Technical aspects in freeze-drying of foods

Drying is a widespread concept in the food industry, typically used to convert a surplus crop into a shelf-stable commodity. With advancement of technology, however, there is interest in moving forward from the traditional convective air drying that is most widely used today for foods, to maintain at a very high level the nutritional and organoleptical properties of the initial fresh product. Freeze-drying (FD) produces the highest quality food product obtainable by any drying method, but it is considered the most expensive operation for manufacturing a dehydrated product owing to high energy consumption and high costs of both operation and maintenance. Microwave freeze-drying (MFD) and atmospheric freeze-drying (AFD) have been developed to reduce the FD energy consumption. The product quality of these two drying methods is similar to FD, due to removal of water content in materials by sublimation in both MFD and AFD. Although a significant amount of scientific research has been carried out in the field of sublimation drying, there are only a few comprehensive summarizations about the various sublimation drying methods. As a result, this review aims to highlight some of the latest and most notable advancements in sublimation-drying of foods, with main emphasis given to recent developments of reducing energy consumption of FD process and suggests future research areas on sublimation-related drying.