Introduction
When space exploration duration exceeds two years, bioregenerative life support system (BLSS) is seen as a potential solution to guarantee a sustainable habitat for astronauts (Bluem & Paris, Reference Bluem and Paris2003). BLSS must, of necessity, involve recycling of resources, regulation of atmospheric gases and provision of suitable food. Supplying food for astronauts remains one of the preliminary issues of space habitation (Berkovich et al., Reference Berkovich, Smolyanina, Krivobok, Erokhin, Agureev and Shanturin2009). In this respect, it is crucial to select appropriate animals species from many varied terrestrial taxa to provide animal food for astronauts (Bluem & Paris, Reference Bluem and Paris2002). To date, the results of numerous investigations on animal feeding have been published. Space animal candidates include fishes, sea urchins, snails and newts (Blüm et al, Reference Blüm, Andriske, Kreuzberg and Schreibman1995; Bluem & Paris, Reference Bluem and Paris2003). However, the edible biomass proportion of aquatic animals is not very high on account of the large amounts of fish bones or snail shells left as wastes in the system. Furthermore, due to the complicated equipment required to rear aquatic species, involving spawning, hatching and development, it is relatively difficult to guarantee that the systems including these species can and will run smoothly for long periods of time and, hence, that they can be maintained (Shimura et al., Reference Shimura, Ijiri, Mizuno and Nagaoka2002). There are serious problems including excretion treatment, noise pollution, growing space deficiency for vertebrates, such as chicken, goats and pigs (Bluem & Paris, Reference Bluem and Paris2002). Because of these factors, and considering the many useful characters of silkworms moth (Bombyx mori L.) larvae under space conditions, as shown in our previous studies, these larvae seemingly have advantages in the BLSS programme (Yang et al., Reference Yang, Tang, Tong and Liu2009). In the system presently used, several ground-controlled mulberry trees (Morus alba L.) lower than 25 cm in height were cultivated to provide mulberry (Fructus mori) as fruit for astronauts and mulberry leaves, inedible for human beings, as food for the silkworms. Meanwhile, previous studies had verified the feasibility of growing lettuce in lunar bases (Liu et al., Reference Liu, Yu, Manukovsky, Kovalev, Gurevich and Wang2008). Thus, we chose stem lettuce (Lactuca sativa L. var. longifolia. lam) as our candidate space plant (Xu & Liu, Reference Xu and Liu2008) with its stem eaten by astronauts, whilst the leaves, which are not favoured by humans, would serve as the insect's foodstuff.
Experiments of feeding design, feeding methods, equipments and animals’ nutrient compositions in a range of animal species (list) in the BLSS have been conducted in previous studies (Gitelson et al., Reference Gitelson, Blüm, Grigoriev, Lisovsky, Manukovsky, Sinyak and Ushakova1995; Shimura et al., Reference Shimura, Ijiri, Mizuno and Nagaoka2002). Although significant attempts were made to optimise feeding methods, measurement investigations of these animals’ development characteristics, such as weight changes, respiration and nutrient composition, have not proved sufficient to date (Yu et al., Reference Yu, Liu and Tong2008). Sustainability of the BLSS, applied during long-term space explorations, needs steady production of safe and nutritious food of high quality (Berkovich et al., Reference Berkovich, Smolyanina, Krivobok, Erokhin, Agureev and Shanturin2009). Therefore, the above-mentioned aspects need to be studied in more detail.
In the present paper, the characteristics of the respiration of silkworm larvae under two main physiological regimes (eating and not-eating) were studied in order to provide data suitable to allow for the placement of silkworms and plants in the BLSS in numbers appropriate to, and in accordance with their O2 consumption rates and O2 production rate in the system. The main constituents of ground and freeze-dried silkworm powder (SP) were examined to determine parameters for system design and calculation, as well as computer simulation and optimisation within the system. Nutrient composition of SP, including protein, vitamins, minerals, fat and fatty acids, were determined to evaluate its quality. Meanwhile, comparisons of nutrient compositions of SP and other potential space animal candidates were performed. Ultimately, this study aims to provide fundamental information and, hence, a practical basis for the establishment of the BLSS in the future. In addition, this kind of ecological engineering method, which is capable of producing animal protein of high quality by processing the inedible biomass of plants, can also be applied in establishing ecological agriculture (Scott et al., Reference Scott, Susan and James2001; Tian, Reference Tian2002; Jay et al., Reference Jay, Eric, Stewart and Bruce2010) and in certain special terrestrial environments, such as isolated military facilities, including warships and submarines, polar regions, deserts, etc. (Olson et al., Reference Olson, Oleson and Slavin1988; Bluem et al., Reference Bluem, Andriske, Paris and Voeste2000).
Materials and methods
Silkworm feeding and harvest
Chinese euryphagous silkworms were raised in artificial cultivation boxes which was 88 cm high, 55 cm deep and 50 cm wide. These boxes comprised 8–10 layers, used to place the experimental samples, i.e. silkworm larvae. Heat and light were provided on the inner walls of the boxes using fluorescent lamps. A humidifier was also used within each box to maintain humidity during the experiments, as required. Cultivation conditions, including temperature, air relative humidity and light intensity, could be set by a control panel on top of the box. Since silkworms in the BLSS need to be fed simultaneously during their five different instars and because their requirements in terms of environmental conditions are different (table 1), we pre-set air temperature and air relative humidity in the cultivator at 25°C and 85%, respectively.
Table 1. Air temperature and relative humidity demands and time periods of silkworms (Hao et al., Reference Hao, Zhang and Tian2003).
For the BLSS feeding, we conceived a new feeding approach, which was different from conventional feeding methods, which involved feeding silkworms with mulberry leaves only (Huang et al., Reference Huang, Yang and Lü2006). Rather, in our new method, the first three instars (from the first day to the 16th day) were fed with mulberry leaves and those in the last two instars (from the 17th day to the 25th day) with stem lettuce leaves. As for biomass change, during the insect’ larval development, ten silkworms were picked out from the artificial cultivation boxes and weighed on a weight-scale table (AB204-S, Mettler Toledo, Zurich, Switzerland).
When silkworms had grown to the third day in the 5th instar, they were directly frozen in the refrigerator under −20°C; 72 h later, they were lyophilized with a vacuum freeze-drier (FD-3-85D-MP, FTS SYSTEM, USA) and ground to produce SP in terms of their optimal nutrient compositions at that particular time (Yu et al., Reference Yu, Liu and Tong2008).
Measurement of silkworm respiration
The respiration characteristics of the silkworms were measured with an animal respiration measurement system (RP1LP, Qubit System Inc., Kingston, Canada). This includes an animal chamber of 218.5 ml, infrared CO2 analyser (range 0–3%), O2 sensor (range 0–100%), data collection equipment, gas pump and a gas flow meter (fig. 1). Oxygen and CO2 concentrations were recorded by data collection equipment and stored automatically on a computer. Silkworms were placed in the animal chamber during the measurements. Given the fact that gas circulation within the equipment was completely closed, in order to guarantee that the physiological activities of the larvae were normal, measurements lasted 20 min and were repeated six times a day. The insect's respiration characteristics under the eating and not-eating regimes were studied during larval development.
Fig. 1. Schematic diagram of animal respiration measuring system.
During the measuring process, CO2 and O2 concentration changes (ΔCO2 and ΔO2) were determined according to CO2 and O2 concentrations at the beginning and end of the test (initial and final periods, respectively). Thereafter, ΔCO2 and ΔO2 were calculated using equations 1 and 2, respectively.
Then, silkworms’ respiration amounts were calculated with using equations 3 and 4.
Silkworms’ respiration quotients were calculated using equation 5.
The computer program SPSS PASW STATISTICS 18 (Wang et al., Reference Wang, Jiang, Wang and Zhao2006) was used to study respiration characteristics of the silkworm larvae.
Nutrient composition measuring methods
Main element compositions
The carbon (C), hydrogen (H), nitrogen (N), sulphur (S) and oxygen (O) concentrations of SP were measured using an element analyser (Vario EL, Elementar, Hanau, Germany). The first four elements were determined following incineration of SP; the last was measured via oxidative pyrolysis. In terms of operation processes, the lyophilized SP was directly put into the equipment. When C, H, N and S contents were tested, the carrier gas was helium (He), whilst the temperatures of combustion tube and reduction tube were 950°C and 550°C, respectively. When O content was measured, the carrier gas was H2 and the pyrolysis temperature was 1150°C. The results, in terms of percentage concentration of each of the four elements tested, were automatically shown on the analyser control.
Main nutrient compositions
The main nutrient compositions, including protein, fat and ash of SP, were analysed using the Kjeldahl nitrogen method, Soxhlet extraction with petroleum ether and a muffle furnace, respectively. The total carbohydrate content was calculated as: 100%–%protein–%fat–%ash.
For the Kjeldahl nitrogen method, a Chinese national standard GB/T 5009.5-2003 was employed. Firstly, samples were pre-treated. This involved placing a sample in the nitrogen apparatus which was heated until all substances present were carbonised. When the colour of the liquid was nearly transparent, it was heated continuously for another 0.5–1.0 h. Thereafter, the liquid was cooled to room temperature, whereupon 20 ml of water was added, and the sample solution was transferred to a volumetric flask. Secondly, an azotometer was set up. Boric acid, used to neutralise ammonia when it was generated during distillation, was added to the reservoir bottle with the mixed indicator showing titration end point. A 10 ml sample was pipetted from the volumetric flask and poured into the reaction chamber. Then, 10 ml of NaOH (400 g l−1) was added and distillation began. Lastly, titration was performed with H2SO4 (0.05 mol ml−1) until the sample showed grey or blue violet in colour. Meanwhile, the control experiment was done. Nitrogen content was calculated using equation 6,
where X, V1, V2, c, 0.0140, m and F represent the protein content of the sample (g per 100 g), the amount of H2SO4 used during the sample titration (ml), the amount of H2SO4 used during the control titration (ml), the concentration of H2SO4 (mol l−1), the amount of nitrogen equivalent to 1.0 ml of H2SO4 standard titration liquid (g), the sample mass (g) and coefficient of transferring nitrogen into protein (6.25 in this study), respectively.
Soxhlet extraction was used to measure fat content (Chinese national standard GB/T 5009.6-2003). The operational process was as follows: (i) the sample was crushed into powder; (ii) a filtration paper cylinder was put into extraction tube of the fat extraction equipment and was connected to the reservoir bottle. After petroleum, ether was added, the equipment was heated in a water bath and extracting liquid continuously refluxed; (iii) the reservoir bottle was removed and the petroleum ether was recycled. Lastly, the reservoir bottle was dried at 100±5°C for 2 h and thereafter cooled in the dryer. Fat content was calculated according to equation 7,
where X, m1, m0 and m2 are crude fat content (g per 100 g), weight of reservoir bottle and crude fat (g), weight of sample (g), respectively.
When ash content was measured, a quartz crucible was heated at 550±25°C for 0.5 h. Thereafter, it was carried out and cooled, and the weight of the quartz crucible was measured. Then, the sample was put into the quartz crucible and carbonised until there was no smoke seen with a low fire, where after it was put into the muffle and burnt at 550±25°C for 4 h. When the temperature of the muffle had dropped under 200°C, the sample was removed and cooled in the dryer. Ash content was estimated according to equation 8,
where X, m1, m2 and m3 are the ash content (g per 100 g), weight of crucible and ash (g), weight of crucible (g) and weight of crucible and sample (g), respectively (Chinese national standard GB/T 5009.4-2003).
Amino acid compositions
Amino acid compositions and contents were tested using an automatic amino acid analyser (L-8900, HITACHI, Tokyo, Japan) according to the Chinese standard method GB/T5009.124-2003. Firstly, a given amount of sample was weighed and hydrolysed with concentrated hydrochloric acid (6 mol l−1) and phenyl hydroxide (0.1% v/v) in the hydrolysis tube. Then, the external reference method was utilised. The pre-treated sample and the mixed standard amino acid liquid were simultaneously poured into the automatic amino acid analyser. Lastly, the amino acids content was calculated according to equation 9,
where X, c, F, V, M, m, 1/50 and 109 represent amino acid contents (g per 100 g); amino acid content of sample liquid (nmol per 50 μl), sample dilution rate, constant volume of sample after hydrolysis (ml), molecular weight of amino acids, weight of sample (g), ratio of transferring the tested results into the amino acids contents of the sample per millilitre and coefficient of transferring sample with the unit of ‘ng’ into that with the unit of ‘g’.
Vitamins
Generally speaking, vitamins A, E and D3 were examined with high performance liquid chromatography. Vitamin B6, folic acid, niacin, vitamin B12, pantothenic acid and free biotin were measured by microbioassay. Vitamins B1 and B2 were tested with fluorometry. Carotene was determined using paper chromatography.
As for vitamins A, E and D3 (Chinese national standard GB/T 5009.82-2003 and GB/T 5413.9-1997), the samples were saponified and the unsaponified parts were extracted with benzinum purificatum. Then, these three vitamins were separated with C18 contract columns of high performance liquid chromatography (L-2000, HITACHI, Tokyo, Japan). Thereafter, they were tested using an ultraviolet detector, according to the internal standard method.
Microbioassay was applied to measure vitamin B6 (Chinese national standard GB/T 5009.154-2003). Firstly, pure yeast (Saccharomyces carlsbergensis) was incubated with three agar medium (5.3 g of pyridoxine Y medium and 1.2 g of agar in 100 ml water) in a thermostat for 18 h or so under 30±0.5°C. Seed cultivation liquid including 50 ng ml−1 B6 was also prepared. The sample was placed into a conical flask with sulphuric acid (0.22 mol l−1). This was then placed into a pressure cooker at 121°C for 5 h and pH of the mixture was adjusted to 4.5. The liquid in the flask was transferred into a measuring flask. Meanwhile, the flask with bacteria was removed from the thermostat and the upper layer of the mixture was discarded after centrifugation. Standard brine (9 g l−1) was added to the remaining part. A standard curve was made and the standard flask and sample flask were removed from the thermostat and the absorbancy of both standard and sample was measured with a spectrophotometer (SP-752, Spectrum Shanghai, China). B6 concentration was calculated according to the standard curve. Folic acid, niacin, vitamin B12, pantothenic acid and free biotin were also measured using this method.
Fluorometry was used to measure vitamins B1 and B2 contents. When measuring B1 (Chinese national standard GB/T 5009.84-2003), B1 in the sample was oxidized into pyrantel pigment by potassium cyanide. This pigment is fluorescent in ultraviolet light. The light intensity is in proportion to B1 content. As for B2 (Chinese national standard GB/T 5009.85-2003), the sample liquid was added with Na2S2O4, which reduced riboflavin into a non-fluorescent substance. Then, fluorescence intensity of the residue fluorescent impurities was tested, the difference between the results of samples with and without Na2S2O4 representing the fluorescent intensity of riboflavin.
Paper chromatography was applied to measure carotene content. Firstly, the sample was put into a conical flask with absolute ethyl alcohol and KOH (1 g ml−1). This reaction was refluxed for 30 min and cooled. The sample was then extracted with petroleum ether. Meanwhile, a standard curve was prepared with β-carotene concentration and absorbency as X and Y coordinates, respectively. Paper chromatography was performed using petroleum ether as solvent; the polarity of carotene was the least whilst the migration velocity of carotene was the fastest among all the substances within the sample mixture; therefore, it could be readily separated along with other phytochromes. Thereafter, the carotene spot was cut out from the chromatography paper, and the concentration of carotene was determined quantitatively under 450 nm wavelength. Carotene content was determined according to the absorbency of the sample in relation to the standard curve.
Mineral elements
Phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn), sodium (Na) and zinc (Zn) were all determined spectrophotometrically.
When phosphorus content was tested, the SP sample was first pre-treated. The sample, along with sulphuric acid (18.4 mol l−1) and 1:4 of perchloric acid (17 mol l−1) and nitric acid (15 mol l−1) (digestive liquid) was put into a Kjeldahl flask. The flask was placed on a digestive furnace, whereupon when the liquid changed from brownish black to colourless, 20 ml of water was added. The mixture was cooled to room temperature and transferred into a volumetric flask. Meanwhile, a phosphorus standard curve was prepared. P content was calculated according to equation 10,
where X, m1, V1, V2 and m represent P content of the sample (mg per 100 g), and P content of the sample was figured out with the standard curve (mg), total volumetric volume of digested liquid (ml), volume of the used digestive liquid (ml) and the sample's weight (g), respectively (Chinese national standard GB/T 5009.87-2003).
The content of the other elements tested was estimated using methods similar to the above and using Chinese national standards GB/T 5009.90-2003, GB/T17 5009.91-2003, GB/T 5009.13-2003, GB/T 5009.14-2003 and GB/T 5009.92-2003.
Fatty acids
These were analysed by gas chromatography (GC-14C, HITACHI, Tokyo, Japan) according to the Association of Official Analytical Chemists Method 996.06.
Fat and fatty acids were extracted by the hydrolytic method. Firstly, fat extraction was performed. Ground and homogenised sample was accurately weighed and put into a labelled Mojonnier flask. Pyrogallic acid, triglyceride internal standard solution (C11:0-triundecanion; 5.00 mg ml−1 in CHCl3) and a few boiling granules were added into the flask. Following this, ethanol and HCl were poured in and the flask was held in a water bath. Next, methylation was performed. For this, the extracted fat residue was dissolved in chloroform and diethyl ether. The dissolved portion was transferred to a glass vial and evaporated to dryness in the water bath under a stream of nitrogen. Next, 1.0 ml of toluene was added and the vial was heated in an oven for 45 min at 100°C. The fatty acid methyl esters (FAMEs) so produced were injected into a gas chromatograph (GC) column for analysis. For this, relative retention times and response factors of individual FAMEs were obtained. At the same time, about 2 μl of mixed FAMEs standard solution was injected. After a series of calculations, including response factors and retention times, fatty acids contents were estimated.
Energy content measuring method
Energy content of SP was measured with an oxygen bomb method according to international standard ISO9831. Firstly, the sample was made into a ball and weighed. Combustion thread was tightly linked to the terminal of the oxygen bomb with a piece of cotton thread connected with combustion thread and the sample. Then, it was burnt in an oxygen bomb calorimeter (IKA-C2000, IKA, Staufen, Germany), which was thermostatically controlled and insulated. The total heat was calculated in accordance with the increase of water temperature in the heat chamber and effective heat ability of the calorimeter. Energy content could be calculated with equation 11,
where Q, C, t0, tn, e1, e2, m represent total combustion energy of the sample with constant volume, effective heat ability of calorimeter, combustion temperature, final temperature, heat calibration including energy released by the cotton thread and combustion thread, energy stored in incomplete combustion in the crucible (33 500 J g−1) and weight of the sample (g), respectively.
Results and discussion
Biomass changes of silkworms
As shown in fig. 2, during the first three instars, silkworm grew relatively slowly, and then entered the exponential growing phase representing the fourth instar, after which body weight levelled off at around 1.85 g (n=10).
Fig. 2. Body weight changes during silkworm's larval development.
Respiration characteristics of silkworms
The respiration characteristics of silkworms were measured and calculated in relation to the silkworm's weight. There were two main physiological regimes, namely eating and not-eating. Amounts of CO2 exhaled and O2 inhaled by the silkworm larvae under these two regimes are shown in fig. 3. Respiration intensities were higher when the insects were eating leaves compared to when they were not. This is clearly because, when they were eating, the larvae had to spend large amounts of energy on chewing, moving leaves and other physical movements. This phenomenon may be equated to that of humans (Gitelson & Okladnikov, Reference Gitelson and Okladnikov1996).
Fig. 3. Gas amounts of silkworms per gram of body weight and per hour (
CO2, not-eating, 
O2, not-eating, 
CO2, eating, 
O2, eating).
Moreover, it was found that there was a significant difference (paired t-test; P<0.01) in respiration intensity between silkworms fed on mulberry leaves and those fed on stem lettuce in the 4th and 5th instar. This might be due to the different nutrient compositions of mulberry and stem lettuce leaves. Mulberry leaves contain mainly carbohydrate (51.8%), crude protein (21.9%) and crude fibre (14.5%) (Liu et al., Reference Liu, Wang and You2010), while the main components of stem lettuce leaves are cellulose (26.4%), ash (19.3%) and crude lipoid (3.5%) (Mao et al., Reference Mao, Zhang and Mao2003). Mulberry leaves also have some exclusive components, including sterol, guaranteeing the silkworm's normal development and metamorphosis and fluorescence substances which help the insects to resist illness, which do not exist in stem lettuce leaves; consequently, the respiration intensities when they ate mulberry leaves were higher than when feeding on stem lettuce leaves.
Generally speaking, gas fluctuations are intensive during their whole larval development. From the results of paired t-tests on gas amounts produced when silkworms either were or were not eating leaves, it was found that a significant difference existed between the amounts of CO2 exhaled when silkworms were under the two different physiological regimes (P<0.01). Furthermore, there was also significant difference between the amounts of O2 inhaled under these two physiological conditions (P<0.01). These results indicate the necessity of simultaneously feeding silkworms during five different instars in order to maintain CO2 and O2 concentrations at relatively steady levels in a closed system, such as the BLSS, where autotrophic creatures, including vegetables and crops, are cultivated in a multistage manner.
As shown in fig. 4, the respiration quotient (RQ) of silkworms of the eating cohort was generally larger than when they did not eat. The reason for this is probably that most of the animals’ physiological activities are derived from sugar metabolism (Xiao & Li, Reference Xiao and Li2009), and silkworms mainly consumed more carbohydrates, whose RQ was one larger than those derived from fat and protein stored in their bodies, when they needed to increase metabolic intensities when eating.
Fig. 4. Silkworms’ respiration quotient.(
eating, 
not-eating)
Main element contents and nutrient composition analysis of SP
On average, the silkworms’ 5th instar is of seven days duration. As the optimal nutrient composition of silkworms is on the third day in the 5th instar, the insects were harvested and processed to produce SP on this day.
Main elements contents
The main elemental content of SP is shown in table 2. An approximate molecular formula for SP can be expressed as: CH1.894O0.586N0.242K0.026P0.011S0.005. This formula has considerable importance in the system design and mathematical calculations, as well as computer simulation and optimisation of the BLSS system.
Table 2. Main element contents of SP.
Main nutrient compositions
The main nutrient compositions of SP and other space animal candidates are shown in table 3. According to the dietary criteria and protein requirements in the recipe applied onboard the international space station (ISS) (WHO, 2007; FAO/WHO/UNU, 1985), animal protein plays a vital role in keeping human beings in good health. Therefore, whether the candidate animals contain ample protein is an important criterion in selecting space animal candidates. Table 3 presents the main nutrient contents of SP and other animal proteins. Though protein content of SP is not the highest, breeding other kinds of animals in space, such as mammals, might cause some serious problems, such as loud noise, space deficiency and foul odours. Furthermore, the time required to grow fish to a stage where it is harvestable is long, whilst their bones are hard to process and its fat content is higher than that of SP. Although fat content of SP is higher than that of snails, a snail's life cycle is three times that of silkworm and its shell is difficult to recycle in the system (Midorikawa et al., Reference Midorikawa, Fujii, Ohira and Nitta1993).
Table 3. Main nutrient contents of SP and other animal proteins (dry).
SP, silkworm powder.
Moreover, energy provided by SP was found to be 359 kcal per 100 g (dry), 3.15 times more than that of fish (Li et al., Reference Li, Chen, Feng, Hao, Cai and Fan2001). Therefore, it may be said with some degree of justification that a moderate amount of SP could provide sufficient energy for astronauts living in space or people living in extreme/enclosed environments on Earth.
Amino acids composition
Essential amino acid content of SP is shown in table 4. Amino acid score is defined as the ratio of the amount of amino acid in 1 g of test protein to the amount of amino acid in the requirement pattern. According to adult requirements of amino acids (WHO, 2007), essential amino acid scores in SP are shown in table 5. All values are above 0.5, with two greater than 1. It may be concluded that amino acid compositions of SP are reasonable.
Table 4. Amino acid compositions of SP.
E, essential; N, non-essential.
Table 5. Essential amino acid scores of SP.
Other valuable nutrient compositions
Besides providing high-quality animal protein, SP also contains several vitamins, mineral elements and fatty acids.
Vitamins
Vitamin content of SP is shown in table 6. It is clear that SP contains abundant vitamins which play an important role in astronauts’ diets. Vitamin A, B2 and E contents are all much higher for SP than for snail, beef or chicken with vitamin B1 also higher for SP than for beef or chicken (Ensminger et al., Reference Ensminger, Ensminger, Konlande and Robson1994). Therefore, feeding silkworms in the BLSS could reduce the necessity for planting fruit and vegetable areas by providing a certain amount of essential vitamins. The last column in table 6 lists some of the daily requirements of astronauts onboard the ISS. Pantothenic acid and free biotin were relatively abundant in SP.
Table 6. Vitamin contents of SP and daily requirements onboard ISS (dry).
ISS, international space station; SP, silkworm powder.
Mineral elements
Mineral element contents of SP and daily requirements onboard the ISS are shown in table 7. Phosphorus and potassium account for the maximum proportion in SP.
Table 7. Mineral element contents of SP and daily requirements onboard the ISS.
ISS, international space station; SP, silkworm powder.
In terms of deep space exploration, minerals are fairly important for an astronaut's health during space flights. Studies show that short-term flights can result in lower phosphorus and sodium but higher calcium concentrations than normal levels in the blood of astronauts; great losses of phosphorus and sodium in human bodies may occur due to hypo-gravity's effects (Wang et al., Reference Wang, Bai and Zhang2004). SP was rich in calcium, with the content much higher than that of fish, chicken, pork or beef (29.78 mg per 100 g; 42.64 mg per 100 g; 23.21 mg per 100 g and 36.55 mg per 100 g, respectively) (Ensminger et al., Reference Ensminger, Ensminger, Konlande and Robson1994) and lower than that of snail (1240.22 mg per 100 g) (Chen et al., Reference Chen, Zhang, Zhang, Zhang and Li1997). However, as mentioned above, the life span of the snail species (Biomphalaria glabrata) tested is much longer; therefore, it is less suitable as a space animal candidate. Besides, it is well known that phosphorus is an essential element in maintaining healthy human brain tissue. As shown in table 7, the phosphorus content of SP is significantly higher than that of snail, fish, pork, beef or chicken (677.29 mg per 100 g; 1007.35 mg per 100 g; 373.42 mg per 100 g; 391.65 mg per 100 g and 736.43 mg per 100 g, respectively) (Ensminger et al., Reference Ensminger, Ensminger, Konlande and Robson1994).
Furthermore, comparing mineral element contents of SP and daily requirements onboard the ISS (Lane & Feedback, Reference Lane and Feedback2002), it may be inferred that small amounts of SP can satisfy astronauts’ needs in terms of phosphorus, potassium and magnesium.
Fatty acids
As shown in table 8, there were 12 fatty acids detected in total in SP. The unsaturated ones are essential for human beings and can lower human beings’ blood lipid and blood pressure and are involved in resistance to certain types of cancers (Drysdale et al., Reference Drysdale, Ewert and Hanford2003; Gui et al., Reference Gui, Dai, Chen, Wang, Qian, Yang and Zhuang2004). The total amount of unsaturated fatty acid is 1.7 times more than saturated fatty acid. Among the unsaturated fatty acids, α-linolenic acid, dihomo-γ-linolenic acid and linoleic acid cannot be synthesized by humans; therefore, they have to be supplied as supplements within food. From table 8, it can be seen that the contents of two fatty acids, α-linolenic acid and linoleic acid, are the highest in SP. It may be concluded that quality of the fatty acids in SP is relatively high.
Table 8. Fatty acid contents of SP.
S, saturated; U, unsaturated (dry).
Conclusions
In terms of the respiration characteristics of silkworms, the amount of CO2 exhaled and O2 inhaled under the two different physiological regimes tested (eating and not-eating) were rather different from each other. The insects’ RQ under the eating regime was larger than when under the not-eating regime. Therefore, it is necessary to feed silkworms in a multistage way in the BLSS.
The SP contained large amounts of animal protein of high quality, and numerous essential minerals, vitamins and fatty acids. In addition, parts of the nutrient compositions possessed medical and sanitary functions for maintaining the health of astronauts. Silkworm was considered to be more useful and important as a source of animal protein than snail, fish, chicken, beef or pork, in terms of nutrient composition, breeding methods, life span, growing space and waste production.
It may be concluded that feeding silkworms with plants biomass inedible to humans is a promising approach to the production of high-quality animal protein and processing of waste in the BLSS, as well as for people living in extreme and/or agro-ecologically impoverished environments.
Acknowledgements
This work was supported by the Ministry of Science and Technology of China Grant (2006DFB81140).
