RESUMEN
Freeze drying (FD) is an important and highly effective technology in food industry for retaining the quality in final dried product. This drying technique is performed at lower temperatures, restricting the damage suffered by thermally sensitive ingredients. However, FD consumes large amount of energy and required more time than conventional drying methods. The utilization of ultrasonic technology (US) as pre-treatment before FD represents a promising alternative in accelerating the drying process, decreases energy consumption and maintaining quality as compared to the non pre-treated sample. This review summarizes research progress and current studies in ultrasonic as pre-treatment for freeze drying (US + FD) technique. The impact of US + FD on phytochemical, color, texture and micro-structure of food are well summarized. The review also suggests that the optimised US treatment parameters are required to improve heat and mass transfer in food samples which help in speed up the drying process and reduction of drying time.