31 Food & Beverage Analysis Using rotational viscometry to determine
fl ow behaviour A suitable instrument is required to determine viscosity at defi ned shear rates as well as to model the various processes food is submitted to. Such a device needs to be suffi ciently versatile to cover the wide viscosity range (e.g., from fruit juices to chocolate cream). Ideally, the instrument should be able to provide the mathematical tools for analysing the registered data, for example, for yield point calculation.
Rotational viscometers come with various measuring systems designed to suit specifi c applications. Instrument software allows the programming of several steps to simulate a complex process and includes mathematical model functions.
This can be shown by presetting a speed ramp from 120 rpm to 180 rpm and evaluating the resulting data with the mathematical model ‘Shear thinning index’. If the resulting index is greater than 1, the sample is shear thinning, while an index of approximately 1 stands for Newtonian fl ow behaviour. To eliminate temperature infl uences from variables such as unstable room temperature, a Peltier temperature device, which kept the sample at a stable 25°C, was used for a test that is reported below.
Figure 5: Rotational viscometer with Peltier temperature device with inserted double-gap measuring system.
Figure 3: Rotational viscometer ViscoQC 100 and ViscoQC 300 with standard spindles.
A majority of rotational viscometers on the market have a motor and a spiral spring. The motor turns a measuring bob immersed into the sample under test. The rotational speed is user-defi ned. To actually turn, the measuring bob has to overcome the viscous forces of the sample, which winds up the spring. The sample’s resistance is registered via the spring as torque (usually in %) and as dynamic viscosity.
In order to cover the vast viscosity range, springs of different mechanical strength and sensitivity are in use for low (L), medium or regular (R), and high (H) viscosity. This means it is important to choose a viscometer model with the correct spring for the intended application.
1- Magnets registering the defl ection of the spring (difference between motor and spindle torque)
2 - Spring 3 - Spring connection to measuring bob shaft 4 - Spring connection to motor shaft 5 - Motor
6 - Measuring bob 7 - Sample
Table 2: ‘Shear thinning index’ analysis and report table of orange juice using a rotational viscometer and double-gap measuring system.
Speed rpm
120 130 140 150 160 170 180
Shear Thinning Index Example 2: Chocolate
Chocolate – especially when it comes to coatings – demands more complex testing to ensure the desired quality. The main goal is to achieve a perfect surface where the coating is evenly distributed, without holes or lumps. The IOCCC (International Organization of Chocolate, Cocoa, and Confectionery) has developed its own method to analyse the chocolate’s fl ow behaviour at varying shear rates followed by calculation of the yield stress. The yield stress is a key parameter for successful conching and perfect coating.
The higher viscosity of chocolate requires a viscometer R- or H-model with a more robust spring. As in Example 1, a concentric cylinder system (here: CC12) was selected because the shear rate is essential for the IOCCC test. A Peltier temperature device ensures that the sample remains liquid at 40°C. In addition to the IOCCC test, the temperature device can be used for a temperature scan to determine the perfect conching temperature.
Figure 4: Spring-type rotational viscometer. Examples for fl ow behaviour analysis
Example 1: Fruit juice Fruit juices can either be clear or opaque liquids when containing fruit particles and pulp. In any case, juices are a low-viscosity application (viscosity approx. 2 mPa·s) that require a viscometer model with a highly sensitive spring. A concentric cylinder system with double-gap provides suffi cient surface for shearing in order to obtain a viscosity value. A comparatively high value (greater than 100 rpm) has to be set for the rotational speed for the same reason.
Concentric cylinder systems have standardised geometries of measuring bob and cup, which allow the calculation of the shear rate. Measuring systems with defi ned geometry are absolute systems: Viscosity values obtained with such a system are comparable to viscosity values from a different system - provided the other system also has a defi ned geometry and, in case of non-Newtonian samples, the test was performed at the same shear rate.
As long as the fruit content is not too high (i.e., the sample does not resemble a smoothie), the fl ow behaviour is Newtonian.
Figure 6: Real-time graph of the IOCCC step test.
The IOCCC test consists of four steps [6]: Step 1 serves to homogenise and control the temperature of the sample. Step 2 and 3 are an upward shearing ramp plus high shearing interval as preparation for Step 4, which is the main measuring step for analysing the yield point (Figure 6).
Dynamic Viscosity mPa·s
2.093 2.093 2.067 2.074 2.066 2.051 2.046
1.0228
Torque %
33.5 36.3 38.6 41.5 44.1 46.5 49.1
Runtime hh:mm:ss
00:01:00 00:01:00 00:01:00 00:01:00 00:01:00 00:01:00 00:01:00
Temperature °C
25.0 25.0 25.0 25.0 25.0 25.0 25.0
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