How do I prevent jams from turning brown?

Turning brown on top is usually a sign of too much oxygen in the jar.

The NCHFP FAQ covers this exact topic (search for "dark"). There are a few primary causes:

• too much headspace, or bubbles left in the jam before processing;
• not enough liquid to cover bits of food/fruit; or
• not enough processing time.

All of these result in the surface of the jam being exposed to oxygen. The first two are fairly direct: there's too much air on top. The third is because processing drives some of the air out of the headspace, leaving lower pressure air, which not only prevents browning but also creates a reliable vacuum seal.

Too much headspace is an easy possibility for you to check. The right amount varies depending on the jar size, but you might well have too much.

The processing time is the really worrying thing to me in your case; you're not processing your jam at all, just packing it in hot and letting it seal. That's actually a good way to get unreliable seals, but it sounds like you're getting lucky and it's working out. (But be really careful to test the seal later just to make sure after it's cooled - push down on the middle and make sure it doesn't pop back up.) Still, I might suggest trying a couple batches of jam with a tested recipe that involves water bath processing just to be sure. It might well solve your problem, and it'll make your canning safer.

What turns your jam brown is the same substance that turns cut fruit brown: Oxygen.

The head space in your jars is filled with air, albeit less than at normal pressure. A jam jar has no genuine vacuum (= nothing there), but low pressure. To decrease the amount of oxygen that can react with your jam, you need to reduce the amount of air in the headspace.

• One way is to fill a bit more jam in your jars (check with the manufacturer on appropriate fill level, different systems have different requirements),
• another to can your jars after filling, which typically expells a bit more air. You can boil already filled and cooled jam jars, but if the surface is already brown, it won't reverse the process.
• A third method is to pour some (1/2 tsp. or so) high-proof alcohol in the jar after filling, light it and close the lid. Don't burn yourself, though. The alcohol will burn off some of the oxygen. If you use too much, the taste will be quite discernible and you might even get some caramelization on the surface, so this is method can have it's disadvantages. The trick is to use the right amount and find the right moment to close the lid so that "all" alcohol burns off yet uses up the oxygen. (And not set the kitchen on fire while doing it ^_^.)

On a more general note, I found using some citric acid (or lemon juice) in my jam helps with thickening and prevents browning. I think it also improves the taste, but that's subjective, obviously.

My mother and my wife have been making jams for a total of about 90 years. Based on what I have seen, I think all jams made with either hot bath or open kettle method oxidize a bit near the top of the jar. Lighter coloured jams, such as peach, show it more obviously. The taste is generally unaffected. We have always eaten these and never had any problems. I don't think you have to tighten the lids super tight, it's more important that the lid and top of jar are clean. I don't believe any bug was let loose to turn the jam a bit brownish. Jars are sterilized in the oven.

Darkening could be caused by several factors including: Stored too warm, too little sugar in the recipe & improper sealing. More details here Canning Questions

Open kettle canning (filling hot jars with hot ingredients) is no longer recommended due to a higher likelihood of spoilage. While your jam may not spoil, you may not be removing enough air to stop oxidation of the jam at the top of the jar. You can reprocess the jam. You will need to put the jam back into a pot and bring to boiling, then process in water bath as described here making sure to leave the proper head space and processing for the proper time for your altitude.

Sterilization of the jars is of the utmost importance. This sounds like some sort of bug has been let loose on your concoctions and has ruined everything.

Some people think that you can sterilize the jars in a dish washer but I would not chance it. I boil my glass jars in a big 5 liter pot of water to ensure everything is sterile.

Also you need to close the lids of your jars super tight. They must form a seal until the day they are to be consumed. Also full them up right to the brim.