Dewatering Potato & Carrot Peel

November 5, 2007                                                                                                                                                                                                ISSUE #193

For several decades, Vincent has attempted to use a screw press to dewater potato peel. Our 1994 Pressing News described the best results that could be obtained. These results were relatively marginal, limiting the market for screw presses. After all, it is hard to justify a screw press if all you are going to do is put half the waste into the sewer and reduce the moisture content of the remaining solids to 82%.

Good results on carrot peel have been even more difficult to come by. Dewatering the pulpy waste from the Grimmway baby carrot factory was successful. However, waste from conventional brush peelers was virtually impossible to dewater in a screw press.

The problem has been that these vegetables contain a high amount of bound water. Some of this water is held within cells, by the pectin. Other water is found in long-chain organic molecules. The mechanical pressure of a screw press is not going the break loose this water. It takes heat, or chemistry, to break down the pectin and organic molecules.

For over sixty years the citrus industry has known that the addition of hydrated lime [calcium hydroxide, Ca(OH)2] causes a chemical reaction in orange peel that allows the waste to be dewatered by pressing. It is thought that the presence of hydrated lime and water breaks down the pectin in the cell walls. The end result is that the cake produced by a screw press will have a solids content half again higher if the orange peel is first reacted with hydrated lime.

It is worth noting that the presence of calcium hydroxide in the waste does not adversely affect its value as an animal feed.

In 2006, a number of tests were run that demonstrated that this same chemistry, that of adding hydrated lime, works for both potato and carrot peel. At one French fry plant, potato peel (with 9% solids content) was mixed with a small percentage of hydrated lime. With the addition of lime, the screw press increased the solids content of the waste to 25%, compared to 13% solids without lime. Similar results have been obtained on carrot waste.

To facilitate on-site demonstrations, Vincent has added lime dosing machines and reaction conveyors to our rental fleet. The lime dosers take 50 pound bags of lime, and generally one bag is used per shift. Bulk bag lime dispensers have been offered for larger scale operations. The reaction conveyor is a 12' long section of 12" diameter screw conveyor, with a mixing section at the inlet. Cut flights and VFD's are used in order to delay the flow of material through the reaction conveyor.