Analytical, Nutritional and Clinical Methods Section Eects of isolation technique and conditions on the extractability, physicochemical and functional properties of pigeonpea (Cajanus cajan) and cowpea (Vigna unguiculata) protein isolates. I. Physicochemical properties Mwanjala A. Mwasaru*, Kharidah Muhammad, Jamila Bakar, Yaakob B. Che Man Faculty of Food Science and Biotechnology, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia Received 25 July 1996; received in revised form and accepted 17 May 1999 Abstract Physicochemical properties of pigeonpea and cowpea protein isolates were determined as a function of extraction technique and pH of the extracting medium. Protein extractability by the isoelectric point precipitation (IP) technique was positively correlated within the pH of the NaOH solution used in the pH range 8.5±12.5. The micellization (MP) technique extracted signi®cantly (P<0.05) less protein than the IP technique when extraction pH of the NaOH was 9.5 or higher, and 10.5 or higher from cowpea and pigeonpea, respectively. The subunit composition and electrical mobility of the isolates were not aected by extraction techni- que and pH conditions. However, it was observed that the IP isolate extracted at pH 12.5 had the lowest proportion of hydrophilic amino acids, suggesting that the pH of the extracting medium exerted a major in¯uence on the hydrophilicity of the isolates. Pigeonpea MP isolate exhibited signi®cantly (P<0.05) higher exposed hydrophobicity than the IP isolates except for those extrac- ted at pH 9.5 and 10.5. However, the cowpea MP isolate exhibited signi®cantly lower exposed hydrophobicity than the IP isolate extracted at pH 8.5 but this was higher than the rest of the IP isolates. For IP isolates, an inverse relationship was apparent between the extraction pH and hydrophobicity. The MP isolates from both legume seeds were signi®cantly lighter in colour than the cor- responding IP isolates and, for the latter, the lightness value (L) was inversely correlated with extraction pH. Dierential scanning calorimetry showed that the MP isolates exhibited higher transition enthalpy (ÁH) than the IP. For the IP isolates, ÁH decreased with increasing extraction pH. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Tropical legumes such as pigeonpea and cowpea are crops that are well adapted to the semi-arid zones of Kenya because of their drought tolerance. Over the last 30 years, the usage of concentrated proteins from plant seeds has increased tremendously because of greater knowledge of their functional properties, processing and nutritive value. While soybeans have had a competitive advantage over other legume seeds, there is a need to develop other sources of concentrated plant proteins (Vose, 1980) which ideally should be crops that are widely grown in tropical countries. Although the chemical composition of pigeonpea and cowpea seeds has been reported in several publications (Jorg & Klein, 1989; Longe, 1980; Mnembuka & Eggum, 1993, 1995; Mosse & Baudet, 1983; Singh, Rao & Subrahmanyam, 1993), little information is available on the eects of extraction conditions on the physicochemical and func- tional properties of their protein isolates. A review of available literature reveals that more eort has been invested in the nutritional and chemical evaluation of these legumes than the studies of those physicochemical and biochemical properties that bear upon the technological transformation and postharvest stability (Hulse, 1991). As pointed out by Mitchell and Ledward (1986), the modern food processing industry is becoming increasingly dependent on the manufacture of fabricated foods rather than the preservation of commodities grown or reared on the farm. Since many past developments of fabricated foods have been as a result of inspired crea- tivity and trial and error manipulation of ingredients 0308-8146/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0308-8146(99)00150-8 Food Chemistry 67 (1999) 435±443 www.elsevier.com/locate/foodchem * Corresponding author at Department of Food Science and Post- harvest Technology, Jomo Kenyatta University of Agriculture and Technology, PO Box 62000, Nairobi, Kenya.