You want the temperature of whatever you are refrigerating to remain constant. While it is possible to keep a product at a remarkably stable temperature, it is likely that the air temperature around it is not nearly so static.

Air vs. product temps
Look at this temperature graph over the course of a single day of an actual walk-in cooler containing perishable foods and controlled by a Freeaire® Cooler Controllertm. Two temperatures are precisely measured by digital temperature sensors connected to the controller. The blue line is the temperature (Fahrenheit) of the air inside the walk-in and the red line is the temperature of the product (in this case with a sensor put inside a bottle of water on a shelf). Every minute throughout the day, the controller records the temperatures measured by the sensors, each resulting line having its own story to tell.

You can see that every 20 minutes or so,  the air temperature fluctuates through a fairly predictable, repeating pattern. All day long, as the Cooler Controller’s thermostat switches the condensing unit on and off, the air temperature simply swings between about 35°F to 39°F.  The air in the room gets colder as it passes through the evaporator coils, which are cooled by the operation of the condensing unit’s compressor and condensing fans. There is a resulting rapid drop in the air temperature during the cooling cycle, ending at 36°, which is what is called the Set Point, when the compressor is switched off. The air continues to get colder as it passes through the evaporator coils as long as the fans keep operating.  When the fans do finally stop spinning, in this case three minutes after the compressor has stopped running, sometimes the air temperature bottoms out at around 35.5° and sometimes it falls all the way to 35°. Every cooling cycle is followed by a warming cycle as the air gradually warms up, and which ends at 39°, when the compressor is switched on again and the pattern is repeated.

There is also one large spike in the graph of the air temperature during the single defrost cycle of the day (around 1 o’clock in the afternoon), in which the evaporator coils are allowed to warm to just slightly above the air temperature to ensure that all frost on the coil is melted.

Over the course of the day, the air temperature varies almost 8°, from a minimum of 35.0° to a maximum of 42.9° during the defrost cycle.

Meanwhile, the red line shows that the product temperature is much more stable than the air temperature, as it is never varies more than 1/2 of one degree, from 37.3° to 37.8°, even during the defrost cycle. The reason why the product temperature is steadier is that air is not a particularly good medium for rapidly transferring heat or cold.  Unlike a denser fluid like water, a gas like air has less mass per unit of volume and that mass has less contact with things that it surrounds. Consequently, the cold from the air is only slowly transferred to the product. On the positive side, the cold in the product is also somewhat slow to be transferred to the air once it is removed from the walk-in cooler and is let to stand in the warm air of the store.  A hand wrapped around a bottle of cold soda will warm the soda more quickly, but by that time, the soda is probably rapidly being consumed and warmed in the drinker’s stomach.

As long as the product remains on the shelf inside the walk-in and was surrounded by air only a little bit colder than itself, its temperature varied very little. That is as it should be, since the whole reason for refrigeration in the first place is to keep the product as close as possible to a particular desired temperature. The product never came close to freezing (32°), which can damage both the product and its container, or to 40°, which is usually considered the maximum safe temperature for the storage of perishable food.  The product was always safe and ready for sale.