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Effect of Particle Size and Moisture Content on Viscosity of Fish Feed

¹Consultant Engineer, FM Insurance Company, Hong Kong, China. ²Formerly with the Grain Science and Industry Department, Kansas State University, Manhattan, KS. ³Present address: U.S. Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, Microbial Food Safety Research Unit, 600 East Mermaid Lane, Wyndmoor, PA 19038. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. ⁴Corresponding author. E-mail: rflores@arserrc.gov. Fax: 215-233-6581. Phone: 215-233-6489.

A model was developed for the influence of particle size on the extrusion of a fish feed and the physical characteristics of the extrudates evaluated. The study was conducted using factorial experiments in a fractional replication design for four variables with three levels, and one-third of the replicates (3⁴ factorial in 27 units) were examined in a laboratory extruder. The torque-screw speed measurement was used to develop a viscosity model equation that considered different shear rates, product temperature, initial moisture content, and particle size. When particle size decreased, the apparent viscosity became smaller. The barrel pressure was important in producing extrudate with a uniform volume over the range of processing conditions tested because it had a strong correlation with the volumetric expansion. The material with lower moisture and larger particles caused the specific mechanical energy to increase. The viscosity model developed in this study can be applied to the development of large-scale extrusion models that determine the effect of particle size on the feed material extrudates.