The CELSS Diet

A continuous supply of nutritious, safe and appealing food is necessary to sustain life. It becomes particularly essential for people who are living and working under unusual conditions where healthful food is a vital part of maintaining peak physical condition. Food also plays an important role in the psychological welfare of crew members by providing familiarity and variety in the diet. The foods that are being developed are being carefully scrutinized for an optimal balance of nutritional quality and acceptability. Processing is necessary to convert crops into palatable, safe and satisfying foods. In addition, processing also preserves food for storage in case of crop failure.

To address the issues of food safety and adequate nutritional adequacy of the diet, we must know the composition of the biomass being produced for human consumption. Such data provide feedback to biomass production investigators as they seek to optimize conditions for plant growth and genetically modify the nutrient composition of plants. The plant composition data are also needed for the development and processing of food products, and for the designing of appropriate diets. Materials must be examined from crops grown under a variety of different conditions for comparison. Once the baseline data have been collected, the focus can turn to more specific areas of concern regarding plant/food composition as it relates to developing safe and nutritious CELSS diets. The goal of this research is to provide information about compositional differences that may be found between field-grown and hydroponically-grown crops and to investigate and identify nutritious portions of these plants that may not be traditionally eaten in Western cultures. The information gathered will not only support efforts in CELSS development but also be useful to growers and researchers in hydroponic environments. Testing of non-traditionally eaten plant parts could result in new food products and increase the value of economically-important crops.

There are several analyses that can be performed on the various plant parts of each of the species of interest. The proximate composition is determined by measuring the contents of moisture, protein, fat, ash, and carbohydrates. There are separate measures of total dietary fiber, fatty acid composition, amino acid composition, nitrate, and nonprotein nitrogen content. In addition, the levels of selected vitamins can be measured and the mineral content of the material determined by inductively-coupled-plasma atomic emission spectrometry. Antinutrients and toxicants can also examined by measuring substances such as solanine, trypsin inhibitors, tannins, phytic acid, lectins, erucic acid and glucosinolates. Research on the toxic and antinutritive factors in these crops, and developing ways to minimize their levels during growth or overcome their effects by processing, has general implications for agriculture and food processing.

A calorically and nutritionally adequate diet is essential for a CELSS to function. Human performance and productivity is dependent on diet. Three types of research studies have relevance to the development of a CELSS diet: 1) animal nutrition studies, 2) nutritional analysis and 3) animal toxicology studies. The objective of the animal nutrition studies is to determine the nutritional adequacy of potential CELSS plants. Such CELSS studies have applications on earth as well. Diets of many developing countries consist solely of limited plant foods. Furthermore, there has been an increase in the percent of total calories from plant foods during the past several decades in the U.S. Therefore, nutrition studies concerning strict vegetarian diets have a world-wide application.

Nutritional adequacy of a food or meal is reflected by nutrient composition as well as nutrient bioavailability. Nutritional adequacy of a diet is often measured by growth and assessment indices. For example, rats can be fed equal amounts of either a controlled diet (known to be adequate in macro- and micro-nutrients) or a vegan (strict vegetarian) diet consisting of various CELSS crops. In addition, mineral, vitamin or vitamin+mineral can supplemented to some of the vegan diets after some time on non-supplemented vegan diet. The animals are fed the diets for several weeks and their growth and health monitored to determine the nutritional value of the diet.

Bioavailability of various nutrient can also be determined by animal feeding studies. For example, the digestibility of proteins and the availability of their component amino acids can be determined by comparing controlled diets and vegan diets. Various amino acid supplements can be added to the vegan diet if the growth of the animals fed the vegan is not comparable to that of the control animals. Nitrogen balance can also be determined by collecting and testing the wastes generated by the animals. The animal feeding studies can also be used to identify any toxic effects of the experimental diets. At the end of a feeding study the animals can be autopsied and various organs can be examined using histochemical techniques.

Rice is an excellent cereal crop to complement legume protein in a balanced vegetarian diet. The hypo-allergenic storage protein of rice grain is tolerated by virtually all people and its use versatility is the best of all the cereal grains. Rice is the one crop that produced year round and was eaten every day during the 2-year mission of the first Biosphere 2 crew. Wheat is another good choice for a cereal crop in a CELSS. The plants can be grown in high density in CEA and the grain is very versatile. Wheat in the form of breads and pastas is a very important and common foodstuff in many cultures. Potatoes, whether white, sweet or both, make good and hearty additions to a CELSS diet. The plants have a very high (80%) edible fraction and the tubers are quite versatile and commonly consumed throughout the world.

Several legume species have been considered for use in a CELSS. Various brassicas (similar to wild mustard) are available as ultra-dwarf, fast-growing oilseed crops, producing oils similar in quality to that of canola. These species can exhibit a very short stature (<20 cm in height), a short time from seed to flower (17 days), and a rapid cropping time (55 days in a controlled environment). Soybeans are a very common crop in commercial agriculture and could be used in a CELSS. The seeds are high in protein and rich in oils. Unfortunately, the plants are relatively inefficient to grow in terms of power, mass and volume. Peanuts have been suggested as legume crop for CELSS. These can be grown hydroponically but the yields obtained to date are not as high as field-grown crops. Peanuts have an interesting flavor and would be a good addition to the vegetarian diet. Cowpea (black-eyed peas) is a good low-fat legume to complement the oily CELSS candidate legumes soybean and peanut. Besides being typically heat and drought tolerant, cowpea is a staple crop eaten in Africa as a dry bean, snap bean, pot herb, or raw salad green. Its harvest index (proportion of edible biomass) is potentially much greater than that of other legume crops under investigation in the CELSS program. Low seed oil content permits cowpea meal to be incorporated into formed or extruded vegetarian food products.

Other crops may be included in the vegetarian CELSS diet. Studies have shown that the more plant foodstuffs included in a vegetarian the more palatable and satisfying the diet becomes. It is likely that tomatoes will be included in a CELSS. Tomatoes are a very versatile seasoning that can be used in stews, sauces, and salads. Lettuces will also probably be included in the diet as these make good salad greens and can be grown efficiently on a CELSS. It is not clear what other vegetables may be included in small amounts to enhance the palatability of a CELSS diet. Herbs will surely be grown. With a limited number of crop species, spices and herbs will be important in making the diet seem more varied. It has been shown that hot peppers are commonly eaten on a daily basis in many cultures and could enrich a CELSS diet. It is unlikely that fruits will be included since many of these grow on bushes or trees that would be inefficient to grow in CEA. Some have suggested that small animals like chickens, goats, or fish be grown in a CELSS. These are inefficient to grow and impose additional problems for a spacecraft. It is very likely that the CELSS diet will be purely vegetarian.

An interesting set of crops that are CELSS candidates are microbial crops. Microorganisms are a good source of single-cell protein since these creatures are about 50% protein. Brewers' yeast is used as a dietary supplement on Earth and could be used in a CELSS. It is possible that the yeast could be fed sugars released from the cell walls of inedible plant biomass during waste processing. Algae represent another possible dietary supplement. Green algae are a good source of protein as well as a good source of essential fatty acids and vitamins. Cyanobacteria (blue-green algae) are an excellent source of nutrition. Many of these species can make Vitamin B12 which plants do not make. In addition, since the algae are photoautotrophic they can help provide oxygen to the atmosphere. Species of cyanobacteria are capable of nitrogen fixation and could provide fixed nitrogen for plant growth media from atmospheric N2. Algae and cyanobacteria also make good back-up systems should the plant growth chambers fail. Although you would not want them as your only source of food, in an emergency they could be grown very quickly and provide needed sustenance for the crew.

The limited number of CELSS crops that will be grown means that creating an interesting and tasty diet will be a challenge. Remember, for a mission to Mars, these few crops will provide the only food for the crew for three years. The foodstuffs chosen for a CELSS will have to be versatile and capable of being converted into different types of foods. For example, soybean can be pressed to release oils. But the soybean meal remaining is high in protein and can be manipulated to give many different textures. The soy milk can be used in place of cow's milk or can be used to make curd in the form of tofu or tempe. Other CELSS foodstuffs will be pushed to their culinary limits. The processing of raw materials into these varied foods will require research and engineering advances. The equipment needs to be compact, light-weight, versatile, and require limited manpower. Many foods may be generated by extrusion, such as pastas and breakfast cereals. Cooking and food preparation in microgravity poses additional challenges. Safe food storage is also an issue if foods are not eaten fresh.

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