February 23, 1999
Rev. 2008, 2022

Many people assume that a second press is used because the first press fails to remove all the free moisture. This is not the case; presses are designed to remove almost all of the moisture that can be removed in a single pressing. However there are applications where double pressing is technically sound.

In the production of juices for human consumption, double or even triple pressing is common. This is because the moisture in the press cake from first pressing contains many of the dissolved solids which are the essence of the juice. To capture these solids, water is added to the press cake. The dissolved solids in the cake diffuse into the water. The water is separated in the second pressing. This water carries with it the dissolved solids from the previous press cake. These are valuable solids which otherwise would be lost. Production of apple juice, pineapple juice, and coconut cream are applications where double pressing is recommended. Usually a concentrate evaporator is used to remove the water which has been added.

There are other applications where a single press designed large enough to remove all the liquid is not economically justifiable. In this case double pressing can justify the cost of operating a second press. An example is found in extracting juice from alfalfa. The macerating action in the first press frees additional liquid, and the air removed produces a press cake with greater bulk density. This allows the second press to get a stronger bite on the press cake from the first press, and additional juice is expelled.

To test for the justification for a second press, we recommend that the customer collect a 5-gallon pail of their normal press cake and run it through a lab press. If they send us a sample, we run it in Tampa at no charge. Alternatively, especially overseas, we have them collect a ton or two of their press cake and run it back through the press which they use in normal production. The trial will tell if it works.

A far more sophisticated application where double pressing is sound is in the production of animal feed from citrus waste.

In 1970 Dan Vincent was awarded a patent covering this double pressing concept. The system described calls for pressing citrus waste in two consecutive presses, positioned in series. The idea was to reduce the moisture content of the press cake going into the peel dryer, thus reducing the amount of fuel required to dry the peel.

The key to this double pressing is to diffuse dissolved solids into the peel. This is done in a diffusion conveyor located between the first and second presses. High Brix (solids) molasses is added to the press cake from the first pressing. After a couple minutes of stirring in the diffusion conveyor, some of the solids in the molasses diffuse into the moisture in the press cake. Equilibrium is reached at a mid-level dissolved solids content, a point which is between the low Brix press cake and the high Brix molasses. When this cake is run through a second pressing, the cake resulting will have a higher Brix. Consequently, that cake has a higher solids content and lower moisture content.

With a lower moisture content it takes less fuel energy to dry the cake into cattle feed. The result is a lower fuel cost per ton of pellets produced.

A mass balance shows us how this works. We start with peel at 80% moisture and 10o Bx. In a 100# pound sample this is 20# total solids and 80# water. The 80# of water, at 10o Bx, has 8.9# of dissolved solids (mostly sugars). The rest of the solids, 11.1#, are suspended (insoluble) solids.

If we press this peel, the cake will still have 10o Bx. By diffusing 50o Bx molasses into the cake, the Brix will equalize at around 20o Bx. When this mass is run through the second pressing, the resultant cake will still have 20o Bx. In effect, water in the first press cake has been displaced with dissolved solids from the molasses.

Some people argue against double pressing because “the second pressing only squeezes out the molasses added in the diffusion conveyor.” This is not an accurate evaluation. Solids from the molasses will have been diffused into the peel. This is seen in the fact that the press liquor from the second pressing will have a lower Brix than the molasses that was added in the diffusion conveyor.

Copies of the 1970 patent are available upon request.

BRIX DEFINITION: DISSOLVED SOLIDS DIVIDED BY THE SUM OF THE DISSOLVED SOLIDS PLUS WATER, x 100.

NOTE THAT SUSPENDED SOLIDS DO NOT ENTER INTO THE EQUATION.