Her keeper, Bob Keesecker, said Duchess didn't seem too stressed about the milestone.
"I told her it was her birthday today and she didn't seem to be overly concerned about it," he said. "I made sure her hair looked good before she went out."
Keesecker said Duchess has quite a sweet tooth and worked pretty hard to get to the fruit in the ice cake.
Zoo officials say Duchess is the nation's oldest captive Bornean orangutan, and is now 10 years older than the 40-year life expectancy of orangutans in the wild.
Duchess was just 2 years old when the zoo opened in 1962, and is one of only a few remaining original animals. She has given birth seven times and lives with one of her daughters, her daughter's mate, and their daughter.
Keesecker said besides a strong devotion to food, Duchess also enjoys painting pictures on canvas and cleaning her own pen with a scrubbing brush and a bucket of water.
Saturday's birthday party included the groundbreaking of a new $4 million orangutan exhibit that will provide a more natural environment for Duchess and her family. Zoo officials hope the exhibit will allow them to add more orangutans in the future.
The new exhibit will be part of the zoo's overall $20 million makeover that includes a recently opened Komodo dragon exhibit and a future Sumatran tiger exhibit.