J. agric. Engng Res. (1996) 65, 335 – 345 Identification and Significance of the Oil-point in Seed-oil Expression M. O. Faborode; J. F. Favier* Department of Agricultural Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria * Department of Agricultural and Environmental Science University of Newcastle, Newcastle upon Tyne, UK (Recei ed 9 October 1995; accepted in re ised form 25 July 1996) The oil-point, which indicates the threshold pres- sure at which oil emerges from a seed kernel during mechanical seed-oil expression, is theoretically related to the kernel density of an oilseed thereby enabling its evaluation from initial bulk properties of a bed of seeds. A method for dynamically determining the applied pressure at the oil-point by precise sensing of the oil (pore) pressure is presented. This measurement permits experimental validation of the hypothesis that the oil-point generally occurs at the point where the bed density approaches the kernel density of the constituent seed or material particle. The ef fect of bulk compression on bulk density, volumetric strain, bed compressibility and specific energy of compression up to the oil-point was examined for rapeseed, soy- bean, groundnut, palm kernel, and cashew and melon seed before and after pretreatment by heating or size reduction. It was found that oil-seeds can be charac- terized objectively as hard or soft using bed compres- sibility and specific energy of compression at the oil-point. In addition the ratio of maximum pore pressure to applied pressure was positively correlated with seed oil content. The ef fect of particle size reduction and heating was generally to increase bed compressibility and lower the applied pressure at the oil point. The results show that the ef fect of pre- treatments on the mechanical properties of seed beds must be taken into account when considering the ef ficacy of mechanical expression of oil from oil-seeds. ÷ 1996 Silsoe Research Institute 1. Introduction The concept of the oil-point in seed-oil expression was put forward in 1989 by Sukumaran and Singh 1 who identified it as the stage in the process at which the oilseed has been suf ficiently squeezed to force oil out onto the surface of the seed. This is similar to the occurrence of the so called ‘‘apparent saturation point’’ in silage dewatering, this being the point at which the juice first appears, during the process. 2–4 Notation B Bulk modulus, MPa b Bed porosity index E Specific energy of compression, kJ / kg F k Porosity f friction ratio H Bed height, m r Compression ration ( /  i ) S Bed compressibility, m 2 / MN u Pore pressure (inter-kernel), MPa V Volume Greek symbols  Density, kg / m 3 » Volumetric bed strain »  Average volumetric bed strain  Applied pressure; MPa Subscripts a applied i initial o oil-point t kernel f friction b base of the bed It is now known that other researchers such as Mrema and McNulty 5,6 had encountered the oil-point in their work without proper acknowledgement since, perhaps inadvertently, they pre-loaded their seeds before monitoring oil flow and yield. It is significant that the oilseeds were indicated not to have been ruptured at the oil point, 1 thus corroborating the hypothesis of Mrema and McNulty 6 that cell wall rupture was not a precondition for oil expression, the cell wall being permeable and having pores or plas- modesmata which possess valve-like characteristics. 7 Sukumaran and Singh 1 developed empirical equations 335 0021-8634 / 96 / 120335 + 11 $25.00 / 0 ÷ 1996 Silsoe Research Institute