Vacuum cooling of lettuce with various rates of pressure reduction

This article discusses the cooling characteristics of lettuce vacuum cooled at different pressure reduction rates. The cooling characteristics importa nt to this study were determined to be the final temperature profile within the lettuce and the product mass loss during cooling. A laboratory-scale vacuum cooler was modified so the retort pressure could be controlled using a data acquisition and control system. Air leaks contro lled by solenoid valves allowed air to enter the retort at a rate such that the pressure of the retort decreased in an exponential manner at different rates to simulate the use of different sizes of vacuum pumps. Two lettuce heads were instrumented per trial, using three different simulated pressure reduction rates. Three replicates were performed for each pressure reducti on rate. Each lettuce head was instrumented with three thermocouples locate d at the center the mass-average temperature location, and just underneath the first leaf An infrared temperature sensor was used to measure surface t emperature. The lettuce heads were weighed before and after the cooling cyc le. Results showed that changing the pressure reduction rate had no effect on t he mass loss, temperature reduction per percent mass loss, or the temperatu re difference between various locations. The significance of these findings is that vacuum coolers with smaller vacuum pumps (slower pressure reductio n rate) can be designed without any changes to the cooling characteristics of lettuce, within the pressure reduction rates used in this study.