Viscosity Testing’s Role in Maintaining High-Quality Food Production
By Mary Vessele, Global Product Marketing Manager, Test & Measurement, at Cole-Parmer
Many factors can impact the quality of foods being manufactured. During the production process, special attention is given to measuring and tracking viscosity, as it is an important indicator of quality.
Viscosity describes a fluids internal resistance to flow. A fluid with a higher viscosity would pour slower and seem thicker than a fluid with less viscosity. When there is a change in a materials property such as molecular weight and density, both of which affect how a liquid flow, the viscosity changes and the quality is altered.
The texture of food is dramatically impacted by changes in viscosity. The chocolate industry is particularly dependent on viscosity testing, as the texture and flow of chocolate can be complex to manufacture and maintain. The thickness of chocolate, when it is in a melted state, will impact the finished solid-state. To achieve the correct density, texture and taste of chocolate, when it becomes solid, careful consideration to the viscosity of the product when it is a liquid is vital.
In the baking industry, viscosity plays a very important role. It will impact the chance of phase separation, affecting the products made from batter and dough. Pinpointing and maintaining an exact viscosity when baking or working with dough ensures phase separation happens at the appropriate time during the baking process, affecting the quality of the baked goods. If a cake batter has an inappropriate low level of viscosity the phases will separate easily at the wrong time resulting in a poor-quality cake. Viscosity testing is a tool used in the production process of just about every type of food and beverage. It is a very important test that offers repeatable and reliable results ensuring that the quality of the product is not only achieved but maintained from batch to batch.
The most common way to measure and track viscosity is with a viscometer. Today’s viscometers are very user friendly and functional thanks to the Rotational Viscometer technology. These viscometers measure a large range of viscosity from 200-106000000 with an accuracy range of ±1% to ±0.2%. The measurement of a product viscosity is based upon immersing a specifically selected spindle within a sample of product. As the spindle rotates at a specific speed, vigorously stirring to create a vortex, the torque required to rotate the spindle at a set speed is measured and is based upon the amount of drag created from the spindle moving in the liquid. Resistance to rotation is sensed by the instrument and the measured torque value is mathematically converted into a viscosity value in scientific units called centipoise (cP) or milliPascal-seconds (mPa.s). Water is the reference material that establishes the benchmark value of 1 cP for its viscosity. All other materials are subsequently measured for viscosity in comparison to water. The rotational viscometer device will record and store up to 10,000 data points. With the software provided, instantaneous viewing of flow curves, plotting up to four data sets at the same time, and graphical presentation of data can be achieved. These results can be downloaded to an excel spread sheet and the user has infinite reporting possibilities. It is very important to track batches to ensure they contain the same viscosity level throughout production to achieve optimal quality.
The flow resistance increases with the speed and size of the spindle. Different samples and materials require different types of spindles depending on how much of a sample is available, the torque range used, temperature needs, and material consistency. Thanks to the complete flexibility and configurability of today’s devices users can interchange different spindles depending on their needs. For thick more viscous samples a parallel plate or cone spindle is used. For thinner, less viscous pourable liquids a concentric cylinder spindle would be appropriate. For gel like material a T-bar is an option. This is a relatively inexpensive process and many sellers of viscosity equipment offer convenient bundles in which the device, spindles, cleaning materials can all be purchased together at a discounted price.
When measuring viscosity temperature becomes a very important player in the game because the sample will have a different viscosity measurement depending on the temperature it is at. For example, honey tested at room temperature may have a viscosity measurement much different then honey that has been cooled in the refrigerator or honey that has been heated. The test conditions to measure viscosity should be tightly controlled because temperature highly effects the results. To achieve accurate viscosity measurements, the food sample should be controlled within -05 C. To help the user maintain accurate temperatures of their samples many viscometers have built in RTD temperature probes and will display the temperature on the unit.
The speed of the rotation and the amount of time the spindle is rotating are other considerations when performing viscometer testing, as samples are impacted by both. The spindle should rotate at least five turns before a value can be taken. Over and under range alarm will sound if your sample is too high or too low for the spindle selected. The amount of time the spindle rotates should be dictated by a defined period of time. Some fluids are more sensitive than others to the amount of time they are sheared which will result in a change in viscosity. Today’s viscometers allow the user to set the speed up to 250 rpm and offer 2600 speed selections. This allows the user complete flexibility for any sample. The display on the device will provide readouts of speed, spindle, viscosity, % of torque, shear rate, and shear stress. When the sample density is entered, the device will even calculate and display kinematic viscosity as well. The device will calculate the shear rate (rotation speed of spindle), measure the shear stress (the torque needed to rotate the spindle) and plot the points on a graph. The slope of that line is the viscosity at a specified temperature. Thanks to the networking capabilities of devices today, all of the information provided on the device monitor can be downloaded to a PC and used in an excel spread sheet. Tracking the viscosity of a sample and being able to produce a detailed report is essential to ensuring the best quality of a product is being produced.
It is very easy to see that viscometers play an important role in the food industry. Viscosity is measured either in process or at the end of processing to qualify the end product for acceptability and quality standards. Many times, samples are selected off the production floor and brought into the QC lab for testing to ensure the quality is maintain throughout the entire process.
Check out this Viscosity Testing Kit for Food from Cole-Parmer
In addition to the food industry, many other industries use viscosity testing to ensure quality is kept intact. The adhesives, chemical, biofuels, paints, petroleum production and pharmaceutical industries use the same rotational viscometers as the food industry. It is clear that rotational viscometers fulfill many needs because they are so versatile, precise, user friendly and produce consistent repeatable results.