Often when people ask if their canned food is safe even though they know or have been told they processed them unsafely, they end by saying "the jars sealed".  Or, they are finding really old home canned jars of food they forgot about, or found, when cleaning out a relative's home. "They're sealed so shouldn't they be safe?" Here is short posting to say that a sealed jar is not a sign of food being processed safely!

A vacuum seal that forms on a canning jar after processing only means that there is a vacuum holding the lid on. To create a vacuum requires enough heat produced in the contents and the lid and then a cooling process that draws the vacuum.  The heat required to form a vacuum seal with today's canning lids is much less than the heat needed to sterilize most foods.  Killing of spores of C. botulinum bacterial spores in low-acid foods, for example takes much more thorough and severe heating throughout the food mass than the heat needed to sufficiently warm the lid and headspace for vacuum to form after cooling.

Even killing of harmful bacteria like E. coli or others of concern in acid foods would take more heating of the jars of food than that needed to cause a vacuum seal to form during cooling. Enough heat has to get distributed throughout all parts of the mass of food in the jar to get to wherever the bacteria might be.

If a food was processed safely, a vacuum seal then needs to be maintained during storage of canned foods on the shelf to keep the food safe. The vacuum seal prevents food from being re-contaminated (and drying out). But a vacuum seal by itself is not a sign that the food inside was processed correctly to make the food safe.