The food synthesizer (or food processor) was a common device used aboard 23rd century starships and starbases for synthesizing foods and beverages. Replacing the older protein resequencer, these food receptacles served as a supplement to the ship's chef and were predecessors to 24th century replicators. Unlike replicators, with which food orders were made by voice command, food orders given to food synthesizers were usually made by program tapes or cards inserted into a slot.
The two prevailing glucose matrices for food synthesis in the 2250s were the Katzman glucose matrix and Gupta glucose matrix. The USS Enterprise used the "Una glucose matrix," though, named for its creator, Number One, which combined the best qualities of each with a more palatable result. (ST: "Q&A")
According to a commercial for Tribbles cereal, food synthesizers were equipped with replicator safety protocols, which made it impossible for anyone to synthesize any type of hazardous foods. Still, overriding the program was possible. (ST: "The Trouble with Edward" credit cookie)
In the mid-22nd century, T'Les had a food processor in her house on Vulcan. When T'Les' daughter T'Pol and Trip Tucker were visiting the residence in 2154, the food synthesizer was broken, so T'Les asked Tucker if he could fix it, but he didn't have an opportunity to reply, due to the arrival of Koss. Later however, he did the repairs and fixed it. (ENT: "Home")
A series of food synthesizers were located in the mess hall aboard the Crossfield-class starship USS Discovery. When Commander Saru offered Michael Burnham some blueberries aboard the Discovery in 2256 and she responded that they tasted different from ones she had eaten in prison, Saru reckoned that was less likely to be "a food synthesizer issue" than a result of the environment she was eating them in, which Burnham casually agreed with. (DIS: "Context Is for Kings")
In 2268, when the Enterprise was infested with tribbles, it was discovered that they had entered into the food processors aboard the ship through one of the ship's air vents. (TOS: "The Trouble with Tribbles"; DS9: "Trials and Tribble-ations")
Nurse Chapel offered the surviving children of the Starnes Exploration Party a selection of cards with differing flavors of ice cream to choose from. All they had to do was pick a card and the computer would mix their favorite combination. (TOS: "And the Children Shall Lead")
Upon taking on a contingent of Klingon officers, led by Kang, aboard the Enterprise, Captain Kirk ordered Johnson to have the food synthesizers programmed to accommodate their guests. (TOS: "Day of the Dove")