Rheometer RHEOTEST® RN and Capillary Viscometer RHEOTEST® LK - Food and Flavoring Products
Measuring tasks in research and development
From rheological point of view food and flavoring products cover a broad assortment of goods. It extends from Newtonian fluids with very low viscosity through viscous-flow or pappy/ paste-like products to nearly solid substances. As is well known, the main tasks in research and development are development of new products and task-oriented improvement of products properties for actual assortment. Rheological properties of products play a decisive part in this connection. They are equally rather numerous based on variety of available goods. Some of the most characteristic rheological criteria are listed later and their acquisition and processing using measurement technique are presented in more detail at the following sheets:
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Regulation and evaluation of raw material quality and its properties during processing
First of all, among them are viscosity, yield point and gelatinization ability |
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Researches of product structure and its change in the course of technological process
At that, the characteristic features are, among others, yield point, stitch resistance to destruction and thixotropic properties |
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Food products development with texture that promotes consumption
(positive organoleptic perceptions during nibble, chewing and swallowing)
On the one hand, the texture quality correlates with exactly defined rheological values such as viscosity, flow limit and modulus of elasticity.
However, it is also evaluated by empiric values that are determined using qualitative measurement methods or methods of modeling.
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Measuring task in the area of quality control
Procedures of measurement and assessment set in quality standards related to an enterprise are intended for quality control of semi-products and end products. Among these are, first of all, physically exact rheological measurements. Determined with the use of rheometer rheological parameters should reflect important properties of a product under control. Also they have to provide good comparability in the whole production range of an individual producer and to be metrologically reliable for supplier and end consumer. It is possible to achieve high reproducibility and good comparability of measurement results only with the help of standard, physically exact measuring procedures. It is necessary to apply for a specific product or for a specific rheological property a suitable algorithm for results processing.
Important rheological parameters are: yield point, zero viscosity or rest viscosity, shear rate-dependent viscosity and end viscosity.
Qualitative parameters, for instance, are: starch gelatinization ability and stitch resistance of yoghurt. The measurement methods for their determination are described below and typical measurement results are presented graphically. Encountered still today, first of all at small enterprises, use of empiric methods and simulation methods that are not based on physical grounds results in very limited comparability of measurement results.
Possible measurement procedures:
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Measurement procedure to characterize starch products gelatinization ability |
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Controlled Rate Tests - CR-Tests |
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Measurement procedure to characterize pappy vegetables and fruits products (apple pulp, ketchup) |
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Controlled Rate Tests - CR-Tests |
| Linear ramps with controlled shear rate in forward and backward direction to research shear rate-dependent and time-dependent flow properties (thixotropy).
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Example: see Figure 2
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Controlled Stress Tests - CS-Tests |
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Measurement procedure to determine rheologic properties of chocolate |
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Controlled Rate Tests - CR-Tests |
| Equilibrium flow curves at controlled shear rate in gradient ranges: 5 ... 60 s-1 according to OICCC-standard
and 0.1 ... 200 s-1 according to Tscheuschner
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Example: see Figure 4 |
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Measurement procedure to determine rheologic properties of yoghurt and cream |
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Controlled Rate Tests - CR-Tests |
| Linear ramps with controlled shear rate in forward and backward direction to research shear rate-dependent and time-dependent flow properties (thixotropy).
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Example: see Figure 5 |
| | Controlled Stress Tests - CS-Tests |
| Linear ramps with controlled shear stress to determine resistance to destruction
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Example: see Figure 6 |
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Exact viscosity measurements of fluids with low viscosity (e.g., wort, beer, milk) |
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One-point measurements with the help of patented capillary viscometer RHEOTEST® LK. |
| 25 ml of investigated material is automatically sucked in capillary made of high quality steel and returns back into measuring cup after viscosity determination. (Result of measurements in 25 s).
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Example: beer/wort and milk |
Determination of starch gelatinization properties
Determination of starch gelatinization properties of flour puts forward particular requirements to rheometry since viscosity value could increase more than 3 exponents of a number. Moreover, gelatinization process depends strongly against temperature and time. To measure gelatinization curves, flour-water suspension with a special sensor is "mixed" at a constant number of revolutions, it is heated and again cooled in a certain way at the same time. Torque is measured. The viscosity is calculated by the microprocessor of the rheometer and is displayed at PC screen. This measuring procedure is used, for instance, for pure, native or modified starches.
Figure 1: Viscosity change at a definite temperature-time profile
Notes:
Viscosity and temperature in maximum of gelatinization are in close dependence on quality of rye and yeast properties. Thus, for example, high viscosity values at high temperatures are characteristic in case of ferments lack.
Instrument configuration
Rheometer RHEOTEST® RN with stand and software for CR-tests and thermostat control
Special measuring system consists of temperature-controlled measuring cup, starch mixer and temperature sensor
Cryostat K8-0 E10
Personal computer with accessories and printer
Determination of flow properties of pappy vegetables and fruits products
Pappy vegetables and fruits products are suspensions of high concentration consist of cell liquid, crushed solid phase and impurities. One increases part of fluid by addition of water or decreases it by concentration depending on purpose of processing. Suspensions with high concentration are non-Newtonian fluids. Effects of structure and orientation change become apparent in investigated material at very high shear rates. Moreover, one could observe destruction of solid particles in investigated material relatively frequent. Apple pulp and ketchup show plastic flow properties with clear yield point and with measurement results that are strongly time-dependent. Good reproducibility of measurement results requires measuring values correction using software during which allowances are inserted for wall effects of shear rate change in clearance stipulated by product.
Figure 2: Shear rate ramps in forward and backward direction
Figure 3: Slow shear stress ramps to measure yield point
Notes:
Rheological properties, especially yield point and viscosity, depend on temperature, solid substances concentration, dissolved solids content in liquid phase (sugar, pectin), of particles form and of solid particle-size distribution, of hardness and deformation ability of solid particles, of interaction and properties of outer surface.
Instrument configuration
Rheometer RHEOTEST® RN with stand and software for CR-/ CS-tests and thermostat control
DIN cylinder measuring system S1, H1, H2 with measuring cup and temperature control vessel
Cryostat K8-0 E10
Personal computer with accessories and printer
Chocolate mass quality assessment with the help of flow curves by OICCC standard and/or in wide range of shear gradient (according to Tscheuschner) (Figure 4)
Chocolate is a multiphase suspension of cacao butter with disperse solid substances of cacao, sugar, powdered milk. Along with concentration and granulation composition of solid substances, humidity and lecithin content also have a profound effect on rheological properties. Rheological properties are characterized by the yield point and viscosity decrease to the equilibrium value of viscosity at high shear rates. In the actual praxis, one carries out measurement of equilibrium flow curves at controlled shear rate according to OICCC standard or Tscheuschner depending on interesting parameter.
Foreseeing in OICCC standard restriction of shear rate (5 ... 60 s-1) assures that at calculation of model's parameters hCA and tCA by CASSON equation the error limit of 3% will not be exceeded. Shortcoming is in the fact that one could not achieve concordance of hCA with equilibrium viscosity h¥ as well as tCA with yield point. It means that actual yield point t0 is frequently only at 25% of model's parameter tCA .
Researches of Tscheuschner made it clear that yield point by chocolate shear is in the range of shear rates 0.08 ... 0.12 s-1.
It follows that shear stress measured at shear rate of 0.1 s-1 is defined as yield point t0 .
Equilibrium viscosity h¥ (viscosity at shear rate® ¥) is nearly attained at shear rate of
200 s-1. The whole coarse of flow function is described by the structure component of
viscosity at shear rate of 1 s-1 and by the exponent of structure destruction kinetics.
Comment:
Measurement with controlled stress (CS-Test) is necessary to measure the yield point t0 . In this case shear stress rises slowly.
Value of shear stress at which the flow of sample begins (shear rate > 0 s-1) is the yield point t0.
Notes:
Structure conditions of solid components exert significant influence on rheological properties of chocolate mass. These structure conditions change during the conche process (considerable shear forces at temperature 60 ... 100 °C) and stabilize again by addition of emulsifier. Maintenance of the correct conditions in the conche process is controlled generally by way of flow curves measurements. It could be fulfilled in laboratory as well as quasi-continuously in the conche, if one uses for that the working place developed by us especially for quasi-continuous control of chocolate mass.
Instrument configuration:
Rheometer RHEOTEST® RN with stand and software for CR-/ CS-tests and thermostat control
DIN cylinder measuring system S1 with suited flow-through sensor and temperature control vessel
Cryostat K8-0 E10
Personal computer with accessories and printer
Check test of flow properties of yoghurt and cream
Taking into account rheological flows, one could segregate yoghurts at stitch resistant yoghurts (their fermentation takes place inside packaging and they preserve theirs gel structure till consumption) and yoghurts obtained by mixing (fermentation takes place in big vessels; pseudo-plastic fluid forms by subsequent mixing of gel). From rheological point of view, one could compare cream with stitch resistant yoghurt.
Figure 5: Shear rate ramps in forward and backward direction
Figure 6: Slow ramp of shear stress to determine stitch resistance of stitch resistant yoghurt and cream
Notes:
Technological parameters of the fermentation process have a decisive effect on the end products quality. It is reflected in the stitch resistance. For yoghurt obtained by mixing, the flow properties must be set in such a way that they are acceptable for further processing as well as for consumer.
Instrument configuration:
Rheometer RHEOTEST® RN with stand and software for CR-/ CS-tests and thermostat control
DIN cylinder measuring system S1 with measuring cup and temperature control vessel
Special rotor with cruciform paddles, optional with temperature-controlled measuring cup
Cryostat K8-0 E10
Personal computer with accessories and printer
Exact viscosity measurements of fluids with low viscosity such as beer and wort
Viscosity is an important qualitative value for assessment of malt solutions. It depends on content and type of dissolved substances. One could influence on content and type of dissolved substances in method of mash preparation by means of changing temperature and time of the process.
Low viscosity of wort is decisive for high velocity of clarification and rapid filtration. Substances that rise viscosity, such as b -glucan and pentosan have a significant effect on foam stability and gustatory sense of beer.
Advantages of RHEOTEST® LK for quality maintenance laboratories:
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The measuring values of viscosity, temperature and the temperature compensated viscosity are output to display, printer or PC after 25 seconds only |
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Very simple control using 4 keys or through PC |
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Electronic viscosity-temperature-compensation: that means the viscosity result at measured temperature is calculated and displayed automatically at your wished standard temperature (e.g. 20 °C). One only have to insert in software menue a calculated temperature coefficient. |
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Measuring system of high quality steel could not be damaged at normal conditions |
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Simple maintenance, calibration and cleaning |
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RHEOTEST® LK could be purchased as automatic, PC-controlled working place for viscosity measurement with sampler for 20 samples |
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Commercial offer includes two versions of viscometer: |
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Main instrument without jacket for temperature control (for measurement conditions at ambient temperature) |
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Main instrument with jacket for temperature control (for measurement conditions beyond the ambient temperature) |
Instrument configuration:
Capillary viscometer RHEOTEST® LK without jacket for temperature control (optional with sampler for 20 samples) with special capillary 6 for beer and wort (viscosity range 1 ... 3 mPas)
Exact viscosity measurements of fluids with low viscosity (for example, milk)
Milk viscosity is a complex property that is especially effected by emulsified and dissolved colloid particles.
Fat and casein content has the most strong influence on milk viscosity. Besides that, viscosity depends on technological parameters also.
Some examples concerning viscosity:
| - condensed milk without sugar (of German producers): |
about 5 to 35 mPas at 20°C |
| - condensed milk with sugar (of German producers): |
about 6000 mPas at 20°C |
| - yoghurt (of South-East Asia producers): |
about 150 mPas at 40°C |
Instrument configuration:
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Main instrument RHEOTEST® LK without jacket for temperature control (optional with sampler for 20 samples) with standard capillary 1 for condensed milk without sugar (viscosity range about 1 ... 20 mPas) or with special capillary 6 (viscosity range about 3 ... 50 mPas) |
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Main instrument RHEOTEST® LK 2.2 without jacket for temperature control with standard capillary 5 for condensed milk with sugar
(viscosity range about 500 ... 10,000 mPas). |
