Effects of initial temperatures on vacuum film cooling and vacuum spray cooling on apple juice and milk

Abstract

This work aims at evaluating the effects of different initial temperatures on vacuum film cooling (VFC) and vacuum spray cooling (VSC) on apple juice and milk as compared with water. For VFC, the maximum cooling rate increased with increasing initial temperatures, while there was no obvious cooling curve pattern for VSC. Results from CCD camera during the VFC process showed that the maximum expansion height increased with initial temperature. For VSC, the mean diameter of droplets of water, apple juice, and milk decreased with decreasing chamber pressure. GC-MS results showed that both VFC and VSC processes significantly reduced the content of volatile compounds in apple juice and milk. The 1-hexanol in apple juice could be obviously recovered in the cold trap, while the cold trap had no obvious effect on the milk. In addition, higher initial temperatures caused more loss of volatile compounds in both apple juice and milk. Practical applications Based on the evaporation of free water in foods under vacuum conditions, vacuum cooling is a rapid cooling method. However, studies of vacuum cooling on liquid food are limited. The current study investigated the effects of different initial temperatures on water, apple juice, and milk vacuum film cooling and vacuum spray cooling, assisted by a high-speed CCD camera for observing the flash evaporation phenomenon, and the effects of flash evaporation on cooling temperature curves and losses of volatile compounds. The results from the current study should provide more information for the food industry in applying vacuum cooling for liquid foods.