IN HIGH, dry Salt Lake City, a salt shaker is a salt shaker. Turn it upside down, and the little grains pour out. In rainy Seattle, when you flip and shake, you are much more likely to hear a "thunk" and get next to nothing. Even around the house, clumping and caking can cause problems. On the production line, those problems get big and expensive.
A small dry soup manufacturer was determined to avoid clumped-up soup base. One day he was processing a base with 3% moisture content. He measured some newly received pepper and found it to also have 3% moisture content. Sure that it was safe to mix the two components, he did so and headed out to lunch. The manufacturer returned from lunch, flipped on his processing machinery and discovered that the whole batch had clumped. He had to throw away the lot. His loss felt even worse as he spent considerable time cleaning up his equipment.
Caking and clumping are ubiquitous problems in the food and pharmaceutical industries. Predicting-and avoiding-the problem means paying attention to three things: time, temperature, and water activity. Over time, free flowing powders will move through the stages of caking. The particles within the powder will get wet, then sticky. Sticky particles will agglomerate, compact, and finally reach the liquefaction stage. The process is affected by particle shape and size, temperature, moisture available within the system, applied pressure, and chemical composition. The problem can be avoided by keeping the powder below a certain water activity.
The soup manufacturer thought he had the process under control. His soup base was dry, the temperature and relative humidity in his processing plant were fine, so why did he end up with a mess? This manufacturer was distracted by moisture content numbers. Just like the fruitcake producer in Lesson Five, he thought he had his eye on the ball, but he was actually out of the game completely. He thought that dry enough for soup base was also dry enough for pepper. But it isn't the quantity of water, it's the quality-not how much there is, but how much is available. And the 3% water in the pepper was free enough to wreak havoc. Had he measured water activity, he would have found that the soup base had a water activity of 0.28, while the freshly ground pepper was at 0.69 aw. When he combined the two, he brought the water activity of the mixture above the critical clumping point.
The critical point at which a product will cake or clump is product-specific. It must be determined by putting the product at different humidities and measuring the water activity at which the product begins to clump. Maintain the product below that value, and you will avoid clumping. As the soup manufacturer discovered, adding ingredients at higher water activities can change the water activity of the product. In addition, high ambient humidity and increases in temperature can raise the water activity of the product.