Strawberry/rhubarb Jam No Added Pectin

Ingredients

To ensure jelling, at least 25% of the berries should be slightly
underripe (underripe berries have a higher natural pectin content):
1 cup rhubarb, sliced lengthwise, then cut into 1 inch lengths
6 cups whole strawberries, washed and hulled
4-1/2 cups sugar

Instructions

Place strawberries in 8 or 12 quart pot. With a potato masher, crush a few of them against the bottom of the pot to get some juice. (it's OK to leave most of them whole) Add the rhubarb and sugar. Stir gently with a long handled spoon. Wait a couple of minutes and then stir again until most of the white sugar grains have turned red. Now turn the heat on high and start stirring. You'll need to continue stirring for quite a while to prevent the mixture from burning against the bottom of the pot. In 7-10 minutes, the mixture should come to a furious boil. If it threatens to overflow the pot, either turn down the heat or, using potholders, lift the pot off the burner for a few seconds. Continue stirring on high heat for 16-24 minutes after the initial rise, until you feel a significant increase in resistance against the spoon as the mixture thickens up. Now take the pot off the heat and fill your pre-heated jars. Attach lids and process in a boiling water canner for 7-10 minutes. Makes 5-6 cups. Natural pectin or "No added pectin" jams are more work, but there's much less sugar used and your jars are completely filled with fruit. This is the route to take if you are tired of seeing clear jelly at the bottom of your jars while the comparatively small amount of fruit floats to the top, i.e., when pectin is added and the amount of sugar is necessarily increased. The above recipe works for essentially all fruits that have some natural pectin. Use the basic 4:3 fruit:sugar ratio and you should be fine. Just adjust mixture boiling time to get the consistency you prefer. Note that you won't really know what the consistency is until the next day, after the jars have cooled, unless you use the freezer plate test. This recipe may not work if scaled up to double or triple batches. There seems to be something special in the boiling of sugar and natural fruit pectin that demands small batches at high heat to get a good jell. If others have successfully scaled up "No added pectin" recipes, please comment.