cocoa liquor, and sugar with cocoa butter, which wereudstudied to underpin the basic mechanisms of the flow properties ofudthe total chocolate.udShear thinning in milk chocolate has been shown to be accounted forudby surface coating and fat release from the cocoa cellularudmaterial.udAnalysis of the sugar and cocoa butter system gave large hysteresisudloops which may be explained as due to agglomeration of the sugarudparticles. The level of hysteresis was found to be related to theudpolarity of the liquid phase, such that a more polar fluid resultsudin less hysteresis.udLaboratory experiments have revealed that the level of work inputudto give permanent viscosity reduction for milk chocolate isuddependent on the measuring shear rate. The level of optimum shearudinput for the measuring range 10 to 130 secud1 is 645 sec for 30udminutes. The apparent viscosity measured at lower shear ratesudrequires much longer ([approx]100 minutes).