Professor Fringle's Algae Cakes were a nutritious snack item made from algae.
In 1931, Cue Ball Donelly drove the delivery truck that delivered the algae cakes to the expo. He couldn't stop eating them, even though he didn't like them, and begged Marty McFly not to tell anyone, since he needed the job as few people would hire a man that used to work for the gangster Kid Tannen.
Marty told him that he wouldn't tell anyone and stated take the algae cakes off his hands. Cue Ball gave him the algae cakes and Marty gave them to the people around Hill Valley High School that were setting up for the Hill Valley Science Expo. The only person who said that she liked them was Edna Strickland, and her future husband, Citizen Brown stated that they tasted like the food Edna served at the Citizen Plus ward in 1986.
Once the expo opened, Ernest Philpott gave out free algae cakes to exhibition guests. Marty was surprised that Philpott was the one who was giving out the algae cakes as he told him the day before that he hated them. Ernest Philpott then recognized Marty as the man that he thought was looking at his wife, Eunice, at the El Kid speakeasy months prior.
Ernest Philpott refused to give Marty any algae cakes, but Marty took a box when Philpott was busy giving a speech about the cakes as part of the exposition's science exhibits. Marty later gave that box to Edna Strickland, using the alias Mary Pickford, when she aged fifty years after traveled back to 1876 in the temporal duplicate DeLorean time machine.