It's important to sterilize jars, lids and filling utensils for cooked jams to prevent mould.
You can pre-sterilize jars & lids by using one of two methods:
Sterilize all jars, lids and filling equipment in a 225ºF oven for 10 minutes, then keep in the oven, with heat turned off, until needed so they stay warm.
Sterilize all jars, lids and filling equipment in boiling water for 15 minutes and keep warm.
Use tempered glass jars with two-piece lids, not leftover pickle or baby food jars. For more than twenty years General Foods Kitchens, now Kraft Kitchens, have successfully pre-sterilized and hot-filled jars for a large number of cooked jams, jellies, relishes, chutneys, marmalades and conserves. Both methods are equally effective at preventing mould growth.
Paraffin Wax Seals Using paraffin to seal is NOT recommended for jams and jellies as mould growth often occurs and will spoil your jam or jelly. If a jam or jelly has mould on it, throw out the entire jar. The mould produces toxins that penetrate deeply into the jam and cannot be removed by simply scraping off the mould. In the past, jam makers needed paraffin seals since they often used leftover food jars for their jams and these did not seal tightly.
Wipe jars of cooled jams and jellies with a clean damp cloth before storing. Label jars with the date and recipe name. Cooked jams and jellies should be used within one year of making. Store opened cooked jams and jellies in refrigerator.
Properly Sealed Jars Once the sterilized, warm jar is filled and two-piece lids are applied, turn the lid as far as it will go. After the jam or jelly has cooled, a jar is properly sealed if the lid is curved down and remains so when pressed. This may take up to 24 hours.
For no-cook jams, simply wash jars and lids or plastic containers with hot soapy water, rinse and fill. Freezer storage prevents mould spoilage.