Dealing with Shear

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Dealing with Shear
November 1995 -- Process Engineering

By: Lynn A. Kuntz
Associate Editor*

*Editor since August 1996

  While food products tend to undergo significantly different processes, most of them will encounter some sort of shear along the way. Sometimes shear benefits the finished product - in the form of aeration, particle reduction or individualization, for example. But often shear results in undesirable changes in the finished product or it throws up stumbling blocks to the process itself.  It is not possible to take shear out of the equation or even to predict its results. Still, knowing what shear is, where it occurs and how it helps or hinders the product or process can give a food designer an edge when it comes to getting a product to the production facility.Shear determination  During manufacturing, products must be moved from point A to point B, and even up to points X, Y or Z. Although the term shear ideally applies to fluid systems, any product that experiences a kind of "flow" - from liquids to large particulates moving down a conveyor - will encounter a force that can be described as shear. Put simply, shear is the friction generated by this movement. In a truly fluid system, the term shear denotes the amount of force needed to move molecules or particulates along a plane. The more force required to move the product, the more shear it encounters.  The shear force of a fluid system can be calculated using a number of factors, including the volume-to-surface ratio, the fluid density and viscosity, and the velocity of flow. From an engineering standpoint, these calculations are invaluable in designing the appropriate system for a given product and in determining if the existing system is appropriate for other products.  Any text dealing with process engineering and fluid handling should cover thoroughly this subject and the appropriate calculations. For reasons of available space, though, such a discussion is beyond the scope of this particular article. Furthermore, without extensive research, the numbers these calculations provide will be meaningful only in relative terms for their effect on the finished product.  Methods exist for calculating shear rate, particularly in mixing and pumping. These will differ depending on the kind of mixer used. A rotor and stator type of mixer needs a different method to calculate shear than a turbine, for example. However, the effect on the finished product can be very different even with numerically similar rates, especially with different types of equipment.  "The process may be imparting the same order of magnitude of shear, but things like duration of exposure to the shear may have a large effect, or the volume of the product exposed to the shear can vary with the piece of equipment used," points out Dale Spridco, product manager, fabricated products and tanks, APV Crepaco, Inc., Lake Mills, WI.Shear misery  It has happened to all of us. A product turns out beautifully on a small scale, but once it goes out to production it is only fit for scrap.  "The analytical methods are very helpful to determine the appropriate equipment and horsepower requirements for a product," notes Spridco. "But the effect on the product is best determined by a reasonably large scale test. We've found that labscale equipment doesn't scale-up very well in terms of production, so we test in the pilot plant - 300 gallons as opposed to 3 liters. In going from 300 gallons to 1,500 gallons, you can scale-up reasonably well with certain precautions - things like tip velocities, for example."  Shear can promote a number of adverse effects. Often the physical force can wear down a product or a particular component. Some of the more common effects include particle identity loss, starch breakdown, and the disruption of networks such as with those formed by proteins. Any type of structural network can be affected. Shear can cause an emulsion (both liquid/liquid and liquid/gas) to break.  "You can destabilize emulsions very easily with sheer," observes Mark Freeland, director of advanced colloids, Rhone-Poulenc Food Ingredients, Cranbury, NJ. "If you use shear to create emulsions, you can go too far and actually churn the product. Ice cream is a good example. Actually, there you use a controlled destabilization to make the product when it passes through the freezer. It's a matter of the amount of shear."  Says Spridco: "In the beverage industry, bi-phase fluids are very sensitive to shear. There you have a gas dissolved in a liquid. Shear will tend to coalesce gas bubbles, so you lose the gas in the fluid."  If the product is moving, some degree of shear is being generated. When designing a shear-sensitive product, it's best to keep in mind the processes and equipment that create the most significant amount of shear and, if possible, minimize them.Mixing. Most food products undergo a mixing or blending stage. To minimize shear, a mixer can use a low-speed, large-diameter impeller or a number of impellers interleaved which gives a slow but steady turnover in the mixing vessel. This actually blends by multiple division as the interleaved blades pass through each other, minimizing the turbulence introduced.  Another method to reduce shear with some higher viscosity products is by using a horizontal agitator, such as food blender or dual ribbon blender. The product is gently rolled over onto itself. Any particulates that settle onto the bottom are scooped up and deposited on the top of the product in a kind of multiple folding arrangement combined with a gentle tumbling action. A low tip viscosity on any blade used minimizes shear to within the area of 500 feet per minute or less.  "The size of the mixture is important in the design of the agitation," advises Spridco. "Even at very low rpm - which you would tend to associate with low shear - if you are using a large-diameter agitator in a large vessel, you still can have some very high tip velocities. Most of the product is subjected to low shear, hut you can still be introducing a lot of turbulence at the tip of agitator due to the high velocity."Pumping and moving. Both the pump itself and any piping or similar conveying method will generate shear. The smaller the inside diameter and the greater the speed of flow, the more shear a product encounters. Any restrictions, reductions in diameter and turns increase the shear. An elbow in a pipe is estimated to produce as much shear as 10 feet of straight pipe because of the turbulence generated. The tighter the radius of the pipe, the more shear produced. Changing the diameter of a pipe will create tremendous back pressure if you're trying to push the same volume through it. T-shaped joints, valves and other common assemblies also increase shear. The more distance a product has to travel the more shear it encounters.  "In many cases the filling machine is far removed from the manufacturing site," Freeland says. "Sometimes the attitude is 'We'll just run a pipe.' That can add to the formulation difficulty. It certainly is difficult to build that into the bench formulation."  For large particulates, a rectangular inlet pump allows product to be introduced directly into the pump without passing through piping. To minimize shear, use as large a volume load pump as practical at low rpm. Very shear-sensitive products require a positive-displacement pump. Even a large-diameter, slow moving centrifugal pump introduces a relatively high amount of shear.  A centrifugal pump introduces turbulence in almost any circumstance at the tip of the vane and it creates shear between the vane and the stator, or the casing. The pump works because of centrifugal force on the fluid. To develop enough pressure to move the product, it must rotate at a speed that is high enough to develop the centrifugal force. The process of rotating the product at those speeds develops shear.Filling. Most filling operations do not depend on a constant gravitationally induced flow, which in itself creates shear. Problems with shear may occur in filling because of valving, stopping and starting the flow stream, and the nozzle that introduces it into the container. There will be some type of surge in the system to accommodate the filling operations from the supply. All of these can accelerate or decelerate the product through a pipe or orifice and introduce shear. Multiple heads, which assure a continuous flow, can help reduce the amount of shear generated during the filling process, but they will not eliminate it.  Other operations inherently create a high level of shear in order to perform a specific process. These include homogenization and milling. The only way to reduce the effect of shear in these cases is to modify the process and, if possible, add any shear-sensitive ingredients after the process is complete. This is also the reason for multi-stage mixing procedures. When high shear is required to hydrate a particular hydrocolloid, for example, a slurry step not only aids dispersion and hydration, but reduces the shear that the rest of the ingredients must encounter.Shear mayhem  Some of the aspects of processing that can cause shear-related problems are indirectly related to shearing of the product. For example, it is difficult to use a common manifold for non-Newtonian products and get flow division through a number of filler heads. Peanut butter, for instance, is shear-thinning. If one leg of the divided flow becomes restricted for any reason, the flow slows down and the viscosity of the product actually increases because of the lower shear rate. As the viscosity increases, the pressure-drop increases and the flow rate decreases.  "It becomes a continuous loop, where the product just comes to a halt in that particular leg," says Spridco. "That means more product gets diverted through another orifice. That speeds up the flow rate, and the product thins out and flows easier. It becomes virtually impossible to balance a system like that for a shear-thinning product. That isn't a surprise when you put a large number of nozzles on a common manifold. In some cases, you almost have to have an individual timing pump for each head."  Friction generates heat, as well as physical shear forces. High-shear processes such as high-speed mixing may generate significant amounts of heat which affects a product, literally cooking it. The effect often depends on the duration the product is exposed to the operation. In many cases high-shear operations require jacketing to control the temperature and dissipate the heat that is built up.Shear fabrication  While most shear issues are process- and equipment-related, some formulation options exist. Correct ingredient selection often can improve the quality of the finished product. When looking at maintaining particulate identity, consider ingredients that have a firmer texture and are more resistant to mechanical abuse, especially when cooking the product. A Granny Smith apple, for example, will hold up much better in a pie filling that is pumped than a Delicious apple. Less tender cuts of meat resist shear better when processed into stew or soup.  One of the most important ingredients to choose carefully in high shear situations is starch. With unmodified corn starch, the swollen granule is sensitive to damage from mechanical sources including shear. The resulting product will exhibit a stringy texture and reduced viscosity. If a starch-based product encounters a relatively high degree of shear, using a cross linked starch provides a degree of protection from deterioration. The degree depends on the level of cross linking. This type of starch actually requires a high level of shear or insufficient hydration is likely to occur.  "If you put starch through a homogenizer, you can rupture the granules," observes Freeland. "The granule will shatter and lose its water binding capacity."  In addition, the rheology of a particular system must be considered. For example, many hydrocolloids are shear-thinning. This means that in the presence of shear they will thin out, but once the shear has been removed they return almost to the original viscosity. This has processing implications. Because the viscosity decreases, less energy is needed to maintain a flow. However, if something happens in the system to make the shear rate uneven, problems such as inconsistent filling can occur because the flow rates vary.  Adding hydrocolloids also may provide a measure of protection if shear causes a network to break down. Xanthan gum can help maintain and stabilize emulsions in the presence of shear, for example. In addition, the waterbinding capabilities of the hydrocolloids may serve to bind any free water that can occur if released during shear and to keep it from separating out.  When adding stabilizers, consider the fact that the more viscous the fluid, the more shear will be generated in order to move the product. This increase in shear may damage particulates that would come through unscathed in a thinner matrix.  Whatever formulation concessions that can be made to decrease the sensitivity to shear should be made. Although it is essentially a production problem, opportunities to change production set-ups are rare unless it involves a new installation.  If the product won't run or if it is unacceptable in the end, it becomes everyone's problem. Keep the sheer scope of your project in mind so you can avoid a shear disaster.Back to top