Infants’ ability to detect salt taste is mainly measured by the quantity the infant ingests, sucking patterns, and facial expression in response to ingestion of water solutions with different concentrations of salt. Because of the nature of these measurements, it is hard to distinguish between infants’ ability to detect sodium and infants’ preference for, or liking of, sodium.
Newborns have a biological need for sodium, which brings up the question of why newborns are not able to show a preferential response to salty taste like they show for sweet taste. The lack of a salt taste response might have an evolutionary reason. Although there is a need for sodium at birth, the first food the infant naturally encounters is breastmilk, which is dominated by the sweet taste and contains a sufficient concentration of sodium for the baby to thrive. Children have an inborn preference for sweet taste, which would lead to a natural acceptance and consumption of breastmilk. This makes an inborn acceptance of salt taste, from an evolutionary point of view, not needed per se.
By the age of approximately 4 to 6 months, when sodium channels have further matured, infants show a preference for salty water over plain water, and salted baby cereal over plain baby cereal, as measured by ingestion. This shift from indifference to the preference of salted water is thought to reflect an unlearned biological response to salty taste, rather than a learned response. This does not mean, however, that the addition of salt to any baby food would ensure an increase in consumption. When salt was added to baby formula, 6- to 7-months-old infants found it less palatable, as measured by frequency of sucks, than baby formula without added salt. Presumably, because the addition of salt made the baby formula less sweet, which was confirmed by a trained adult sensory panel. Alternatively, but not mutually exclusive, the addition of salt created an unknown flavor combination, which infants rejected because of its novelty.
In summary, infants’ ability to detect salt taste develops postnatally such that infants younger than about 3 months of age are most likely not able to detect salt taste. Once infants can detect salt taste they show a preference for salt taste in water. There is no prior exposure to salt taste needed for infants to prefer salted water, which suggests an unlearned biological response to salt taste.