Food Preservation: What makes the jelly stiff or too tough?

What makes the jelly stiff or too tough?

Overcooking could cause the jelly to be stiff. Cook the jelly mixture to a temperature of 8 degrees higher than the boiling point of water or until it "sheets" from a spoon.
The jelly could be stiff if the is too much pectin in the fruit. Use ripe fruit or decrease the amount of pectin if you are using a commercial pectin product.
Jelly could be tough is there was too little sugar in the juice and you had to cook it longer to obtain the correct consistency. When pectin is not added, try to use 3/4 cup of sugar to every 1 cup of juice for most fruits.