New canning jars out of the box are not sterile. They can be contaminated by microorganisms that cannot be seen with our bare eyes, accumulate dust, or other small bits of debris even while being wrapped in plastic. That is why it is important to prepare your jars before canning. Sterilizing canning jars sounds scary, but it’s really very easy.
The safest way to sterilize canning jars is to boil them. Fill your canner with water and bring to a boil. This will take some time because the canner can fit a lot of water so be sure to plan ahead when you do this. You also always want to check your jars to see if there are any chips or defects, if so discard the jar or re-purpose those jars for non-canning uses.
Whether brand new or re-used many times over, you should always clean jars just prior to filling them when canning. Wash jars in a dishwasher or by hand. Never use a brush with abrasive bristles though, like wire or steel wool. Those could damage or weaken the glass. Since you’re going to be putting them into boiling water, then it’s crucial that you rinse them in hot water, and put them into the pot or canner right away. Don’t allow the glass to cool or it can cause the jar to crack.
In order to actually sterilize jars, they need to be submerged in boiling water for 10 minutes. When the process time for canning a food is 10 minutes or more (at 0-1,000 feet elevation), the jars will be sterilized during processing in the canner. Therefore, when process times are 10 minutes or more at this altitude, pre-sterilization of jars is NOT NEEDED.
When you are ready to fill the jars, remove the jars one at a time, carefully emptying the water from them back into the canner. This will keep the hot water in the canner for processing filled jars.