Download BIOLOGICAL FOODS DORIS HOWES GALLOWAY University of

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 atmosphereregenerative 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, %, inLeaves
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
a
Fiber
8 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 minimumweight, 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