Here is the FDA document that discusses acidity: http://www.fda.gov/Food/FoodSafety/Product-SpecificInformation/AcidifiedLow-AcidCannedFoods/default.htm
Shoot for a pH of lower than 4.6.
How do you measure pH? Canning Across America recommends against using pH strips to gauge pH. Use a meter like one of these: pH meters. You'll see a wide variety of prices and reviews, so do your research before you buy.
For more information on acidity, check out the sources at the bottom of the post.
Here are lists of high acid and low acid fruits. High acid fruit can usually be made into jam without adding extra lemon juice. Low acid fruits need the lemon juice or can be mixed with high acid fruit to lower the pH.
High Acid Fruits
- green apples, tart apples
- tart plums
Low Acid Fruits should be canned with extra acid added for shelf stability and food safety, according to the FDA.
- sweet apples
Here's another site with a lot of great info on fruit acidity: http://foodpreservation.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=foodpreservation&cdn=food&tm=211&f=10&su=p284.13.342.ip_&tt=2&bt=1&bts=1&zu=http%3A//www.ext.nodak.edu/extnews/askext/jamjelly/4341.htm