At a temperature of -196 degrees Celsius, liquid nitrogen freezes food very rapidly upon contact. Freezing food with a conventional freezer can form large ice crystals within the products that degrade their integrity and nutritional value. Smaller ice crystals are formed when flash freezing with nitrogen, keeping foods intact. The low temperature also effectively kills any food-borne pathogens or parasites.
Things You'll Need
Acquire liquid nitrogen from a cryogenic laboratory supplier or a welding supply company. The amount of nitrogen needed varies depending on the quantity of food, although a four liter Dewar flask usually suffices for domestic applications.
Prepare the food and arrange all items in a specific order. For instance, if preparing to freeze dry fruit, peel if necessary, cut to the desired size, remove the seeds and store in an airtight container until needed. Place items in the work area within reach and in the order of their intended use. If working from left to right, place the gloves and goggles first, the food items second, the nitrogen and stainless steel bowl third and the freeze dryer last. The food vacuum can remain stored away until needed. Having all the items and equipment in place before beginning reduces the risk of an accidental spill and promotes efficiency.
Place the food items in the stainless steel bowl. While wearing gloves and safety goggles, carefully and slowly pour liquid nitrogen over the food, then wait until the residual nitrogen has evaporated before proceeding.
Check to make sure the chamber of the freeze drying machine is clean and dry, then place the frozen food in the chamber and securely close the lid.
Activate the freeze drying machine. This initiates a vacuum pump that lowers the atmospheric pressure inside the chamber to below .06 ATM (standard atmosphere). Refrigeration coils then lower the temperature inside the chamber to -50 degrees Celsius. The device gently heats the food, purifying the ice crystals and beginning the drying process. This can last from a few hours to several days, depending on the amount and size of the food items. Some devices allow for the removal of the item during the drying phase to determine a product’s readiness, while others use an indicator light to signal completion.
Put freeze dried food items in plastic bags used specifically for vacuum sealing. Place the bag in the vacuum sealing device, shut the closure and activate. When finished removing the air from the bag, the machine will seal it and indicate the end of the process, usually with an indicator light. The nearly oxygen-free environment of a vacuum sealed bag helps to achieve the maximum benefits of liquid nitrogen use.
References and ResourcesTLC: How Freeze Drying Works
Williams College: Liquid Nitrogen
Science: Food Preservation
ResourcesCole-Palmer: Cylindrical Dewar Flasks With Handles
Brnuerosci: Dangers of Liquid Nitrogen