When I am canning jam, after filling the jars, can I process the filled jars in a boiling water bath in two batches, or must all the jars be processed immediately after filling?
There is a simple rule to water based canning:
Cold jars go into cold water, hot jars into hot water.
The reasons behind this rule are
- It prevents breakage due to thermal shock.
- It should ensure correct canning/heating times.
So fill all your jars at once (or you might run into a problem like in this post), process the first batch. For the second batch you could either keep the jars in warm water or well covered, then no significant adjustments are necessary (perhaps a few minutes to make up for the heat loss), or leave them to cool and adjust the temperature of the water for the second round. Then the second batch will need slightly longer because you need to heat the water and the jars. Canning time starts once the entire pot has reached the prescribed temperature (hence never cool jars into hot water, times will be too short, leaving you at risk of food poisoning!).
You can process in two batches. Occasionally a jar may not seal properly, you can reprocess it with in 24 hours, according to freshpreserving.com/fresh-preserving-solver.
What happens if a seal fails?
Use food or refrigerate immediately. Only If the food was properly processed the first time, you may correct and reprocess if done within 24 hours. If an ingredient was left out, an ingredient was added, the wrong processing method was used, or the food was not processed long enough, you only have TWO hours to reprocess correctly.
This would apply to your situation as well. I would fill the jars and get them ready to place in the water bath, then process as soon as the first batch is done. They should still be quite warm which will help insure good vacuum and seal.