It is best to add sour cream shortly before serving so that the borscht will keep longer. The sour cream can simply be served on the side, to be spooned in at the table. Or you can add sour cream (about 1 heaping tablespoon per cupful of borscht) to the soup in a jar. Close tightly and shake vigorously to blend. Fluffy, dry, hot boiled potato is wonderful in the middle of this ice-cold soup.