Article written by Lim Kong Sun
Freeze drying or lyophilization is a process in which water is removed from a product via sublimation. This is done by freezing the product in subzero temperatures prior to drying, which is done in a vacuum. Freeze drying is the preferred method of food preservation as it retains much of the shape, colour, flavour and nutritional content of the product. Having minimal or no water content, freeze dried products have long shelf lives of up to several years, with little or no preservatives. Apart from allowing easy rehydration later on, freeze dried products are lightweight, hence giving easy and economical transporting.
Used mainly for fruits such as durian, freeze drying not only maintains the original aroma and taste of the fruit, but also gives the final product an interesting texture, likened to a crispy and flaky biscuit. The final durian product can be eaten on its own or either used in confectionery making, such as chocolate. These products are highly sought after in countries where durians are not endemic yet in high demand such as China, as they last longer, but still having the very same smell and taste as the original fruit.
The freeze drying process begins by cutting the durian flesh into pieces and laying them onto trays. It is important to ensure that the durian pieces are spread out evenly with spaces in between them in order to increase surface area and prevent sticking. The trays are then kept in the freezer, allowing water in the product to solidify to ice. A factor which would affect process times would be sugar content of product, in which a higher sugar content would result in longer freezing (and drying) time due to change in physical bonds in the product.
After a certain period of time, the frozen durian is then transferred into the vacuum dryer. The vacuum drying process has to be ideally below 133 pascal to eliminate risk of liquid water formation during the freeze drying process as liquid water will have negative impact on final product quality. As heat (by warm water providing the infrared radiation) passes through the trays on the radiator plates, the solid ice sublimes to vapour and deposits as ice once again on the condenser, thus preventing liquid from entering and damaging vacuum pumps. Freeze drying leaves the durian product with little or no water content at all, hence, upon unloading, the freeze dried durian would need to be packed immediately due to its hygroscopic character. The final durian product (close-up view is pictured above) is now ready for direct consumption or other downstream applications. Click here to see a typical operation of the freeze drying of durian.