This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
Fruit darkens at the top of the jar:
a. Liquid didn't cover the fruit--pigments become oxidized.
b. Fruit not processed long enough to destroy enzymes.
c. Air left in jars permits oxidation (bubbles or too much headspace).
Fresh fruit exposed to air oxidizes.
d. Exposure to high temperatures and light during storage.
Color changes in canned apples, pears, peaches, quinces: Pink, red, blue
or purple color--natural enzymatic reaction (not harm- ful) which may occur
during cooking, or a result of a chemical reaction between fruit pigments
and metal ions (iron and copper). Use soft water, stainless steel cookware,
plastic or wooden utensils.
Fruit floats in the jar:
a. Fruit is lighter than syrup. Use lighter syrup, cook fruit before
b. Improper packing. Pack fruit tightly without crushing. Use hot pack
c. Fruit is overprocessed. Too much heat destroys pectin and acid, so
the fruit loses its shape and floats.
d. Fruit is packed too loosely.
a. Overpacking. Heat penetration is poor and food does not become
b. Poor selection of fruit (over ripe, wrong pH, large bruises).
c. Underprocessing. Food is not sterilized.
d. Unsanitary conditions. Microorganisms are not removed from the food or
larger numbers are added during preparation. Clean up as you go. Wash
equipment, utensils and hands in hot soapy water.
Prepared by Susan Brewer/Foods and Nutrition
Specialist/Revised, 1992 ----