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12344Re: [John Muir Trail] Score! Permit granted. & 2.5 lb. a day...wet?

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  • John Ladd
    Feb 2, 2011
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      Don asks

      I'm having a hard time envisioning a freeze dried product having any more air in it than the same non freeze dried product. I stopped using freeze dried food for a number of reasons, but entrained air was not one of them so I am curious.

      Sorry if I sound pedantic, but I've thought about this a lot.

      Most of the freeze-dried product I see is cooked first, and then dried.  Results in a lot of trapped air because removing the water from something pre-cooked leaves little air voids where the water used to be. It's "sublimated" out. Same issue as instant rice. Or at least used to.  I haven't used freeze-dried in years.

      Traditional foods aren't usually cooked first so they don't get a bunch of added water, which later needs to be replaced by air.  Compare rice and instant rice.  Traditional dried hams (Proscuitto) vs. commercial cooked hams.

      Wikipedia article has some reference to this

      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.

      For more, see


      You can usually figure if something has entrained air by just hefting it in your hand.  Things that don't have entrained air have a heavy feel.  Like a bag of cream of wheat.  Things with a lot of air in the substance or between pieces feel light, like a bag of Oatmeal.  Or compare a bar of baker's chocolate to a KitKat bar - rice to instant rice - couscous to Top Ramen, etc. - a bag of sugar to pieces of candy.  Counter-intuitively, you want things that feel heavy but have no water content so that every ounce is nutrition but you aren't filling your bear can with 50% air - like peanut butter or olive oil or even lard, if you are willing to go that far.

      Also it seems like some of the freeze dried stuff I see has stiff pieces that are shaped in ways that leaves voids between the pieces.  A lot of traditional backpack foods have small particle sizes to make them cook more quickly but also makes the pieces pack more tightly.  Or they are sfter so that the air can be pressed out of them by just smushing them hard (like dried apricots).

      John Curran Ladd
      1616 Castro Street
      San Francisco, CA  94114-3707

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