Modeling energy requirements in planetary ball milling of rice grain

Abstract

Modified rice flours were obtained by high-impact milling. A factorial design was performed in a planetary ball mill varying rotational speed (450–650 rpm) and milling time (10–40 min) while milling energy (0.3–4.3 kJ/g), particle size distribution (PSD) of rice flour, and changes in starch structure were recorded. The effect of rotational speed and time on milling energy was satisfactorily modeled by the Response Surface Methodology. Modified flours presented bimodal PSD unlike that of the control flour, which was monomodal with a lower size dispersion. A significant reduction of D[4,3] (mean diameter) was observed as the milling energy increased. However, the effect of milling energy on PSD was dependent on milling speed. Classical milling equations provided a satisfactory fit of energy-size records at low speed (450 rpm), but at higher speed, a poor adjustment was obtained due to the increasing relevance of heat dissipation. The empirical milling equation based on linear or exponential relationships between milling energy and size reduction ratio provided a good fit for experimental data. Significant correlations between milling energy and damaged starch, crystallinity, and gelatinization degree were found. The proposed mathematical models are a contribution to the milling process design for specific modifications of rice flour.