• Do not use overripe fruit. Canning doesn’t improve the quality of food, so if you start out with low quality, it will only get worse in storage.  Plus
• Do not add more low-acid ingredients (onions, celery, peppers, garlic) than specified in the recipe. This may result in an unsafe product.
• Don’t add substantially more seasonings or spices, these items are often high in bacteria and excess spices can make a canned item unsafe. I doubt whether increasing a spice from 1 teaspoon to 2 in a batch of 7 quarts will have any adverse effect, but use some common sense and don’t go overboard.
• Do not add butter or fat to home-canned products unless stated in a tested recipe. Butters and fats do not store well and may increase the rate of spoilage. Adding
  butter or fat may also slow the rate of heat transfer, and result in an unsafe product.
• Thickeners - With the exception of “Clear-Jel” which has been tested in USDA and university food labs, do not thicken with starches, flour, or add rice, barley or pasta to canned products – this applies to both savory products (such soups and stews), sauces and pickled items. Items that thicken products will absorb liquid during processing and slow the way in which the food heats. Under-processing and unsafe food could result.
  ClearJel Starch, 1 lb. has been tested in pie filling recipes.

• DO add acid (lemon juice, vinegar or citric acid) to tomato products when directed in the recipe. In 1994, food scientists proved the risk of botulism poisoning from canned tomato products, and acid is now added to canned tomatoes, even to those canned commercially. Lemon juice is widely available, but will add a sharp note to canned tomatoes; citric acid will change the flavor less noticeably, and vinegar is part of many recipes anyway. If necessary, you can balance the tart taste by adding sugar.
• Heat process (water bath canning or pressure canning, as called for in the recipe) all canned items that will be stored on the shelf. Some recipes, especially those
  for jams and jellies, instruct you to simply seal hot-filled jars, or to invert the jars as the final step. I know of no reputable source (university food science departments, the USDA, FDA, National Home Canning Center, etc.) that recommend either “open-kettle canning” or inverting jars as the final step, as unsafe final products may result.
• Never process the jars in any oven (electric, gas or microwave). Steam canning is also, pretty broadly NOT recommended.  There ARE a couple of manufacturers selling steam canners, but you’ll find virtually no credible authorities recommending them, for a variety of reasons, starting with basic heat transfers properties of steam vs. water.
• Increase water-bath processing times at altitudes of 1,000 feet or more to compensate for the lower temperature of boiling water at high altitude. I’ve tried to be sure to include the conversion charts in all recipes for this.
• To prevent darkening: Some peeled or cut fruits (such as peaches, apples, nectarines) will darken when exposed to the air. Any of these simple treatments will help prevent darkening:
  • Use a commercial ascorbic acid mixture like “Fruit-Fresh”, which is available at the grocery and drug stores. Sprinkle it over the cut fruit and mix well. OR
  • Put the cut fruit in a solution of 1 teaspoon ascorbic acid (vitamin C, available in a powdered form from the drug store) and 1 gallon water. Drain before canning.
  • Put the cut fruit into a lemon juice solution (3/4 cup lemon juice to 1 gallon water). Drain fruit before canning.
• Canning jars. Use standard mason / Ball / Kerr (etc.) jars for home canning. Commercial food jars that are not heat-tempered, such as mayonnaise jars, often break easily  Sealing also can be a problem if sealing surfaces do not exactly fit canning lids. Be sure all jars and closures are perfect. Discard any with cracks, chips, dents or rust. Defects prevent airtight seals.
• Do not use jars larger than specified in the recipe, as an unsafe product may result. It’s almost always ok to go smaller.  Generally speaking, quart jars are the largest size you should use.

• To remove scale or hard water films on jars, soak several hours in a solution of 1 cup vinegar (5 percent) per gallon water. Keep the jars warm until ready to fill (to reduce breakage from thermal shock).
• Prepare the two-piece metal canning lids by washing them in water and following the manufacturer’s instructions for heating the lids (some need to be covered with hot water  for a minute or more – in steaming, but not boiling water)
• The flat lids can be used only once, but the screw bands can be reused as long as they are in good condition. Read Do not reuse lids from commercially canned foods.
• Check jars for seals within 24 hours of canning. Treat jars that fail to seal properly as if they are fresh (refrigerate and eat soon).

Article from: pickyourown.org