This summer we canned mango jam.  Now, about two months later, we noticed that all the jars have discolored fruit at the top.  We followed instructions for the recipe and filled the jars 1/4 inch from the top and placed them in a water bath canner for a full ten minutes.  The fruit has settled a bit and now there seems to be more space between the lid.

I have read that discoloration is probably due to either not filling the jar to the desired level or not processing long enough.  We did both of these according to the instructions.  What could have happened and should we throw all of our jam out?  We wanted to give some away as gifts but they do not look nice anymore.   We will keep some of them if it is still safe to eat.

-- Kiyo

I checked with Jarden Home Brands, marketers of Ball and Kerr brand home canning products, regarding this question.  Here is what they advised:

"Food darkening in the top of a jar can be caused by the food not being processed long enough to destroy enzymes.  Another cause can be that too much air was left in the jar due to the headspace.  Air left in jars permits oxidation.  Since the consumer stated there was some settling of the jam, this seems to be the likely cause.

The consumer did not mention if the jars had good vacuum seals.  Leaving too much air inside the jar may lead to a low vacuum seal or no seal at all.  If the seal has broken on these jars, the contents should be discarded.

Products that darken at the top of a jar is not necessarily a sign of spoilage.  Unfortunately, we have not tested a mango jam recipe.  We are unable to say a recipe is safe if we have not tested it."

I would add these comments:

• Processing time is measured from the time that the water comes to a full boil after the jars are put in, not from the time they are placed in the processing bath.   The time stated in most recipes is for use at sea level.  Because the boiling point of water is lower at higher altitudes (see High Altitude Cooking) the processing time needs to be increased accordingly.  For jams and jellies the processing time needs to be increased by 5 minutes for altitudes between 1,000 and 6,000 feet; and 10 minutes above 6,000 feet.  Note that this is a general rule for jams and jellies, and may vary depending on what is being canned.  Always follow canning instructions carefully.

• It is also important, in processing, to have one or two inches of water above the tops of the lids in order to ensure that the entire contents are uniformly heated.  Jars should not be touching, both to avoid having them bump against each other and crack, and to allow the boiling water to circulate well.

• Headspace is measured from the top of the food, or liquid, to the rim of the jar.  The amount of headspace required depends on the type of food being processed, and the method used.  Too little headspace may cause food to expand and be forced out of the jar, under the lid.  This can result in failure of the lid to seal properly.

• After filling jars, it is important to wipe the lip of the jar with a clean, sterile cloth to remove any food particles that may also interfere with the seal of the lid.  The screw bands should be tightened only finger tight.  If they are over-tightened, the air will not vent during processing, and the food may discolor with storage.

• As to whether the jam is still good to eat, it is hard to say.  General safety rules suggest not eating anything if the lid is bulged, if the contents spurt when the lid is removed, if there is any sign of air bubbles or of mold, of any color, any unpleasant or unusual odor, or any unatural color.  The problem with these recommendations, though, is that some spoilage, particularly in low-acid foods, may be completely undetectable, except with laboratory testing.  The best rule, as much as it may hurt given the effort to make the jam or other canned food, is "When in doubt, throw it out."

Resources:
National Center for Home Food Preservation at the University of Georgia.

Complete Guide to Home Canning and Preserving (Second Revised Edition) US Department of Agriculture

Canning & Preserving for Dummies

Joy of Cooking: All About Canning & Preserving

One final note:  the fundamentals of canning were rewritten in about 1994, based on new knowledge and up-to-date research.  Heirloom recipes may not benefit from this new information and should be used with caution.