SP-202 Aerospace Food Technology

 

BIOLOGICAL FOODS
 
DORIS HOWES GALLOWAY
 
University of California

 

[141] Several biological systems of varying complexity have been proposed to fulfill the triple role of food production, atmosphere regeneration, and waste removal in spacecraft. It should be noted that production of food in spacecraft (in contrast with that in planetary stations and fixed bases) is thought of only in terms of a multiple role, otherwise food would be carried aboard as it is for submarines. (I suppose a case could be made for the recreational value of gardening, but it probably assumes an exceptional breed of spacemen. ) All of the bioregenerative systems have drawbacks, but some might have advantages over a purely chemical system.

The system most studied is based on growth of green algae, usually Chlorella. In this scheme, carbon is recycled by photosynthetic reduction of carbon dioxide; the nitrogen and minerals of human excrete are utilized to support growth of the microscopic green plants and thereby cycle these nutrients as well. Higher plants can function similarly but less efficiently, in that their rate of growth is slower and a larger portion of plant tissue is not capable of photosynthesis. The most likely candidates among higher plants are duckweed and other fairly primitive plants and a few of the more traditional food plants that have a large leaf surface and reasonable growth rate, such as endive, Chinese cabbage, radish, and sweet potato.

The only bacterial system given serious consideration so far involves coupling an autotrophic hydrogen-fixing bacterium, Hydrogenomonas eutropha, with electrolysis of water to return breathable oxygen and food in the form of bacterial cells. Other suggestions have utilized as yet uncharacterized bacteria in conjunction with chemical atmosphere-regeneration schemes. In one, a methane-fixing organism would be used with the Sabatier carbon dioxide removal system. Other researchers have proposed bacterial conversion of formose sugars or fatty acids from chemical food-synthesis systems. In all cases urine contributes the nitrogen needed for bacterial growth.

Two different fungal systems are potentially useful. The simpler forms, molds and yeasts, can grow in media containing urine and feces with sugars added. Mushrooms may be grown on cellulose and nutrients from human wastes. These systems use oxygen and produce carbon dioxide but could be linked with a chemical atmosphere-regenerative system or be used to process further the inedible portions of higher plants.

The most elaborate schemes anticipate using algae or higher plants as food for one or more animal intermediates. Water fleas, fish, rabbits, and fowl all have had proponents among scientists who seek stability in ecologic diversity and who hope to provide more acceptable and nutritious food in this way. The very complex systems are probably best reserved for planetary [142] habitation or major space laboratory stations where they can serve a dual role as biological test subjects before they are eaten.

Typical compositions of leaves, algae, fungi, and bacteria are given in table I. All of these items are quite high in protein content on a dry-matter basis. Their ratios of carbon to nitrogen are much different from that in normal human diets. In general, the higher the growth or cell-division rate of the organism, the higher the protein (and nucleic acid) content. These rapid growth rates are necessary if the systems are to recycle oxygen effectively within reasonable weight and volume limits.

 

TABLE I. PUBLISHED TYPICAL COMPOSITIONS OF BIOMASSES

Dry solids

Amount, %, in-

Leaves

Algae

Fungi

Bacteria

.

Protein (N x 6. 25)

20 to 40

40 to 60

30 to 50

65 to 85

Lipids

5 to 9

4 to 10

2 to 7

6 to 8

Ash

9 to 15

6 to 9

6 to 9

2 to 4

Carbohydrate (by diff. )

40 to 60

25 to 45

40 to 55

5 to 20

Fiber

a8 to 15

2 to 9

1 to 10

Nil

a Includes leaf ribs but not stems and roots.

 

 

Leaves, algae, and fungi are rich in carbohydrate, but in the cases of the microorganisms variants or methods are known for increasing the content of lipids. The usual means of changing composition within a given strain is by altering the nutrient medium. Estimates of ash content vary, often because mineral-rich media are not thoroughly removed when the cells are harvested.

All biomasses contain some indigestible solids, usually in the form of polysaccharides or complex carbohydrates. Leaves and algae are particularly offensive in this respect. Sometimes this indigestible material interferes with absorption of nutrients within the cells. This occurs when the indigestible material is included in the cell wall and the cells are not ruptured before consumption. If the unabsorbed residues reach the lower ileum and colon where they can be acted upon by bacteria, they produce both excessive intestinal gas and a number of short-chain organic compounds that have a laxative effect. This may cause poor absorption of the diet in general.

Hydrogenomonas accumulates lipid if deprived of nitrogen or oxygen, as do a number of other bacteria. This lipid is chiefly a polymer of beta-hydroxybutyric acid, which we have shown cannot be absorbed from the animal intestine. In this instance there is no interference with protein digestibility, most probably because the lipid is intracellular.

This brief discussion of composition serves to illustrate two basic judgments to be made before qualifying biomasses as human food. The first is to assess the closeness of fit between the [143] composition of the product and nutritional needs of man (i. e., the C/N ratio). The second is to detect the presence of substances that have no nutritional value but that do have physiologic effects (e. g., cellulose). Neither of these factors can be established conclusively on the basis of present knowledge of either nutritional needs of man in space or attributes of the biomasses. But some informed guesses are permissible, and these must be made to set the direction of research programs that will supply the facts by the time such systems are absolutely required, perhaps by about 1984 for the Mars mission.

Distorted ratios of carbohydrate, protein, and fat would be present in the diet if major dependence for food were placed on a chemical system (high carbohydrate or low-fat protein), a biological system (high protein, low fat or carbohydrate), or a minimum-weight, stored-food system (high fat, low protein and carbohydrate). We have explored the tolerance of healthy men to these patterns, in those cases for which published information was inadequate. On the basis of our own and other studies we have concluded that dietary protein may vary between 45 and 300 g/day, provided that quality is assured in the former case and adequate water intake in the latter. Fat tolerance is in the range of 200 to 250 g/day, provided that all the fatty acids are not saturated and long chain. A minimum of 7 g of some oils could meet the accepted minimum need for essential fatty acids. Maximum capacity to absorb and metabolize carbohydrate has not been determined, but the amount which can be absorbed is much greater than 600 g/day. From several lines of evidence, the least amount of carbohydrate that will prevent ketosis is about 70 g/day.

The protein quality of biomasses (the digestibility and amino acid balance) is obviously important in view of these tolerance limits. In comparison with animal proteins, the amino acid pattern of leaves is best among those of the biomasses. The other products are somewhat poorer, particularly with respect to methionine, but compare favorably with the milk protein casein and good quality plant proteins, such as soybean. Studies recently completed in our laboratory have indicated that slightly less than 50 g of protein from ethanol-extracted Chlorella (courtesy of Dr. R. L. Miller, Brooks AFB), commercial Torula yeast, and casein will support nitrogen balance in man, in contrast to 35 to 40 g of egg protein. In rat studies bacterial protein also compared favorably with casein. Thus, any of the biomasses could theoretically meet all of the protein needs of the crew.

One of the most important of the nonnutritional factors that may limit consumption of unprocessed cells is the amount of purines present as nucleic acids. Purine is degraded by man to uric acid which is sparingly soluble in tissue fluids and may precipitate as stones in the urinary tract or crystals in the joints. Unfortunately, high levels of dietary protein also increase urinary uric acid, presumably by stimulating endogenous synthesis. The biomasses contain roughly 1 g of ribonucleic acid (RNA) per gram of protein. To be perfectly safe, it might be necessary to limit intake of cells to 20 to 40 g of protein per day, the amount depending on individual tolerance limits. If consumption is increased to the maximum allowable protein intake, the least amount of processing that could be considered is removal of purines from the cells; this would require a new direction in food technology.

[144] Nucleic acids are by no means the only undesirable coincidental compounds present in biomasses. The list includes among others the carbohydrates alluded to earlier, pigments, minerals, nitrates, glycosides, amines, and steroids. Many are innocuous at low dosages but harmful to lethal at high-intake levels. Recently, we found that men cannot tolerate even very small amounts of either of two bacteria tested H. eutropha and A. aerogenes. Subjects became acutely ill from a few grams of dry cells, with symptoms reminiscent of food poisoning.

On the basis of the present most optimistic view, consumption of crude biomasses is limited to the function of providing protein (plus a few vitamins and minerals) to accompany a chemically regenerated or stored-aboard diet high in fat or carbohydrate (table 11). Regeneration of this order of magnitude (7 g of nitrogen and 260 Kcal) is of doubtful value in a spaceship. After they have been fully processed to remove nucleic acids, fiber, toxins, and other unwanted substances, leaves, algae, or yeast could form the major portion of a diet. Bacteria could provide about one-half of the needed food, on the basis of the composition of present candidates. The processing steps would be quite extensive and it would be a challenge to produce edible products recognizable as food and acceptable to the crew.

 

TABLE II. AMOUNT OF BIOMASS USEFUL IN SPACE DIETS

Biomass

Leaf

Algae & yeast

Bacteria

Yield from crude product

Protein/man/day, g

~ 25-30

~ 40

?

Energy/man/day, Kcal

~ 260

~ 280

?

Yield after complete processing

Protein/man/day, g

185

300

300

Energy/man/day, Kcal

2800

2200

1360


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