THE INFLUENCE OF THE PROCESS PARAMETERS AND THE FORMULATION ON THE CONTINUOUS FOAMING OF A MODEL FOOD

This study concerned the combined influence of process parameters and formulation on foaming of a model food. Experiments were carried out on an agitated column with a narrow gap equipped with a torque meter. The model food is glucose syrup with a newtonian behaviour, in which two surface-actives agents were added: tween 80 and gelatine, alone or combined. Viscosity (mu) and surface tension (sigma) were measured by a stress constant rheometer (SR5) and by the tensiometer K12. Foaming was studied by varying gas to liquid flow ratio (G/L) and rotation speed (N). At the exit of the column, foamed materials were characterized by their overrun and by the distribution of bubbles sizes by images analyses. The average size was defined using Sauter mean diameter (d(3.2)). Torque measurement permitted to estimate the mean velocity gradient during agitation (gamma). Results showed that for a ratio of G/L equal to 1, only the formulation containing 0.25 % (w/w) gelatine permitted to obtain 100 % overrun. This value was limited to 40 % in the presence of tween 80 or a mixture of tween and gelatine. For these formulations, an increase in gas flow rate didn't improve overrun. Overrun decreased by increasing rotation speed, except for the formulation with gelatine. In this case, overrun increased and passed by a maximum when rotation speed increased from 400 to 1600 rpm. Sauter mean diameter was relatively independent of gas flow rate but it decreased when rotation speed increased. For the formulation with gelatine, d(3.2) decrease from 56 mu m to 29 mu m when rotation speed passed from 400 to 1600 rpm. For this formulation, d(3.2) was linked to rotation speed via the Weber number which remained constant (We = 0.81 +/- 0.028) over the range of rotation speeds studied.