After the introduction of the OICCC standard recently, Rheological tests are a must in the industry for quality assurance of the chocolate.
They include measurements at different shear rates up to 60s-1 to be performed after a defined pre-treatment of the chocolate.
With the computerised rheometers available today the measuring conditions and procedures can be preset so that most of them can be carried out automatically by the push of a button.
This will reduce the time effort for routine tests drastically and the saved work capacity can be used otherwise.
However, even when working accurately and, on the first glance, with identical pre-treatment of the samples the results can differ considerably especially in comparison with tests performed by someone else and with different instruments.
Some of these measurement-related aspects should be introduced and explained. However, the dependence of the flow properties of chocolate on their composition will not be discussed here.
From the Rheological point of view chocolate is not an easy fluid: it is a suspension of cocoa, sugar and solid milk particles in a continuous fat phase.
The mix of the substances and their interactions are strongly dependent on production and pre-treatment.
The yield point of chocolate e.g. increases with longer tempering time since the cocoa butter is already partly crystallising (i.e. physical networks are forming which have to be broken off before a possible shearing).
In order to obtain comparable results special attention has to be paid to the correct reproducibility of the pretreatment of the sample (there are tolerances even within the OICCC standard which can have their effects on the results, e.g. the cooldown/ tempering times when filling the chocolate into the rheometer).
The differences occurring for thixotropic chocolate are especially dramatic: Here, the viscosity depends on the stirring time (shearing time).
However, allowing sufficient time for preshearing can eliminate errors (the necessary time can be determined by test with a constant shear rate).
Two successively recorded flow curves of a sample of a nougat cream available on the market (measuring time 3 min each) which also had a different history is shown.
A slight decrease of the viscosity can be detected which can be the result of the small thixotropy but also of sedimentation.
The range of computerised rheometers is available from Rheology Solutions .