The cold chain is not a single piece of equipment but a series of links joined together: pre-cooling after harvest or slaughter, the processing plant, the cold storage warehouse, the refrigerated vehicle, the distribution depot, the supermarket cabinet and the refrigerator at home. The chain is as strong as its weakest link; even if the product stays at the right temperature for ninety-nine per cent of the journey, a pallet left waiting in the sun on a loading dock does not give back the shelf life that was gained. Time spent in the warm adds up: bacterial count and quality loss are not reduced by cooling afterwards, they only slow down.
In figures: the rate of the reactions that cause spoilage roughly doubles or triples with every 10 K rise in temperature. Fresh meat that keeps for two weeks at 0 °C spoils in a few days at 10 °C; spoilage bacteria that take roughly half a day to double at 4 °C do so in about an hour at 20 °C. In frozen product every period above −18 °C enlarges the ice crystals; snow inside the pack and pieces clumped together are the sign that the chain has been broken somewhere. Refreezing thawed product harms both quality and safety.
The places where the chain breaks are usually well known: loading and unloading docks, product loaded without being cooled, a vehicle body that has not been pre-cooled, a store overfilled so that the air path is blocked, a room whose door is left open and a cabinet that warms up during defrost. The solutions are well known too: an enclosed dock with dock shelters, a dispatch area at 0/+5 °C, high-speed doors and strip curtains, and having the product cooled by the store, not by the vehicle. A refrigerated vehicle is designed not to cool product but to keep product that arrives cold in that state; its cooling power is limited accordingly.
The proof is the record. In stores the temperature is typically recorded every 5–15 minutes, an alarm is raised when the limit is exceeded, and the records are kept to be shown at inspection; in transport, data loggers placed on the pallet do the same job. Remote monitoring systems carry these records to the phone and the cloud. In the international carriage of perishable foodstuffs, the insulation and refrigeration class of vehicles is defined by the ATP agreement. Records and limit values are part of the HACCP (hazard analysis and critical control points) plan; you can find the correct temperatures of products in the food storage conditions guide.