Mock orange juice (aka “Carolene”) was a recipe used during rationing in the U.K. in World War Two. It produces a liquid whose taste is reminiscent of orange juice.
By the morning, the sugar will have extracted a clear liquid from the slices of swede. The liquid has a faint turnip smell, but a slightly orangey taste.
Mixed with carrot juice, it made for a slightly sweet, slightly tangy, slightly orangey tasting drink.
The liquid can be used in recipes calling for orange juice.
Here is the process, illustrated briefly, by Ruth Goodman in Episode 8 of “Wartime Farm” (BBC, 2012).
The relevant section starts at 13:26 in the video. Embedded link valid as of June 2018.