Filling hot, and what the temperature is for
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What the heat is actually doing
Product going in hot carries heat into the headspace and against the underside of the closure, which is the coolest part of a filled jar and the part a water bath reaches last. Filling cool leaves that volume to be dealt with entirely by the process, and in a thick sauce the process is already working against conduction.
It is a minimum, and it is recorded
Your filed process will state a minimum fill temperature. That means the last jar of a run as well as the first, which is the one people miss: a kettle cools while you fill, and the tail of a long run can go in below the figure the process assumed. Measure during the run, not at the start.
- Check the kettle temperature periodically through filling, not once.
- Hold the product at temperature rather than letting it coast down.
- Record the temperature at the start and the end of each run.
- Stop and reheat rather than finishing a run cool.
Why thick sauces lose heat in ways thin ones do not
A thick product does not circulate in the kettle either. The top can be at temperature while the bottom scorches and the middle sits cooler, which is the same conduction problem as the jar, one step earlier. Stirring is part of holding temperature rather than part of mixing.
What hot filling does not do
It is not a substitute for the process. A hot-filled jar that was not then processed to its filed time is not a shelf-stable product, whatever the seal looks like. The two work together and neither is optional.
The record matters as much as the number. A run filled correctly but unrecorded is, to anyone checking, a run you cannot show was filled correctly.
Fill temperature is also the easiest control to lose when a run is going well, because nothing visibly changes as the kettle drifts down and the jars keep looking the same. That is why it is measured on a schedule rather than watched, and why the last jar of a run is the one worth checking.