Michigan State University Extension
Preserving Food Safely - 01600549
Because the products contain fruit pulp or pieces of fruit, they tend to stick to the kettle during cooking and require constant stirring to prevent scorching.
Wash the fruit and remove the stems and blossoms. Peel peaches, pears, tomatoes, pineapples and quinces. Then shred the pineapple after removing the core. For tomatoes, cut slits and squeeze out the seeds; cut large tomatoes in quarters and leave small ones whole. Thinly slice pears, peaches and quinces after halving and coring and/or pitting. Take the pits from sour cherries. Leave strawberries and raspberries whole.
With Added Pectin When powdered pectin is used in making conserves and marmalades, combine powdered pectin with unheated crushed fruit. Mix well. Bring to a full boil with bubbles over the entire surface. Add sugar and boil hard for 1 minute.
Without Added Pectin Conserves, marmalades and preserves made without added pectin require longer cooking than those with added pectin. The most reliable way to judge doneness is to use a thermometer. Before making the product, take the temperature of boiling water (212 degrees Fahrenheit at sea level). Cook the fruit mixture to a temperature 9 degrees Fahrenheit higher than the boiling point of water. For a softer product, shorten the cooking time; for a firmer one, lengthen the cooking time.
It is important to stir the mixture thoroughly just before taking the temperature, to place the thermometer vertically at the center of the kettle, and to have the bulb covered with fruit mixture but not touching the bottom of the kettle. Read the thermometer at eye level.
If you have no thermometer, cook products made without added pectin until they have thickened somewhat. In judging thickness, allow for the additional thickening of the mixture as it cools. The refrigerator test for jelly may be used.
Process all these products in a boiling water bath canner.