Dealing with Shear

  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."

  "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.

  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.

  "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.

  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.