Everyone, especially those in the food industry, has experience with mixing. All of us have at least some experience in the kitchen, and many of us have extensive experience. Commercial process mixing can be very different from kitchen or laboratory mixing for several reasons. In the kitchen or development laboratory, quantities are small and distances are short. Almost any mixing occurs quickly, especially in comparison with production scale equipment and processes. In the kitchen or laboratory, mixing intensity usually can be increased or decreased to achieve good results. Small-scale results can be easy to observe even as the mixing takes place. Kitchen and laboratory mixers are usually designed to handle a wide variety of food properties.
Everything that seems simple in the kitchen or laboratory becomes more difficult at the production scale. Even when successful mixing is done and observed carefully in small-scale development, scale-up to reproduce those results at a production scale can be difficult. Some knowledge of mixing and a little creativity can overcome many of the common problems.
The following discussion of mixing includes many of the common problems experienced in production mixing for foods. The discussion also provides ideas on how the problems can be solved, or at least improved. If the process results do not make products that meet your quality standards, something needs to be investigated and changed.
Obstacles to Improvement
One of the biggest obstacles to improving mixing is the requirement to use existing equipment, as opposed to buying new equipment. This limitation becomes an even bigger challenge when the equipment is more than 25 years old, which is common, or when used equipment is purchased. Old equipment, even if it has been well maintained, may not match the current product or process requirements. Used equipment is probably chosen because of price or availability rather than needed performance.
The upside to these equipment problems is that many facilities have a variety of mixing equipment with different sizes, mixers, and heating capabilities. The first step in solving these practical limitations is understanding the capabilities of each piece of mixing equipment. Total tank volume is important, but knowing the minimum and maximum practical operating volumes is more important. Trying to mix a batch that is too small for a mixer can be as bad as mixing a batch that is too large. Mixing intensity is usually inversely related to batch size. The same mixer used in a small batch should provide more intense and rapid mixing than in a large batch, even if the mixer does not have a variable speed capability. Batch size will also affect the mixer’s influence on surface motion.
Good surface motion may be an advantage for ingredient addition. However, a deep surface vortex becomes a problem if it draws air into the product, especially if the product tends to foam. Antifoam may solve some problems but not all of them. A surface vortex should never extend more than halfway from the surface to the mixing impeller. Vortex depth is strongly influenced by the depth of the liquid over the impeller nearest the surface. If all these ideas seem obvious, unfortunately they are often overlooked or not communicated to the people who need to know.
Solving problems or making improvements in existing equipment requires a little creativity. For example:
- Changing the order of addition can improve mixing or reduce batch time. Ingredient additions to a batch with a changing viscosity will be easier if the additions are made when the viscosity is low. Two liquids with different viscosities can take a long time to blend, even if they are mutually soluble, like corn syrup and water. If the more viscous liquid is added to the less viscous liquid, the operation should be easier and faster than if the low viscosity liquid is added to the high viscosity one.
- Adding minor ingredients to a batch can be similar to combining different viscosities. To be sure that minor ingredients, such as nutrients, stabilizers, flavorings, emulsifiers, and preservatives, are well mixed, they should be added to low-viscosity liquids whenever possible. In some cases, pH adjustments are necessary to cause a viscosity change. Reactive ingredients may be adversely affected when an acid is added for any reason.
- Similar suggestions apply to mixing batches of bulk powders. Free-flowing powders are the bulk solids equivalent to low-viscosity liquids. Ingredients should be added while the blender is running and the powder is free flowing. Once ingredients such as water, oils, or moist ingredients are added, powders are likely to become more cohesive and difficult to blend. Very minor ingredients, less than half of a percent of the formula, should be preblended in a portion of a major free-flowing ingredient.
- Another solution to some mixing problems is to change to different forms or concentrations of problem ingredients. Different particle sizes or granulations may make powder addition to liquids easier. These steps may solve initial production problems but may create operational problems if the changes are not controlled during ongoing production. Sometimes an ingredient change happens unintentionally, such as when going from product development to production. Other problems may not appear until after the product has been in production for a period of time. Consistent results almost always depend on consistent ingredients, equipment, and procedures. A process needs to be sufficiently robust to avoid process problems caused by minor ingredient changes.
- In any mixing application, an optimal mixing time probably exists. Too little mixing time may not yield uniform results. A mixing time that is longer than essential may be more than just a waste of time; overmixing may cause product degradation. During initial production runs, you should carefully observe and probably sample to get an idea of how long a batch needs to mix to get a quality result.
- The time required for batch uniformity in similar equipment should be inversely proportional to the rotational speed of the mixer. Large batches often take longer to mix than small ones just because the mixer rotates at a slower speed. Similar uniformity is often achieved after a certain number of mixer rotations, not the mixing time. For those cooks familiar with old cookbooks, some recipes called for a certain “number of stirring strokes” for proper mixing, which is similar to the number of mixer rotations. The rotational speed of a mixer should be known and not subject to the whims of the operator.
From Product Development to Production
Many process problems develop in the transition from product development to commercial production. Scale-up is not a single procedure that always works. Successful scale-up will depend on different methods for different products. Some knowledge and observation of a specific formulation can make scale-up from the development lab to production more reliable.
The ingredient formulation is rarely the only factor in the production of a failed or successful product. Food has many subtle characteristics that define the success of the product, even if the “product” is an intermediate ingredient on the way to a consumer product. The start for successful scale-up begins in the development lab. The new or modified formulation must first meet basic customer requirements. Then, the combination of the ingredients must establish the expected quality standards. Scale-up to production must also provide important or unique information about the ingredients and process. Those observations made during development or known to the developer need to be communicated to production.
One of the best pieces of scale-up advice is: Make your mistakes on the small scale and your money on the large scale. This means that not all formulation or preparation problems in the development lab are real failures. Some “failures” may be useful learning experiences that should be noted and understood. Successful scale-up may be just a matter of avoiding the causes of failures. Observing the effects of undermixing or overmixing also may help to plan for conditions to be avoided in production.
Not all production problems are mixing problems. Ingredient addition, transfer pumping, final screening, and heat transfer can all contribute to production problems. Knowing a few reasons for scale limitations can be important. Processes associated with area will cause more problems after scale-up, because area does not increase at the same rate as volume. For the scientists, area increases as length squared and volume as length cubed. For the practical minded, this means that if volume increases by a factor of eight, the area only increases by a factor of four. This surface area effect means that the addition rate for ingredients in the large scale should be proportionately slower than in small-scale development. The rate of addition should be in proportion to the increased surface area, not the formula weights, which increase as a function of the volume. Heating for cooking also takes longer in production, not just because of a larger volume, but also because the heat transfer surface area is less in proportion to the volume.
Viscosity is always an important factor in mixing. Simply described, viscosity is the resistance to flow of a liquid, or what appears to be the thickness of a fluid. The real problem in food is that viscosity is almost never represented by a single value, other than possibly for low-viscosity water-like liquids. Low-viscosity liquids usually are easy to mix and less likely to cause problems. Many factors affect the observed viscosity of a fluid. Understanding some of the factors affecting viscosity can be a useful tool in understanding food quality and production. Temperature is an obvious factor, both with respect to viscosity and quality. Higher temperatures almost always result in lower viscosities, which makes higher temperature liquids flow more easily. Easier flow may be good or bad, depending on the desired performance of a product.
What makes viscosity difficult to understand are the factors that affect it other than temperature. After temperature, the most common effect on viscosity is caused by shear rate. Shear rate depends on the relative motion internal to a liquid. Equipment such as mixers and pumps create shear gradients in a fluid because some mechanical parts of the equipment are moving, while others are not. This difference in velocities is what causes shear rates in a liquid. The effects that shear rates have on viscosity depend on the physical and chemical properties of the liquid. Some fluids are shear thinning, which means that the viscosity is reduced when the fluid is in motion. This effect will also be observed when the viscosity is measured. A lower viscosity may be observed if the measurement is made with an instrument that turns faster or causes the liquid to move quickly.
Shear thinning behavior also may be time dependent. That means that the longer a food is sheared, the lower the apparent viscosity becomes. The reduced viscosity may be only temporary so that the fluid returns to the unsheared condition after it stops moving. In other cases, shear may cause a permanent breakdown of the original viscosity. The performance effects of shear on viscosity may be observed in food behavior as coating ability or mouth feel. Shear effects occur most often in fluids with droplets or particles dispersed in them. Some concentrated multiphase fluids, such as starch solutions, may exhibit shear thickening behavior, where the apparent viscosity increases as the fluid flows.
Another viscosity effect observed and even desired in some food products is yield stress. Yield stress causes a fluid to behave like a semi solid until a minimum amount of force is applied. Common examples of yield stress fluids are ketchup and mayonnaise. Once the initial resistance to flow is overcome, e.g., a hit on the bottom of the ketchup bottle, the food flows as a viscous liquid, e.g., as when the ketchup splashes on your shirt. Another viscosity property important in foods is viscoelasticity, as in items such as bread dough, taffy, and gels.
When fluid viscosity is affected by shear rate, mixing becomes more difficult than when shear rate is not a factor. Understanding, measuring, and observing viscosity is necessary for the success of both processes and products.
Perhaps the most common problem with mixing that affects food quality and production is inconsistency. Mixing is a chaotic process; the chaotic flow patterns are the fluid motion effects that cause mixing to take place, but that chaos should not have a significant effect on process inconsistencies, unless something is done to cause problems. If ingredient additions land in a location with insufficient surface motion, such as near the tank wall, inconsistent mixing results may occur. One way of overcoming ingredient addition in a poor location is to use a funnel or chute to direct where the ingredient addition lands. A funnel may even be used to control the rate of addition for ingredients. Control of ingredient addition can overcome some inconsistency problems caused by different equipment, different operators, and different procedures.
The best way to get control of inconsistency is through process documentation. Documentation needs to be more than just records for quality control. Quality control typically checks incoming ingredients and finished products. The missing information may be in what happens between the ingredients and the products. Most operations pay attention to the measurement of ingredient quantities and sometimes the order of addition, but process records should also track which equipment was used, who operated the equipment, and how long each step took.
Record keeping needs to follow the production process. Procedures for combining ingredients need to be defined and followed in production. Product development information may influence the choice of production equipment and scheduling. Once production begins, the planned steps need to be followed. Any necessary or incidental deviations should be recorded. At the end of the process, some measure of quality should verify whether or not the desired properties were achieved. If quality problems are observed, the records of the actual process may provide insights into possible causes. Elimination of batch-to-batch differences must be achieved for continued product success. The usual packaging and shipment samples with batch records will provide traceability and identification. If that information is linked to the production procedures, many problems can be identified and corrected for future production.
If problems develop, the missing information may not be just in the written records of what was done or not done during mixing. Today’s technology provides powerful and available tools to make better observations. Photos and videos are extremely effective ways to observe vague or transient problems. A photo of the liquid level at each stage of batch loading, especially in different mixing equipment, may reveal reasons for the success or failure of mixing.
A 30-second video of a liquid surface or powder batch during mixing can provide information about both properties and processing. Process viscosity or bulk powder behavior can be difficult to sample and measure with instrumentation. Even the desired quality standards may fail to capture the process conditions. For example, quality control may be done at a standard or final temperature, while the actual conditions in the process equipment may be different. Sometimes a photo or short video is all that is needed to communicate important mixing or product behavior. A video of a previous batch may show similarities or differences with current conditions. Careful observation of a video timed with a stopwatch may even provide a way to measure the rotational speed of a mixer without a tachometer.
If the storage and handling conditions for ingredients are subject to question, consumer weather instrumentation may provide temperature and humidity information compatible with a digital computer. Humidity can always affect the handling of powdered ingredients. One of the most commonly overlooked measurements is the initial temperature of the ingredients or process water.
Food ingredients and processes typically are cost sensitive. Cost limitations are justification for greater creativity in the use of technology. For instance, if a photo or video of a mixing operation may be beneficial, an expensive camera is probably not the best option. People take cell phone pictures of their food at restaurants all the time. Why not record what food looks like when it is being made? Videos at each ingredient addition or process change may provide a more complete view of the process steps. Time stamps on photos or videos may provide information about how long the process took. Every photo taken with a cell phone has a date and time in the details about the photo. For a continuous video of the process, a basic security camera with an overhead view and a digital recording device may provide information about the entire process at a minimal cost. Observation and recording can be as extensive as appropriate to monitor success in production. If you are still experiencing inconsistencies, you have not identified important differences in the process.
Mixing is an empirical process, which means that results are obtained by observation. Sophisticated computers and instrumentation may provide more detailed information than is necessary for success. If you experience process problems, learn about your products and operations through observation. Many problems and potential improvements may become obvious. Remember, you can’t keep doing the same thing and expect different results. Something needs to change.
Dickey is a consultant with MixTech, Inc., which specializes in all types of mixing processes and equipment for both liquids and powders. Reach him at firstname.lastname@example.org.