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Time-temperature distribution studies during preconditioning of extruded pet food.
T. ZHOU (1), S. Alavi (2), C. Stark (1). (1) Food Science Institute, Kansas State University, Manhattan, KS, U.S.A.; (2) Department of Grain Science and Industry, Kansas State University, Manhattan, KS, U.S.A.

Preconditioning is an important step during extrusion of pet foods as it improves process efficiency, product quality and very importantly microbial safety. The high-intensity-preconditioner (HIP) presents a significant development in which two independently driven shafts offer control of both speed and rotational direction as steam is introduced. The primary focus of this study was residence time distribution (RTD) of a dog food formulation during HIP operation. Plastic beads were introduced as tracer and RTD calculated based on relationship between tracer concentration and time. In the first experiment, both shafts had clockwise rotation; speed of the small shaft was varied between 200-400 rpm while keeping large shaft speed constant at 200 rpm, and that of the large shaft varied between 200-300 rpm while keeping small shaft speed constant at 200 rpm. With increase in small shaft speed, average residence time decreased from 185s to 106s and RTD had narrower spread. The latter validated the primary mixing function of the small shaft of HIP. Residence decreased from 185s to 100s with increase in large shaft speed, but the difference was much higher relative to change in speed of the small shaft. This validated the conveying function of the large shaft of HIP. Residence time calculated from ‘dead stop’ method (ratio of weight of material retained in HIP to throughput) was higher, although same trends were observed. In the second experiment, rotational directions of the two shafts were varied independently with their speeds constant at 200 rpm. RTD and residence time varied significantly with direction; longest retention time (220s) was observed when both shafts had clockwise rotation, and shortest retention time (159s) when both had counter-clockwise rotation. As a next step, a time-temperature distribution (TTD) study was designed for HIP using a specially fabricated time temperature indicator (TTI). Intensity of time-temperature history had a good correspondence with residence time and preconditioning temperature.

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