Dolphins are aquatic mammals belonging to the infraorder Cetacea. The five primary tastes are sweet, salty, bitter, savory (umami), and sour. These tastes are detected by the tastes receptors present in the taste buds of the tongue. However, not all animals possess the receptors for all five types of tastes.
Research showed that the genes that code for the receptors such as sweet, bitter, umami, and sour are nonfunctional in Dolphins. Therefore, dolphins are unable to taste sweet, bitter, umami, and sour. However, dolphins can only taste saltiness.
Source: Feng et al., Genome Biol. Evol. 6(6):1254–1265