Prior works: Space Food I: Physicochemical Manufacturing, Space Food II: Microbial Manufacturing, Space Food III: Botanical Manufacturing.
Now that we've looked into what can be made and recycled in many different ways, let's decide how we should do it for real. Let's first look at daily requirements of nutrients for humans to see which diet components are the most massive, and use that to decide what we can bring to space and what we should grow or manufacture along the way. I'll mostly mention men rather than women throughout this essay, not because only men go to space, but because men generally have higher dietary intake requirements and I'm designing a diet for the worst case scenario. Men are the worst case scenario.
I. Manufacturing fatty acids and carbohydrates as calorie sources
Calorie sources are the hardest nutrient to bring because of their large mass. An adult man needs about 2,500 kcal/day. Suppose we brought a person-year's worth of calories to space in the form of table sugar (for a Terran year that is, not a Martian year). Table sugar provides 4 kcal/g, everyone says with only one significant digit, so that's 625 g sugar/day or 228.281 kg sugar / person year. That's a lot of mass to throw from a gravity well! That's 503 lb sugar / person year in clown units.
Manufacturing calorie sources chemically is a pretty good deal for space. Compared to growing your calorie sources, you don't have to tend to plants and wait for them to grow and then possibly deal with low food yields and waste biomass, and also you don't have to count on your ability to keep edible microbial populations productive and free of infection while growing them in a tin can (spaceship or planetary habitat) with a crew of astronauts who are basically tubes of poop covered in yeast.
The best physiochemically manufactured calorie sources we found in Space Food I were synthetic fatty acids (which can be esterified with glycerol to make Imhausen margarine) and the highly edible polyol carbohydrate glycerol (which is already a normal part of human diets as a component of triglyceride oils and fats).
Margarine made from plant oils provides 7.18 kcal/gram (very calorie dense), which means you'd need to eat 348.19 g plant margarine / day to meet calorie requirements on an all margarine diet, and Imhausen margarine (the fatty acids of which are made, more or less, by the catalytic oxidation of alkanes produced through the Fischer-Tropsch process) is probably comparable to plant margarine. In clown units, that's 0.7 lb or 1.4 cups of margarine per day. Half a cup of margarine for a meal is doable for motivated individuals. For reference, half a cup of butter with a meal is common for arctic sledders (alongside other things like freeze-dried spaghetti and hot chocolate). I don't know much about human consumption of long-chain fatty acids that haven't been esterified (e.g. esterified with glycerol in Imhausen margarine) if you're interested in skipping a step in the manufacturing. However, if you want to separate out and consume independent quantities of fatty acids and glycerol, I do know that fatty acids can be esterified with other alcohols to make highly non-toxic food compounds (generally with really nice fruity flavors). Some highly edible fruity esters were covered in Space Food I.
Now for Glycerol. Glycerol is part of Imhausen margarine, and its low toxicity is part of why triglycerides like fats oils and margarine have low toxicities; with great generality, esterifying a fatty acid with glycerol lowers its toxicity (i.e. increases its LD50). Glycerol is also sweet (and a little spicy in my experience), so making it and keeping some separate from the margarine might make a diet of manufactured calorie sources more varied in taste and therefore easier to consume, psychologically. Glycerol provides 4.32 kcal/g, comparable to table sugar, so a typical man would need to consume 578.7 grams of glycerol (or 459 ml at a density of 1.26 g/ml) per day to meet calories requirements on an all glycerol diet. That's 1.28 lb or 1.9 cups in clown unit. Now, the point of including both glycerol and Imhausen margarine is that you can eat however much of each you want (since your body will sometimes crave carbohydrates or fats more than the other), so you shouldn't actually need to eat 2 cups of glycerol syrup per day. But even if you had to eat that much, then again, that would be palatable. I don't know of anyone who has actually eaten that much glycerol in a day, and I wouldn't be surprised if there were some negative health consequences, but it's doable psychologically.
So we know how much glycerol we'd need to eat for caloric requirements even if we didn't have margarine and I'm pretty sure that's it's psychologically feasible to eat that much. But how much glycerol should we actually eat to balance calorie intake against the fact that's it's still an alcohol, if a highly edible one? Let's look at how much glycerol humans normally consume in terrestrial diets for a lower bound. A typical American diet gets about 27% of its calories from triglycerides. That means 675 calories worth of triglycerides per day. Natural triglycerides are always reported with only one significant digit as having 9 kcal/g, so if you're eating 675 calories of natural triglycerides, that works out to 75 grams daily. 75 grams is already far below the 578.7 grams of glycerol daily to meet nutritional requirements, but our lower bound gets worse because glycerol only makes up a fraction of triglycerides. Natural triglycerides contain long chain fatty acids, often with chain lengths of 18 or 16 carbon atoms. Let's use triolein (glycerol trioleate) as the average natural triglyceride. Triolein's molecular weight is 885.432 g/mol, compared to 92.09382 g/mol for glycerol. which means glycerol makes up 10.4% of triolein, which means a typical American diet with 75 grams of triglycerides only has about 7.8 grams of glycerol per day. *wipes brows* We can assume that it's safe to consume 7.8 grams of glycerol per day, which is 34 kcal out of 2,500 daily. Lol, maybe I should increase the lower bound by looking at healthy populations that don't get 50% of their calories from sugar and starch, as Americans do? Or just look directly for studies on how much glycerol humans have been fed without ensuing health problems.
Okay, that's calories covered, and despite that disappointingly low lower bound on glycerol, I think it's actually an adequate menu of calorie sources. Remember: the margarine would be fine by itself without separate glycerol, and the amount of margarine you'd need to eat per day will drop somewhat when we include amino acid sources. Also, the things in this diet are replenishable, but there's nothing preventing you from bringing some normal rations alongside for creature comfort. You can dial back this diet as much as you want and bring as many bags of groceries as your space agency can afford, and this diet will just help to stretch the Terran supplies a little longer.
II. Growing spirulina as an amino acid source:
The next most massive nutrients to tackle for space diets after calories are the essential amino acids. Can we just bring them? How much do we need?
Dietary guidelines commonly lump amino acid requirements together into one number - required complete protein - with a value around 0.8 g complete protein / kg body weight. For the standard-issue 70 kg man (154 lb), that means 56 g complete protein / person day, which is 20.4 kg protein / person year (or 45.1 lbs / person year). That's not a crazy amount of mass to bring to space for short term missions, but if you're looking to save more launch weight or you're looking at long term missions or you just want a backup protein supply when your groceries run out on Mars, fortunately there's an alternative.
The alternative is not manufacturing. There are nine to fifteen amino acids that would need to be manufactured (details in Space Food I) and some of them are moderately complex organic molecules that require lots of steps to build, with attendant yield losses and workups to food purity at each step, and this is made even more difficult by the harsh reaction conditions (e.g. cyanide is a common reactant in amino acid synthesis), and for some of the amino acids, I just couldn't find patents or published reactions pathways, not that that would stop a competent organic chemist. Manufacturing protein at scale by physiochemical methods in space is just not practical. And there's a better alternative anyway.
Despite my comments about the difficulties of growing microbial food in the introduction of the section on calorie sources, the cyanobacterium spirulina is a good deal for space. Some of its virtues:
1) It's a complete protein with all the essential amino acids in good quantities per gram.
2) It grows in alkaline environments, which keeps down the risk that your culture will get an infection.
3) It includes at least some of every vitamin besides D (which our bodies can make themselves under UV light) and B12. It also includes some choline, which isn't always called a vitamin depending on the decade your textbook was published, but choline is also an essential nutrient.
4) It's photosynthetic, so it can remove carbon from waste CO2 in the crew's habitat's atmosphere.
5) It even contains small amounts of carbohydrates, dietary fiber, and triglycerides. The triglyceride composition varies a lot between samples (particularly the oil composition changes with growing temperature), but the oil definitely contains some polyunsaturated omega 3 and omega 6 fatty acids, though possibly not any of the essential ALA or LA, and not enough for dietary requirements. Literature concerning spirulina oil often focuses on the high quantities of GLA (gamma linolenic acid) relative to other biological sources. GLA is a conditionally essential fatty acid that the body can also use to make arachidonic acid, a major component of cell membranes. So the fatty acids in Spirulina oil might not be essential, but they are highly nutritious.
6) It grows as-well-as-or-better in urine fortified with iron than it does in commercial spirulina growth medium solutions, which means it can be an effective part of a waste water recycling system. Astronauts make urine and you've got to do something with it. You might as well turn the nitrogen into protein.
Spirulina's flavor is a little bit unpleasant if you're not used to eating seaweed and I don't know anyone who's tried getting all of their required daily protein from it. But we should start trying it immediately. We should immediately investigate whether people can learn to like this as a major component of their diets, and not just as a nutritional supplement in the form of pills and smoothie powders sold at health food stores. Even the Aztecs and the Kanembu who historically harvested spirulina from tropical soda lakes ate more of it than that. Other than taste, which I really think motivated individuals can deal with, I once heard an anecdotal report that someone felt sluggish after eating spirulina, so maybe astronauts would have to be screened for spirulina allergies before being put on this diet. But those are basically the only downsides in exchange for protein and oxygen and wastewater treatment, alongside some vitamins and fiber and conditionally essential fatty acids.
How much spirulina should be in the diet and how does that reduce the amount of margarine that has to be eaten? At the start of this section, we found a daily requirement of 56 g complete protein / person day. Everyone lists complete protein with only one significant digit as providing 4 kcal / g, which means if we get the recommended required amount of protein of 56 grams daily from spirulina, then that accounts for 224 kcal out of the 2,500 kcal diet, or about 9% of the calories. Margarine is more calorie dense than protein, so it's less than a 9% reduction of margarine by mass, but it's something. Diets in the United States typically contain more like 100 g protein daily, which means 400 kcal protein / day, or 16% of daily calories. If Imhausen margarine provides 7.18 kcal/g, then we can eat 51 grams less than the 348.19 g/day pure margarine diet by supplementing 100 g of protein from spirulina.
The USDA says spirulina only provides 3 kcal/gram, not 4. Maybe 4 kcal/g is for steak and 3 is for dry cyanobateria. Or maybe they're using the wet weight and the dry weight is closer to 4 kcal/g? I don't know. Let's suppose it's the dry weight and dry spirulina just isn't as calorie dense as meat. That means we'll have to eat a little more spirulina to get equivalent protein and calorie requirements to the numbers in the last paragraph. Like 4/3 more, yeah? That would be 133 grams of spirulina daily instead of 100g, or 0.29 lbs or or 4.7 ounces. The volume of that depends on whether it's loose powder or compacted pills or dihe cakes or whatever. If we assume the powder has a density like flour, 0.593 g/ml, then that's 224 ml seaweed bacteria powder, or 7.6 fl oz, or a little under a cup per day. Totally doable.
Other microorganisms can be used to make other food substances like sugar and vitamin B12 and essential fatty acids, but they're harder to grow. Why? Other microbes need special growth media besides waste water and iron and other microbes require non-alkaline solutions, which means a greater risk of infection. And lots of other microorganisms are not photosynthetic, which means you have to make digestible calories for them somehow. Is there any point trying? Of the three (sugar, B12, and essential fatty acids), sugar isn't essential, B12 is needed in such small quantities that it's easy to bring lots of it to space for long missions, and that just leaves essential fatty acids. So...
III. Essential Fatty acids
There are two essential fatty acids, LA and ALA. We can make them by physiochemical methods, but it's not easy or practical. Making the essential fatty acids LA and ALA chemically is nowhere near as easy as making inessential fatty acids to be used as a calorie source in Imhausen margarine. So, can we just bring them, and if not, can we grow them?
We need less ALA than LA: 1,600 mg ALA / person day = 584 g ALA / person year. That's 1.3 lb of oil for a person-year's supply. No problem. We can bring that.
We need like ten times as much LA as ALA. In particular, we need 7,0000 mg LA / person day = 6,205 g LA / person year = 13.7 lb LA / person year. That's still not much, honestly.
If you're not bringing your essential fatty acids to space with you, a medium-term solution is to find an oleaginous (oil-producing) photoautotrophic microorganism that can make ALA and LA. The best one I know is Chlorella vulgaris, the second most famous photosynthetic microalgae dietary supplement after spirulina. It makes both LA and ALA, and quite a bit: "The freshwater species C. vulgaris had the highest levels of linoleic and linolenic acids, followed by the marine species T. chuii." (ref: "Lipid content and fatty acid profiles in ten species of microalgae" (Ohse et al., 2015)).
This is kind of a medium-term solution because Chlorella is generally grown in a growth medium such as Bold's Basal Medium that has lots of interesting chemical salts and trace elements and I think even the chelating agent EDTA, but I don't know how to grow Chlorella once you run out of those. Like, obviously it's possible, because it grows in the wild without Bold's Basal, but how do you get the right balance of boric acid and potassium and everything else from the crew's waste streams? The oil is ultimately made form CO2 and water, and not the salts of the growth medium, so in principle, if you only harvested oil, you wouldn't run out of growth medium, but I think in practice, you'll get some salt and algae in your oil, and that will have to be dealt with. Hopefully by the time you run out of Bold's Basal, you'll have a garden set up that can give you seed oil or you'll have an aquaculture system with clams or something. Or another ship comes in two to four years when the planets align and you get some more supplies.
I'll keep looking for other oleaginous photoautotrophic microorganisms that make LA and ALA, and I'll keep looking into easy way to grow Chlorella, but it's not a huge issue if nothing works out better. Bringing your oil works for a long time and growing Chlorella works for even longer. Maybe urine can be filtered through some semi-permeable membrane to make something that look's like Chlorella's preferred freshwater environment.
Chlorella is also cool because it's one of the few things that makes vitamin B12 and isn't a disease producing pathogen. But it's not all that necessary because...
IV. Vitamins
Vitamins are easy to bring to space. A daily multivitamin masses, what, 3 grams per tablet? That's 1,096 g / person year or 2.4 lb / person year. And I bet most of that is the weight of the excipient and we could make vitamins smaller if mass was a bigger issue than the convenience of getting it from the bottle to your mouth. Bring vitamins with you to space.
V. Fiber
Spirulina has a little fiber. Wikipedia lists 3.6 g dietary fiber per 100 g dry spirulina, which is about how much spirulina this diets recommends eating per day. However, recommendations for dietary fiber are higher than 3.6 g / person day. The Mayo Clinic goes as high as 38 g / person day for men. I don't know very much about fiber as it acts on the body; I speculated about a ton of chemical that we can synthesize that could serve similar functions in Space Food I, with no definite conclusions. Supposing we don't synthesize our fiber chemically, how terrible would it be if we had to bring it to space with us? 38 grams / person day * 365.24 days / year = 13.9 kg / person year, or 30 lb. It's not a ton, but it's more than the essential fatty acids combined, so it's not great either.
So what options are there? First idea: we could use acetobacter to make cellulose from ethanol. We can make ethanol from scratch, so that's pretty close to adequate, though I'm sure acetobacter needs some trace nutrients besides the ethanol. Otherwise we could find some fast growing plants and eat the cellulose and hemicellulose of the inedible parts. Mix some bamboo powder in with your margarine or something like that. It sounds a little wrong, but I'm pretty sure that would work out fine. Or some other fast growing grass that does better in cold climates, maybe something with the C4 pathway. Let's see, bamboo, corn, and kudzu are the fastest growing plants, right? And kelp, which is plant-like, but it needs a water tank full of nutrients, and that's a waste of space ship water. I'll keep thinking about 'em.
VI. What about eating *edible* plants?
Plants are a little hard to grow. They take effort, they take time, sometimes the seeds just don't sprout, sometimes they grow for a while but then a fungus takes over, sometimes you don't get good yields of the edible parts and you're not sure if garden critters ate them or if something went wrong with the growing or what. In contrast, a really great thing about plants is that they're an efficient way to recycle waste biosolids and they remove carbon from waste CO2 gas. If a planetary space habitat is pulling in carbon, hydrogen, oxygen, nitrogen from the environment, then it doesn't need to be as aggressive in recycling all of its waste elements into things like margarine and glycerol, and in that case, growing plants sounds really cool, right? Plants make normal solid food and they smell nice and they look pretty and some people like taking care of them, and they give us vitamins and oil and sugar and fiber. Plants are wonderful. But they're a less reliable source of food than physiochemical manufacturing and microbial culturing. I hope people grow plants on Mars for lots of reasons, from science to cuisine to human psychological well being to the advancement of a large self-sustaining Martian population. But this diet doesn't depend on them for edible nutrients for reasons of reliability and speed. Although, to be fair, bamboo sprouts are edible, so there's still a little bit of plant life in this diet.
So if we go to Mars, you bring the vegetable garden seeds and I'll bring the margarine machine, the green goop, the essential fatty acids and/or second bottle of green goop, the tiny vitamin tablets, and the bamboo. Deal?
Cool. And that's it. Those are all the things we need to eat. Don't forget your UV lamp and you'll be fine.
