If two different people measure the viscosity of the same chocolate and they uncritically just measure the viscosity at 40ºC (104ºF), they will come up with two different chocolate viscosity values, depending on their chosen shear rates. Milk, or other liquids, would have come back with identical viscosity results. Hence, measuring single shear rate viscosity in chocolate does not provide sufficient information.
Many producers have used an elderly method when evaluating chocolate flow properties for a long time. This single-speed evaluation method was originally created so that everyone with even a simple viscometer could use it. However, there is a relation between production type and chocolate flow, hence more shear rates are needed to describe the link between chocolate shear rate and the measured viscosity. During chocolate moulding, the chocolate speed is low (0.1-5 [1/S]) while the chocolate moves relative fast at the blowing stage of enrobing (>25 -30[1/S]).
Simple instruments and measuring methods all look at limited numbers of shear rates, often only one. Using the single-speed method, it is possible to obtain good pumping information of the chocolate but it doesn’t give any indication of moulding performance, enrobing layer thickness or feet forming. There is no proper viscosity overview when relating measurements to practical production situations such as moulding, shell moulding and enrobing.
Out of the depositing and vibration aspects of chocolate moulding, vibration has the most crucial impact on the final product. During depositing, the chocolate moves relatively fast, while vibration makes the chocolate move quite slowly and spread out into all impressions of the mould. To properly evaluate a moulding chocolate you need to imitate this slow movement by selecting a fairly low shear rate. This low shear rate viscosity is very important and a vital point during tablet production - the lower viscosity, the better flow.
You require a rheological method that, by making one set of measurements, is going to provide sufficient information to define the type of critical chocolate speed that is needed to link to shell thickness formation during shell moulding. It also needs to imply feet forming properties, chocolate ability to flow into a specific mould design, chocolate depositing and chip forming as well as air release.
As an example, it is possible to demonstrate how two milk chocolates containing 29.8% and 34.8% total fat can still work in very different ways during moulding and enrobing production, despite having an identical single-speed viscosity at shear rate 7.0 [1/S].