A mixture experimental design was used to evaluate the combined effects of non-fermented (PDPF) and fermented (F-PDPF) peanut flours, added feed moisture content, and extrusion temperature on physical properties of extruded cornstarch-peanut flour-based snacks. Moisture content, expansion ratio, bulk density, modified Kramer compression-shear force and colour of extrudates were significantly affected by the amount of PDPF and F-PDPF in the formulation. Oil absorption by extrudates was only influenced by the type of peanut flour. Within the range of (per 1-kg batch) 0.30-0.34 gg-' added water, 0.16-0.2Og g-' PDPF and/or F-PDPF, and extrusion temperatures of 125 to 150°C, extrudates with a wide range of functional characteristics can be produced. GA 30223-1797 (U.S.A.) 0 199.5 Blackwell Science Ltd 37 38 W Prinyawiwatkul et al.in the form of partially defatted peanut flour (PDPF) if it is processed from edible-grade peanuts under sanitary conditions. It has a relatively high protein content, bland flavour and light tan colour which facilitate its incorporation into a wide range of food products (Prinyawiwatkul, 1992). In studies of peanut proteins modified to improve their functional properties in food systems, fungal fermentation with Rhizopus microsporus var. oligosporus gave a product (F-PDPF) with changed functional properties (Prinyawiwatkul el al., 1993) which would alter physical properties of products in which it is incorporated.Our objective was to evaluate the combined effects of PDPF and F-PDPF, added feed moisture contents, and extrusion temperatures on physical characteristics of cornstarch-peanut flour-based extruded snacks.
Materials and MethodsPreparation of raw materials PDPF and F-PDPF. Peanuts (cv. Florunner, medium size 199 1 crop) were roller blanched (model EX, Ashton Food Machinery Co., Inc., Newark, NJ) to remove testae and then partially defatted in a hydraulic laboratory press (model M, Fred S. Carver Inc., Menomonee Falls, WI). Peanut press cake was ground into flour without or after fungal (Rhizopus microsporus var. oligosporus) fermentation; procedures and chemical and physical properties are fully described in a previous report (Prinyawiwatkul et al., 1993).Cornstarch. Food grade cornstarch (code 2200, Cargill Corn Milling, Cedar Rapids, IA) had a composition of 11.33% moisture, 0.12% fat, 0.62% protein, 0.10% crude fibre, 0.85% ash, and 86.98% carbohydrate (by difference).
Feedstock mixturesFeedstock mixtures were conditioned by adding 0.3Ck0.34 g g-' of tap water per 1-kg batch, mixing thoroughly, storing overnight, then remixing in a commercial blender (model 34BL22, Waring Products Division, New Hartford, CT).
Extrusion conditionsThe pilot scale single screw extruder ("Yellow Jacket", Wayne Machine and Die Co., Totowa, NJ), with a barrel of diameter 19 mm and length 475 mm, a cylindrical 7 mm die and a 3:l compression ratio single screw, had three electrically controlled heating zones of which only zone 2 (compression section; 125°C) and zone 3 (metering section; 125, 137, or 15OOC...