Tips for successful freezer jams from Oregon State University Extension Service:
• Use 1- to 2-cup glass or rigid, plastic, freezer containers with tight-fitting lids. There is no need to sterilize containers or process the jam. But containers should be washed in hot, soapy water, or run through the dishwasher.
• For best flavor (and to ensure a proper set) use fully ripe fruit.
• Use one of the regular commercial pectins which includes recipes for freezer jam. Follow directions exactly and do not reduce the sugar.
• For any given recipe that you’re using, never substitute one pectin for another; they are not interchangeable and each is prepared a different way.
• Do not double the recipes. Doubling recipes could result in a runny product, and it’s hard to dissolve the large quantity of sugar, so sugar crystals could grow during storage, which would result in a grainy jam.
• Measure accurately and follow directions exactly. The most common reason for failure is inaccurate measuring. Because freezer jam uses large quantities of sugar, it’s a good idea to keep a record of each cup as it is measured.
• Leave at least 1/2 inch head space to allow for expansion of the jam as it freezes.
• After thawing and opening the container, store in the refrigerator. Remember, the product is not cooked so it will ferment and mold quickly if left at room temperature for extended periods of time.
• For low sugar freezer jams, consider using a Ball canning product called “Fruit Jell — Freezer Jam Pectin (check Bi-Mart, Walmart and WinCo) and Sure-Jel Lite.