What melachah is it to make jam on Shabbos?
If the fruit is cut finely on Shabbos, it is the melacha of
mechatech (chopping; cutting). If the fruit are crushed on Shabbos,
it is the melachah of tochen (grinding).
However, as the melachat of cooking food מלאכת בישול is
forbidden during Shabbat, it is not possible to make jam as the
fruit must be cooked.