I was demonstrating the wet-bulb temperature with a friend, and explaining the dry bulb and what happens when you are cooking chicken using a dry bulb temperature of 170°C. The dry air doesn’t do much expect help dry the food, and it isn’t until the final stages of drying that the surface of the food becomes dry. In the meantime as evaporation occurs the chicken stays cool (under 100°C), and it isn’t until the juices from the center no longer reach the surface (keeping it wet) you form a crust. When the final stages of drying begin known as the falling-rate period, the temperature of the product increases, causing water to move from the interior to the surface as it evaporates.

There is a time when the crust forms and beneath the crust the food is still moist and evaporation continues. But this all depends on how a chef controls his cooking process. The longer the time, the drier the food potentially. The issue is when you use dry heat, you risk a higher core temperature as the wet-bulb reaches 100°C.

In BBQ or in a dry oven you have the “stall” which happens during cooking. This is when the core temperature stops rising and may even fall slightly before it climbs again. This happens when the wet bulb temperature of the oven falls, and the core heats up drying out. Essentially the core is over heating and the food is drying and the ambient moisture in the oven decreases.

It takes time to understand how dry heat versus wet heat works. The core of the chicken stalls if basted frequently and you help prolong the drying keeping the surface wet.

The three stages that occur in core temperature are attributed to the settling period, the constant-rate period, and the falling-rate period. The settling period is when the temperature at the surface quickly rises from its starting point up to the wet-bulb temperature, stalls and remains there until the surface dries substantially.

After the initial settling period, the wet-bulb temperature slowly increases again until enough water evaporates from the food, the humidity in the oven increases, and frees the wet-bulb temperature to inch upward.

The final stages is the constant-rate period, when evaporation takes water particles from the food into the air of the oven’s chamber, and at the same time water is just as quickly replenished by liquids moving through narrow channels of the foods structure. In addition the water molecules are moving from a high are of concentration to an area of low concentration driving juices to the surface from within the moist interior. During the constant-rate period, the core gradually begins to dry, and its temperature rises to equal the wet-bulb temperature in the oven. By this time you are likely over cooking the food.


Categories: Kitchen Facts, Life Cycles