Uniformly mixing ingredients may be a challenge.

Engineers at Rutgers University have demonstrated that stirring a mix of granular ingredients faster or longer during processing won't always give you a smooth blend. In fact, you may even end up separating ingredients you thought were thoroughly mixed.

While these findings won't change the way cooks at home blend flour, salt and sugar to bake a cake, they could help manufacturers make better products with less manufacturing waste. While mixing dry ingredients would seem to be a simple undertaking, creating uniform batches on a large scale can be a challenge for industry. The consequences of uneven blending could range from a box of raisin bran without enough raisins to pills that don't have the safe or effective amount of active pharmaceutical.

A large number of food products, including breakfast cereals and bakery goods, require the blending of granular ingredients of different size, density or shape. Inadequate blending or subsequent segregation can lead to products that do not meet quality specifications and thus must be reworked or discarded. At the same time our understanding of mixing and segregation of granular materials is lacking in many respects, and this often leads to a trial-and-error approach to process development and manufacturing. In their research the scientists were able to identify patterns of granular motion in a bench-top experiment that promoted layer formation and interfered with achieving uniform mixing. It is hoped that the understanding gained in the bench-top experiment can be translated to industrial mixers with the goals of improving product quality and decreasing waste.

   The most striking finding by the investigators involved a tendency
   of fine particles to separate into distinct layers under conditions
   that would otherwise seem to cause thorough mixing.

At lower mixing speeds, fine glass beads of different sizes started to mix uniformly. But as the speed increased, the beads started forming distinct layers. The number of beads and their thickness varied with rotation speed. The researchers were able to identify patterns of granular motion that promoted layer formation and interfered with uniform mixing.

While the researchers did their work under controlled laboratory conditions, they believe they have uncovered principles about mixing behavior in solid particles that could be useful not only for manufacturing, but also in understanding and manipulating particle flow on a larger scale, such as rock slides and avalanches. Just as the earlier studies of fluid mixing have helped us understand how river water mixes when it reaches the ocean, researchers believe that their understanding of granular mixing could help colleagues better understand how particles and fluids mix during food processing operations.

Further information. Ben Glasser, Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Rd., Piscataway, NJ 08854; phone: [phone]; fax: [phone]; email: [e-mail].