Milling - A further parameter affecting the rapid visco analyser (RVA) profile

Rapid Visco Analyser (RVA) profiles were recorded for raw maize grits and t wo extruded nonexpanded pellets based on wheat and maize. Large differences were found between the profiles obtained when an impeller mill was used to prepare the samples compared with a disk mill. The differences were relate d to differences in particle properties of the ground products (particle-si ze distribution, particle shape, and protein content). Generally, milling t he samples with the impeller mill resulted in greater starch conversion tha n with a disk mill. For raw maize grits, this was shown by X-ray diffractio n, differential scanning calorimetry (DSC), and alkaline viscosity measurem ents. Several other laboratory mills were tested and all produced particula tes with a sieve range of 125-212 mum that had substantially differing RVA profiles. Cooling the sample during milling did not nullify the milling eff ects. All the laboratory mills produced <20% of the particulates of the siz e range required for the RVA analysis. The mill used for sample preparation can exert a significant effect on the RVA for both raw and processed cerea l samples, even if measurements are made on a defined sieve fraction.