Recent Developments in Freeze Drying of Foods

Freeze drying is a drying technology involving cryogenic temperature and extremely low pressure, to achieve high product quality. The process depends on dehydration by sublimation of a frozen product. Because of the absence of liquid water and the low temperatures needed for the process, most of the deterioration and microbiological changes are stopped, which results in a final product with good quality. The freeze-drying technique is a combination of freezing and drying processes. Earlier efforts were mainly aimed at optimizing the drying process without considering the possible influence of the freezing step. During freezing, a solution undergoes various physical changes, including a supercooling state. The degree of supercooling of a solution dictates the ice crystal properties (size, number, and morphology) during freezing, which impacts the subsequent drying process including the resistance to water vapor flow. Freeze drying coupled with other processing technologies such as infrared, microwave, ultrasonic, and pulsed electric field shortens the drying time, speeds up the drying rate, and saves energy. This chapter discusses the combination of freeze drying with other techniques and its effect on the freeze-drying process. In addition, it describes studies on the applications of freeze drying for processing of fresh produce, meat, poultry, seafood, and coffee over the past decade.