Food Powder Flow in Extrusion: Role of Particle Size and Composition

Innovations in food extrusion technology are enabling its rapid expansion and applicability in diverse areas related to bioprocessing and value addition. This study relates raw material particulate rheology to the granular flow in a single screw food extruder. Raw materials based on corn (i.e., meal, flour, and starch), wheat (i.e., farina, flour, and starch), and sucrose (i.e., granulated, superfine, and powdered) were used as model particulate systems for the study. Various particulate-scale characteristics and flow parameters of these nine materials were determined using a powder rheometer, a promising new offline tool. Properties such as basic flow energy, specific energy, cohesion, stability index, flow function, and effective angle of internal friction were good indicators of flowability in an extruder. Corn meal exhibited lower energy requirements and a higher propensity for flow than corn flour (6.7 mJ/g versus 10.7 mJ/g, and “free-flowing” versus “cohesive,” according to Flow Function classifications), with wheat farina showing similar results when compared to wheat flour (5.8 mJ/g versus 7.9 mJ/g, and “highly free-flowing versus “cohesive,” according to Flow Function classifications), although both wheat systems showed comparatively lower energy requirements than their comparable corn systems. Sugar, being of a different base material and particle shape, behaved differently than these starch-based materials—flow energy decreased and propensity to flow increased (51.7 mJ/g versus 8.0 mJ/g, and “free-flowing” versus “highly free-flowing”). This large energy requirement for coarse sugar particles may be attributed more to particle shape than composition, as the sharp edges of sugar can interlock and increase restriction to movement through the sample. The starch-based results were validated in a particulate flow study involving the above model systems (corn meal, corn flour, wheat farina, and wheat flour) in a pilot-scale single screw extruder. Visualization data, obtained using a transparent plexiglass window during extrusion, confirmed that the flours exhibited higher flow energy requirements and a lower flow factor when compared to the coarser-particle size corn meal during extrusion, seen by the increased peak heights and barrel fill.

Characterization of bulk and shear properties of basmati and non-basmati rice flour

BACKGROUND

Flours are often unstable in relation to their flow performance, which is evident when a free-flowing material ceases to flow and the processing, handling, and production parameters depend on the inherent powder characteristics and their bulk behaviour. The present study was conducted to compare the flowability of basmati and non-basmati rice flour affecting bulk handling, which could be related to its particle size, shape and surface roughness (measured by atomic force microscopy) as well as bulk and shear properties, depending upon the processing conditions.

RESULTS

Particle size (171.1-171.9 μm) of both samples was not significantly different. However, the flowability of the non-basmati rice flour was significantly affected by its particle shape (circularity 0.487), surface roughness (124.23 nm) and compressibility (25.32%) in comparison to basmati rice flour (circularity 0.653, surface roughness 113.59 nm and compressibility 21.09%), making it more cohesive than basmati rice flour. Also, basic flow energy was significantly higher in non-basmati flour, thus requiring more energy (147.54 mJ) to flow than basmati rice flour (130.15 mJ).

CONCLUSION

Overall, flowability was analysed by applying three different pressures (3, 6 and 9 kPa), among which non-basmati rice flour was found to be less flowable (flow function coefficient (FFC) 2.33 at 9 kPa) in comparison to basmati (FFC 3.35 at 9 kPa), making bulk handling difficult. This study could be useful in designing processing equipment, hoppers and silos for rice flour handling. © 2017 Society of Chemical Industry.

Significance of composition and particle size on the shear flow properties of wheat flour

BACKGROUND

Size-based fractionation of flour particles is an important process in wheat milling. Inter-particle cohesion could affect the dynamic separation process and result in loss in throughput. This study quantifies the effect of particle properties that includes physical and chemical characteristics on the shear flow behavior of wheat flour.

RESULTS

The cohesion and flow function values of wheat flour at three moisture contents (10%, 12%, and 14%), three particle sizes (75-106, 45-75, and <45 µm) and three sifter loads (0.5, 1.0, and 1.0 kPa) were significantly different (P < 0.05). The triggering moisture content and particle size where the flowability shifts from 'easy flowing' to 'very cohesive' were found to be 12% (wet basis) and 45 µm, respectively.

CONCLUSION

The high correlation observed between the chemical composition (damaged starch, protein, and fat) on the physical (bulk and tapped densities) and flow properties (cohesion, flow function, and angle of internal friction) demonstrates that chemical composition significantly contributes towards the differences in dynamic flowability of wheat flours. In addition, fat composition had a significant effect on the differences in flowability of wheat flours due to the increased inter-particulate cohesion. © 2016 Society of Chemical Industry.