Flavor is the unified perception created when the brain integrates signals from multiple sensory systems, primarily gustation (taste) and olfaction (smell). This unified sensation appears to arise from within the mouth, guiding our decisions about what to consume. Understanding the distinct pathways taste and smell take is necessary to appreciate the complexity of flavor.
The Independent Roles of Taste and Smell
Gustation, or the sense of taste, is strictly limited to the detection of five basic qualities: sweet, sour, salty, bitter, and umami. These taste qualities are detected by specialized receptor cells located primarily within the taste buds on the tongue. When a chemical compound in food is dissolved by saliva, it stimulates these receptors, sending signals to the brain that identify the fundamental taste.
Olfaction, the sense of smell, operates by detecting volatile chemical compounds that become airborne. The olfactory receptors, situated in the olfactory epithelium in the upper nasal cavity, are capable of recognizing thousands of different odorants. When a person actively sniffs an external object, this process is called orthonasal olfaction. This form of smelling is what we use to detect a scent in the environment, such as the aroma of coffee before we drink it.
The complexity of smell allows for a much broader range of perception than the five limited taste categories. While both senses rely on chemoreceptors, taste detects chemicals dissolved in fluid using three separate nerves, whereas smell detects airborne chemicals using a single pathway.
Retronasal Olfaction: The Key to Flavor
Flavor is the encompassing sensory image that results from combining taste with smell and other sensations. The brain creates this holistic perception by integrating the signals in a region called the orbitofrontal cortex. For example, the difference between a strawberry and a cherry lies not in basic taste qualities, but in the aromatic compounds detected by the olfactory system.
The mechanism responsible for this integration is retronasal olfaction. When food is chewed and swallowed, volatile compounds are released from the food inside the mouth. These odorants travel up the back of the throat, through the nasopharynx, and into the nasal cavity, reaching the olfactory receptors from the rear. This is distinct from orthonasal olfaction, where odorants enter through the nostrils from the outside environment.
Retronasal olfaction continuously supplies the olfactory receptors with new odorants as a person chews, creating a dynamic and evolving flavor experience. This internal route allows the brain to experience the smell of the food as originating from the mouth, merging it seamlessly with the gustatory signals. The result is a unitary sensation that is far more detailed than either taste or smell alone.
Factors That Alter Flavor Perception
The dependence of flavor on retronasal olfaction is most evident when the process is disrupted. A common cold or allergies cause nasal congestion, which blocks the passage of volatile compounds to the olfactory receptors. When this retronasal pathway is obstructed, only the basic five tastes can be perceived, making food seem bland or flat.
The overall flavor profile is also influenced by sensory input from the trigeminal nerve. This nerve transmits information about texture, temperature, and chemical irritants like the burn of chili peppers or the cooling sensation of menthol. These somatosensory cues combine with taste and smell to finalize the perceived quality of the food.
Sensitivity to both taste and smell commonly declines with age, significantly altering flavor perception. Olfactory impairment is prevalent in over 60% of people aged 80 to 97, though the number of taste buds may not decrease until after age 75. This reduction in olfactory function, which is the major component of flavor, can lead to reduced enjoyment of food and potentially inadequate nutritional intake.