INFLUENCE OF EXTRUSION CONDITIONS ON EXTRUSION SPEED, TEMPERATURE, AND PRESSURE IN THE EXTRUDER AND ON PASTA QUALITY

A fully instrumented pasta press was used to analyze the effects of ex perimental parameters (hydration, temperature, and shearing) on the pa sta-making process and on quality characteristics of spaghetti. The ro tational speed of the screw determined the flow rate of the extruder a nd influenced the extrusion speed, as well as the open surface of the die. The differential speed (ratio between the shortest and longest st rands during the same extrusion time) was decreased by the presence of a trefilette at the end of the screw. From an energy viewpoint, the p rocedure was very susceptible to the variation of parameters. For inst ance, the average specific mechanical energy transferred to the produc t for pasta extrusion was about 70 kJ/kg, but it could vary in a 1:5 r atio according to the extruding conditions, especially dough hydration and dough temperature, which both determine dough viscosity. These tw o parameters also had a great effect on pressure in the barrel and at the die. The presence of a pre-die plate seemed to exert a greater inf luence on the running of the screw, especially the fill rate and press ure at the end of the screw, than did the extruding conditions at the die level. Production factors were found to have little influence on t he color of pasta. They did, however, affect the cooking quality. Incr easing the amount of hydration of the semolina and the rotational spee d of the screw enhanced cooking quality. A large open surface of the d ic improved the surface condition of cooked products but decreased the viscoelastic properties. An excessive increase of pasta temperature d uring extrusion appeared to be the main factor in the degradation of c ooking quality. Therefore, a control of pasta temperature at the die w as proposed as the simplest method to guarantee the quality of finishe d products.