Kinetics of colour changes during extrusion cooking of maize grits

Abstract

Yellows maize grits were extrusion cooked in a conical, counter-rotating twin-screw extruder, at different barrel temperatures (140–180°C), feed moistures (13–17% wet basis), feed rates (38–52 kg/h), and screw speeds (60–80 rpm). Residence time distribution was measured by a dye tracer technique. The kinetics of colour changes during extrusion cooking was studied using the CIE Lab colour parameters ( L * a * b * scale) as physical parameters. The lightness ( L *) and redness ( a *) were markedly dependent on barrel temperature and feed moisture content, whereas screw speed was not significant. The yellowness parameter ( b *) was only slightly dependent on extrusion variables. A zero order rate equation was used to model the kinetics of colour changes. The kinetic rate constants for the change in lightness and redness were dependent on product temperature and feed moisture content. The activation energy, according to the Arrhenius equation, was 65 and 74 kJ/mol for lightness and redness, respectively. The lightness parameter ( L *) showed marked changes due to extrusion cooking and was the best parameter in the modelling of extrudate browning kinetics.