High moisture extrusion with twin screw extruder - Fate of soy protein during the repetition of extrusion cooking.

Abstract

Defatted soy flour (DSF:NSI ca. 35; crude protein 55 w/w%) was extruded three times to investigate the effects of longer residence time and shearing on the strength, fine structure and protein component of extrudates. The extruder used in this study was twin-screw, fully intermeshing type (Mitsubishi FT-60, L/D = 20) equipped with a long cooling die and operated at 60 rpm, 20 kg/hr feed rate within 130 .apprx. 150.degree. C. DSF was moistured to 60% and adjusted its pH to 7 with NaOH. The extrudates could be prepared without any puffing and found to have its apparent residence time 2.5 min by the color distribution method. The major factor for DSF texturization was found to be the temperature and its critical value was around 140.degree. C, which agreed well with DSC of DSF. The repetition of extrusion did not disturb or destroy the protein matrix in the extrudate but improved or reproduced it, indicating that the second factor was the residence time. Even after three times of extrusion cooking at highest temperature (150.degree. C), the extrudates showed negligible loss of amino acids. These results suggested that DSF could be processed like the thermoplastic resin.