Sour jam recipes? Is it possible?

Mrs G has turned our kitchen into something reminiscent of the boiler room of a 19th century transatlantic liner. She’s made some wonderful jams, but is failing at one particular experiment…

Has anyone made a truly sour lemon jam? Something that doesn’t taste sugar-sweet and candied, but that will give the eater a good pucker-cramp behind the jawbone?

Don’t know about lemon, but our local marmalade, Frank Cooper’s Vintage Oxford, is distinctively bitter. I presume the fruit:sugar ratio is extremely high, and it really works. This compares to the Knott’s Berry Farm jam that I avoided for breakfast yesterday when I saw that the #1 ingredient was high fructose corn syrup (I opted for honey instead).