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Pectin packages always explicitly state that jelly/jam recipes cannot be scaled up.
My own empirical evidence is limited but I did have a doubled batch not set so I no longer double the batches.
What is it about
((pectin + acid + sugar + water) * boiling) = jelly
that makes scaling impossible?
I haven't made jam in years and don't know first-hand, but this website suggests it won't scale as the jam won't cook as well. This seems predicated on the idea that your cooking vessel remains the same size but the batch is larger.
From the linked site:
Most jam recipes already call for you to use the widest pot you have, for maximum surface area. This large surface area leads to faster evaporation of water. Fast cooking leads to the freshest tasting, best textured jam.
However, if you double the amount of jam in your pot, you greatly increase the cooking time, because there’s so much more product in the pot that needs to be cooked down. This can lead to rubbery batches, burning and jam that doesn’t set.
This is also supported by another site that claims doubling is difficult "due to inherently uneven heating of home cookware".
Per nico's comment, it sounds like doubling/scaling is possible - perhaps give it a try in cookware appropriate to the doubled batch size.
Because of time considerations ?
I made a 1 and 1/3 recipe last night which seemed to work well even though I accidentally made a couple mistakes (like I forgot to put in the last 2/3 cup of sugar & mixed half a pkg of 1 brand pectin with an eyeballed "half" of different brand that I had just bought...etc.)
Larger scaling, like double, I have never tried, but I thought it was discouraged because of the timing with hand canning: You will either have to leave the extra jam in the pot twice as long with the fire off, potentially getting cool; or keep it cooking the whole time, and it may over-thicken.
The ingredient ratios themselves really can't be all that exacting, or recipes would routinely fail due to years of slightly sweeter, juicier, or tart-er fruit, or different varieties of trees, etc. Not to mention forgetting abt. 6% of the sugar, which didn't seem to matter at all.
Scaling is difficult, as the ratio of ingredients changes with the batch size. Acid and pectin levels need to be altered to create proper gel formation.
I have finally decided that doubling up on the recipe simply will not yield the best end product. I believe you have to cook it too long to get to 220 and sometimes it may not even get there. The ratios of sugar, pectin and fruit is not consistent when doubling either. I have thrown so much jelly and jam out so will never make large batches again. I have even tried to start over and add pectin but often I believe I have cooked the living daylights out of my fruit and it still does not turn out the way I want it to! So it is simply not worth it. I make jelly to sell so I try to decrease my labor and increase my income but it never works that way! Quality more important that quantity!
