Yaakov was different than his father and grandfather. "Emes" is the unification of heaven and earth and lasts forever. The final end is "Resurrection of the Dead" where death has been removed. Reb Tzadok only ate at a siyum Shas (completion of a Talmudic tractate) so he made a siyum everyday in the morning and another at night! Eating on Shabbos is crucially important since it is a tikkun (fixing) for the sin of "Aitz Hades" (eating from the Tree of Knowledge).