Acidic Reaction

• The tart taste of a plum illustrates the amount of acid the fruit contains. Acids are powerful substances that can eat their way through many materials, including some metallic surfaces, such as aluminum pots. Cooking plums for jam creates a reaction between the acid they contain and the surfaces of an aluminum pot they come into contact with during the cooking process. When this happens, some of the metal leaches out and a metallic flavor is imparted to the jam. This can be avoided by cooking the fruit in a non-reactive pan.

Non-Reactive Pans

• The acids in plums do not react with stainless steel, glass, copper or enamel-coated cast iron, which offer cooking pots in many different sizes. Glass cookware is generally not available in sizes that lend themselves well to making jam, as the process calls for a pot that is large and deep enough to handle the volume of hot boiling fruit and sugar and prevent boil-overs, which create a sticky mess on the stove. Ideally, the pot you choose should be only half-full once you have added the fruit, liquid and sugar.

Making Plum Jam

• You can make plum jam to use right away or to process and store for several months. In either case, the basic cooking process is the same, and the difference lies only in how you bottle the final product. You can also make jam with or without pectin, which is the natural gelling substance found in most acidic fruits and some tomatoes. Preparing the fruit involves washing it, removing the stem ends, cutting it in halves or quarters, and discarding the pits. If you are not using pectin, lemon juice is added to the fruit, sugar and water to ensure that enough natural pectin is present for proper gelling. For jams made with pectin, which is available as a powder or liquid, and as regular-set and quick-set, follow package directions for specifics on when it should be added to the fruit.

Considerations

• The insides of an aluminum pot that has been used for cooking fruit often shows the effects of the acid in the form of a discolored section that is darker and sometimes pitted. Over time, this pitting can results in holes that make the pot unusable. No matter what type of cooking pot you use to make plum jam, scorching can be a problem during cooking. Stir the hot fruit often with a long-handled spoon, scraping the bottom and sides to prevent sticking.