Pectin thickens jams and jellies quicker than preparing jam without it. However, you can make jam and jelly without pectin. You just need to cook it longer, on the right temperature and combine it with the right ingredients (in this case, sugar and lemon). If it's still not thick enough, add a little cornstarch mixed with a teaspoon of water to it while it's cooking. As it starts to cool, it will thicken.