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Based on research performed in 2010 at the Fraunhofer Institute in Germany where they have a replica airplane cabin and can simulate different altitudes, cabin pressure, cabin external wall temperature, relative humidity, noise level, vibration, light, air circulation, etc... food scientists discovered that high altitude (low cabin pressure) and dry air essentially degrades the human sense of smell and taste to a state equivalent to having a bad viral cold.
And everyone knows how food tastes with a bad viral cold.
Researchers found that at about 8,000 feet where air pressure is much lower than at ground level, the detection and recognition thresholds of odorants become much higher confirming that the sense of smell becomes more impaired as the pressure decreases. When it comes to taste, the threshold for detecting salty and sweet was also much higher while bitter was not noticeably affected. The threshold for monosodium glutamate (MSG) and umami (or savoriness), was only slightly impaired at low pressure.
Humidity is also much lower in airplanes hovering around 30% which can dry out the nose and dull the olfactory nerve endings in the nose essential for tasting flavor in a food.
White noise from the drone of the airplane engines can also adversely affect perception of taste and smell independent of air pressure too. [link]
A Feast for Research. Fraunhofer IBP.
Why Airplane Food Is So Bad. The Atlantic 5/19/14
Airplane noise and the taste of umami. Flavour 2014, 3:2 http://www.flavourjournal.com/content/3/1/2