Cats have relatively few taste buds compared to humans (470 or so versus more than 9,000 on the human tongue). Domestic and wild cats share a taste receptor gene mutation that keeps their sweet taste buds from binding to sugary molecules, leaving them with no ability to taste sweetness. Their taste buds instead respond to acids, amino acids like protein, and bitter tastes. Cats also have a distinct temperature preference for their food, preferring food with a temperature around 38 °C (100 °F) which is similar to that of a fresh kill and routinely rejecting food presented cold or refrigerated (which would signal to the cat that the "prey" item is long dead and therefore possibly toxic or decomposing).
Why are cats so picky about what they eat?
Cats' taste buds respond to acids, protein, and bitter tastes. The taste buds do not respond to sweetness. Additionally, cats prefer food with a temperature near 100 °F (38 °C). For wild cats, this temperature is associated with fresh food as opposed to food that may be decomposing.