Determination of the product refrigeration load in vacuum cooling of lettuce

This article discusses the relationship between product refrigeration load and vacuum rate for the vacuum cooling of lettuce. A theoretical relationsh ip is developed based on evaporation temperature of water as a function of pressure and the rate at which the pressure decreases in a vacuum cooler. I t was assumed that the temperature of the product followed the evaporation temperature of water The slope of the temperature-time function was used as a measure of the rate of heat transfer from the lettuce, and of the produc t refrigeration lend. A laboratory-scale vacuum cooler with air leaks contr olled by solenoid valves, was modified so the retort pressure could be cont rolled using a data acquisition and control system. Air entered the retort at a rate such that the pressure of the retort decreased in an exponential manner at different rates to simulate various sizes of vacuum pumps. Lettuc e was instrumented with thermocouples to measure the temperature throughout the cooling process. Using the collected data from different vacuum rates, the maximum slope of the temperature-time data was determined and used to calculate the maximum product refrigeration load. These results were compar ed to the theoretical predictions. An empirical model was developed to pred ict the peak refrigeration load based on the experimental data. It was obse rved that there was a heat transfer lag when the vacuum rate was high, thou gh the lag decreased when the rate of vacuum application was reduced. Resul ts from this study allow for the peak product refrigeration load to be pred icted based on the volume of the retort and the mass of the lettuce to be c ooled.