Lunar Homesteaders are going to produce lots of food (we hope!). They are going to need an efficient way to safely store the excess food. Freeze drying (aka lyophilisation or cryodesiccation) is a dehydration process that uses low temperature and vacuum to remove the water from food. This produces a higher quality product, with a longer shelf-life, than traditional dehydration using heat . The cool thing is that freeze drying can be used for more than food preservation. Pharmaceuticals, medical products, and industrial chemicals can also have their shelf-life extended by freeze drying.
Another advantage of freeze drying is that it greatly reduces the mass of the food. This will be a huge benefit when Homesteaders want to sell food to other settlements.
However, freeze drying requires more technology than heat dehydrating. We’re going to have to figure out how to make this work using only locally available resources and tools. It’s probably going to be a difficult challenge.
Expanded Data (all the data we can find)
Advantages of freeze drying:
- Final residual water content is 1% to 4% .
- The original shape and color of the food is maintained . Does not usually cause toughening or shrinkage .
- Nutrients are retained because of the very low temperatures .
- Only needs water (hot works better) to reconstitute
- Long shelf life – up to 12 months when properly packaged and stored . I’ve seen claims of up to 25 years. We’ll need to research this.
- Fewer crumbs
- Flavors, smells, and taste are generally unchanged .
- Rehydration is quicker, easier, and more complete because of the microscopic pores left in the material when the ice crystals sublimate .
Disadvantages of freeze drying:
- Water isn’t the only component that will sublimate. Acetic acid (vinegar) and alcohols could be lost, leading to undesirable results .
- Energy intensive – freezing, heating, creating a near vacuum.
- Microbial growth – The low temperature process doesn’t kill all the potential pathogens or spoilage organisms. The lack of moisture can only inhibit growth. Pathogens will begin reproducing once moisture is introduced.
- Silicone oil leakage – Silicone oil is commonly used to heat or cool the shelves in the freezer-dryer . The thermal cycling can cause oil to leak on to the food. Mass spectrometers are used to identify silicon oil vapor coming from leaks . Local production of silicon oil could also be a problem.
Freeze drying steps
- Adding components
- Increasing surface area
- Freezing and annealing
- The material is cooled below it’s triple point (the temperature where the solid, liquid, and gas phases of the material can coexist). This ensures that sublimation will occur instead of melting.
- Because the preservation of structure is important for food, the material must be rapidly frozen to prevent the formation of large ice crystals.
- Temperatures are usually between −50 °C (−58 °F) and −80 °C (−112 °F) .
- The freezing phase is the most important step in the process . This phase impacts the “speed of reconstitution, duration of freeze-drying cycle, product stability, and appropriate crystallization” .
- Primary drying
- Pressure is lowered to a few millibars .
- Just enough heat is applied for the ice to sublimate . The heat necessary can be calculated using the sublimating molecules’ latent heat of sublimation . Heat is introduced by conduction or radiation as the near vacuum makes convection ineffective .
- 95% of the water is removed at this step .
- Released water vapor is captured using a cold condenser chamber and/or condenser plates .
- This step can take up to several days .
- This is the most energy intensive step .
- Secondary drying
- Removes the unfrozen water molecules from the material.
- Governed by the material’s adsorption isotherms .
- Temperature is raised to break any physico-chemical interactions that have formed between the water molecules and the frozen material .
- Pressure is lowered to encourage desorption (typically in the range of microbars, or fractions of a pascal) .
- Once the freeze drying process is complete the vacuum is usually broken by nitrogen or other inert gas .
- The material is then packaged, usually with the inert gas .
- Vacuum chamber
- Holds the shelving and the material.
- Is insulated to keep the interior cold.
- Process condenser
- Refrigerated coils or plates. Internal or external to the chamber.
- Traps released water.
- Should be 20 °C (68 °F) less than the product during primary drying .
- Shelf-fluid system
- Needed to heat the product for the sublimation of water.
- Refrigeration system
- Cools the shelves and process condenser.
- Vacuum system
- Produces a vacuum of 50-100 microbar . A two-stage rotary vacuum pump is commonly used . Multiple pumps are needed for large chambers.
- Control system
- Controls the other components. Needed to keep conditions constant over the hours or days necessary to complete the product.
Types of freeze dryers
- Contact freeze dryers
- Uses conduction of heat from the shelf-fluid system to heat the material.
- Simplest type
- Downside is that the material is only heated on the side contacting the tray. This can be mitigated by increasing the surface area contact or placing the material between layers of heating plates .
- Radiant freeze dryers
- Uses infrared radiation to heat the material.
- Allows for very uniform heating of the surface but minimal penetration .
- Microwave-assisted freeze dryers
- Microwaves heat the material, allowing for deeper penetration . This helps speed up the process.
- More tech reliant and a very complicated process .
- Microwaves can generate an electrical field that causes the gases in the chamber to become a plasma . The plasma can burn the sample.
Resources (not used but potentially relevant)
- Chuño – a potato freeze dried without technology (en.wikipedia.org/wiki/Chuño)
- Meals for Mars (www.mealsformars.com/)
- Elements (getyourelements.com/) – Freeze dried meals
- Harvest Right (harvestright.com/product/home-freeze-dryer/) – Home freeze dryer
- DIY freeze dryers