The critical distinction between the temperature of air surrounding frozen products and the actual temperature within those products — a gap that can exceed 9°C after routine freezer operations and that standard temperature monitoring systems are unable to detect.
Why This Distinction Matters
Food safety regulations — including R638 and Codex Alimentarius CAC/RCP 8-1976 — specify product temperature requirements: -18°C storage, with maximum -12°C tolerance during brief distribution periods. But virtually all commercial freezer monitoring systems measure air temperature.
Air temperature and product temperature are not the same measurement. They diverge dramatically during and after any thermal disturbance: door openings, loading events, equipment failures, or defrost cycles.
The Physics
Air has negligible thermal mass (1-2 kg in a typical freezer, specific heat 1.005 kJ/kg·K). Product has enormous thermal mass (200-500 kg of frozen food, specific heat 1.5-2.5 kJ/kg·K). The ratio ranges from 150:1 to 1,000:1.
This means air temperature responds to changes in minutes while product temperature responds in hours. After a 30-minute door-opening event:
Air recovers to -15°C within 10-15 minutes of door closing.
Product surface recovers within 30-60 minutes.
Product core remains at -6°C to -10°C for four to eight hours.
During the entire recovery period, every air sensor in the freezer reports compliance. Product is not compliant.
The Implication for Monitoring
An air sensor confirming -15°C tells you the compressor is running. It does not tell you the product is at -15°C. This gap is bridged by glycol-buffered probes — sensors designed to respond at food-like timescales rather than air timescales.
Measuring the air temperature in your oven doesn’t tell you if the roast is done. A meat thermometer does. Your freezer has the same problem, and the glycol probe is the meat thermometer.
The Implication for Receiving Inspection
When accepting frozen goods from suppliers, the air temperature of their vehicle or cold store tells you nothing about the product they’re handing you. An IR thermometer reading of product surface temperature provides screening information. But surface temperature lags core temperature — a product with -10°C surface may have -6°C core after warming, or -14°C core during re-cooling. As detailed in our receiving inspection article, surface readings reveal the minimum extent of a problem, not the full picture.
Related Terms: Glycol Buffered Probe, Thermal Mass, Temperature Excursion, Temperature Monitoring System
Related Articles: Your Freezer Says -15°C. Your Product Says -6°C. Both Are Correct. | The Receiving Inspection You’re Not Doing