Vacuum cooling process of Cos Lettuce (Lactuca sativa var. longefolla)

Abstract

The research was aimed at studying the optimum vacuum cooling process of cos lettuce precooled from initial core temperature of 19-21 °C to the final core temperature of 4±1 °C. The experiment was carried out in a rectangular vacuum chamber having a volume of 14.98 m3. In the experiment, the final pressures for vacuum-cooled cos lettuce were set at three different levels namely, 5.5, 6.0 and 6.5 millibar. The pressures were again experimented with three levels of holding time; 20, 25 and 30 minutes. The study results illustrated that the optimum operating process parameters for precooling cos lettuce to 4±1°C with initial core temperature of 19-21°C were at the final pressure of 0.60 kPa and the holding time of 25 minutes. The total cooling time was 39 minutes and the energy consumption was 4.17 kwh per 200 kilograms of cos lettuce. The electricity cost was 0.067 baht per kilogram of fresh weight. The weight loss during precooled cos lettuce with the previous optimum parameters was 2.58%. The effect of final pressure on cooling time, weight loss percentage and temperature variation of cos lettuce were also investigated. Weight loss occurs during vacuum cooling since the chamber cooling effect directly comes from water evaporation from the produce. Weight loss percentage was closely related to the holding pressure and holding time. For the same holding time, the lower pressure was used, the higher weight loss was experienced. For the same pressure, the longer holding time was used, the higher weight loss was found.