J. agric. Engng Res. (1990) 45, 31-43 Processing Factors Affecting Yield and Quality of Mechanically Expressed Groundnut Oil K. A. ADEEKO,* O. O. A.JIBOLA The effects of particle size, heating temperature, heating time, applied pressure, and duration of pressing on the yield and quality of mechanically expressed groundnut oil were investigated. Shelled groundnuts were manually cleaned and reduced to two particle sizes (finely and coarsely ground). The milled samples were then heated at 70°, 90°, 115 °, 135°, and 160°C for 15, 25, 35 and 45 min. Pressures of 10, 15, 20 and 25 MPa were applied for 10 min during pressing. Generally, oil yields from coarsely ground groundnut were higher than those from finely ground samples, but the free fatty acid values were lower. Increasing the temperature did not improve the oil yield after 25 min of heating. Oil yield increased with pressures of up to 20 MPa beyond which the yield either levelled off or decreased. The rate of oil expression was increased by an increase in temperature, time of heating, and particle size. An increase in the heating temperature and time increased the free fatty acid, peroxide value, and the colour intensity of the oil expressed. 1. Introduction Groundnut is the third most abundantly cultivated oilseed in the world and over the years, it has played an important role in the economy of Nigeria. It is estimated that 800000 to 1 200000 ha of groundnut are grown annually in Nigeria. 1 Some of the harvested nuts are consumed as roasted groundnuts, but a larger percentage is used for production of vegetable oil. In West and Equatorial Africa, small scale processors obtain oil from oilseeds either by aqueous extraction or by hand pressing. 2 Low yields and poor quality oils are obtained due to the inefficient technologies employed for extraction. With increasing cultivation of the crop, it becomes necessary to improve the processing techniques in order to increase oil yield and quality. Mechanical expression is one of the ways by which oil is removed from oilseeds ° and this can be accomplished by either hydraulic or screw presses. Although the screw press is more efficient, the hydraulic press is more common with small and medium scale processors due to relatively lower initial and operating costs. There is, therefore, a need to study the effects of processing factors on oil yield and quality when the hydraulic press is used. Pre-pressing conditions such as particle size, heating temperature, heating time, and moisture content are known to affect the yield and quality of oil during expression. 4 Efficient mechanical expression, therefore, involves a careful establishment of optimum processing conditions for different seeds because the best preparation is somewhat different for each oil-bearing seed. 4 The purpose of size reduction is to expose a greater area of oil bearing cells to moisture and heat during cooking. However, excessive size 5 reduction is not desirable as it reduces expression efficiency. Ward observed that coarsely breaking groundnuts into quarters or eighths produced a more easily extractable cake than did grinding. Flaking soybean prior to expression gave a higher oil yield than did * Department of Agricultural Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria Received 23 June 1988; accepted in revised form 1 June 1989 31 0021-8634/90/010031 + 13 $03.00/0 © 1990 The British Society for Research in Agricultural Engineering