Home Freeze Drying
Home Freeze Drying
Recently there was an article brought to my attention. It was about a home freeze drying experiment. In essence the experiment was to slice the material thinly. The slices are spread in a single layer on a tray or pan. This is stuck in your freezer. Then you wait and wait and wait while the liquid slowly sublimates out. This could take weeks to accomplish for a very small batch. Most of us are familiar with the end result. We know it as freezer burn in extreme cases.
While this may work. I doubt it would be worth the effort. First it would not produce worthwhile amounts. Ties up or causes access problems with limited freezer space for weeks on end. The final product would be of poor quality.
Iíll admit that I havenít tried this yet myself. Therefore I havenít the necessary timing and the amounts used to impart. But I have studied how it is done commercially. I have devised some work-a-rounds so it could be done at a home or cottage industry level. The theories behind my suggestions are sound and should be easily accomplished.
While most foods can be dried in the conventional manor. There are some that will only work by being freeze dried. Both methods of drying can also be use to process, cooked foods as well as those that are raw. There is also difference in texture and form using this method. Also there is a difference in flavor between the different styles. Reconstituted freeze-dried foods are more like fresh foods or like it was just prior to the drying process than those that are dried normally. Freeze-drying is especially good for combination products like complete meals, especially stews and the like. Such stews could then be either vacuum packaged or bulk stored. A soup base or ersatz bullion can be made through freeze-drying. A stock or soup base is cooked. Then it is cooked down or to use the cooking term reduced by simmering driving out most of the water. It would be reduced almost to the point where it is in danger of burning. The reduced stock would then be poured into a thin layer onto a sheet and frozen. The ice sheet of stock would then be broken into conveniently sized pieces to fit the drying racks. Then vacuum dried. The result would be powdered for storage. Of course you would need to figure out the proportions of dry base and water to make soup.
This is the how and why it is done commercially. First whatever is to be dried is sliced thinly or chopped into a fine dice. The size is as small as possible that is practical for the end application it will be used for. By being as small as practical make both the freezing process and the eventual drying much quicker. The product is spread evenly in a single layer on trays or sheets. Then is placed in a blast freezer. These freezers operate at about 50 below rather than the 0 of a home freezer. This quickly freezes the product. By rapid freezing the ice crystals that are formed in the food are smaller. This leaves the food with a better texture. The smaller crystals also speed the drying process. That is because there is more surface area per volume of ice. The trays are then placed in large chilled vacuum chambers. The vacuum causes the water to sublimate out of the food at a lower temperature and at a quicker rate. Sublimation is the process where a solid goes directly into a gas without going through a liquid phase. Again the speed of processing helps preserve the quality and texture of the food.
While we preppers cannot duplicate all of the techniques and equipment. We can come fairly close on our small scale with a little finagling.
Most of us donít have access to a blast freezer. So if all else fails we might get by with the slow process of using the home freezer. Call it about 24 hours for a single batch. But as I said we are going a little out of bound. We are going to engage in a little down home engineering.
Many of us do or have done a bit of home processed bulk storage. While nitrogen is the preferred and commercially used gas for an inert storage atmosphere most of us donít usually have access to it. So we usually go with the next best method. By that we put a chunk of dry ice in the bottom of the buckets and flood the container with CO2. One thing we know from handling dry ice is that it is very, very cold. Because we have used it we know how and where to get more. If you donít, look in the yellow pages. So we might just be able to approximate flash freezing or at least a lot faster than by normal methods. The simplest freezer that I can envision would be an ice chest. Although you can get as elaborate as you wish or can afford. A layer of dry ice is placed in the bottom of the chest. A layer of food then follows this. Finally covered with another layer of dry ice. These alternating layers of food and dry ice can be continued until the cooler is full. Finally the lid is closed. Then you wait until the food is frozen. Before continuing on to the drying stage. The amount of time will need a little trial and error to get the amount of time needed for it to work. It shouldnít be very long. You may even be able to freeze more than a single chest load with the same dry ice.
You will not want to simply toss the food loosely into the chest. You will want to make it into some form of package for ease of handling. I can see 2 ways, 3 if you use a combination of both, of making the packets. Odds are you might be able to come up alternate methods. The first up is the all foil method. First you take a sheet of foil that will fit into the cooler and cover it with a single layer of food. Then you cover it with a second sheet or a continuation of the first sheet folded over the food. If you crimp the edges together this should make a fairly neat and easier to handle. The second style is to use cookie sheets. The ridged cookie sheets should make handling even easier. Cover the first sheet with food like above. The second cookie sheet should nest into the first so that the bottom presses directly onto the food. The combined method uses a cookie sheet is used for the bottom and then is covered tightly with foil. With these sandwiches the food is in essence surrounded by and in direct contact with the dry ice through the highly conductive metal.
If cookie sheets are used there mass will retain a lot of heat. Therefore they can be pre-chilled in the freezer before assembling the packets. This would reduce the freezing time and save on the amount of dry ice needed. Another method of saving would be to assemble all the food packets for the first load prior to unpacking the dry ice. If subsequent loads are going to be processed those packages can be made up while the first load is freezing. By pre-packing there will by no delays or interruptions while packing the freezer. It makes for more efficient use of the expensive dry ice and speed up the entire process.
You might also want to do your bulk packing at the same time and find a way to vent the CO2 into the buckets from the cooler. Since CO2 is heavier than air a tube or several tubes could be run from the bottom of the cooler. The cooler would be placed above the buckets. The tubes would then run to the bottom of the buckets. The reason I say several tubes is you could probably process several buckets at the same time. When the buckets are full of CO2. Simply pull the tube out and seal the bucket. While CO2 is heavier than air it is not so by much. You have to handle the bucket carefully to prevent spilling the CO2 and mixing in air. That way the expensive dry ice would do double duty. Waste not want not.
Like the commercial process one of the keys is to slice the food as thinly as possible or to use a fine dice. The smaller the pieces are the faster the process will proceed. The faster things are accomplished the higher the quality of the end product you will produce.
OK, now that we have the food frozen. It can be temporarily stored in a regular freezer while it is awaiting its turn at being vacuum dried.
The next step in the in the process is drying the food. Therefore we need a way to place the food in a vacuum. The closer to an absolute vacuum that we can get the faster and the more complete the process. The big boys use huge massively built chambers to be strong enough to withstand the outside pressure. We canít compete on that scale. By doing it in small batches we can come close. The smaller chambers donít have to be nearly as strong.
The vacuum sealing machines made for home use seem to be a good starting point. You cannot do it in the plastic bags. As they draw down upon the food there is no place for the moisture to go. What you need rigged chamber to hold the food. Also in a ridged chamber the food can easily be spread and separated. As we know from conventional drying. That if we pile the food into a big lump it wonít dry evenly or very well. Each piece needs to be surrounded by air or in this case vacuum. While you doesnít have to be extremely fussy. You want the pieces in essence a single layer with each piece apart from the others. You donít have to go in and measure with a micrometer and make sure that things are exactly space. The occasional piece touching another doesnít hurt.
If you have a vacuum sealer or have seen their commercials then you know that some brand sell and can be connected to that machines ridged containers. Some also have the provisions to seal regular canning jars. In that method a cap that is connected to the machine is placed over a jar with a lid place. After the air is pumped when the cap is vented the canning lid seals the jar. These canisters and canning jars are for vacuum storage of loose materials or things that would be crushed in the typical vacuum bag that is the staple of these machines. These foods would be things like cereals, chips, coffee and flour. These containers or canning jars can work as vacuum drying chambers.
While this might suffice for our purposes. I have seen and read an article years ago about a home brewed system that can accomplish the same function. The writer designed a system for sealing canning jars. There is nothing to say we couldnít use the same basic design as a vacuum drying chamber. It might be even better for the job at hand.
A piece of PVC pipe is used that is just large enough in diameter and tall enough to hold a canning jar. About 6 inch PVC would be about the largest practical size that could be used. The top and bottom of the tube are closed with steel plates. I donít remember how thick they were in the article but Ĺ inch should do nicely. The bottom plate can be either sealed on permanently or a gasket can be used. The reason you might want the bottom plate removable is for easy cleaning. As the top plate has to be removable so you can put things in and take them out again it needs to be sealed with a reusable gasket. 2 or 3 holes need to be drilled and threaded into the top plate. One hole is to attach the chamber to the vacuum pump. If a shut off valve is included in this line the pump can be used for other purposes. The most likely reason would be so you can pump down other chambers at the same time. The second hole is to install a vent. You will need a vent to equalize the pressure so you can open the chamber when the job is finished. The last hole is to install a vacuum gage. While not absolutely necessary the gage can be very useful to allow us to keep track of what is going on inside the chamber. Actually any or all of these functions and attachments can be combined into any hole. You could in reality get by with a single hole.
There is also the possibility of constructing a larger chamber. You could then load several jars at once or process a larger amount of food. Furthermore you could construct several chambers. That way you can run several batches while waiting for them to dry. Although if going with a larger design you would want to upgrade to steel pipe over PVC for added strength. When using steel pipe you could weld the bottom plate onto the pipe. I say pipe rather than building a box because the round shape is inherently stronger.
You could use your vacuum sealer or buy a dedicated vacuum pump to run the system. But by our very nature we want to make do with what is available. Also to do it in the most cost effective manor. I understand that that with a few minor modifications that a car air conditioning pump can function as a vacuum pump. One of these could be easily picked up at a junkyard at a reduced price. Or from a car that is already owned that the AC no longer works in for free. The AC unit or other vacuum pump can be run from a gas engine or an electric motor. For difficult times the pump could even be rigged to be powered with a hand crank or be rigged for pedal powering.
Of course you will need to construct something to hold several layers of food in the chamber. The assembly will need to be similar to the stacked trays in a conventional dehydrator. The best material for making the trays is plastic fine mesh screen. For ease of handling you can either make a rack that the shelves can be slipped into. Another way would be to construct trays that lock together to form a stack. These would need to be sized to fit into the chamber. By being either a stack or rack system the entire load of the dryer can be inserted and removed as a single package.
As the water sublimates into the chamber the vacuum will begin to drop. That is why the gage is so helpful. You will probably need to redraw the vacuum several times during the drying process until complete. This may take several hours or even days. How long it takes depend upon the total amount of food loaded and the size of the food pieces being dried. Another factor will be the strength of the vacuum and how often it is drawn back down. You would have to experiment to find out the expected time to process a batch. Also different foods will probably require different amounts of time. A drain valve place at the bottom of the pressure vessel might be a nice addition. That way you can drain any water that condenses on the sides and runs to the bottom.
You can even emulate the pros and chill the vacuum chamber. The easiest way would be a bucket for a single chamber. A horseís watering trough could accommodate a dozen or more chambers at the same time. Fill the tub or bucket with ice and water. To make it even colder add some salt to the bath. Just the way you do it with a home churn ice cream freezer. Depending upon the time it takes you might need to add more ice periodically. On thing you want to make sure is that the water level does not rise above the tops of the chambers. It wouldnít do to open the chamber and flood it with water. First it would defeat the drying. Second you would add a dose of salt to the food. While you could pull the chamber completely out to empty the food. If you are going to process more that one batch it is easier to leave the chamber in the bath and reload it in place. If using a chilling bath you may need to weight the containers so they donít float.
ďA word of caution.Ē Cooling down PVC can make it brittle. This could cause the chamber to fail.
For the freezing I would suspect that it would only take minutes or probably less than an hour to accomplish. That is unless you are using very little dry ice or a relatively large batch. It is that cold.
Figure that a 6-inch by roughly foot tall chamber loaded as much as possible with food. If pumped down fairly often would probably only take a few hours. Again it would take some experimentation to find out for sure. Over exposure to either the cold or vacuum wonít hurt. Running things too short of a time would result in ruined food due to spoilage. Worse yet if you stored it than ate it. Food poisoning isnít fun and could be fatal.
When experimenting, you will want to do small batches to get the timing down and reduce waste.
I would recommend starting with the vacuum part of the setup. Even if things fail with the freeze drying. The chamber and pump can be used for other things. Therefore it will not be as much of a waste of resources. Since you wonít be store the first small batches. But will be reconstituting them to see how it works. You can do the freezing in your home freezer. Then once you are satisfied that you have that part down pat. You can move on to the rapid freezing portion. Then you can determine is the results are worth the extra expense.
That should give you a way to freeze dry food at the home level. It might also spark some other ideas and methods to do the same thing.