Canning 101: Low and High Acid Foods

Different foods require different methods of canning. Some can up with just a boiling water canner, others require the high heat of a pressure canner. Low acid foods have a pH value higher than 4.6. Unless you are adding lemon juice or citric acid to increase the acidity level then they must be processed with a pressure canner. Low acid foods include red meats, seafood, chicken, milk, and all fresh vegetables except for most tomatoes.
Acid foods have a pH of 4.6 or lower. They include fruits, pickles, jams, marmalades and fruit butters. They can be processed in a hot water bath canner.

Here's a brief list to give you an idea of what's appropriate for either method:

Hot Water Bath Canning
Lemon
Apple
Apricot
Orange
Strawberry
Cherry
Pear
Plum
Peach
Tomato

Pressure Canning
Okra
Pumpkin
Carrot
Asparagus
Beans
Onion
Corn
Pea
Green Bean
Mushroom
Olive
Meat

To ensure a good quality canned product you'll want to pick produce at the peak of it's quality and can them within 6-12 hours of picking. Peaches, apricots, plums, pears, and nectarines can be ripened for a day or two between harvest and canning. Foods from your own garden or local farmer's market (that are usually picked the day before or morning of) will provide the best quality for canning.