Buy heat packs ten at a time and this doesn't come up. Buy two hundred and keep them in a store room, and it's worth two minutes of your attention — because the thing that makes them useful is also the thing that decides where they live.
They react with water. That's the product
An unused pack is classified UN2813, Class 4.3 — a water-reactive solid. That sounds alarming written down and it isn't, particularly. It's the formal way of saying what the packaging already tells you: add water and you get heat.
The composition is calcium oxide 80~95% with aluminium 5~20%, which is quicklime and a bit of metal. Quicklime slakes when it meets water, and slaking throws off heat. In a tray, with a measured water pack, that's steam within 10 seconds and dinner. In a cardboard box that's sitting in a puddle, it's the same reaction with nobody watching.
What that actually changes
- Keep them dry. Not dry-ish — off the floor, away from a chiller that drips, not under the air-con outlet, not against an outside wall that sweats.
- Keep the carton sealed until you need them. The pouch is the barrier and a torn one in a humid store room is a slow problem.
- Don't stack them next to the mop bucket. This is the one that actually happens.
- Store them away from heat sources, which is on the label anyway.
Singapore humidity alone won't set a sealed pack off. Standing water will. The distinction is worth holding on to, because the first one makes people over-cautious and the second is what catches them out.
What it doesn't change
You don't need a licence, a cabinet or a separate store. You're not handling a hazardous substance in any sense a kitchen would recognise — it's quicklime in a pouch. A dry shelf is the whole requirement, and most caterers already have one.
The classification matters more for moving them than for keeping them. If you're shipping a pallet somewhere, your freight forwarder will want the number; for a box in the back of a van, it's a non-issue.
Buying in volume
Packs run $1.80 each from 10 pieces, $1.70 from 30, $1.60 from 100 and $1.50 from 200. They're single use and each 4000ml tray takes two, so a caterer doing four 15-tray jobs a month gets through 120 in a month.
The tempting move is to buy 200 and bank the $1.50. Before you do, look honestly at where they'd live for the next few months — because the saving between 100 and 200 is thirty dollars across the box, and a damp store room will cost you more than that in packs that have already reacted quietly in their carton.
When not to stock up
If you cater occasionally, don't. Buy at the tier your actual volume reaches and keep the cash. The packs don't go off on a dry shelf, but a hundred spare pads sitting in a cupboard for a year is money you could have spent on the job in front of you — and if your storage is a corner of a wet kitchen rather than a dry store, buying fewer more often is simply the right answer.