Why boiling?

What is the reason to bring your product to boil right before you put it into jars for processing? I do it, because that is what all the instructions say, but I don't know why I do it. Is it a product safety issue? Or a thermal shock issue?

For instance, I made apple pectin yesterday. The stuff boiled on my stove for 3 hours before I strained it. But after I strained it, I had to bring it back to boiling just to put it into the jars. Even when I make applesauce, the stuff boils for a while before I strain it. But then I have to reheat (and dirty another pot) simply to put it into the jars.

It is a safety issue. Re-heating to boiling eliminates any bacteria the food may have picked up during the interim steps (straining, milling, the air etc.). But the main reason is that processing times called for in the recipes are based on you inserting near-boiling hot food into the canner. Those processing times would be invalidated otherwise.

In other words, food that was boiling when jarred comes quickly back up to processing temps of 212 or 240 (PC) in the jar. That same food, if jarred at room temperature, would take many more minutes to get back up to the processing temperature. So if the recipe calls for processing 10 mins. in a BWB you'd have to double that time or more if you filled the jars with room temperature food.

This is why we only fill enough jars for one canner load at a time and we fill them quickly. Then you reheat the food and fill the jars for the next load, etc. You don't fill all the jars and let the ones that don't fit just sit on the counter and wait.

OK?