Many factors can impact the quality of manufactured foods. During the production process, special attention is given to measuring and tracking viscosity as it is an important quality indicator.
Viscosity describes a fluid’s 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 material’s property such as molecular weight and density, this affects how a liquid flows and how the viscosity changes, and the quality is altered.
How viscosity affects food quality
The texture of food is dramatically impacted by changes in viscosity. The chocolate industry is dependent on viscosity testing as the texture and flow of chocolate can be complex to manufacture and maintain. For example, 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 as 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 inappropriately low level of viscosity, the phases will easily separate 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 rotational viscometer technology. These viscometers measure a large range of viscosity from 200 to 106 million, with an accuracy range of ±1% to ±0.2%. The measurements of a product’s viscosity are based upon immersing a specifically selected spindle within a sample of product. As the spindle rotates at a specific speed, a vortex is created by vigorous stirring. The torque required to rotate the spindle at a set speed is measured, based upon the amount of drag created from the spindle moving in the liquid. The instrument senses the rotation 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 1cP for its viscosity. All other materials are subsequently measured for viscosity in comparison to water. The rotational viscometer will record and store up to 10,000 data points. With the software provided, users can instantly view 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 a 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.
Spindle design determines flow resistance
The flow resistance increases with the speed and size of the spindle. Different samples and materials require different types of spindles depending on available sample size, the torque range used, temperature needs, and material consistency. Due to the complete flexibility and configurability of today’s viscometers, users can interchange different spindles depending on their needs. A parallel plate or cone spindle is used for thicker, more viscous samples. For thinner, less viscous pourable liquids, a concentric cylinder spindle is appropriate. A T-bar is an option for gel-like material. This is a relatively inexpensive process and many sellers of viscosity equipment offer convenient bundles in which the device, spindles, and cleaning materials can all be purchased together at a discounted price.
Consider spindle speed and time during testing
The speed of the rotation and the amount of time the spindle rotates are other considerations when performing viscosity testing, as samples are affected by both factors. The spindle should rotate at least five turns before a value is taken, and the amount of time the spindle rotates should be dictated by a defined period of time. In many rotational viscometers, an over and under range alarm will sound if the sample is too high or too low for the spindle selected. 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 2,600 speed selections. This allows the user complete flexibility regardless of the sample. The display provides readouts of speed, spindle, viscosity, percentage of torque, shear rate, and shear stress. When the sample density is entered, the viscometer will even calculate and display kinematic viscosity. The instrument can also 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. Viscometers with networking capabilities allow the user to download information to a computer for use in spreadsheets. 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.
The importance of temperature control
When measuring viscosity, temperature becomes a very important player in the game because the sample will have a different viscosity measurement depending on its temperature. For example, honey tested at room temperature may have a viscosity measurement much different than honey cooled in the refrigerator, or heated honey. The test conditions to measure viscosity should be tightly controlled because temperature highly affects the results. To achieve accurate viscosity measurements, the food sample should be controlled within +-0.05°C. Many viscometers have built in RTD temperature probes and will display the temperature on the unit to help the user maintain accurate sample temperatures.
Viscosity testing for quality control
It is easy to see that viscometers play an important role in the food industry. Viscosity is measured either during processing 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 quality control lab for testing to ensure the quality is maintain throughout the entire process.
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.
Mary Vessele is the product marketing manager, Test & Measurement at Cole-Parmer.