Impacts of cellulose fiber particle sizes and starch types on macro and microstructural properties during extrusion processing

Sravya Kallu

Incorporation of insoluble fiber into direct expanded extrudates often leads to products with poor sensory attributes, which are less preferred by the consumers. It has been reported that the insoluble fiber, when incorporated into starch products, acts as an inert material, leading to a decrease in the expansion with an increase in fiber level. In addition to the percentage inclusion of the fibers, properties of the fiber also have significant effects on the extrudate quality. The objective of this study was to understand impacts of the particle size of cellulose fibers and native corn starches on macro and microstructural properties of extruded products. Fiber with three different particle size distributions (<125, 150-250, 300-425 µm) and four types of starches with different amylose contents (0%, 23%, 50% and 70%) were investigated. The feed moisture content (18 ± 0.5% w.b.) and extruder temperature (140oC) were kept constant and only the extruder screw speed was varied (100, 175 and 250 rpm). Extrusion process and product responses were significantly (p<0.05) affected by the change in amylose content of the starch. As the particle size of fiber decreased, there was an increase in the expansion ratio. Expansion ratio significantly (p<0.05) increased with the addition of small particle size fibers for starches with higher amylose contents (50%, 70%) compared to their respective control starches. This trend was in contrast with the lower amylose starches. Further, the Water Absorption Index (WAI) values increased with the addition of fibers of all sizes and at all levels for the high amylose content starches. This trend was opposite for extrudates with 23% amylose content starch. Microscopic images showed breakdown in low amylose starch extrudates (0%, 23%) with the addition of fiber, which can be correlated with the reduced expansion from the addition of fiber. A decrease in the cell sizes and accumulation of fiber was seen as the fiber inclusion level increased from 5% to 10%. The research results suggest the presence of active molecular interactions between the fiber and the starch under specific extrusion processing conditions. Further understanding of these interactions can help in the development of high fiber extruded products with acceptable sensory characteristics.

Impacts of cellulose fiber particle sizes and starch types on macro and microstructural properties during extrusion processing

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