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On the first glance, it seems that most small organisms do not exhibit any interest in consuming any live or dead organic matter if it is completely dehydrated. Regardless if the food consists of protein, fats or carbohydrates, or any mixture of them, it appears to me that removing water makes bacteria, mould or even insects not consider it food.

Here I consider organic matter which can last for years without being subject to decay by small live organisms:

  • ghee, made from butter by boiling off all water, lasts for a year or more
  • freeze-dried food, which still can have 0.5% to 3% moisture, but can last 20 years
  • honey, supposedly can last forever

While the humble homo sapiens will happily put freeze dried milk in his mouth, most small organisms seem not to have any interest in organic matter with ultra-low moisture:

  • bacteria
  • mould
  • and to a certain extent even insects

For example I noticed ants will quickly gather around any drop of fat used in cooking, which has moisture and other impurities, but will ignore clean, dry ghee.

Why is moisture removal such an universal protection against small organisms and where exactly is the boundary between organisms which can't consume dry matter and the ones which can? Are there any known microorganisms which will consume completely dry organic matter?

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  • $\begingroup$ Just a hypothesis: a lot of these small organisms who are not interested in completely dehydrated "food" would also be ones who don't explicitly drink water (at least, not enough). Pretty much all organisms need water to live, they also lose water to the environment via urination, perspiration, or other pathways. For organisms that don't explicitly drink water, their major source of water would come from the food. So if the food doesn't contain enough water, they don't bother. $\endgroup$
    – Kal
    Aug 18, 2020 at 4:20
  • $\begingroup$ For microorganisms, the lack of water in the food may inhibit their ability to even break down the food, mechanically (too hard) or chemically (lack of solvent to facilitate chemical reactions). $\endgroup$
    – Kal
    Aug 18, 2020 at 4:29

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TL;DR If it doesn't last forever this is some organisms consuming it. It is a matter of ecological niches, what are the requirements (humidity, temperature, etc.) to live and reproduce

I believe we should separate the discussion in two: small organisms and microrganises

Microorganisms

Microorganisms that consume our food need to get there, either they got there from the air or they are already there. Other vectors can be insects, animals, water and soil but since this discussion is about food and we tend to keep our food protected from those vectors I'm going to ignore them.

Most of the microorganisms of interest (Bacteria, Archaea, Fungi & Oomycetes) exist in air as spores. This are analog to seeds, they are alive but inactive and need to arrive to a good environment to sprout. Spores can survive for many years, even millions of years("Revival and identification of bacterial spores in 25- to 40-million-year-old Dominican amber". Science. 268 (5213): 1060–1064.). This survival mechanisme includes dehydration and therefore rehydration is necessary for sprouting.

In addition some of these (mainly oomycetes) need free-water to live & reproduce while others require some levels of moisture.

Dehydrated organic material tends to be hygroscopic. Thus if the non-spore form of the microorganism lands on it the organism will probably loose water and die.

Microorganisms, like every organism, have niches they occupy. One of the characteristics of these niches is the temperature. Freeze-drying includes freezing which lowers the temperature below the survival temperature of many microorganisms and preserves the food. Microorganisms that grow in low temperatures grow slower so it will take more time for us to notice their presence. They still require water so the combination of freezing and dehydration provides a good protection.

Honey has a antibacterial activity in it preventing growth of microorganisms. In addition honey contains a lot of sugar and other molecules making it hypertonic to most microorganisms causing them to loose water if they land on it. Also his high viscosity prevents the free movement of microorganisms.

Small organismes

Many small organismes still need water to live and reproduce (like nematodes) Insects are highly specified to their environment and food intake. They might not be able to consume dehydrated food. In addition, as mentioned in the comments by Kal

a lot of these small organisms who are not interested in completely dehydrated "food" would also be ones who don't explicitly drink water (at least, not enough).[...] For organisms that don't explicitly drink water, their major source of water would come from the food. So if the food doesn't contain enough water, they don't bother.

Ants and other insects do eat dehydrated organic material like seeds, but it still has around %10 moisture Oil is not hydrated but liquid, since it is oil and not water

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