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In the last two years Vincent screw presses have been used in testing sweet sorghum at a dozen sites. This work is driven by an interest in fermenting the sorghum juice for the production of cellulosic ethanol. Customers include several universities, entrepreneurs seeking a feed for ethanol plants now using corn syrup, and misguided farmers trying
to make fuel for their farm machinery. Quite a bit of experience and data has been accumulated.Sweet sorghum is of interest because it can have Brix in the range of 18 Bx to 22 Bx. However, all of our first-hand experience has been with cane in the range of 7 Bx to 14
Bx. This is almost always attributed to the maturity of the material being pressed.
We think of sweet sorghum cane as consisting of three elements: the stalk, the leaves, and the tassel. In one set of tests, we measured the following split:
Stalk: 72% by weight
Leaves: 23% by weight
Tassel: 4% by weight
Clearly the tassel is not significant. It becomes of interest only when the plant has been de-tasseled because of a need to prevent the spread of a disease, Ergat, an airbornepathogen. This disease, a recent arrival in the States, results in a catastrophic reduction of Brix.
Much debate has centered on the presence of leaves. It has been thought that the leaves should be removed because juice from the stalk is absorbed into the leaves, and is thus lost, during the pressing operation. After all, the Spaniards 400 years ago burned the leaves off sugar cane stalk before they crushed it. The truth seems to be that the pressing operation is made more difficult with the presence of the leaves; it takes a tougher press to handle it.
To our surprise, when we pressed leaves we found that the resultant juice had just about the same Brix as the juice from the stalk. In our case, we were pressing stalk with 7 Brix juice and its leaves gave us 6 Bx juice.
There has been a focus on putting forage harvested material into the screw press, as compared to running whole cane into the press. The torque requirement of thescrew press goes up dramatically, by a factor of three, when whole cane is fed into the press. We attribute this to the fact that the press is doing the work of the shredder(or harvester) in addition to the normal pressing action. (The press definitely shreds the whole cane, usually more so than what is achieved by the harvester or shredder.)
We ran tests pressing the three components separately. The results were:
Stalk: 64% juice yield by weight
Leaves: 25% juice yield by weight
Tassel: 4% juice yield by weight
Overall, juice yields run from 50% to 60% when whole cane is run through a screw press. This is consistent with the sum of the yields from each of the components, which tells us that overall yield is not improved by removing the leaves. Most people do not seem to agree with this.
Researchers have run tests comparing roll crushers (roll mills), like those used in the sugar cane industry, to screw presses. In general the roll crushers seem to work better on whole cane, but poorly on forage chopped material.
Reportedly the juice degrades immediately in forage chopped material. We have definitely seen a reduction yield from forage chopped material as compared to pressingwhole cane. (This assumes that the press has enough power to adequately press the whole cane.) Pressing 6 billets of stalk gave us the best results.
One operation has developed equipment to chop the whole cane into 3 to 4 billets, and then blow away the leaves. Reportedly this gives extremely high yield of high quality juice.
We measured the bulk density of pieces of stalk, and found it to be a surprising 57 pounds per cubic foot (compared to 62.4 pounds per cubic foot for water). This told usthat there is very little air entrained in the pith of the whole stalk.
We found that the press liquor produced in pressing sweet sorghum has very low insoluble solids, 1% or 2%. This is a severe contrast to alfalfa, where the press liquorwill have 4% insoluble solids. In any case, the press liquor is filtered, with the separated solids being re-admitted to the screw press.
We found that material harvested shortly after a heavy rain had little extra moisture content. The effect on pressing results was minimal. The plant must be like the humanbody: the water does not soak in when it stands in the rain.
We compared twin screw to single screw presses. The greatest advantage of the twin screw is that the torque within the press is balanced. Single screw presses have a strong tendency to rack when subjected to the high torque requirements. As would be expected, the shredding action which occurs within the press is better in the twin screw press. The advantages of single screw presses are lower initial cost; simpler maintenance considerations; and reduced susceptibility to damage by tramp material.
ISSUE #203