Recipes for grape jam (e.g. from Gourmet) commonly say to separate the skins, puree the skins for inclusion in the jam, cook them, cook the pulp, and remove the seeds with a food mill.
Is there any reason one couldn't instead just cook the entire grapes for long enough for everything to intermingle (i.e. long enough to fully cook the skins), then use a food mill to remove the seeds and skins?
I did actually do two batches with the two methods within a few weeks of asking the question.
Just cooking it all together, then removing the skins and seeds with a food mill, is definitely a lot faster. Removing the skins first then pureeing, so that you only remove the seeds later... well, though it is easy to remove the skin from each individual grape, time-consuming is an understatement unless you're making small quantities. It took me a few hours for (if I recall correctly) 10-15 pounds of grapes.
Including the skins has a moderate effect on the texture - it's slightly thicker and more opaque. You get the feeling there's just more stuff in it. As long as you're cooking it fairly long, though, there's not a very noticeable difference in flavor. On the other hand, if you're using a recipe with pectin instead of cooking it down until it sets, I definitely prefer having the skins in, for both flavor and texture.
So there's some personal preference involved; I do rather like the texture with the pureed skins. But if you're doing large quantities, and the grapes are on the small side, and you don't have help, you might want to skip it.
Most of the recipes I see that call for the skins to be pureed and added into the jam call for a longer time on the stove for the skins, so they probably need more time on the stove in order to achieve a proper texture and flavor. That Gourmet recipe also calls for maceration in order to develop more flavor.
The seeds are removed after the jam is cooked (not before as you imply in your question), because grape seeds are not something you want to crunch on while eating your jam and toast.
I assume you could mash all of the grapes, cook, and then mill, however - that might not cook the skins enough, and they would probably ultimately be removed from the final product, which would be different from the recipe above which includes them in the jam.
Removing concord grapes from the skin is incredibly easy, all you have to do is squeeze the grapes. Sure it's a bit time consuming, but it's not as bad as pitting cherries!
