Bostwick Limitations Rotational Technology
Rotational Methodology
Testing Tomato Sauce Conclusion
For years, the accepted test methodology for measuring the viscosity of tomato-based products has utilized the Bostwick Consistometera reliable, easy-to-use instrument, but one with limited capabilities.
The Bostwick Consistometer consists of a level, stainless-steel trough with two compartments. The first compartment, which holds the sample to be tested, is separated from the second compartment by a spring-loaded gate. The second compartment is 24 cm long and has graduated, parallel lines at 0.5 cm intervals. The measurement consists of releasing the gate and letting the fluid flow freely under gravity down the slope. The distance the fluid flows from the gate after 30 seconds is measured in centimeters as the Bostwick Consistometer reading.
The Bostwick Consistometer presents two problems. First, the instrument is limited in the range of products it can test. For example, it is not suitable for high-solids tomato paste because the paste does not flow far enough in 30 seconds to give measurable differences between samples. Secondlyand more importantlythe results from this instrument cannot be converted into fundamental rheological parameters because surface tension, wetting power and factors other than viscosity may also be involved. There is also no way to control sample temperature.
These limitations do not come into play when using a rotational viscometer. Typically, a highly accurate viscosity profile of tomato sauce can be obtained using a Brookfield rotational viscometer in conjunction with a small sample adapter and a circulating water bath to maintain the appropriate temperature. The chamber in the small sample adapter requires only 11 mL of product, making it more suitable for rapid temperature equilabration. The rotational viscometer measures viscosity by determining the viscous resistance of the fluid. This measurement is obtained by immersing a spindle into the test fluid. The viscometer measures the additional torque required for the spindle to overcome viscous resistance and regain constant speed. This value is then converted to centipoise and displayed on the instrument's LCD readout.
A sample of tomato sauce was analyzed to determine the product's viscosity profile. The test was conducted at a temperature of 25°C. An up/down speed ramp was performed from 10 to 100 RPM, giving a viscosity range of from 3,800 to 632.5 cP, over shear rates from 3.4 to 34.0 reciprocal seconds. The test data obtained for tomato sauce shows that this product exhibits a marked shear thinning viscosity profile over the test conditions. Rotational viscometers are a necessity for any contemporary high-tech food laboratory.
