How to Tackle Quality Problems Related to Mixing

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

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.