How to Tackle Quality Problems Related to Mixing

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.

Inconsistency

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.