created by the air velocity from the blower. A lower enrobing viscosity, (at high shear viscosity) ensures a thinner chocolate enrobing layer while a higher enrobing viscosity achieves a thicker chocolate enrobing layer. The low shear viscosity is going to inform if the enrobing chocolate is going to develop feet, when travelling down the cooling tunnel. It can predict if a rippled chocolate decoration formed during blowing will stay put. If the low shear viscosity is above a certain critical minimum level, the top layer chocolate will stand up keeping it’s original rippled “blowing” shape. Flow curve measurements Two evaluation methods provide this additional information and allow for a better understanding of practical production situations – flow curve and three-step shear rate measurements. Flow curve measurements are a form of viscometer programming done using a viscometer with a rotational spindle and stationary cup to create a well defined chocolate shear rate. When rotating the spindle, hereby making the chocolate flow, the viscometer can detect the force required to create the flow. By selecting a series of different shear rates, the flow curve covers different production related chocolate speeds – slow, medium and fast. Different instruments have different detailed ways of doing this but all imitate a number of different critical chocolate production steps. The force (in Pa or dyn/cm2) required to create the chocolate flow is displayed against the shear rate (speed of the chocolate [1/s] or the spindle speed: rpm). The Casson yield value is the force required to start flow of chocolate and the Casson plastic viscosity is the force required to maintain a constant flow in chocolate, also referred to as Figure 3: A medium shear rate covers the Swiss roll in a relative thick layer of chocolate in the double chocolate curtain of an enrober. Figure 4: 2 flow curves with the same “flow” at shear rate 11.5, but very different yield values and plastic viscosity. 200 150 p on la c sti co vis y sit (sl op e) Shear stress ss Ca 100 s Cas on p isco ic v last sity (slo pe) 50 Casson yield values 0 Casson yield value Shear rate “shear thickening factor”. The two flow curves shown in Figure 4 (above) determine differences between the chocolates: The green curve chocolate has a higher Casson “yield value” than the blue (force required to start flow). The blue curve chocolate has a higher Casson “plastic vis- cosity” (curve steepness), than the green curve chocolate. The green curve chocolate is going to coat with the thinnest layer of chocolate, due to the lowest enrobing viscosity/plastic viscosity. The blue curve chocolate is going to be the most liquid chocolate in difficult tablet moulding, due to Palsgaard Technical Paper - January 2012 The importance of expanded rheology information and emulsifier functionality in chocolate production 4
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