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We are going to China and then onto Mongolia from Australia. We would like to take dehydrated prepackaged meals to Mongolia and would like to know if this is permissible.

While the Customs Clearance for International Passengers lists animal and plant products as goods which have to be declared (item 5), they're described as items which will be "dealt with according to current regulations."

More informative is Travel China Guide, the largest online tour operator in China.

Its list of Prohibited Items describes (emphasis mine):

6. Animal and animal products:
   
   a. All living animals (excluding dogs and cats) and their sperm, zygotes, embryos and other genetic things.
   
   b. Uncooked or cooked meat (including internal organs); marine animal products;
   
   c. Milk of animal and its products, including raw milk, fresh milk, yoghurt, cream, butter, cheese and other dairy products which are not heat-treated as well as whey powder, silkworm chrysalis, silkworm eggs, animal blood and its products, products of aquatic animals and so on.
   
   d. Eggs and its products, including fresh eggs, preserved eggs, salted eggs, egg liquid, egg shells, mayonnaise and other egg products which are not heat-treated.
   

As dehydrating uses a heat treatment that effectively destroys disease-causing micro-organisms, your items should be acceptable. However, to avoid misunderstanding, it would be preferable that they be commercially produced, and remain sealed in their original and labeled packaging.

Although not a government source, Travel China Guide may be able to further clarify, as it organizes excursions such as treks to Outer Mongolia. The contact page includes an email format, as well as toll-free numbers (USA & Canada, UK, Australia).