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April 15, 2013
Extraction of juice from fruit and vegetables requires a focus on the amount of juice which can be recovered. No one wants to throw out good juice with the pomace. This need becomes a focus point of screw press design and operation.
The longer a material is in a press (the greater the residence time), the more time there is for liquid to drain out. It follows that the lower the screw rpm, the greater is the juice yield. Nowadays we always recommend running a press with an inverter VFD so that the performance can be optimized: the operator picks the slowest speed at which the inlet hopper will not overflow.
Since the operators always pick a higher speed so that they will not have to bother with an overflow condition, our customers are using level controls on more and more of our presses. That way the press can be set to always run as slow as possible.
The greater the air pressure on the discharge cone, the more the screw must squeeze the material in order for the material to get out of the press. If, however, the material starts to channel out one side of the cone, then juice yield can go down slightly. The condition is easily corrected by reducing either the cone pressure or the rpm of the screw.
The quickest way to get more juice yield is to run the press cake back through the press a second time. If this works, the customer sometimes buys a second press which they mount in series before the first one.
I say "before" because the customers usually have a tight-squeezing Series CP or VP press to start with. When they go to double pressing, we supply a less expensive Series KP press for the first, soft, squeeze. That first pressing gets out the easy juice so that the second press can work harder.
Customers who say they are after higher juice yield usually mean that they want to increase the recovery of dissolved solids from the material being fed into the press. Most commonly the way to do this is to add some hot water to the press cake from the first press before it is put it into the second press. Triple pressing in this fashion is common with deciduous fruit juice and coconut meat, and somewhat with pineapple juice. These customers have evaporators which they use to bring the Brix back up to where they want it.
Steam addition sometimes works even better than adding hot water. In any case, presses can be equipped with passages drilled in the resistor teeth so that fluid (hot water, steam, CIP caustic solution, aqueous alcohol, supercritical CO2, whatever) can be injected into the press.
Issue 255