MODELING, MONITORING AND CONTROL STRATEGIES FOR HIGH-TEMPERATURE SHORT-TIME PASTEURIZATION SYSTEMS - 2 - LETHALITY-BASED CONTROL

A lethality-based control system was designed to provide accurate cont rol of a high temperature shout time (HTST) pasteurizer and to process milk products with a lethality equivalent of 161 degrees F (71.67 deg rees C) or above for 15 s. This control system provides significant fl exibility in operating the process and optimizing functional propertie s of the food components. Multivariable control of an HTST pasteurizer is implemented by using product total lethality to determine the cont roller set-points. The equation that relates the temperature and flow rate combinations to the product total lethality, 161 degrees F (71.67 degrees C), 15 s, was modified to permit overprocessing levels specif ied by plant personnel. By using this equation and the set-point value selected for the other variable, set-point values for the temperature or the flow rate controller were computed. The flow and temperature c ontrollers are integrated into a real-time monitoring and control syst em. The monitoring and control system includes the multivariable contr oller, the lethality rate calculation module, statistical monitoring o f the total lethality, product flow rate, hot wafer outlet temperature , and holding tube exit temperature measurements, and the display scre ens for visual inspection of the monitoring tools. This study attempte d to achieve compliance of the HTST process operation with the recomme nded Pasteurized Milk Ordinance by providing a margin between the alar m limits of the monitoring chart and the safety limits. (C) 1998 Elsev ier Science Ltd. All rights reserved.