Dehydrated and Freeze dried foods
Drying food can be done by two processes. Both strip the water out, but the details differ. The cheaper method is to use gentle heat and air flow to evaporate the water off the surface of the lumps. This produces sun-dried things and is also used in domestic dehydrators. The expensive method is the opposite: the food is frozen way below zero very quickly, and then put in a vacuum. The ice evaporates straight into a gas (sublimes), giving the ""freeze-dried"" products. The difference is that ordinary drying lets the soluble bits of the food migrate to the surface of each lump, and they can form a very tough, almost waterproof, skin there. With the freeze-drying the soluble bits stay exactly where they were, and the result is a porous object which absorbs water readily and looks a bit like a sponge.
What difference does this make to a bushwalker? Well, the waterproof layer from ordinary drying can easily survive ordinary cooking. Bits of it go through your stomach into your bowels, where they finally rehydrate. But down there they are beyond the digestion system, so they tend to ferment instead. Much gas is produced, which leads to the well-known bad dehi smell in the tent in the middle of the night (with matching noises). For some this is a problem ...
The freeze-dried foods do rehydrate quickly - often more quickly if allowed to soak in cold water for 5 minutes first. There are several brands of meat and meals made in Australia/NZ using this process: Settlers Beef Mince, BackCountry (NZ), Adventure Foods (ex-Army) and Surprise vegetables. There also some imported brands, but they do not include any meat dishes because of some strange Customs ruling.
Dehydrators: drying your own food
Given the comments above about drying food and safety, this is an area for a little caution. Maybe the best advice is to practice at home, eating the results at home first. There seem to be various home dehydrators available in departments stores around Australia. They all operate on blowing hot air over the food: none of them make 'freeze-dried' food. .
Various readers have asked about powdered egg. Sometimes a bushwalking shop will carry this, but these days the trend is for them to sell only pre-packaged foods. So you will probably have to either buy from a manufacturer or dry some yourself.
Do a dozen or more at a time. Separate the whites and yolks. Whip the whites into a stiff meringue adding 1 teaspoon cream of tartar per dozen eggs. Meringue, when ready, is like a stiff whipped cream. Place this meringue on a plastic film covered tray and dry it at 110 to 120°F [around 46C]. It dries in about an hour. When dry, crush the whites into a powder and package them separately.
Whip the yolks until they are smooth and pour this puree on a plastic film covered tray. Dry as a leather. When it feels dry, crush it and grind it into a powder. Redry this powder on a tray. When finally dry, grind it again and package it separately.
To cook one egg, take 1 tablespoon of both the dried yolk and white and mix with 3 tablespoons of water. Let this sit 5 to 10 minutes until it becomes thick. These are used as fresh eggs, and they taste like fresh eggs.
Most walkers take powder or dried milk with them, but experiences are extremely varied when it comes to mixing that powder with water to make milk. Some walkers have a lot of trouble getting the lumps out, while other walkers just chuck the lot in a cup, stir, and it's done. Is there a secret here? Oh yes indeed! Full cream powdered milk can be hard to mix with cold water; the fats in it get in the way and stop the water from mixing easily. It is best to make a paste first, grind that around to get rid of the dry or half-dry lumps, then dilute it down. Or you can heat the water up, and the fats in the powdered milk then dissolve reasonably well. But skim milk powder mixes very easily even in very cold water. The author uses skim milk at home, and takes skim milk powder walking. Currently that's 'Diploma Skim Milk Powder', and it mixes with just a few stirs. 'Dutch Jug' is another skim milk powder which has given fairly good mixing.
However, in 2009 we discovered ""Nestles Sunshine Instant Full Cream Milk Powder"" (in a tin) which does seem to dissolve just as easily as the best skim milk powders. The word 'Instant' has a red background in the tin. You should note that many things, like Instant Pudding etc, do turn out much better with full cream milk rather than skim milk.
Then there is custard: this needs powdered milk. Made with skim milk custard is not all that inspiring: you really need full cream milk. Fortunately you have to heat the water to make custard, so as long as you stir carefully it will dissolve. Of course, hot custard over fruit cake in the evening, at the end of a meal, is a needless indulgence.