I'm often asked why processing times for canning recipes are important.

Whether you use a water bath or pressure canner, the process time ensures that the very core of your jar of jam, beans, meat - whatever you are canning - is able to reach a temperature that kills harmful microorganisms.

Suppose you are canning green beans in quarts, and according to the recipe you should process beans in a pressure canner for 25 minutes at 10 pounds of pressure.  The canner has reached 10 lbs., and you start the timer for 25 minutes.

Now suppose the phone rings - your child wants a car ride instead of riding the bus home (oh, this sounds like MY life!)  Really, you can't trim a minute off the timer!  Finish the full 25 minutes.  Because the extreme heat you're applying to the jars hasn't been applied long enough to kill all the bacteria at the center of the jar. 

If you shorten the processing time - what could happen?  Well, sometimes you get lucky.  But weeks later, you could detect a funny smell from the basement where the jars are stored.  It's the mold, yeast, or bacteria you didn't kill in the beans, having a "party" in the jar!  They have thrived in the moist, nutritious environment, and now they will emit nasty gasses and possibly explode the jar.  Worse case: botulism exists in the beans, waiting to sicken and possibly kill anyone who eats them. 

Wow.  Process your jars completely!  Beginners: I recommend the Ball Blue Book or try this USDA link: http://foodsafety.psu.edu/usda/1PrinciplesHomeCanning/RecommendedCanners.pdf