Dry the strawberries carefully because when left wet, the strawberry preserves will not set easily.
If your strawberry box has a few moldy ones, throw them away not the entire box. Cut away the watery patches from the overripe ones and you can cook them. This way you reduce food waste.
It is possible to reduce sugar in this recipe, but no more than 1/4 of the whole, because the preserves will be runny and not balanced. If you choose to alter the amount of sugar, you should add 1 grated Granny Smith apple, which is high in pectin, directly into the strawberry-sugar mixture while boiling.
You should make sure that your jars are clean to maintain the freshness of the strawberry preserves. Sterilize your jars by washing them (and their lids) thoroughly in hot soapy water. Rinse them well. Heat oven to 100°C (212°F). Place the jars and their lids on a baking sheet and put them in the oven for 20'. Using a funnel, ladle the strawberry preserves into the hot jars, filling to just below the rim. It is important that the preserves is still hot when filling the jars. Cover with a lid. Immediately flip the jar upside down. Leave to cool completely. When cool it creates a suction that pulls the lid down. When you open the lid, you can hear a pop. The jam can be stored for up to 1 year (it will be long gone before that!) in a cool dry place. Refrigerate after opening.
If you use white sugar, not overripe berries, and boil for 25' over medium-high heat, the strawberry preserves will have a lighter red color.