In the first century, wine was kept in leather bags made of animal skins (mostly goats or sheep), which were used in their entirety, so that the bags reflected the form of the animal. New wine would then ferment in these bags, generating internal pressure. Old wineskins typically could not withstand this internal pressure and burst, while new, more elastic wineskins would expand with the fermenting wine.
During the first century wine was not preserved in flasks, but rather in leather bags made of animal skins (mostly goats or sheep), which were used in their entirety, meaning that the bags themselves reflected the form of the animal. New wine would then be fermented in these bags which generated internal pressure. Old wineskins were typically not able to withstand this internal pressure and burst, while new wineskins were better suited to this end because of their elasticity.1
New wine ferments and therefore creates expanding pressure, which old wineskins—that is, leather skins which had been sewn up and made watertight so they could hold liquids—tended to be dry and hard, and therefore burst as they were unable withstand the expansion.2
16 Personne ne met une pièce de drap neuf à un vieil habit; car elle emporterait une partie de l'habit, et la déchirure serait pire.