Edrena, jams that begin as red except for raspberry, tend to fade
towards brown. We hope to have strong color for 3 months on normal red
jams. In the past holding strong color in plum and white nectarine has
forced us to eliminate these jams from our line. Apricot and peach jams
always hold their color. Over cooking the fruit will cause immediate
darkening. Another factor with color is the ripeness of the fruit. We
work with fruit that begins in frozen form, if we over defrost we lose
color in our jams. Over ripe fruit will fade more quickly.
As far as added sweetner is concerned, there is a balance to find.
Fruit that is too dilluted with sweetner will also dull color, yet a
good balance of sugar and acid will hold color. We try to keep our
acid/ph (we use frozen single strength lemon juice) below 3.8. There
are commercial jams from Europe that advertise right on the label that
their fruit was never boiled. This is a secret in making high quality
preserves. -commercialcanner