Importance of Blackberry jam

Blackberry jam is produced by Tom by hand at home, a jar of which is carried by Alistair through the war as a reminder of better times and a physical reminder of the time that will come after the war. The jar of jam is given to Alistair as a gift, with Alistair promising only to open it at war’s end. Alistair zealously guards and carries the jar with him every step of the way, almost deciding to eat it at one point. He accidentally leaves it behind on Malta, where it is then cared for by Simonson, who intends to return it to Alistair when Simonson returns to London.