We recently sent out a newsletter, Four Kinds of Water, that helps explain why it is not possible to take potato waste down to 50% moisture in a screw press. The problem is that potato has a lot of water that is chemically bound to other molecules. The water cannot be released without the addition of heat, as occurs in a rotating drum dryer.

If a screw press could reduce the moisture content to a low number, then there would be no need for a dryer. Yet the opposite is true: dryers are more common than screw presses.

Citrus peel presents a good illustration. The peel starts out at 82% moisture, and it is taken down to 10% in making by- products. If the plant is making cattle feed, they first add caustic lime (calcined calcium carbonate) to the peel to break down the pectin molecules. With this procedure it is possible to press citrus peel to as low as 62% moisture. The rest of the moisture reduction is achieved by adding heat in the form of fuel burned in a dryer.

If we try to press to less than 62% moisture, the peel just extrudes through the screen in the form of frit. Potato behaves very similarly; it comes through the screen looking like mashed potatoes. It is not possible to separate the water from the solids with the mechanical action of a screw press.

Another citrus peel by-product is pectin peel. This is dry peel in which the pectins have not been broken down. It is not permissible to add lime to peel that is used in this process, so instead water is used to wash the sugars out of the raw peel. This drives up the moisture content from 82% to over 90% (after screening). Next the peel is run through a screw press, and the best we can do is get down to 86% moisture. Thus the pectin peel goes into the dryer at 86% after pressing, which is wetter than it started.

Processors would love to run the moisture down to 50% in their screw presses because it would save them a fortune on their fuel bills. But it just can not be done.