A logger sits in the middle of the load and shows what the carrier did; what one pack on the shelf went through is what a TTI shows. Through an irreversible change that speeds up with temperature, the label combines the temperature the product saw and the time spent at it in a single visible result. The Codex code of practice for quick frozen foods lists indicators on the outside of cartons and pallets as a way to catch temperature abuse at transfers where no monitoring record exists.
| Type | What it shows | Example |
|---|---|---|
| Threshold indicator | Whether a set temperature was exceeded | Frozen food carton |
| Polymer-based TTI | Cumulative time-temperature exposure | Diacetylene-ink vaccine vial monitor (VVM) |
| Photochromic TTI | Charged blue by UV light, fades to white with temperature and time | Trials on fresh poultry |
The end point is matched to shelf life. Under the WHO PQS E006/IN05.4 specification, a VVM30 reaches its end point within 30 days at 37 °C, a VVM7 within 7 days and a VVM2 within 2 days, and the type is chosen to match the stability of the vaccine. Food works the same way: an end point looser than the shelf life leaves the label clean throughout, while one too tight runs out while the product is still saleable.
The label sits on the pack surface and cannot say at which transfer the product warmed up. That is why it does not replace the air temperature records required by Turkish Communiqué 2014/48 on frozen foods; it sits on top of them.
