Jellies are so rich in sugar that they are protected from bacteria and
yeasts, but they must be covered carefully to protect them from mold
spores and evaporation. The following methods of covering jellies are
all good:
Have disks of thick white paper the size of the top of the glass. When
the jelly is set, brush the top over with brandy or alcohol. Dip a disk
of paper in the spirits and put it on the jelly. If the glasses have
covers, put them on. If there are no covers, cut disks of paper about
half an inch in diameter larger than the top of the glass. Beat together
the white of one egg and a tablespoonful of cold water. Wet the paper
covers with this mixture and put over the glass, pressing down the sides
well to make them stick to the glass; or the covers may be dipped in
olive oil and be tied on the glasses, but they must be cut a little
larger than when the white of egg is used.
A thick coating of paraffin makes a good cover, but not quite so safe as
the paper dipped in brandy or alcohol, because the spirits destroy any
mold spores that may happen to rest on the jelly. If such spores are
covered with the paraffin they may develop under it. However, the paper
wet with spirits could be put on first and the paraffin poured over it.
If paraffin is used, break it into pieces and put in a cup. Set the cup
in a pan of warm water on the back of the stove. In a few moments it
will be melted enough to cover the jelly. Have the coating about a
fourth of an inch thick. In cooling the paraffin contracts, and if the
layer is very thin it will crack and leave a portion of the jelly
exposed.