Sterilizing jars can be a necessary procedure for preserving food in jars. It helps ensure the destruction of microbes that could either degrade the food, or render the food dangerous to eat, or both.
Modern lab research has shown that it is not necessary to sterilize:
- if you will be using a pressure canner to process fruits, meats or vegetables;
- jars for items that will be processed 10 minutes or longer in boiling water (though many people like to sterilize them anyway first as a “Hail Mary” measure);
- jars for no-cook jams that are stored in refrigerator or freezer: just put the jars through the dishwasher, or wash by hand with hot water and soap.
Suggestions that sterilisation of jars at home can be done by baking them in an oven, microwaving them or putting them through the dishwasher are actively discouraged by food authorities. Sterilisation requires temperatures above 212 F / 100 C for a period of time. You can’t guarantee that that condition has been met with the above methods. Only boiling water or a pressure canner can absolutely guarantee that temperature in a home environment.
Wash jars first with soap and hot water, then rinse well, or, put through dishwasher. Put jars standing up in boiling water. The water should be 1 inch (2 1/2 cm) above the top of jars.
At altitudes of less than 1,000 feet (305 metres), boil the jars for 10 minutes. For every 1,000 feet (305 metres) above that, boil an additional minute.
Remove jars one at a time from the water as you need them. Good, sturdy tongs are ideal for fishing them out safely.
Don’t empty the pot of hot water — you’ll need it for the processing step — why boil a whole pot of water all over again?
Put food in jars, leave 1/2 inch (1 cm) headspace. Make sure that the jar rim is clean, and that there are no air pockets in the food in the jar. Put the lid on, tighten it, then put the jars back in the pot of boiling water, and boil them (aka process them) for the amount of time called for by your recipe.
Don’t retighten the lids after processing, or you may break the seal.
It is not necessary to boil the self-sealing lids; in fact doing so make cause the seal to fail. They have a rubber gasket on them that melts during processing, creating a seal: you don’t want this rubber to melt too soon. Instead, put the lids in a bowl, and pour boiling water over them.
Save your antique preserving jars with the pretty glass lids for display.
If gas bubbles form in your jars of canned food, or there is seepage, something has gone wrong despite your best efforts. Destroy the food.
“Complete Guide to Home Canning,” Agriculture Information Bulletin No. 539, USDA (Revised 1994).
Michigan State University Extension. Preserving Food Safely – 01600946. 3 August 1999.
Schroepfer, Mary. Sterilizing Jars. Missouri University Extension. September 2006.
Van Wyk, Magdaleen. The Complete South African Cookbook. Struik, 2007. Page 340.