Microcoagulation of a Whey Protein Isolate by Extrusion Cooking at Acid pH

A whey protein isolate (WPI) was coagulated by thermomechanical processing in a twin screw extruder. Nonaggregated semi-solid spreads were obtained only in the pH range 3.5–3.9, at ca 20% protein (77% water), a barrel temperature of 90–100°C and a screw speed of 100–200 rpm. WPI extrusion-coagulated at pH 3.9 displayed a high nitrogen solubility (NSI) (43–47%). Electrophoresis indicated that the β-lactoglobulin constituent was entirely soluble in 1% SDS, while scanning calorimetry revealed about 82% protein unfolding. WPI extrusion-coagulated at pH 4.5–6.8 displayed lower NSI (25%), were less soluble in 1% SDS, were 88% unfolded and had grainy texture. Light microscopy, centrifugation in glycerol solutions, and laser diffractometry indicated the acid spread (pH 3.9) was composed of small coagulated particles, mean diameter 11.5 μm (volume basis).