Plankton's chum is the signature ingredient in all of Plankton's menu items. As one of several major factors that causes the Chum Bucket to attract no customers, the taste is highly considered foul in contrast to Mr. Krabs' Krabby Patty. Although mentioned in previous episodes, chum first appears in the episode "Friend or Foe."
Most of the time, the chum bears a reddish-brownish color similar to that of real-life beef patties.
During Plankton's childhood, he and his former best friend, Mr. Krabs, saw how their classmates' favorite burger joint gained "li-spect" and wealth. Pursuing their own yet combined goals, they attempted to open their own restaurant with their own recipe and burger, which turns out to take no attention. Their burger poisoned Old Man Jenkins, resulting in an argument that sparked their hostile relationship.
Mr. Krabs made his own burger, the Krabby Patty, and Plankton used chum for his burger. Plankton's creation proved foul even to those who could eat anything. On the other hand, Mr. Krabs' tasted quite good.
Role in series
Plankton continues to use chum in most of his products, despite its foulness and overall toxicity, as shown by many having vomited after eating it. In later seasons, his chum is still inedible but another quality in his restaurant, such as the Chum Caverns or a popular slogan, will draw in customers.
As revealed in "Jailbreak!," chum can actually be used to help pull off crimes and as a result, the criminal community holds Plankton in high regards.
In "Free Samples," it is revealed that Patrick is the only one who can eat chum compared to anyone else who eats it. Also, in "Chum Caverns," Patrick eats a lot of Plankton's chum, though he does not know the food item is sold at the Chum Bucket.
Despite the fact that the ingredients for chum (as explained by Plankton in the episode "Spongicus") are only just that, chum, many other things have been seen in it, such as an eyeball, as seen in the episode "Drive Thru."
In real life, people use chum for bait when they catch fish.