6 months ago
Last edited at 12:36AM on 6/24/2013
Hi Dolly59 there are actually a couple of reasons. First, they are sealed in air tight containers (prior to being opened) the way this is done is the jar with the jelly or pickles in it is heated with a lid on it then taken away from heat. As the jar cools a vacuum is created inside the jar and the lid is sucked down tight. This is one reason new jars can be hard to open at times, the vacuum holds the lid down. This mean there is no air in the jar so bacteria can not grow to spoil the food.
Cooking the food prior to jarring it also helps prevent spoilage.
The last reason is preservatives. This does not mean chemicals. Sugar, Vinegar, and Salt are all preservatives. In Jams and Jellies Sugar and sometimes citric acid are the preservatives in the jelly, in Pickles Vinegar is the most common preservative but salt and sometimes sugar plays a role and occasionally there are other things used like alum or lime, all of those thigns keep the food from spoiling even after the jar is opened.
Jams, jellies and pickles are typically preserved in air tight containers, meaning they can keep fresh for a very long time. The lack of air means bacteria and other pathogens cannot grow on the surface of the food. These foods are also stored in preservatives, whose work is to preserve the food. Read on from nchfp.uga.edu/publications/usda/GUIDE%201%20Home%20Can.pdf.