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close this bookFood, Nutrition and Agriculture - 11- Edible Fats and Oils (FAO - FPND - FAO, 1994)
close this folderGhani: A traditional method of oil processing in India
View the document(introduction...)
View the documentTraditional ghani technology
View the documentCrushing oilseeds
View the documentOil yield
View the documentRecent evolution of oil processing
View the documentAdvantages and disadvantages of ghani crushing
View the documentBibliography

Crushing oilseeds

In the crushing of 10 kg of sesame seed in a ghani, about three-fourths of the material is placed in the pit and the rest is evenly laid out all around the flat rim (Patel, 1943; Nag, 1982). The animal is prodded and allowed to perambulate for a few minutes until pulverized seed is found to climb the walls of the pit. The animal is halted, and 180 ml of water is sprinkled around the chest and 120 ml poured into the pit. A further 5 minutes of pestle rotation will cause about three-fourths of the seed to be pulverized, after which another 300 ml of water is poured evenly around the pithead. The material built up in the chest is raked using a crowbar, and the pieces are broken up by hand and cast into the pit. After the animal has resumed movement, the rest of the seed is evenly pushed in all around. The operator now tests the solid material by balling it in his or her palm; if it crumbles too easily, more water is needed. The layer of built-up material is again broken up, and brisk ambulation is resumed. After about 45 minutes, a sudden release of frothy oil floods the surface. Another 100 ml of water is sprinkled over the oil, the animal is stopped and the oil is allowed to settle. A final quantity of about 20 ml of water is now brushed over the compacted cake surface using the edge of the palm, after which the animal makes a few more rounds. The operation is stopped, the two curved pieces are detached and the pestle is lifted out and laid aside. If the ghani has a drainpipe, it is unplugged and the oil is drawn into a vessel. Otherwise the oil released into the pit is mopped up with a piece of cloth and wrung out by hand into vessels. While the cake is still hot and before it has set really hard, it is prised out as thick slabs from the chest using a crowbar.

Rape and mustard seeds need more water during crushing than sesame, and copra rather less. The oilcake is not raked during linseed crushing, but only at the very end. Safflower seeds are always very carefully decorticated by passage between grinding stones, sieving and winnowing: only practically pure meats are subjected to ghani crushing. During crushing of groundnuts at least part of the shells are retained in the ghani so as to ensure formation of a granular and compact cake.

At the point of maximum contact, the pressure in a ghani is about 10 kg/cm2 (Gujarathi, 1982), about one-third of that in a small screw-press and about one-tenth to one-hundredth of that in a large modern expeller. The pressure in the ghani is largely determined by the weight placed on the load-beam, usually 115 to 160 kg, which is transmitted by way of the curved piece to the top of the pestle.

The fit of the pestle within the pit is important. Experiments have shown that an inclination that exceeds 21° from the perpendicular causes so much lateral pressure that the mass will not climb the walls of the pit. Too much dead space in the pit will have the same effect.

The phased additions of about 7,5 percent water during ghani operation have a major role. The first addition provides the pestle with a grip on the dry oilseed, and the friction produces heat. The second portion, with the heat present, cooks the ground seed. This is analogous with what happens in the stack precooker in modern screw-press operation. Protein is denatured and coagulated, and as the moisture level reaches a critical point, oil is rather suddenly displaced from the cells. (In the Russian Skipin process, in which oil is extracted by displacement with hot water, this critical moisture level has been ascertained to lie within the limits of 14 to 18 percent (Alderks, 1948)). The cake at this stage turns granular and cohesive, and will not reabsorb the expelled oil. After the oil has appeared, the third addition of water serves to hydrate and coagulate gums and phospholipids. This phase is analogous to modern oil degumming. The last brushing with water serves to clear surface oil on the cake and give it a sheen.