Handling properties of cereal materials in the presence of moisture and oil

A powder flow analyzer attached to a Texture Analyser (Stable Micro Systems, UK) was used to compare the flow behaviour of four cereals systems: maize and wheat (in both starch and flour forms), as functions of particle size and distribution, water content and the addition of different types of oil. It was expected that the smaller the particle size the higher the tendency to stick (because of less free volume between the particles), but that was not the case. The results showed that wheat starch used, with bigger particle size than maize starch, had higher cohesion properties and as water content increases the cohesion increases by the same magnitude. This was attributed to the differences in granular shape as well as protein quantity and quality. Caking strength for both starches was influenced by the water content; in particular at 30% water content (w/w), neither cohesion nor caking indices could be measured for wheat starch because of the high stickiness of the particles.Although the two flours had particles of very similar sizes, with differences in the distributions only, maize showed higher cohesion indices compared to wheat flour. These values decreased with increasing water content. The caking property for maize was not significantly affected by water content with values of approximately 100 ± 5. The caking strength increased for wheat flour from 8 to 500 as moisture increased from 12.5 to 30%. This was ascribed to the differences in hydration properties of the two flours. For wheat flour and as the water content increased, gluten started to form and would require more than 30% to form a homogenous, visco-elastic mass.Generally, cohesivity and cake forming ability were affected by water content as well as the physical state of the oil i.e. by the solid/liquid ratios. As water content increases, wheat starch showed the greatest packing and cohesive behaviour, with and without the oil, while maize flour exhibited the weakest packing and cohesive properties.

The way cereal powders pack and flow can be measured by caking strength (CS) and cohesion index (CI). Water is known to act as an adhesive between particles by creating liquid bridges and can increases cohesiveness, but simple relationships between the measured flow parameters and water content were difficult to define. The wheat samples, starch (WS) and flour (WF) seemed more influenced by high water contents (30% wet basis) than the maize (M) to the extent that no measurable flow values could be obtained for WS at the 30% water.Figure optionsDownload as PowerPoint slide