Crystals in the bottle

The main cause

Finishing above the window. A batch taken past the upper limit is supersaturated at room temperature, and supersaturated solutions do not stay that way; they crystallize as soon as something gives them a site to start on. That might be a speck of sediment, a scratch in the glass, or a cold night in a warehouse.

Cold makes it worse

Less sugar stays dissolved at a lower temperature, so a syrup that is fine in a warm kitchen crystallizes in an unheated storeroom or in a vehicle overnight in winter. A batch that was marginal on density fails in storage rather than on the shelf you tested it on.

It is a defect, not a hazard

Crystallized syrup is safe and can be redissolved with gentle heat. That is a reasonable answer for your own kitchen and not one you can offer a retail buyer, who sees sediment in a premium product. For a product with your name on it the cost is the same as a safety failure even though the cause is not.

Where the sediment connects

Unfiltered mineral sediment gives crystals somewhere to start, so a batch that was both slightly over-density and poorly filtered fails faster than either fault alone. Filtering is a quality step with a shelf-life consequence attached.

The fix is upstream

Aim at the middle of the window rather than the top of it. A producer who finishes at the ceiling to maximize yield is converting a small gain in volume into a crystallization risk on the whole batch.

What to tell a buyer

If a retailer reports it, replace the stock and find out which batch it was rather than explaining that the product is still safe. That is true and it is not the conversation; the buyer is describing something their customer returned. The useful response is the batch code and a reading that tells you whether the rest of that run is on its way to the same place.

References