How to Tackle Quality Problems Related to Mixing

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.