have you ever experience jars cracking from the vacuum? I’d be surprised if that could actually happen… But I could imagine a jar cracking if you will piping hot jam into a cold jar.

Regarding the apples: Different amounts of sugar? (sugar influences gelling) Different volumes? (the one batch was perhaps heated longer than the other) Did you add anything but apples, sugar and water – for instance something that could have changed pH? (pectin jelling is to some extent sensitive to pH)

Can somebody give me a ground-up description of the reaction that produces the jel in jelly? I am intrigued – two consecutive days, same apples, and two completely different jellies – one dark coppery colour as set slow but clear, one pale yellow gold and produced idiotic amounts of foam and trying to set in the pan even before I could get it in the jars. What did I do?

Can somebody please give me the reaction in equation form, so I understand this a little better?

Of course, I always poured boiling-hot jam into scalded jars (you have to boil water for the lids anyway, right?), clapped the [metal] lids on immediately and inverted them. (I don’t do the boiling water bath and I’m still alive twenty years later….I also simply scald out my jars while they’re sitting on the counter and then cover them with a clean towel. Uses much less water than immersing and boiling them.)

FWIW, I’ve never had a failed seal and never had a spoiled/moldy jar, no matter how many years I kept it.

Regarding the sterilization of jars using dry heat at 120-130 °C I’ve been asked about how long they should stay in the oven. This is a difficult question, for a true sterilization (which can not be achieved with boiling water alone!) you would need pressurized steam for at least 15 minutes at 121 °C (also known as autoclaving). This corresponds to as much as 2 hours at 160 °C or 1 hour at 170 °C when dry heat is used. However, the idea is only to obtain the same degree of “sterilization” that boiling in water for 10-15 minutes would give. My guess would be that if you leave the jars for 1 hour at 120-130 °C it should be fine. And you could of course also turn up the temperature even more, but from practical experience, this doesn’t seem to help since the jars are not the weakest part of the chain (the lids, the filling procedure and the closing of the jars is).

2) This would be to allow a skin to form and to avoid condensation. Less water activity = less mold growth. If you close the jar immediately you have to turn it upside down. But if one for some reason (esthetics?) would like to keep the lid and the surface of the jam “clean”, you should wait a while before closing the jars.

4) Closing the jar immediately + turning upside down will be best to avoid growth of mold (unless you opt to sterilize the whole jar in boiling water using a home canner or simply a large pot with boiling water). I have tried this, but the main drawback is that it really doesn’t look very nice… It probably works better for jelly than for jam.

Alt. 3 (letting a skin form before closing) works fine as long as you wet the lid with alcohol. And if sugar contents is > 50% this will also slow down growth of molds considerably.

1) “Place enough jars in a cold oven and heat to 120-130 °C to sterilize them”:

Is this to avoid the glasses cracking? Are you certain that this is necessary with today’s glass quality? Also: don’t forget to do the same with the lids. Sterile jars won’t help if the lids are infected. The ethanol surely does help, but you can save some ethanol by heating them. I often use those jars with aluminium lids that pop up when you open them the first time (that is: I reuse jars from jam I’ve bought). If the lid draws/pops in when the jam cools and stays so, the vacuum is sufficient. Else, eat the jam right away (surely not a bad suggetion

2) “Leave to cool for 10-15 minutes and then screw on lids”:

Why? Leaving them open will result in (contaminated) air flowing into the jar. Closing immediately would certainly avoid this.

3) Foam:

I guess you’re right as long as the sterilisation is done properly. However, there are probably a large number of culinary presitions going around here as this is an old art (science?), and if one lets the jam cool before closing the lid, the foam will cool even more rapidly and contaminated air entering from the surroundings might infect the foam. The old rule of turning the jars upside down might help this because the hot jam will sterilise the lid and upper parts of the jar (see below). Also, foam might funtion as an insulating lid, both during cooking and storage, whatever effect that might have.

4) Turning the jar upside down:

Why not just store the jar upside down to be sure? I’m not a microbilologist, but after a chat with a biologist friend, we place our bets on alternative 2. Alternative 3 is also tempting, since a more viscous solution should leave the jars less prone to drawing in air, either due to dried and cracked seals (why are wine/champagne bottles stored lying on the side?) or because air/airborne microorganisms diffuse(s) more rapidly through air than a viscous jam. However, the moist atmosphere might be good enough for keeping the seals in good shape anyway (biologist comment: atmosphere above jam will funtion as a “moisture chamber”).

The idea of heating the jars in the oven instead of boiling water is also quite appealing.