New protein texturization processes by extrusion cooking at high moisture levels

Abstract

Although primarily used at low moisture levels, extrusion cooking with twin‐screw extruders also applies to food mixes with 40–80% moisture. Such levels reduce or prevent viscous dissipation of energy and product expansion, but facilitate operations such as fat emulsification, sterilization, protein gelation, restructuring and shaping of comminuted meat or fish, and microcoagulation and/or fibrillation of specific protein constituents. Fibrous gelled structures are obtained from (a) defatted soy flours or concentrates; (b) wheat gluten; and (c) mixes containing 70–80% surimi plus 30–20% soy concentrate or gluten, by extrusion cooking at 50–70% moisture, at barrel temperatures above 140°C, using long cooling dies. The continuous gelled bands are highly resistant to stretching in the longitudinal direction. They display a multilayer and partly fibrous structure. An extruded crab analog with a finely fibrous texture, made from Alaska pollack surimi plus egg white and 1% starch, is already commercialized in Japan. Three main phenomena are involved in the formation of these structures: (a) protein plastification (“melting”) within the extruder, which requires a barrel temperature of 140–180°C (at 60–70% water), a residence time close to 150 s, and high shear forces; (b) stability of extrusion conditions, without surging (often difficult to achieve); (c) presence of a long cooling die, exerting essential functions: upstream pressure generation, enabling high temperature, channel filling, and extrudate continuity; progressive reduction of the mechanical and thermal energy of the food mix, permitting proper shaping, while avoiding product expansion; alignment of dispersed protein particules along the shear gradient in the narrow die channel, leading to layer and fiber formation. The restructuring of mechanically deboned meat can be achieved through extrusion cooking (40–50% water) in the presence of high levels of binders such as starch or cereal flours. Such experiments were carried out without a cooling die and without fiber formation. Emulsified and gelled cheese analogs, with textures ranging from hard blocks to soft spreads, can be obtained with a single extruder pass, starting from cheddar or from caseinate plus various fats. The extents of fat emulsification and of casein “reassociation,” and the melting ability, depend on composition and process parameters. The extrusion of fibrous products similar to mozzarella‐based string cheese is attempted. Fat substitutes with a smooth and spread‐like consistency are also prepared by thermomechanical processing of caseinate and/or whey proteins in a twin‐screw extruder at an acid or neutral pH. Shear forces and several biochemical mechanisms concur to induce the “microcoagulation” of β‐lactoglobulin as small‐size particles (<20 μm). Other potential moist extrusion processes include the coagulation of skim milk powder into an insoluble acid casein coprecipitate, the encapsulation of small molecules into protein gels, and the formation of edible thin protein films.