Michigan State University Extension
Preserving Food Safely - 01600546
The biggest problem in making jelly without added pectin is to know when it is done. It is particularly important to remove the mixture from the heat before it is overcooked. Although an under-cooked jelly can sometimes be recooked for a satisfactory product, there is little that can be done to improve an overcooked mixture. Signs of overcooking are a change in color and the taste or odor of caramelized sugar.
When cooking jelly, remember that it should be boiled rapidly, not simmered.
Three methods that may be used for testing doneness of jelly made at home are described below. Of these, the temperature test probably is the most dependable.
TEMPERATURE TEST: Before cooking the jelly, take the temperature of boiling water with a jelly, candy or deep-fat thermometer (212 degrees Fahrenheit at sea level). Cook the jelly mixture to a temperature 8 degrees Fahrenheit higher than the boiling point of water. At that point, the concentration of sugar will be such that the mixture should form a satisfactory gel.
Because the boiling point at a given altitude may change with different atmospheric conditions, the temperature of boiling water should be checked shortly before the jelly is to be made. The bulb of the thermometer must be completely covered with jelly and not touching the pan.
SPOON OR SHEET TEST: Dip a cool metal spoon in the boiling jelly mixture. Then raise it at least a foot above the kettle, out of the steam, and turn the spoon so the syrup runs off the side. If the syrup forms two drops that flow together and fall off the spoon as one sheet, the jelly should be done.
REFRIGERATOR TEST: Remove the jam mixture from the heat. Pour a small amount of boiling jam on a cold plate and put it in the freezing compartment of a refrigerator for a few minutes. If the mixture gels, it is ready to fill.