Mixing of dye in a model scald tank

USDA, Russell Research Center, Athens, Georgia, USA. jcason@ars.usda.gov

A model scald tank was constructed to study the mixing pattern of water in a poultry scalding system. Tank dimensions were approximately 6 m long by 10.5 cm wide with a water depth of 18 cm. Water was vigorously agitated with compressed air delivered through a 1.9-cm polyvinyl chloride pipe on the bottom of the tank. Food coloring was added to the tank at a single point, and water samples were taken at distances of 0, 0.5, 1.0, 1.5, and 2.5 m every 30 s for 10 min, with 0 or 10 L/min water flow through the tank. Dye concentration was determined spectrophotometrically. A chain drive was then installed above the tank with aluminum paddles (area about 25% of tank cross-sectional area) attached to the chain every 15.2 cm to simulate the movement of carcasses through the water at 140 carcasses per minute. Food coloring was added to the tank, and water samples were taken every 15 s for 2.5 min, with 0 or 13.5 L/min water flow through the tank. A computer program based on perfect mixing of water in small slices or cells within the tank was adjusted until predicted dye movement matched sampling data, with correlations of 0.91 or better at all sampling points. For scalder designs with uniform mixing of water, the computer model can predict mixing patterns, including counterflow conditions in a single tank, well enough to yield realistic residence time patterns for bacteria suspended in scald water.