Why is a smile felt when a delicious drink is made revoltingly sweet?

It is proposed that the contraction of the zygomatic muscle in the cheek seen in newborn infants during sleep is recruited in the waking state into the reflexive movements of suckling, evoked by touch and reinforced by the sweet taste of human milk. The sweetness comes from synergy among mono-hydrogen glutamate ions, immune-protective glycoproteins, the milk sugar lactose and related antimicrobial oligosaccharides. Neonatal smiling at sweetness encourages persistence in collaborating with the baby in breastfeeding and thereby helps to reduce the risk of fatal infection and malnutrition. Adults also see the neonatal smile as ‘sweet’ in the interpersonal sense of that word. Even sensual pleasure has been attributed to the baby’s as yet little differentiated awareness. The facilitation of suckling by sweet taste is replaced by acquired ingestive responses to drinks and foods that differ greatly in strength of sweetness in multimodal conjunction with particular levels of other sensed qualities -- seen, heard, touched, tasted, smelt and kinaesthetically perceived. Nevertheless, the congenital disposition to smile at a sweet taste can come to awareness in adults when a repulsively strong sweetness is incorporated into an otherwise delicious drink.