turn to ashes in your mouth

turn to ashes in your mouth

become bitterly disappointing or worthless.
This phrase alludes to the Dead Sea fruit, a legendary fruit which looked appetizing but turned to smoke and ashes when someone tried to eat it. The fruit are described in the Travels attributed to the 14th-century writer John de Mandeville .
1995 Guardian Those who marvelled at the phenomenal climbing feats of Pedro Delgado in the 1988 Tour found words such as ‘heroic’ and ‘Herculean’ turn to ashes in their mouths during the probenecid (a masking agent) scandal.
See also: ash, mouth, turn