*Correspondent: Department of Food Science & Engineering, Taegu University, 15 Naeri-Ri, Jinryang, Kyungsan, Kyungpook 712–714, Korea. Fax: +82 53 850 6509, Phone: +82 53 850 6535. Normalized particle residence times as affected by process parameters in a vertical scraped surface heat exchanger Jun H. Lee 1 * & Rakesh K. Singh 2 1 Department of Food Science & Engineering, Taegu University, Korea 2 Department of Food Science, Purdue University, West Lafayette, IN, USA Summary The effects of process parameters and their two-way interactions on the normalized particle residence times (NPRTs) of 1.0–2.0 cm potato cubes in 0.4–1.2% aqueous solutions of sodium carboxymethylcellulose (CMC), flowing at 453–599 mL s -1 in a vertical scraped surface heat exchanger, rotating at 60–160 rpm were investigated. Minimum and maximum NPRTs and the standard deviations of the mean values were not significantly affected by the conditions studied (P  0.05). Mean NPRTs were sig- nificantly influenced by process parameters, including the concentration of the carrier, mutator speed and particle size, as well as two-way interactions between flow rate, muta- tor speed and particle size (P  0.001). The orientation of the heat exchanger was an important factor influencing the forces acting on the particles. Keywords Aseptic processing, orientation, potato cubes, sodium carboxymethylcellulose, two-way interaction. Introduction Aseptic processing has been successfully applied as an economical and efficient means for the destruction of micro-organisms in liquid foods such as fruit juices and various viscous products. However, application of this technology to the processing of food suspensions containing large particles remains limited because of difficulties in defining reliable process conditions that ensure bacteriological safety and optimum product quality. Previous studies indicated that the most important parameters affecting the heat transfer, and thus subsequent sterilization efficiency, in aseptic processing of large particulate foods were the fluid-to-particle convective heat transfer coef- ficient and the particle residence time distribution. Residence time is the length of time that a unit of product spends within the system’s boundaries. The residence times in the heating, holding and cooling sections of an aseptic system are required for the calculation of product sterility and the design of the process and equipment. Previous studies on particle residence time dis- tribution in a holding tube (Richardson & Gaze, 1986; McCoy et al., 1987; Nesaratnam & Gaze, 1987; Berry, 1989; Dutta & Sastry, 1990a,b; Hong et al., 1991; Pannu et al, 1991; Yang & Swartzel, 1991, 1992; Palmieri et al., 1992; Tucker & Richardson, 1993; Sandeep & Zuritz, 1994; Baptista et al., 1994; 1996; 1997; Salengke & Sastry, 1996) and in a scraped surface heat exchanger (Taeymans et al., 1985, 1986; Alcairo & Zuritz, 1990; NFPA, 1990; Lee & Singh, 1991a,b,c; 1993) have demonstrated the influence of process conditions on the particle residence time distributions. They have indicated that process parameters, such as the size, shape, con- centration and density of particles, and the flow International Journal of Food Science and Technology 1998, 33, 429–434 © 1998 Blackwell Science Ltd 429