Yes and no. If you're using a traditional jam recipe, then yes. And that's why jam sugar is needed for such recipes. It contains the pectin needed to make a gel like consistency to the cooked fruit. Adding the lemon juice to traditional recipes is a must, because this raises the acidity level of the jam mixture, which in turns helps the pectin to create the gelatinous texture of jam. In non-traditional/healthy jam recipes, instead of pectin and sugar, chia seeds are used to thicken the fruit, and therefore mimic the texture of regular sugar jam.