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Monday, February 23, 2009

The Basics about Freeze-Dried and Dehydrated Foods

A reader recently wrote this comment:

As I am very new to this and have had a hard time wrapping my brain over freeze-dried and dehydrated foods. I understand I need to add water to "revive" them. If I buy a #10 can of Freeze-dried ground beef (servings 24 1/2 cups) how long will the food be good? Right now we are two people but we could quickly become 8. But I don't want to he to eat ground beef every meal. All information about freeze-dried or dehydrated food would be appreciated. February 18, 2009 5:45 PM

Our info is towards the bottom but first, here's the specifics from what we've researched on the internet for people new to using dried foods:

Freeze-Dried Food:
Link: Freeze-drying (also known as lyophilization or cryodesiccation) is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. Freeze-drying works by freezing the material and then reducing the surrounding pressure and adding enough heat to allow the frozen water in the material to sublime directly from the solid phase to gas.

The freeze-drying process: There are three stages in the complete freeze-drying process: freezing, primary drying, and secondary drying.

(1) Freezing
The freezing process consists of freezing the material. In a lab, this is often done by placing the material in a freeze-drying flask and rotating the flask in a bath, called a shell freezer, which is cooled by mechanical refrigeration,
dry ice and methanol, or liquid nitrogen. On a larger-scale, freezing is usually done using a freeze-drying machine. In this step, it is important to cool the material below its eutectic point, the lowest temperature at which the solid and liquid phases of the material can coexist. This ensures that sublimation rather than melting will occur in the following steps. Larger crystals are easier to freeze-dry. To produce larger crystals, the product should be frozen slowly or can be cycled up and down in temperature. This cycling process is called annealing. However, in the case of food, or objects with formerly-living cells, large ice crystals will break the cell walls (discovered by Clarence Birdseye). Usually, the freezing temperatures are between −50 °C and −80 °C. The freezing phase is the most critical in the whole freeze-drying process, because the product can be spoiled if badly done.

Amorphous (glassy) materials do not have an eutectic point, but do have a critical point, below which the product must be maintained to prevent melt-back or collapse during primary and secondary drying.
Large objects take a few months to freeze-dry.

(2) Primary drying
During the primary drying phase, the pressure is lowered (to the range of a few
millibars), and enough heat is supplied to the material for the water to sublimate. The amount of heat necessary can be calculated using the sublimating molecules’ latent heat of sublimation. In this initial drying phase, about 95% of the water in the material is sublimated. This phase may be slow (can be several days in the industry), because, if too much heat is added, the material’s structure could be altered.

In this phase, pressure is controlled through the application of
partial vacuum. The vacuum speeds sublimation, making it useful as a deliberate drying process. Furthermore, a cold condenser chamber and/or condenser plates provide a surface(s) for the water vapour to re-solidify on. This condenser plays no role in keeping the material frozen; rather, it prevents water vapor from reaching the vacuum pump, which could degrade the pump's performance. Condenser temperatures are typically below −50 °C (−60 °F).

It is important to note that, in this range of pressure, the heat is brought mainly by conduction or radiation; the convection effect can be considered as insignificant.

(3) Secondary drying
The secondary drying phase aims to remove unfrozen water molecules, since the ice was removed in the primary drying phase. This part of the freeze-drying process is governed by the material’s adsorption
isotherms. In this phase, the temperature is raised higher than in the primary drying phase, and can even be above 0 °C, to break any physico-chemical interactions that have formed between the water molecules and the frozen material. Usually the pressure is also lowered in this stage to encourage desorption (typically in the range of microbars, or fractions of a pascal). However, there are products that benefit from increased pressure as well.

After the freeze-drying process is complete, the vacuum is usually broken with an inert gas, such as nitrogen, before the material is sealed.

At the end of the operation, the final residual water content in the product is around 1% to 4%, which is extremely low.

Properties of freeze-dried products
If a freeze-dried substance is sealed to prevent the reabsorption of moisture, the substance may be stored at
room temperature without refrigeration, and be protected against spoilage for many years. Preservation is possible because the greatly reduced water content inhibits the action of microorganisms and enzymes that would normally spoil or degrade the substance.

Freeze-drying also causes less damage to the substance than other
dehydration methods using higher temperatures. Freeze-drying does not usually cause shrinkage or toughening of the material being dried. In addition, flavours and smells generally remain unchanged, making the process popular for preserving food. However, water is not the only chemical capable of sublimation, and the loss of other volatile compounds such as acetic acid (vinegar) and alcohols can yield undesirable results.

Freeze-dried products can be rehydrated (reconstituted) much more quickly and easily because the process leaves microscopic pores. The pores are created by the ice crystals that sublimate, leaving gaps or pores in their place. This is especially important when it comes to pharmaceutical uses. Lyophilization can also be used to increase the shelf life of some pharmaceuticals for many years.

Dehydrated Food:
Link from - Drying is a method of food preservation that works by removing water from the food, which prevents the growth of microorganisms and decay. Drying food using the sun and wind to prevent spoilage has been known since ancient times. Water is usually removed by evaporation (air drying, sun drying, smoking or wind drying) but, in the case of freeze-drying, food is first frozen and then water is removed by sublimation.

Bacteria and micro-organisms within the food and from the air need the water in the food to grow. Drying effectively prevents them from surviving in the food. It also creates a hard outer-layer, helping to stop micro-organisms from entering the food

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Okay... that's the technical definitions and information about freeze-dried and dehydrated food. Practical applications?

Can I freeze-dry food at home?
First, it would be very difficult for you to freeze-dry your own food. Dehydrating is more than satisfactory, cost efficient, space-efficient, and lasts a while. But if you really have the bug... I found this link: with this information:

An Experiment in Freeze-Drying:
You probably don't have a good vacuum chamber at home, but you almost certainly have a refrigerator. If you don't mind waiting a week, you can experiment with freeze-drying at home using your freezer.

For this experiment you will need a tray, preferably one that is perforated. If you have something like a cake-cooling rack or a metal mesh tray, that is perfect. You can use a cookie sheet or a plate if that is all that you have, but the experiment will take longer.

Now you will need something to freeze-dry. Three good candidates are apples, potatoes and carrots (apples
have the advantage that they taste okay in their freeze-dried state). With a knife, cut your apple, potato and/or carrot as thin as you can (try all three if you have them). Cut them paper-thin if you can do it -- the thinner you cut, the less time the experiment will take. Then arrange your slices on your rack or tray and put them in the freezer. You want to do this fairly quickly or else your potato and/or apple slices will discolor.
In half an hour, look in on your experiment. The slices should be frozen solid.

Over the next week, look in on your slices. What will happen is that the water in the slices will sublimate away. That is, the water in the slices will convert straight from solid water to water vapor, never going through the liquid state (this is the same thing that
mothballs do, going straight from a solid to a gaseous state). After a week or so (depending on how cold your freezer is and how thick the slices are), your slices will be completely dry. To test apple or potato slices for complete drying, take one slice out and let it thaw. It will turn black almost immediately if it is not completely dry.

When all of the slices are completely dry, what you have is freeze-dried apples, potatoes and carrots. You can "reconstitute" them by putting the slices in a cup or bowl and adding a little boiling water (or add cold water and
microwave). You can eat the apples in their dried state or you can reconstitute them. What you will notice is that the reconstituted vegetables look and taste pretty much like the original! That is why freeze-drying is a popular preservation technique.

Most of the "dried" food you get from suppliers are freeze-dried. It's a faster process on the commercial level.

Using Dried Foods:
The basic principles of using freeze-dried food and dehydrated foods are the same. Once you open a can or jar of freeze-dried or dehydrated food, you need to treat like any other food exposed to "regular" air. You need to reseal the un-used portions either in the same container, or put in a separate container you can seal (like with a sealing machine). Or refrigerate the unused portion. The key to making it last in it's dried state is keeping moisture out of the food. Moisture promotes the growth of bacteria, therefore, the lack of moisture means it will be edible and not poisonous for a great while.

Recently we opened some dehydrated peas that we bought 2 or more years prior. They were just as crispy as a new container. Most suppliers will put a time limit on how long their products will be good for, unopened and opened. Be sure to make a note of product recommendations. Don't come to us because you open a container, let it sit out, get wet, and then gets filled with bacteria that makes it inedible. Keep containers closed!

What about the big #10 cans of Freeze-Dried Meat?
One can should last you a while. When we open one that size, we use what we need, then seal the rest as smaller portions (about 4 servings per - we are a family of 3 and one eats enough for 2!) in smaller containers with moisture absorbers. We seal these tightly, mark when it was opened and what the original product label was, and keep track to make sure these are used up before opening another of the same item.

You don't need to exclusively use dried chicken dices until it's gone then start on the dried ground beef or sausage. Just be sure to keep adequately sealed and tracked.

How Do I Rehydrate:
Most people prefer to "rehydrate" (add water back in) the food before using. It will depend on how much moisture was taken out as to how much water or other liquid and time you'll need to rehydrate.

You should be using these products now anyway, so experiment! Take some freeze-dried (usually cooked) chicken dices and time them against rehydrating raisins or onion dices. Make notes in your "stored foods cookbook" so you won't have to keep experimenting everytime you want to cook with them.

Use water, stock, apple juice, etc. to rehydrate. Pour off the extra or use in your recipe.

You could also rehydrate as you cook. For instance, if you are making spaghetti sauce, add the freeze-dried ground beef to the sauce with just a bit more water. The meat will absorb the sauce, making the meat even more tastier and the sauce a little thicker.

Same with adding onions and garlic to your sauce. When I make spaghetti sauce, I start off with a little olive oil, dried onions, dried garlic, then add the spaghetti sauce. When it looks like the onions and garlic have rehydrated, I'll add a bit more water.

You're worked with pasta, right? It's basically the same thing, except you don't need to boil your fruit and veggies, unless you want to lose many of the nutrients to the boiling liquid.

Is Rehydrating Necessary before Using?:
No. Not necessarily. My Very Hungry Tween Son (VHTS) loves eating dried/freeze-dried mango, blueberries, blackberries and peas just as-is. I personally love eating dried banana chips or strawberry slices. Of course, one must drink plenty of water or other liquids because as it digests, it will rehydrate a bit inside of you, which could cause constipation.

How to Dehydrate:
Dehydrating is an excellent way to preserve your harvest, or even what you get on special at the farmers market or at the super grocery store. We talk about dehydrating in this blog from time to time. We try to give specific information for specific food items.

I hope this answered your questions about freeze-dried food and dehydrated food. Please feel free to ask more questions.


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