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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?

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Sounds strange. I generally don't put any pectin in jam and it scales well, so I don't see how adding extra pectin should pose a problem. – nico Apr 3 '12 at 16:22
    
Adding pectin or not depends on the fruit you are using, as some are naturally containing loads of pectin (e.g., apples) while others do not (e.g., raspberries). Furthermore, there are fluctuations in pectin levels within each kind of fruit, depending of ripeness. – Mien Apr 21 at 9:07
up vote 4 down vote accepted

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.

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Your first website applies only to no-added-pectin recipes. When adding pectin the jam does not have to be reduced and so surface area is irrelevant. The second link that talks about uneven heating does apply and is interesting. Thanks. – Sobachatina Apr 3 '12 at 21:00
    
Note that I have tried to scale recipe up to a certain point. I have never done 50 kg of jam in a batch, of course. But for "normal" usage (say, from 500g to 5kg of fruit) I did not have any problems. – nico Apr 6 '12 at 6:14
    
Couldn't you put it in a baking sheet and put it in the oven at low to dehydrate it without over-cooking? – Chloe Apr 22 at 0:25

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.

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3  
Well, the idea of scaling is keeping the ratios constant... – Stephie Apr 21 at 7:53

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