Also Known As Dew Point, Moist Air, Hygrometer, Psychrometric Chart
What is Relative Humidity?
Relative humidity (RH) is a measure of the moisture in the air. The higher the RH%, the more moisture is available in the surrounding air. It is the ratio of the partial pressure of the water vapor in a mixture to the saturation pressure at the same temperature.
This aspect of psychrometry (the science of measuring water vapor in the air), is particularly important to baking. It determines the amount of moisture in the air, which affects processing conditions of dough and batter products.
How Relative Humidity Works
Relative humidity is the ratio of actual saturation to vapor saturation. The equilibrium of the ratio is reached when there is a flat surface of water, with an equal number of evaporating water molecules to condensing water molecules.
- The amount of vapor, or humidity, in the air affects baking times and overall quality of the product.
- Humidity slows the baking process of bread, slowing moisture evaporation and gluten coagulation in the crust.
- It can make products tough and overdone as well.
However, some air vapor such as steam, will keep the surface moisture from evaporating too soon, making the dough flexible longer and affecting the crust color.1 Bread moisture content also varies greatly from the outside to inside, with it ranging from around 11 percent in the outside layers to over 60 percent in the center.2
The amount of moisture in bread is affected while it is in the proofing stage, with the level of outside heat and moisture controlled. The lower the humidity at that point, the crustier the surface layer will become while baking.3 For every degree Celsius dough temperatures at proofer exit vary from normal, the oven must make up three degrees to attain consistent product.
Relative Humidity should be monitored during every step of the baking process, from storing to proofing to baking. Measurements can be made using the wet bulb/ dry bulb method or an electronic RH logger with digital displays.
Wet Bulb/Dry Bulb Measurements: these can be performed with a data recorder or the same thermal profiler one would use to profile ovens.5
- Wet bulb is one thermocouple with a wet fabric ‘sock’ placed over the junction tip.
- Dry Bulb is a 2nd thermocouple as-is, rendering ambient temperature.
- The logged values are referenced on a Psychrometric Chart to render relative humidity trending.
The RH% can be read from the following psychrometric charts for bulb readings:
Electronic RH Sensor with data logging capability5
- The availability of inexpensive instruments which read both relative humidity and temperature can render a real-time display of relative humidity for the purpose of monitoring proofers, dough and mixing rooms.
- RH sensors include the ability to log data over time, which means that a full analysis of the conditions during fermentation are achieved in a graph to ensure a proofer’s calibration, and its capability of delivering consistent dough properties to the oven in all seasons of the year.
Relative humidity is important for adjusting HVAC systems in the production, proofer and oven areas. Most dough fermentation rooms require over 75% RH while proofers need a RH over 80% to prevent a skin from forming during the final proofing stage.4
Most natural convection ovens bake with a high humidity of 90-95 percent with high temperatures. Forced convection ovens have lower humidity levels, between 30-60 percent, with lower temperatures. The desired humidity and temperature levels will depend on the product being baked.
Please note that at oven temperatures, or any ambient conditions greater than the boiling point, Wet/Dry bulb water vapor (moisture) content measurements must use the Absolute Humidity algorithm.5 Relative humidity is an inappropriate unit of measure above 1000C/2120F.5
- Purlis, Emmanuel. “Optimal Design of Bread Baking: Numerical Investigation on Combined Convective and Infrared Heating.” Journal of Food Engineering 137 (2014): 39-50. Web.
- Woodhead, Ian, John Christie, Kenji Irie, and Richard Fenton. “A Water Content Sensor for Baked Products.” 2014 IEEE Sensors Applications Symposium (SAS) (2014): n. pag. Web.
- Hirte, Anita, Rob J. Hamer, Marcel B.j. Meinders, Kevin Van De Hoek, and Cristina Primo-Martín. “Control of Crust Permeability and Crispness Retention in Crispy Breads.” Food Research International 46.1 (2012): 92-98. Web.
- Yi, Byong H. Fundamentals of Food Biotechnology. Oxford: Wiley Blackwell, 2015. Print.
- Pearce, Ray. “RE: Relative Humidity.” Received by Katie Jones. 10 Feb 2017.