Packing hot, and what it rules out

What the hot pack does

It carries the syrup into the container at a temperature that deals with surface contamination and leaves a sealed, hot-filled container that pulls a slight vacuum as it cools. The product was already safe on sugar concentration; the hot pack is what keeps the surface of it that way after the lid goes on.

Material is decided here, not later

Fill temperature is the first question a container has to answer, and most plastic bottles made for pourable food are not built for it. On our own shelf for pourable food that split is exact: of 323 bottles, 37 are glass and those same 37 are the ones the catalog marks hot-fill capable. If you are hot packing, your choice is those 37 rather than the 323.

Fill level and headspace

Leave the headspace the container is designed for and keep it consistent. Too much air in a hot-packed bottle is a larger cooling vacuum and more oxygen against the product; too little risks overflow as the hot syrup expands. Measuring the fill by weight rather than by eye is the way this stays consistent across a run.

Cool it promptly

A case of hot bottles packed tight cools from the outside in, and the ones in the middle keep cooking themselves. The result is darker syrup and a flatter flavor in part of a batch, which looks like an inconsistency problem rather than a cooling one.

Record the fill temperature

At the first bottle and the last. A seal failure or a mold complaint that affects only the end of a run is a fill temperature story nearly every time, and without that column you will be looking at the closure instead.

References