Tip 1: Applesauce canning is the same as for any "acidic" fruit, like peaches and tomatoes. You want fruit that isn't spoiled -- although you can cut out bruises of course, and use the rest of the apple.
Tip 2: Use "real" canning jars. Yes, you can save money with glass mayonnaise jars and such, but the point of the real ones is, they are thicker glass and designed to take some heat. Especially for your first few tries, spend a few dollars on the real ones. You also need two other items shown here: rings, and lids. You can buy them together in one package. The rings are reusable forever (or close to it), but the lids are meant for one use only. See the red rim of rubbery material? That, and the precise shape of the lid -- those are the parts that make the airtight seal. Re-use the lids and you're asking for failed seals. The lids are cheap. Your time (and food) isn't! Make sure to wash everything before use. I like wide-mouth jars, myself.
Tip 3: I sort of quarter my apples, leaving out the cores -- which avoids (harmless) black specks fro the cores that way -- and simmer them in a heavy-bottom stock pan. Another good pan for this is the enamel-lined cast iron kind. Keep the heat modest, to avoid scorching. Add a little water. Stir often.
Tip 4: I've almost always used a "Foley mill" (shown here) to turn the soft apples to mush and remove the skins and any stray seeds. Mine usually lasts 15-20 years or so ... this is my second since 1984. So it's a good investment. You can experiment with other food mills if you like. Of course, if you choose to peel your apples, you can just stir the "mush" and not put it through a food mill at all. (And that's how to make "chunky" applesauce: peel, cook, and do not squish it!) But I'm usually looking for efficiency when I'm doing applesauce, so the Foley mill suits me. The hardware store has it -- or purchase online.
Tip 5: You MUST sterilize the jars. Here's how canning works: Water (or applesauce) when you boil it reaches 212 degrees F. That's hot enough to kill a lot of bacteria, but not some of them, and not the molds. By trapping the heated fruit inside the canning jar, you build up pressure that lets the temperature rise even higher than 212, and kill off the nasties. BUT if you don't sterilize the jars first, you risk having extra populations of bad buggies that won't die. I put mine into the canning kettle, with water up to about 3/4 of their sides, inside them too, and bring to a boil, with the lid on. After 5 minutes, they are in good shape for filling. Use tongs to lift them out, letting the water from inside them drain back into the kettle as you take the jars out. Place them on a dishtowel-covered bread board or countertop -- this prevents them getting shocked by the temperature changes they are going through -- and in a moment they'll be dry and hot and ready to fill.
Tip 6: Don't boil the lids! But they will be a bit closer to 100% sealing if you warm them in a bit of the hot water dipped out of the boiling canning kettle.
Tip 9: Set the lids into place and then screw on the bands LOOSELY. When you lower your jars to be submerged in the canning water (which should come up to half an inch OVER the tops of the jars; I usually keep a kettle boiling in the background in case I need a bit of extra water), air inside the jars will be forced out through the seal area -- it's a one-direction process. Then as you "can" -- that is, bring the water back to a boil and time carefully from the start of the boil (always check the right amount of time for the size jars you are using; it's likely to be about 30 minutes for quarts) -- the steam pressure builds up inside the jar (that's why you're using real canning jars, and new lids) to kill any harmful organisms and make the food safe to store at room temperature. It does help to have one of these special canning tongs thingies, but I canned without it for 20+ years -- a couple of potholders and a pair of cooking tongs will get you through.
Tip 10: When the time is up and you take out the jars, handle them gently and put them back onto that towel-covered board. PROTECT them from drafts and bumps for a few hours. You can screw the bands down a bit when you first take them out. But not too tight. After the jars have cooled (next morning), you can actually take the bands off -- the lids will be firmly sealed to the jars. Taking the bands off, and washing and drying them before putting them away, will help keep them in good shape for the next round. On the other hand, if your jars are likely to get bumped in cupboards or carrying, leave the bands in place. You can wipe the jars down with a clean damp cloth once they are cool, too.
You asked about how to be sure the jars have sealed. Here it is: Once the jars have cooled for a couple of hours, the center of each lid should be sucked down toward the fruit, making a dimple. Good! If instead the center is popped up, press it gently and see whether it stays down. If it does, and is still down in the morning (when the jars are cool), you're fine. But any jars where the lid dimple has NOT sucked down by morning should be considered "not sealed" -- just pop them into the fridge and use the contents within the next week. It's fine for eating, but it won't "keep."
Add your labels (be sure to add the year!) and enjoy having a stable, safe, and delicious food in your cupboard, for later in the season!
