What about quicklime (calcium oxide)?

It's used here in Argentina to make pumpkin "sweets". (I'm not sure the correct translation.) It's a lot of 1 inch cubes of pumpkin, boiled for some time in sugar syrup. [1] Without treatment the cubes will disintegrate while boiling. If you put them for one day in water with quicklime, the cubes get a hard wall and they keep their integrity while boiling, but the interior is soft.

Drinking the water with the quicklime is dangerous and it's discarded. Eating directly the quicklime is even worse. But quicklime is useful for cooking.

[1] Some random recipe I got in Google (autotranlation) https://cookpad-com.translate.goog/ar/recetas/94999-zapallo-... (spanish) https://cookpad.com/ar/recetas/94999-zapallo-en-almibar