FmGrowit's estimate of ~45 psi is how the math works out when using Killebrew's description of the St. James Parish Perique presses of his day--mid 19th century. My own Perique press has no way to measure the pressure. It's just very snug.
Using a lever arm cheese press, which I have calibrated, I have seen a dark cellular exudate produced on the surface of color-cured leaf that has been subjected to as little as 1.5 psi, which is consistent with istanbulin's experience. If you've ever bruised a leaf of cabbage or lettuce, then you are familiar with the glossy appearance. There is no question in my mind that pressure disrupts the laminar cells. It is also clear that such a cellular disruption does not decrease the tensile strength of the leaf.
A separate issue is the exposure of undamaged leaf to a hypotonic solution. Immersing leaf in plain water presents an osmotic gradient that tries to equalize the concentration of intracellular constituents with the extracellular environment. However, the cell membrane prevents all but the tiniest of molecules from passing in either direction. The result is that water is drawn toward the interior of the cells. BUT, the intact cell wall (a feature of plant cells, but not of animal cells) serves to minimize swelling of the cells. Some form of damage to the cell walls--either by pressure or by "decay"--is needed to allow the oxidative enzymes to reach the exterior of the leaf.
I am not disputing the pressures involved in conventional Perique making but merely trying to separate out the effects of pressure and anaerobic environment. 45-50 psi is consistent with what I have seen and calculated from the literature. Bob, the clamp you are using is rated at 400 lbs so if you use that number, the pressure you are achieving is about 29 psi. Of course what you are actually getting is unknown but at least this gives you a ballpark number to start with.
I find it it difficult to believe that 1.5 psi is sufficient to rupture cell walls. (Biology is not at all my strong suit so take these comments with that big grain of salt). This equates to a water column of a little over three feet and as we all know, plants grow considerably taller than that but surface tension and other mechanical effects have a considerable effect so column height probably means nothing. Also, as I said, I quick review of literature seems to indicate much higher pressures are needed. Have you tried your calibrated press on cabbage or lettuce leaves to see what happens?
I just did a quick and dirty test on a Maryland leaf. I took my 2 x 4 inch press block and literally stood on it with the high case leaf sandwiched in plastic between the block and the base. After 5 minutes of balancing there, the leaf was not changed at all. Since I am about 160 lbs, this gives about 20 psi. If I did not mark the leaf surface where the block was, I would not know where the pressing was. On another section of the same leaf, I used the same arrangement but I used a medium duty F clamp rated at 1000 lbs and tightened it as much as I could to the point where I was noticeably bending the bar. After 5 minutes at a calculated 125 psi, there was some darkening on about 20% of the the pressed surface but as I examined it, the darkness all but disappeared. After about 1 hour, the darkened area (with the leaf maintained in high case) is gone and, outside of a bit of flattening of the leaf, the pressed area looks like the rest of the leaf. Was this cellular disruption? Beats me. Could have been a moisture effect too. i also took a section of the leaf and froze it for 1 hour. Warmed up between my hands then placed back into the cut out area, it looks identical to the virgin leaf but I suspect that will change with time.
What does the above mean? Nothing yet. As I said, a quick and dirty test. But it is instructive to me. It leads me to believe that the pressure in the Perique process is not a big deal but this remains to be seen. Perhaps some cell disruption is necessary, perhaps not. I think I will add on another test to my above series where I will place the leaf in the jars in high case, then go through 2 or 3 freeze/thaw cycles to provide some disruption, then evacuate the jar and compare the results to an unfrozen sample. Will any of this work? Who knows. That is what testing is for.