This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
Foods considered high acid (pH lower than 4.6/4.7) can be boiling waterbath
canned. This next chart is reworked from _Putting Food By_.
Foods at pH 2.0-3.0 - lemons, gooseberries, underripe plums
Foods at pH 3.0-3.5 - ripe plums, underripe apples, ripe oranges and
grapefruit, strawberries, rhubarb, blackberries, cherries,
raspberries, blueberries, very underripe peaches and apricots
Foods at pH 3.5-4.0 - ripe apples, oranges, grapefruit, overripe
blackberries, cherries, raspberries, and peaches, ripe apricots,
under-ripe pears, pineapple, sauerkraut, (other pickles?)
Foods at pH 4.0-4.6 (BORDERLINE) -tomatoes, figs Above 4.6 or so, must
be pressure canned.
Foods at 4.6-5.0 - some tomatoes, depends on the variety. (Green
tomatoes are below 4.6). pimentoes, pumpkin. USDA suggests that
pumpkin butter cannot be canned safely.
Foods at 5.0-6.0 - carrots, beets, squash, beans, spinach, cabbage,
turnips, peppers, sweet potatoes, asparagus, mushrooms, white potatoes
Foods at 6.0-7.0 - peas, tuna, lima beans, corn, meats, cow's milk,
salmon, oysters, shrimp.
Above 7.0 - hominy, black olives (each are lye cured). Leave these to
Check the list of pH readings for common foods and condiments below. Need
to also consider the size of your jars (half gallon size jars are made, but
you probably shouldn't can with them), the physical properties of your food
and the gooeyness of what you are canning. Pumpkin/squash purees and
butters, and refried beans probably shouldn't be home canned--it will take
a long time to get the center of the jar hot enough. However, squash and
pumpkin chunks (raw pack) can be pressure canned. [Interestingly, spinach
and chard shouldn't be home canned because the leaves will condense into a
mass, insulate the center of jar, and form a dangerous cold spot--Louis
Nagel.] Also, cream soups and cream-style vegetables shouldn't be home
canned for the same reason that pumpkin butter shouldn't be canned.