Tomato Waste

February 19, 2007

Searching journal notes for the last fifteen years showed only one year in which Vincent Corporation has not been involved with tomato waste. Many times we have sent out rental presses to dewater tomato waste, only to have them return following mediocre performance at best. Cooked tomatoes would press, but raw tomatoes would not.

It was not until December of last year that truly successful results were obtained. First, it was determined that reacting the waste with lime increases moisture removal. Second, the use of a VFD in the auto-reversing mode greatly improved press performance.

Driving the press with a VFD set for auto-reversing (pattern mode) was indispensable in most of our tests. The problem previously was that the tomato material would blind (cover over) the openings in the screen, preventing the press from functioning. With the press set to run backwards for a few turns every thirty seconds, the screen stays clear and the press operates continuously, unattended.

What we have found is a way to run raw tomato waste so that approximately 50% by weight comes out as press liquor. The 50% by weight that comes out as press cake will still be damp to the touch. However the cake will be stackable, and little water will run from the pile.

Fresh tomatoes are generally 94% moisture. The great majority of this water is organic, bound water, which cannot be separated by applying pressure in a screw press. Running a moisture analysis on pressed material will show that the press liquor is about 4% solids and the press cake is 90% moisture. This does not sound like much, but one must keep in mind that 50% has been converted to free-running liquid.

If cooked tomato waste is pressed, the press cake will come out much drier, typically only 65% to 75% moisture content. The proportion of the flow that comes out as press liquor increases correspondingly. This improvement occurs because the heat of cooking breaks down some of the organic molecules, increasing the amount of free liquid.

Even better dewatering of tomato waste is achieved if hydrated lime [calcium hydroxide, Ca(OH)2] is added to the waste in the proportion of 0.5%. The lime, much like cooking, breaks down organic molecules to increase the free liquid in the mass. Pressing raw tomato waste that had been mixed with lime resulted in 75% to 80% of the mass coming out as press liquor. The press cake was noticeably firmer, measuring only 60% moisture content.

Issue 184