Food systems on space exploration missions

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STS-110 crew eating on board the ISS.

The food system for long-duration space missions not only provides nutrition to the crew, but it also provides a form of psychological support by providing a familiar element in an unfamiliar and hostile environment. It can be brought with the crew on board their spacecraft, delivered beforehand, delivered on resupply mission or be supplied by a bio-regenerative system at the exploration site.

A conceptual rendering of a greenhouse on the surface of Mars.

Productive, reliable, and safe human space exploration depends on an adequate food system to provide the crew with safe, nutritious, and acceptable foods for up to five years with minimal impact to mission resources. The complexities of food structure make food an ever changing, dynamic system. Food that is shipped beforehand or brought with must be shelf-stable since refrigeration for food storage will not be available. Currently, shelf-stable foods have an approximate shelf life of 1.5 years. Preservation and packaging technologies must advance for mission lengths to increase.

Freeze-dried strawberries

Current Food System[edit]

The International Space Station (ISS) is currently using a prepackaged food system with occasional fresh food supplementation provided by resupply missions. The probability that this food system will be inadequate increases with mission length and distance. Prepackaged food may need to be shipped ahead of time due to upmass constraints.

Researchers have identified several possible risks to adequate food systems on space exploration missions. Amongst these are delivering the required level of nutrition throughout the mission, maintaining the nutrition and acceptability of the food, as well as developing technologies that efficiently balance appropriate vehicle resources such as mass, volume and crew time during exploration missions with the safety, nutrition and acceptability requirements.[1]

Radiation[edit]

Radiation increases once out of low Earth orbit (LE)O) and may affect the nutritional content and acceptability of the food.

Advanced Food Technology Project[edit]

The objective of the Advanced Food Technology Project (AFT) is developing capabilities and technologies in support of human space exploration.

Space food safety[edit]

Food safety is defined by the absence of a health risk due to physical, chemical and microbiological contamination.

The Johnson Space Center (JSC) Microbiology Laboratory monitors the microbiological safety of flight foods and ensures that preparation and packaging procedures result in products that conform to established microbial standards for flight foods. Microbiological safety is ensured through the Hazard Analysis and Critical Points (HACCP) system and Good Manufacturing Practices (GMPs).

Good Manufacturing Practices include:

  • employee training
  • employee qualifications
  • sanitation
  • record keeping
  • process validation
  • facility maintenance and verification
  • equipment maintenance and verification

Pouch integrity is tested and checked for swelling after the food is made shelf-stable (through thermostabilization, irradiation or dehydration). These tests determine if adequate heat was applied to commercially sterilize the pouch and its contents according to NASA Evidence Category IV). Rehydratable and natural form foods are then tested for viable microorganisms before flight (NASA STD-3001, Vol. 2[2]).

Table 1. Microbiological Testing for Flight Food Production
Area/Item Microorganism Tolerances
Food Production Area Samples Collected* Limits
Surfaces 3 surfaces sampled per day 3 CFU/cm2
(Total aerobic count)
Packaging Film Before use
Food Processing Equipment 2 pieces sampled per day
Air 1 sample of 320 liters 113 CFU/320 liters
(Total aerobic count)
Food Product Factor Limits
Non-thermostabilized** Total aerobic count 20,000 CFU/g for any single sample (or if any two samples from a lot exceed 10,000 CFU/g)
Coliform 100 CFU/g for any single sample (or if any two samples from a lot exceed 10 CFU/g)
Coagulase positive Staphylococci 100 CFU/g for any single sample (or if any two samples from a lot exceed 10 CFU/g)
Salmonella 0 CFU/g for any single sample
Yeasts and molds 1000 CFU/g for any single sample (or if any two samples from a lot exceed 100 CFU/g or if any two samples from a lot exceed 10 CFU/g Aspergillis flavus)
Commercially Sterile Products (thermostabilized and irradiated) No sample submitted for microbiological analysis 100% inspection for package integrity
*Samples collected only on days that food facility is in operation.
**Food samples that are considered "finished" product that require no additional repackaging are only tested for total aerobic counts.

Evidence from ground-based testing[edit]

Incidences of gastrointestinal distress have been recorded by crewmembers during missions, but none of these cases have been attributed to a food borne illness.[3][4] Instances of spoiled food packages on orbit have been recorded once a year on average and have not been documented to result in food borne illness (Evidence Category III). The crew is trained to identify bloated packages or spoiled foods and they are instructed to discard them. Passage of this inspection does not ensure that the food is safe.

There have been instances where rehydratable foods did not pass microbiological specifications due to contamination from mold, yeast or bacterial pathogens detected during preflight testing. Dr. C. Mark Ott from the JSC Microbiology Laboratory reported that 51 out of 7221 products failed to meet the microbiological specifications (Table 1) between 2007 and 2011 and hence were not approved for Shuttle and ISS flights. Though only a small number of the samples failed, even one contaminated food lot can result in crew illness and possibly death during a mission (Evidence Category I). The use of HACCP, good manufacturing practices, standard operating procedures, and finished product testing of processed and prepackaged foods should prevent food borne illness events during space missions, but the rare occurrence of spoiled food on ISS suggest that there is always a small risk of food borne illness during flight.

In context of exploration missions[edit]

Safety issues may become more important for prepackaged foods during long-duration exploration missions. If prepackaged foods are pre-positioned on the Mars surface, then the food packaged may be compromised prior to the crews' arrival. The possibility of food borne illness will also increase with the implementation of a bio-regenerative food system on an extraterrestrial surface. Fresh food, bulk ingredient, processing and meal preparation will provide the crew with more variety and the potential for increased quality and nutrition, but food safety and availability will no longer be ensured as it is through ground-based processing, packaging, and safety testing (Category IV). It is necessary to reach a certain temperature/time combination to ensure safety. Heat and mass transfer are affected by partial gravity and reduced atmospheric pressure. Consideration must be given to the changes in environment and the processing equipment and procedures that will be required to ensure safe food processing on an extraterrestrial surface.

If fresh fruits and vegetables are consumed without a heating (cooking) step, there is potential for microbial contamination, food borne illness, and death, as demonstrated by the commercial produce-related Escherichia coli outbreaks in recent years (Evidence Category III).[5][6] The possibility of produce contamination followed by illness in a closed environment with carefully controlled procedures has not yet been evaluated. It is essential to identify sources of contamination during food production, processing, and preparation in a controlled closed loop system, and determine safety procedures and testing methods to prevent possible food borne illness. Mission loss or major impact to crew health would likely occur if this risk is not quantified and reduced.

Recent evidence indicated that consumption of probiotic bacteria promoted human health (Evidence Category I).[7][8][9][10] Investigations into the effects of probiotic strains on human immunity during spaceflight might lead to incorporation of some strains into the space food system. If probiotics are incorporated, protocols will be required to ensure pure bacteria cultures are safely added and meet shelf life requirements.[11]

Nutrition[edit]

Figure 1: Analytical nutritional profile (solid circles) compared to estimated nutritional profile (dotted circles) of 87 NASA space food items. Nutritional profiles were generally underestimated by the Genesis R&D computer software.

There are two components to adequate nutrition: necessary nutrients and caloric energy. Since consuming sufficient calories without adequate nutritional intake impacts performance and can lead to diminished health, it is essential that spaceflight crews are provided with a variety of foods to meet nutritional need throughout their mission(s) (NASA STD-3001, Vol. 2[2]).

Table 2: Nutrition Composition Breakdown
Nutrients Daily Dietary Intake
Protein 0.8 g/kg
And ≤35% of the total daily energy intake
And 2/3 of the amount in the form of
animal protein and 1/3 in the form of vegetable protein
Carbohydrate 50-55% of the total daily energy intake
Fat 25-35% of the total daily energy intake
Ω-6 Fatty Acids 14 g
Ω-3 Fatty Acids 1.1 - 1.6 g
Saturated fat <7% of total calories
Trans fatty acids <1% of total calories
Cholesterol <300 mg/day
Fiber 10-14 grams/4187 kJ
Fluid ≥ 2000 mL
Vitamin A 700-900 μg
Vitamin D 25 μg
Vitamin K Women: 90 μg
Men: 120 μg
Vitamin E 15 mg
Vitamin C 90 mg
Vitamin B12 2.4 μg
Vitamin B6 1.7 mg
Thiamin Women: 1.1 μmol
Men: 1.2 μmol
Riboflavin 1.3 mg
Folate 400 μg
Niacin 16 mg NE
Biotin 30 μg
Pantothenic Acid 30 mg
Calcium 1200 – 2000 mg
Phosphorus 700 mg
and ≤1.5 × Calcium intake
Magnesium Women: 320 mg
Men: 420 mg
And ≤350 mg from supplements only
Sodium 1500 – 2300 mg
Potassium 4.7 g
Iron 8 – 10 mg
Copper 0.5 – 9 mg
Manganese Women: 1.8 mg
Men: 2.3 mg
Fluoride Women: 3 mg
Men: 4 mg
Zinc 11 mg
Selenium 55 - 400 μg
Iodine 150 μg
Chromium 35 μg

Acceptability[edit]

Food acceptability is defined by and can be affected by several different sensory attributes (appearance, flavor, texture, aroma and serving temperature) as well as variety and usability attributes. Astronauts rate the sensory attributes of products using a 9 point Hedonic scale (with 9 being the highest).[12] Foods must have scored 6 or higher to be included in the astronauts' personal preference containers.

ISS crewmembers have consumed foods up to 3 years post-processing and have reported a decrease in the acceptability of these foods with some being described as not consumable (Evidence Category III). Inadequate food acceptability decreasing food consumption and may affect crew nutrition and psychosocial health, limiting crew ability to complete mission critical tasks.[13] A decrease in food acceptability and quality may be an indication of nutritional degradation.[14]

Quantifying changes[edit]

Due to complexities of food structure, among other variables, food is a very dynamic system. Quantifying changes with kinetic models can be very difficult because of this. Vitamins lost due to leaching, nutrients lost during thermal processing and the potential for increases in nutrient bio-availability as the food matrix is broken down during processing create an ambiguous picture of the actual nutritional content of processed foods. Literature attempts to quantify these changes, but answers are not always obvious.

Kinetic data have previously been determined for the loss of several nutrients under predetermined processing and storage conditions, but the rate constants provided are specific to the food and the testing parameters (Evidence Category I).[15][16][17][18][19][20][21] Use of these models would only provide a rough estimate of remaining nutrition if kinetic models were prepared using this data.

Thirteen representative thermostabilized space foods were evaluated using ASLT to assess the potential of the current food system for use during long-duration exploration missions. The sensory, quality and nutrition of each product was determined at regular intervals over three years of storage at 40°F (control), 72°F (storage temperature of actual flight food) and 95°F (accelerated temperature)(Evidence Category I).[22]

Specifically, egg products did not respond adequately to the thermostabilization process and were found unsuitable immediately after production.

There were considerable losses in folic acid, vitamin B and vitamin C. These losses often correlated with unacceptable change in flavor or color. Other vitamins appeared to be maintained throughout shelf-life. Low temperature storage (40°F) maintained product quality throughout the study. The changes in quality and nutrition were used to determine the shelf-life of each item.[22]

The shelf-life values were extrapolated to NASA's 65 thermostabilized items (figure 2).

Figure 2: Number of acceptable thermostabilized space foods decreases by 90% over 5 year shelf life.
  • Meat products and other entrées were projected to maintain sensory quality the longest without refrigeration (over 3 years)
  • Fruit and dessert products were projected to maintain sensory quality for 1.5 to 5 years
  • Starch and vegetable side dishes were projected to maintain sensory quality for 1 to 4 years
  • Approximately 10% of the 65 thermostabilized items are estimated to have a shelf life of 5 years or more
  • 45% of the 65 thermostabilized products are estimated to have a shelf life of more than 3 years

The most common determinants of shelf-life appear to be the development of an off-flavor or off-color over time. Analysis of 13 thermostabilized products suggest that new processing and storage technologies must be investigated in order to improve quality and extend the shelf life of food products for use in long-duration missions.[22]

Rate equation[edit]

Nutritional and quality changes in food occur due to chemical reactions. The general rate equation of this change is:[23]

 - {d[A] \over dT} = k [A^n]

Where:
A = quality attribute being tested
T = time
k = rate constant
n = reaction order

Most quality reactions in food in the zeroth or first order. Zeroth order reactions (when n = 0) cause enzymatic browning, non-enzymatic browning and lipid oxidation. First order reactions (n = 1), cause protein deterioration, most vitamin deterioration as well as microbial growth. Second order reactions (n = 2) happen very infrequently. In limited oxygen, the degradation of vitamin c is of the second order.[24]

The Q10 value and shelf-life[edit]

The Q10 value is a measure of how the reaction rate changes for every 10°C change in temperature and is defined as:

 Q_{10} = \frac{\text{Shelf Life at temperature T°C}}{\text{Shelf Life at temperature ( T°C + 10°C )}}

If a reaction that changes the product color in half the time as the higher temperature in the denominator, then the Q10 will equal 2.[25] A particular food may have different Q10 values depending on the type of reaction that occurs (lipid oxidation, Maillard browning, etc.). Since there are no definitive Q10 value for any particular category of food, this number is not simple to estimate and foods must be tested. Typical values are located in Table 3.[25]

Table 3: Q10 values for various food preservation methods
Food Preservation Method Q10
Thermally Processed 1 - 4
Dehydrated 2 - 10
Frozen 3 - 40

Product shelf-life can then be projected by using:

 t_s = t_0 e^{-aT}

Where:
ts = shelf-life desired
t0 = shelf-life at a reference temperature
a = slope of the line equal to  \ln {Q_{10} \over 10}
T = temperature difference between ts and t0

Shelf-life information may be collected faster using accelerated shelf-life testing (ASLT) and the Q10 value.

Accelerated shelf-life testing[edit]

Accelerated shelf-life testing (ASLT) requires three different storage temperatures;

  1. a control temperature
  2. the expected storage temperature
  3. an elevated storage temperature to accelerate reaction rates

The elevated temperature reaction rates can be used to determine the Q10 value.[25] Researchers must keep in mind that these evaluated temperatures cause changes in the food that normally would not occur under normal circumstances (melting, protein denaturation, increased water activity, etc.).[23]

Packaging evolution and resource utilization[edit]

During the development of a space flight food system, many resources must be considered. These include:

  • mass
  • volume
  • crew time
  • water use
  • waste disposal capacity

Ineffective use of vehicle resources will decrease the possibility of mission success.

Astronaut Sandra Magnus and cosmonaut Yury Lonchakov, both Expedition 18 flight engineers, works with food storage containers in the Zvezda Service Module of the International Space Station.

Due to the lengthening of mission duration and lack of refrigeration, foods are required to be shelf-stable. The production of byproduct water from fuel cells on the Shuttle brought about the development of freeze dried foods. Apollo-era hard plastic spoon bowls were reduced and replaced with a clear flexible plastic laminate. Rigid cans were replaced with a flexible laminate with an aluminum foil layer for thermostabilized foods. These new flexible packages reduced mass and volume requirements during stowage.[26]

Food packaging is a major contributor to mass, volume and waste allocations for NASA missions. Packaging is integral to maintaining the safety, nutritional adequacy and acceptability of food, protecting it from foreign material, microorganisms, oxygen, light, moisture and other modes of degradation. Higher packaging barrier properties equate to greater food protection from oxygen and water ingress. Oxygen ingress can result in oxidation of the food and loss of quality or nutrition. Water ingress can result in quality changes such as difficulty in rehydrating freeze-dried foods and increased enzymatic and microbiological activity.

Galley tray used aboard the Space Shuttle. Image taken at the Astronaut Hall of Fame in Titusville, Florida.

Clear, flexible, plastic laminate is currently used for freeze-dried and natural form foods. This packaging enables a visual product inspection. This clear plastic laminate is also able to be thermoformed and thermosealed without flex cracks that are common with foil laminates. That being said, the clear packaging does not have adequate oxygen and moisture barrier properties to allow for an 18 month shelf-life for ISS missions. Foods are overwrapped with a second, opaque foil containing package that has higher barrier properties. The packaging materials used for the thermostabilized, irradiated and beverage items contain a foil layer that protects the food from oxygen and moisture beyond the required 18-month shelf-life.

Image taken in the Food Tasting lab in building 17: Bags of International Space Station food and utensils on tray.

The oxygen and water vapor permeability of current NASA food packaging materials are listed in tables 4 and 5.

Table 4: Oxygen Permeability of Packaging Materials (CC/100IN_2/DAY)
73.4°F @ 100% Relativity Humidity
Overwrap 0.0065
Thermostabilized and Irradiated pouch <0.0003
Rehydratable Lid and Natural Form 5.405
Rehydratable bottom (heat formed) 0.053
Table 5: Vapor Permeability of Packaging Materials (G/100IN_2/DAY)
100°F @ 100% Relativity Humidity
Overwrap <0.0003
Thermostabilized and Irradiated pouch 0.0004
Rehydratable Lid and Natural Form 0.352
Rehydratable bottom (heat formed) 0.1784

A significant resource concern lies with the mass of the system. Mass of the food is dependent on the type of food taken and the quantity required per crew member.

The Apollo food system provided 0.82 kg of food per crew member per day.[27] Thermostabilized foods were included starting in 1968. These were preferred to freeze-dried options, which justified the weight increase. By Apollo 14, food averaged 1.1 kg per crew member per day.[27]

The Apollo food system still contained a significant number of freeze dried foods since water from the fuel cells was available for rehydration (Evidence Category III). Current ISS crew members receive approximately 1.8 kg of food per person per day (this number includes packaging). Due to crew preference, a higher percentage of the food is thermostabilized (compared to Apollo era missions). This contributes to the weight increase.

Since the ISS used solar power instead of fuel cells that produce water, there is little mass advantage to using freeze-dried foods.

The average caloric requirement for each crew member is now 3,000 kcal, as opposed to 2,500 kcal provided for the Apollo crews. The actual, number of calories allocated for each crew member is now based on the actual caloric needs of each crew member, according to weight and height. This has caused a food weight increase (Evidence Category III).

Issues with waste[edit]

Food packaging produces a significant amount of waste. In confidential crew debriefs, NASA Mir crew members stated that over-wrapped foods created a trash management problem. This was due to the fact that rehydratable and natural form foods had two food packages per food item.

Foods on Shuttle missions were not over-wrapped, but trash was still significant:

  • 60% of the waste mass on STS-99 was generated from the food system, including food, drinks and packaging.
  • 86% of the waste mass on STS-101 was generated by the food system[28]
  • Food waste on STS-51D was analyzed and showed 12.2 kg of uneaten food and 10.8 kg of food packaging
  • 85% of trash by volume on STS-29 and STS-30 was food packaging and 7% was food[29] (Evidence Category III).

See also[edit]

References[edit]

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  2. ^ a b "NASA SPACE FLIGHT HUMAN SYSTEM STANDARD - VOLUME 2: HUMAN FACTORS, HABITABILITY, AND ENVIRONMENTAL HEALTH" 2. 25 September 2009. 
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 This article incorporates public domain material from the National Aeronautics and Space Administration document "Human Health and Performance Risks of Space Exploration Missions" (NASA SP-2009-3405).