A couple of potential issues. If using powdered pectin there is 1) no idea how much to add and if too much is used you can end up with rock hard set, and 2) it will change the flavor a bit. Best bet is to instead use extra seeds and pith from an additional grapefruit in the cheesecloth bag.

If using liquid pectin then I would add no more than 1/2 dose. JMO

Dave

Thanks, Dave,

That's really helpful. I wish I'd thought about this yesterday when I juiced a bunch of lemons and threw away the seeds! We still have more grapefruit from which I can use the seeds.

I have a follow-up question (sorry): The grapefruit is just the fruit (no peel, no pith), and I have less than a tablespoon of seeds to 4.5 lbs of fruit and juice. Does that sound like it would set up?

I have looked at tables listing fruit by natural pectin levels, and I saw grapefruit has been listed as both "none" and "high" (with the latter, I assume, being if you include the skins and seeds, and the former being if it is just the fruit)

Yeah it is the pith that really makes a difference pectin-wise. I can't say for sure if it will set or not with only fruit and juice but odds are high that it won't. All the citrus jam/jelly recipes I know call for peel and pith added. But all you can do is give it a try. Got any other fruit you can add for more pectin? Say some apple peels, maybe some oranges?

If not then I guess I'd use the box pectin. Worst that can happen is it will be a hard set and you can then thin it down after opening the jars.

Dave