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I have heard that no known long term shelf stable form of vitamin C is known.

I find this surprising, is it really true? What are the best options for astronauts to have vitamin C for long term (>1 year missions) ex: traveling to Mars.

I did some research on this site and found that they have successfully grown some high vitamin C foods in space: Has a source of vitamin C ever been successfully grown in space?. But I think solely relying own growing food at the right time is risky, and we should have some long term shelf stable option for emergencies.

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    $\begingroup$ Where did you hear that "...no known long term shelf stable form of vitamin C is known"? Please cite your sources. $\endgroup$
    – Stu Smith
    Aug 12 at 5:06
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    $\begingroup$ Here’s the first article (it’s like hit #3 on google) I found to corroborate it, Ctrl-C for Vitamin C and there’s a section describing NASA having sources which only lasted up to a year: google.com/amp/s/www.eater.com/platform/amp/2015/9/17/9338665/… $\endgroup$ Aug 12 at 5:10
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    $\begingroup$ I (and this is biased) feel like I’ve read the statement “no long term sources of vitamin C” many times in other articles as well. $\endgroup$ Aug 12 at 5:10
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    $\begingroup$ From a manufacture's website I found Vitamin C pills (nominally) degrades (at most) 10% every 2 years (under normal storage conditions). So doubling the intake compensates for 10 years of degradation. $\endgroup$ Aug 12 at 7:08
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    $\begingroup$ "No stable form of vitamin C" doesn't make much sense. Ascorbic acid is a very simple chemical - in fact it was the first vitamin whose chemical structure was determined and the first to be synthesized by non-biological methods (100 years ago). Of course it is biologically quite active, and it decomposes in the presence of water, but the pure anhydrous chemical could probably be stored for decades if not centuries at low temperatures. Modern mass production uses biological fermentation, but that is because it is cheap, not because it is necessary. $\endgroup$
    – alephzero
    Aug 12 at 20:05
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False premise: Even the article you are citing does not support your statement.

Vitamin C is very much storable in the long term. It's just that the raw form of most of the natural foods that contain vitamin C are not a suitable medium in which to do this storage in the long term.

Yes, it does state that "Vitamin C, for one, breaks down easily in foods" in "some astronaut food staples". It is a matter that needs some attention, meaning they need to provide supplements or select some food forms where the vitamin C (and vitamin B12, and folic acid, and vitamin A) last longer.

However there are food types and storage methods that do preserve these quite well.

For vitamin C, it is the presence of oxygen and water and/or heat in the stored food that degrades the vitamin C. A suitably dehydrated product, or one where the food is stored in oil, will retain its vitamin C indefinitely. (Well, longer than your 5-year requirement). Vitamin C stored as a supplement of anhydrous ascorbic acid powder and stored in a freezer will last, literally, centuries.

Obviously fresh produce is preferred where at all possible, and palatable forms of stored food are needed, and the final nutrient balance of the food must be correct, but the situation is a matter of optimizing the best method, not an insurmountable obstacle.

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Not to disagree with PcMan's answer as marked accepted, but adding some more information that might be interesting and could pass as an answer in its own right :-)

Thing is, before space exploration, people explored the oceans. Some voyages also figured in the >1year range. Big problems due to lack of vitamin C: the legendary and notorious scurvy...

People like James Lind and captain James Cook way back in the 1700's figured to take along citrus, which worked very well for reasons we now take for granted. But only on shorter voyages, due to spoilage. Cook however is reported to have figured out that sauerkraut also worked to prevent scurvy, and the preservation method made it to keep much longer than fresh produce: a 3 year voyage was made possible.

While sauerkraut has a lower Vitamin C content than e.g. citrus, it still contained enough to keep those sailors healthy. Vegetable preservation via fermentation is found in numerous traditional cultures (see eg. Korean kimchi) and it is quite conceivable to apply similar fermentation techniques to vegetables with a higher Vitamin C content (bell peppers and chillies, broccoli, etc.)

So while the vessels and technology change, the problems stay the same, and the solution could be the same.

Of course, fermented vegetables may also have a few drawbacks with regards to space travel, such as:

  • mass of the still-hydrated vegetable matter plus the brine it is packed in (although that could be recycled for water needs). Every gram of mass costs megabucks to get into orbit. Dehydrated food and laboratory-prepared/purified supplements will probably be with astronauts for some time yet even if for this reason alone.
  • would the relevant space agency be OK with all those microbes on board (even though they are touted even by conventional medicine for they benefits to gut health)? Can't really zap them either since the microbes are what preserves the food and its nutrient content (by outcompeting putrefying microbes).
  • the fermentation process seems to defy mass production (mass-produced sauerkraut etc. is obviously available on the market, but often pasteurized, laced with preservatives, or prepared via another process involving heat and vinegar, which I think may be detrimental to the nutrient content). And because of the nature of human-induced fermentation, there are bigger variations in the final product, with the odd batch going bad. (A random guy on the internet who's made some sauerkraut in his kitchen isn't going to know all the figures or have done analyses, which is left as an exercise for the space agency interested in this route ;-) )
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