This recipe uses 1/3 of a cup of sugar, which is really the bare minimum you can go before you change the texture of the jam. See, the sugar also helps to solidify the jam, just like the pectin does. If you reduce the sugar, you risk not having enough to make a solid jelly. The sugar also helps to preserve the fruit. So, if you don't use any (or enough), you run the risk of the fruit growing mold in the jar and/or just reducing the shelf life of the jam. Also, keep in mind that you want to use regular sugar in this recipe. You can't substitute a sugar substitute for sugar in a no pectin jam. If you can't have sugar, you'll want to look for a no sugar needed pectin for your jam. Find out more about that here.