SP-202 Aerospace Food Technology

 

FOOD SYNTHESIS BY PHYSICOCHEMICAL METHODS
 
JACOB SHAPIRA
 
NASA Ames Research Center

 

[133] As space missions become longer and longer, it is obvious that at some point a system that will at least partially recover useful foods from metabolic products will offer net mission advantages. A number of predictions have been made as to the mission duration that would be required before regeneration of food would be expected to result in savings. By using only very fragmentary information, the General Dynamics Co. ln 1966 concluded that, for a 6000-man-day mission (i.e., a Mars mission with a 10-man crew), physicochemical regeneration of carbohydrates would result in savings in weight and volume of the food supply system (refs. 1 and 2). In a similar study, the Lockheed Missiles and Space Co. came to a very similar conclusion (refs. 1 and 3).

 

RATIONALE FOR SPACE DIETS CONTAINING REGENERATED NUTRIENTS

 

Food in its most basic sense is any substance taken into and assimilated by a plant or animal to keep it alive and enable it to grow. The substances themselves, depending upon the source, are generally very complex mixtures of organic materials and inorganic salts. However, the major materials required by man are relatively limited in number and are composed primarily of protein, fat, and carbohydrate.

The protein components of our diet are a large number of complex polymers of approximately 20 simple organic compounds, amino acids, of which only 8 are essential to a man since they cannot be synthesized by the body. The minimum requirement for protein has been variously estimated to be between 50 and 75 g/day.

Fats are mostly composed of glycerol combined with long-chain saturated and unsaturated fatty acids. Only a few of the polyunsaturated fatty acids are considered to be essential to humans and they are required in very small amounts, perhaps as little as 1 to 2 g/day.

The carbohydrates in our diet are polymers of relatively simple organic compounds, primarily the hexose sugar glucose. It is not known whether there is a minimal requirement for carbohydrate. However, a diet which contained exclusively protein and fat might be expected to cause difficulties in metabolism because of the very high nitrogen load and the ketosis associated with very high fat diets. In addition, our diet contains relatively small amounts of various salts, nucleic acids, vitamins, and trace elements.

In the typical American diet the major chemical components are as shown in table I. Not that one-half the calories are derived from the hexoses present in the carbohydrates, about one-third the calories are from fatty acids in the fat, and the remainder are composed of the amino acids in the protein and the glycerol content of the fat. The minerals, vitamins, and other components of the diet contribute virtually no calories.

 

[134] TABLE I. MAJOR CHEMICAL COMPONENTS OF TYPICAL AMERICAN DIET

Material

Weight/day,g

Kcal/day

.

Hexoses

315

1260

Amino acids

90

400

Fatty acids

90

900

Glycerol

10

40

.

505

2600

 

 

It should be emphasized that it makes no difference to the body whether these substances come from a food of natural origin or are synthesized by in vitro biological or physicochemical methods. The main consideration is that the material be safe and acceptable as food.

Equations can be written for the catabolism of food substances by the body. In the case of (1) protein (meat), 12) fat (tripalmitin), and (3) carbohydrate (starch), these equations on a per mole carbon basis are, respectively,

 

(1) C1.00H1.67O0.22N0.27 + 1. 00 O2 -> 0.80 CO2 + 0.30 H2O + C0.20H1.07O0.32N0.27

 

(2) C1.00H1.92O0.12 + 1.42 O2 -> 1.00 CO2 + 0.96 H2O

 

(3) C1.00H1. 67O0.83 + 1.00 O2 -> 1.00 CO2 + 0.83 H2O

 

A net equation can be written for the catabolism of the diet shown in table I as follows, again on a per carbon basis:

 

C1.00H1.74O0.46N0.08 + 1.12 O2 -> 0.94 CO2 + 0.72 H2O + C0.06H0.30O0.09N0.08

 

It is seen that 94 percent of the carbon of our food is exhaled as carbon dioxide and that 83 percent of the hydrogen is converted to water. Only relatively small amounts of material are excreted in the urine and feces.

Now let us postulate a system in which the carbon dioxide and water would, by chemical means only, be converted into carbohydrate. And further, let us postulate that this carbohydrate would comprise about 85 percent of the diet. The remainder of the diet would be composed of other essential components of the foods more difficult to synthesize such as protein, fat, vitamins, and the like which would be carried along on the mission. Catabolism of such a diet by the body is shown by the following equation:

 

C1.00H1.67O0.72N0.04 + 1.01 O2 -> 0.97 CO2 + 0.75 H2O + C0.03H0.17O0.05N0.04

 

It should be noted that an even greater proportion of this diet is converted to carbon dioxide and water than that of a typical diet and that, for all practical purposes, excretion products other than carbon dioxide and water can be discarded from a regenerative system. More than sufficient carbon dioxide and water are produced to permit resynthesis of the 85 percent of the diet which is carbohydrate. Such a diet containing 85 percent carbohydrate should be safe and acceptable and in fact may be healthier than the current American diet with its excessive fat and protein.

Serious consideration has been given to the problem of synthesis of protein (ref. 4) and fat (ref. 5) in the aerospace environment. Unfortunately, it appears that very complicated [135] processes will be required for their synthesis, and in all likelihood automatic systems would not be economical even for long-duration space missions.

 

SELECTION OF PURE NUTRIENTS

 

The hypothesis is certain carbohydrates or carbohydratelike nutrients present in our diets can be made a major fraction of regenerated food. Any such substance must be safe and acceptable as food, comprise a significant portion of the diet, and be readily synthesized with high reliability (ref. 6).

During normal metabolism, large food molecules are broken down to successively smaller molecules which might be synthesized relatively easily. It was hoped that some of these might be tolerated when ingested in large amounts. This did not prove to be the case. For example, the trioses, glyceraldehyde, and dihydroxyacetone which arise from the catabolism of glucose could be tolerated by rats in only small amounts.

The literature was examined for reports of compounds which could be consumed in very large amounts for prolonged periods. There are few such compounds. The known toxicology of one of these, glycerol, is compared with that of the normal blood sugar, glucose, in table II (ref. 7).

 

TABLE II. TOXICITY OF GLUCOSE AND GLYCEROL

Compound

Animal

Route

LD, mg/kg

.

Glucose

Rabbit

Oral

20 000

Glucose

Rabbit

Intravenous

17 000

Glucose

Dog

Oral

10 000

Glycerol

Rabbit

Oral

27 000

Glycerol

Mouse

Oral

31 500

Glycerol

Rat

Oral

27 500

Glycerol

Guinea pig

Oral

7 750

 

In several species, it can be seen that gylcerol administered orally is probably no more acutely toxic than glucose, which is known to be highly acceptable as a large percentage of the diet. Other low-molecular-weight compounds which have been reported to have low toxicity are diglycerol, triglycerol, polyglycerol, propandiol, and triacetin. This last compound is the simplest evenchain fat and arises from the esterification of glycerol with acetic acid.

Glycerol has been administered to both normal and ill individuals in large amounts for extended periods. In the classical study of Johnson, Carlson, and Johnson (ref. 8), 14 subjects each consumed 110 g/day of glycerol for 50 days. This amount of glycerol represented about 20 percent of the calorie requirements of the subjects, and no detrimental effects were observed. In the same study, animals were fed even larger amounts of glycerol for 50 weeks; again, there was no evidence of toxicity.

[136] In recent years, there have been reports concerning the administration of glycerol to over 1000 patients with glaucoma (ref. 9), increased intracranial pressure (ref. 10), and diabetes (ref. 11). Patients have consumed as much as 300 g/day, which is more than one-half their food requirement. It is apparent that glycerol can safely be made a substantial part of the diet, whether it comes from a natural source, such as fat, or is synthesized from metabolic products.

The evidence for the safety of ingestion of propylene glycol, triacetin, and some other compounds by humans is limited. However, they are generally recognized as safe by the U. S. Food and Drug Administration (ref. 12). These materials have been tested rather extensively on animals and there is good reason to believe that they can also be safely consumed in significant amounts by humans.

The situation with the formose sugars which arise from the self-condensation of formaldehyde is more tenuous. All studies thus far reported indicate that the unpurified mixture causes a gastrointestinal disturbance when fed to animals. This may be due to the presence of a limited number of components of the mixture whose formation can be avoided by appropriate choice of conditions and/or catalyst. Alternatively, undesired components could be removed from the crude product by fractionation.

 

SELECTION OF PHYSICOCHEMICAL PATHWAYS

 

The starting materials available for the physicochemical syntheses are carbon dioxide and water. There are currently available prototype apparatuses for electrolysis of water in either the liquid or gas phase to produce oxygen, which can be recycled through the spacecraft cabin, and the byproduct hydrogen (ref. 13). A process also fairly well developed utilizes this hydrogen to produce methane and water (ref. 14). The water is of high purity and can be either electrolyzed to oxygen and hydrogen or consumed by the crew. The methane may possibly be cracked to produce carbon and hydrogen, although this reaction appears to be difficult to accomplish in practice.

Accordingly, the methane produced as the byproduct of the atmosphere control system was considered to be available for food synthesis. The pathway envisioned for the synthesis of glycerol and the formose sugars was:

 

Accordingly, the methane produced as the byproduct of the atmosphere control system was considered to be available for food synthesis. The pathway envisioned for the synthesis of glycerol and the formose sugars was

 

Thus, the methane would be converted to formaldehyde (HCHO) which could be condensed directly to formose sugars or condensed to trioses which would be catalytically reduced to glycerol. Possible pathways leading from methane to propylene glycol, acetic acid, and other simple molecules which might be used as food will not be discussed. However, it should not be difficult to conceive of methods for accomplishing the desired conversions.

It is of interest to write a completely balanced set of equations describing some of these conversions:

 

[137]

Reaction

[Delta] H

.

24 H2O -> 24 H2 + 12 O2

+1638.2 Kcal

6 CO2 + 24 H2 -> 6 CH4 + 12 H2O

- 362.4

6 CH4 + 6 O2 -> 6 HCHO + 6 H2O

-468.6

6 HCHO -> C6H1206

-135.2

.

6 CO2 + 6 H2O C6H12O6 + 6 O2

+672.0

 

The net equation for the synthesis of a hexose (via the formose reaction) is identical to the net equation of photosynthesis, although it should be emphasized that photosynthesis proceeds by a quite different and considerably more complex pathway. It should also be noted that the sole energy-requiring reaction in the sequence is the electrolysis of water required for the recovery of oxygen. The remainder of the reactions are exothermic. Further, the reverse of the net equation is the action that occurs in the body during catabolism of carbohydrate. There are always sufficient starting materials produced to close the cycle, even neglecting the carbon dioxide and water produced from the stored components of the diet.

 

SYNTHESIS OF FORMALDEHYDE FROM CARBON DIOXIDE AND HYDROGEN

 

A NASA contractor, the General American Research Division of the General American Transportation Corp., is currently in the process of assembling a breadboard prototype apparatus which will accept carbon dioxide, hydrogen, and oxygen as starting materials and produce only formaldhyde and water (ref. 15). All intermediates and byproducts are recycled. A representation of this apparatus is shown in figure 1. In the main recycle loop on the right side of the figure, methane is oxidized at 675° C in a reactor containing sodium tetraborate coated pellets. Conversion during each pass was relatively low, but with a recycle ratio of 35 the yield was approximately 35 percent. The recycle gas composition was 30 percent methane, 10 percent oxygen, 45 percent nitrogen, 0.2 percent nitrous oxide catalyst, 15 percent carbon oxides, and 1 percent hydrogen.

In the recycle loop shown on the left of figure 1, a small fraction of the main loop gases is processed in a Sabatier catalytic reaction wherein byproduct carbon oxides are reconverted to methane. The initial feed of carbon dioxide also enters the system in this loop.

The crude laboratory system required about 50 W to compensate for insulation losses and other inefficiencies. However, no external heating would be required if the combined heat exchanger and insulation system were more than 85 percent effective. The first laboratory system could produce approximately 40 g/day of formaldehyde, but subsequent prototype systems will produce appreciably more.

 

SYNTHESIS OF GLYCEROL FROM FORMALDEHYDE

 

Various methods have been evaluated for the synthesis of glycerol from formaldehyde (ref. 15) and considerable progress has been made toward implementing the scheme requiring conversion of formaldehyde to trioses and their subsequent catalytic reduction to glycerol (ref. 16).

 

 


[
138]

Figure 1. Apparatus for producing formaldehyde and water.

Figure 1. Apparatus for producing formaldehyde and water.

 

Extensive studies have been made related to the selection of optimum conditions for the condensation and selection of the best heterogeneous catalyst. Several catalysts based upon calcium oxide or ferric oxide on alumina have been found. Glyceraldehyde was found to be desirable as a cocatalyst because it greatly reduced the induction period for the autocatalytic reaction and had a desirable directive effect on the products formed. The most suitable hydrogenation catalyst was found to be ruthenium on carbon. There is a continuing effort to develop a laboratory prototype apparatus that will continuously convert formaldehyde to glycerol.

 

SYNTHESIS OF FORMOSE SUGARS FROM FORMALDEHYDE

 

The formose reaction whereby formaldehyde condenses to produce a complex mixture of sugars has been investigated intermittently for over 100 years (refs. 6 and 17). Recently, a new type of reactor was developed for the synthesis of formose which permitted much greater control of the reaction than had previously been possible and also permitted the collection of data relevant to the kinetics of the reaction (refs. 18 and 19).

By using a 500-ml stirred tank reactor maintained at 60° C, it is possible to convert up to 900 g/hr of formaldehyde into formose sugars. The concentration of formaldehyde has been varied between 4 and 30 percent in aqueous solution and usually with a 0.1 molar ratio of the catalyst calcium hydroxide. Depending upon space velocities, conversions of 30 to 100 percent [139] can be obtained reproducibly. A method has been developed that permits facile examination of the formose produce for its composition (ref. 20).

The observed kinetics can be explained by using rate expressions similar to those employed for analysis of heterogeneously catalyzed reactions. Complexing-decomplexing steps in the homogeneous system are equivalent to adsorption-desorption steps in the heterogeneous system (refs. 19 and 21). It appears that decomplexing of the product may be the rate-limiting step, whereas the distribution of products is governed by the nature and concentration of the catalyst.

 

FABRICATION OF FOODS

 

It should not be expected that crews of long-duration space missions will readily consume the pure nutrients synthesized onboard without modification or the addition of flavorings. However, it is not difficult to envision using glycerol, which is quite sweet, and sugars in a variety of acceptable foods. For instance, they might be used as sweeteners for coffee and tea; alternatively, they might serve as the basis for flavored soft drinks. If, or rather when, it becomes possible to convert these materials to higher polymers such as starch, the only major limitation will be in the ingenuity of the cook. One can readily foresee starch-based foods such as potato soup, pancakes, and pasta based on regenerated materials becoming quite acceptable food items.

 

REFERENCES

 

1. NASA Ames Research Center: The Closed Life Support System. NASA SP-134, 1967.

2. Drake, G. L.; King, C.D.; Johnson, W.A.; and Zuraw, E.A.: Study of Life Support Systems for Space Missions Exceeding One Year in Duration. Contractor Rept. (NAS 2-3011), General Dynamics Co., 1966.

3. Jagow, R.B.; and Thomas, R.S., eds. : Study of Life Support Systems for Space Missions Exceeding One Year in Duration. Contractor Rept. (NAS 2-3012), Lockheed Missiles & Space Co. (Sunnyvale, Calif. ), 1966.

4. Fox, S.W.: Prospectus for Chemical Synthesis of Proteinageous Foodstuffs. NASA SP-134, 1967, pp. 189-200.

5. Frankenfield, J.W.; Kaback, S.M.; Skopp, A.; and Shapira, J.: Synthetic Fats as Part of a Closed-Loop Life Support System. J. Spacecraft and Rockets, vol. 4, no. 1671, 1967.

6. Shapira, J.: Space Feeding: Approaches to the Chemical Synthesis of Food. Cereal Sci., vol. 13, no. 58, 1968.

7. Spector, W. S., ed.: Handbook of Toxicology. Vol. l. W. B. Saunders Co. (Philadelphia, Pa. ), 1956, pp. 151-152.

8. Johnson, V.; Carlson, A. J.; and Johnson, A.: Studies of the Physiological Action of Glycerol on the Animal Organism. Am. J. Physiol., vol. 103, no. 517, 1933.

9. Consul, B.N.; and Kulshrestha, O.P.: Oral Glycerol in Glaucoma. Am. J. Ophthalmol., vol. 60, no. 900, 1965.

10. Cantore, G.; Guidetti, B.; and Virno, M.: Oral Glycerol for the Reduction of Intracranial Pressure. J. Neurosurgery, vol. 21, no. 278, 1964.

[140] 11. Freund, G. The Metabolic Effects of Glycerol Administered to Diabetic Subjects. Arch. Inter. Med., Vol. 121, No. 123, 1968.

12. Anon.: Regulations on Food Additives. Pt.121, Ch. 1, Title 21, Sect. 121.101, U. S. Food and Drug Admin.

13. Wydeven, T.; and Johnson, R.W.: Water Electrolysis: Prospect for the Future. J. Eng. for Ind. Vol. 90, No. 531, 1968.

14. McDonnell Douglas Astronautics Co.: 60-Day Manned Test of a Regenerative Life Support System with Oxygen and Water Recovery. NASA CR-98500, 1968.

15. Budininkas, P.; Remus, G. A.; and Shapira, J.: Synthesis of Formaldehyde from CO2 and H2. Paper 68-0615, Meeting Soc. of Automotive Engrs. (Los Angeles, Calif., Oct. 7-11, 1968).

16. Weiss, H.A.; Ramsden, H. E.; Taylor, W.F.; and Shapira, J.: Physicochemical Food Synthesis for Life Support Systems. I. Research on a Process for the Synthesis of Glycerol. Abstract AGFD-5, Abstracts of the 157th Natl. Meeting, Am. Chem. Soc., 1969.

17. Shapira, J.: Design and Evaluation of Chemically Synthesized Food for Long Space Missions. NASA SP-134, 1967, pp. 175-187.

18. Weiss, A. H.; and Shapira, J.: The Kinetics of the Formose Reaction. Abstract C-65, 155th Natl. Meeting Am. Chem. Soc. (San Francisco, Calif., 1968).

19. Weiss, A. H.; LaPierre, R. B.; and Shapira, J. Homogeneously Catalyzed Formaldehyde Condensation to Carbohydrates. J. Catalysis, 1969 (in press).

20. Shapira, J.: Identification of Sugars as their Trifluoroacetyl Polyol Derivatives. Nature, Vol. 222, No. 792, 1969.

21. Ugolov, A. M.; Adamovich, B. A.; Krylov, O. U.; Sinyak, Y. E.; Uspenskaya, V.A.; and Shulgina, I. L.: Synthetic Monosaccharides for Nutrition of Man in Space. Abstract of Presentation, XII COSPAR Plenery Meeting, 1969.


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