ISS astronauts, on average, consume only 80% of their daily caloric requirements. Despite decades-long efforts to keep the food appealing, crew members just don’t eat enough in space. Factors affecting multisensory perception, including the five following, contribute to lower energy intake during missions.
1. The Fluid Shift
In the microgravity of space, the body’s fluid redistributes. This brings extra fluid up to the head and into the nasal passages, causing cold-like congestion.1 Odor receptors in the upper nasal cavity become hindered, lessening perception of flavor along with smell. Astronaut Scott Parazynski, who has been on five shuttle missions, describes it as “the same as having a cold or allergies… a stuffy nose definitely dampens your sense of smell and consequently your sense of taste.”2
Parazynski reported a new-found love for shrimp cocktail in space, despite not enjoying it Earth-side, due to the spicy horseradish sauce that cuts through congestion. Clayton Anderson, a mission specialist who spent over 150 days on the ISS, said that the congestion diminished after several days but periodically returned. While Parazynski developed a fondness for horseradish-covered shrimp, Anderson enjoyed tvorog, a tangy Russian version of cottage cheese.2
2. Ambient Smells
Parazynski refers to the ambient smell of the ISS as “sterile,” along a with a mix of other distinctive smells like no-rinse shampoo.2 Other astronauts have described the smell of space with terms surrounding an odd combination of sweetness and machinery.3 Although many don’t describe the smell negatively, these distinctive odors interfere with the usual taste of foods.
Smells of space, as described by astronauts:
- Brake pads
- Burnt steak
- Burnt almond cookies
- Welding fumes
3. Higher CO2 Concentrations
Carbon dioxide makes up around 5% of exhaled breath. A crew of six astronauts, when breathing normally, would produce approximately 6 kg CO2 in 24 hours.1 Clearly, CO2 cannot be allowed to accumulate without removal, as that would make the station uninhabitable. The Environmental Control and Life Support System (ECLSS) does remove most of the CO2 by-product, but restoring cabin levels of CO2 to Earth levels would be too great a drain on resources. ISS CO2 levels hover around 0.4%, or nearly 10 times higher than Earth levels of 0.04%.1
Enough CO2 is removed to keep astronauts from experiencing any known detrimental health effects. However, the elevated CO2 is thought to affect both olfactory and taste receptors, causing possible changes in food flavors. CO2 may alter flavors by activating sour taste receptors, similar to the effect of carbonated drinks.
4. Recycled Water
On the ISS, around 93% of water used is recycled.1 Using wastewater from urine and showering, along with water vapor from the cabin atmosphere, cuts down on transportation of water between Earth and the station. While useful in terms of resources, this causes a higher concentration of some chemical compounds that might alter the flavor. Samples are frequently sent back to Earth to be scanned for possible toxicity. Recycling efforts on the ISS reduce the amount of water used by an astronaut to around one gallon per day.5
Astronauts’ opinion on the water flavor is mixed. When presented with samples at NASA, journalists described only vague differences, one being a “tang of iodine.” Complaints from the crew are rare; astronaut Paolo Nespoli even described the water as “better than on Earth, despite being recycled.”1 Some crew members drink the recycled water plain, while others use flavoring.
Regardless of water taste, different concentrations of chemicals in the water may interfere with the taste of foods that are consumed along with it. Freeze-dried foods are common space fare, and these require rehydration using the station’s recycled water. The water’s interaction with dehydrated meals may cause the food to taste different than samples tested on Earth.
5. Background Noise
While it’s fairly common knowledge that the human sense of taste is heavily influenced by smell, more recent studies show that noise may also have a strong impact on taste perception. In a 2011 study, Woods et al. performed lab experiments in which 48 participants were asked to rate sweetness and saltiness of food while being subjected to background noise. The noise was altered in terms of loudness, ranging from 45-55 dB (quiet) to 75-85 db (loud).4
When subjected to the loud background noise over headphones, participants consistently rated sweet foods as less sweet and salty foods as less salty. The most profound change was among sweet foods; however, participants’ “liking” of the food did not change significantly with loud vs. quiet noise. Interestingly, rice cakes’ “crunchiness” was rated higher when consumed with loud background noise.
What does this mean for the ISS crew? Aboard the station, background noise levels are approximately 70 dB, reaching peaks over 90 dB. Astronauts are consistently exposed to noise levels considered “loud” in the Woods et al. experiments.4
On shorter missions in previous decades, schedules were too packed and the novelty of space was too exciting for food to take center stage. For longer ISS stays, however, keeping the crew fed and happy – and avoiding menu fatigue – is central to their well-being and ultimately, the success of the mission. On hypothetical upcoming journeys to Mars and beyond, international space agencies will be as concerned with the Taste Race as the Space Race.
1. Serge, Pieters & D, Jonathan. (2020). Factors affecting flavor perception in space: Does the spacecraft environment influence food intake by astronauts?. Comprehensive Reviews in Food Science and Food Safety. 19. 10.1111/1541-4337.12633. / 2. Romanoff J. When it comes to living in space, it’s a matter of taste. Scientific American. March 10, 2009. https://www.scientificamerican.com/article/taste-changes-in-space/ 3. The Smells of Space. Australian Academy of Science. Updated April 10, 2017. https://www.science.org.au/curious/space-time/smells-space / 4. Woods et al. (2011). Effect of background noise on food perception. Food Quality & Preference,22,42–47. / 5. Sullivan K. How the ISS recycles its air and water. Popular Science. Published June 21, 2019. https://www.popsci.com/how-iss-recycles-air-and-water/