Hello! Making freezer jam is a wonderful way to preserve the fresh flavor of strawberries, and it is completely understandable that you want to avoid making it overly sweet.
Traditional freezer jam recipes typically require a 1:1 or even higher ratio of sugar to fruit (often 4 cups of fruit to 4 to 7 cups of sugar) because standard regular pectin relies on high concentrations of sugar to set properly. If you simply reduce the sugar in a standard recipe, the jam will end up runny and syrup-like.
To achieve a balanced, less sweet jam that still sets nicely in the freezer, I recommend using a low-sugar or no-sugar-needed pectin (such as Sure-Jell Less Sugar or Pomona's Universal Pectin). With these pectin types, you generally only need between 1 and 2 cups of sugar for every 4 cups of crushed strawberries, rather than the usual 4 to 7 cups. This allows the natural brightness of the fruit to shine through without cloying sweetness.
Another helpful tip is to add 1 to 2 tablespoons of freshly squeezed lemon juice per 4 cups of crushed berries. The acidity not only helps balance the sweetness and brighten the berry flavor, but it also aids in preservation.
Be sure to leave about a half-inch of headspace at the top of your containers before sealing them, as the jam will expand slightly when frozen.
