Why is oven temperature important?
Oven temperature causes physical transitions and chemical reactions to take place in the dough/batter. The following stages are temperature-dependent, and participate in the sequential transformation of bread dough.1
1. Development (also known as oven spring)
As temperature increases, the free water/alcohol mixture in the product vaporizes, fermentation gases (CO2) dissolved in the liquid dough phase become less soluble, and are released into the cells, causing them to expand in response to the rise in pressure.
As a result, cells increase in volume by retaining gases due to their deformable nature as these are surrounded by the continuous gluten/starch soft matrix. This results in a large reduction in the density of the dough as the product gradually develops an aerated structure. Note that the size to which the gas bubbles can grow is limited by the ability of the gluten/starch film surrounding them to stretch without rupturing.
In this stage, the product undergoes a series of irreversible chemical and physical transformations. Oven spring is accompanied by the following changes and conditions:
- Killing of yeast cells at 50–60°C (122–140°F)
- Maximum enzymatic activity at 60°C (140°F). The enzyme-driven reactions that convert starch into sugars and break proteins into amino acids increase with heat, so they increase most near the dough surface.
- Starch gelatinization. It starts at 55–65°C (130–150°F) as granules become fully swollen with local free water.
- Denaturation of gluten proteins at 50°C (122°F) and coagulation at 70–80°C (160–180°F). As a consequence, gluten becomes increasingly tough and stiff as it irreversibly forms a gel.
- Above 85°C (185°F), starch looks glassy, and gluten looks rubbery. This is the start of the dough-crumb transition process (setting).
- Inactivation of naturally-occurring and added enzymes inside the dough (70–85°C) (160–185°F).
2. Drying (reduction of dough/batter moisture)
- Under the action of the heat transfer mechanisms, high temperatures develop inside the baking chamber (200–300°C) (390–570°F), and water molecules at the dough surface absorb latent heat and start to evaporate.
- Due to the low humidity of the air inside the baking chamber, a water vapor pressure (air moisture concentration) gradient is created. Liquid-state water starts to diffuse, and migrates from the product core to the surface, where it evaporates and is lost to the oven atmosphere.
- The loss of moisture from the dough piece is dependent on the baking chamber temperature, colligative properties of the free water in the product, heat transfer methods used, and the humidity of the oven.2
3. Color formation
The external surface of the product is exposed directly to the high temperatures of the oven, and readily absorbs the heat from the energy sources. These high temperatures trigger non-enzymatic reactions that give rise to the desirable brown crust:
- Maillard browning takes place above approximately 105°C (220°F) and requires the presence of a reducing sugar (glucose, maltose, or lactose) together with an amino acid, the type of which determines color and flavor.
- Sugars caramelize at 160°C (320°F). This reaction will happen only in the presence of water.3
Oven temperature vs. internal product temperature
- In most baking processes, the dough pieces are placed into the oven at an initial temperature of 20–30°C (70–86°F). The oven temperature is usually set constant at 150–300°C (300–570°F), and baking usually takes 5–25 minutes.
- Because baking takes place at atmospheric pressure, and moisture escapes freely from the product without leaving it completely dried, the internal temperature of the food does not exceed 100°C (212°F) (boiling point of water).
- The core temperature of the product reaches 90–97°C (194-207°F). The thicker the bread, the longer it takes for conduction heat transfer to reach the center of the product and increase its temperature.
- Through baking temperature/time profiling, it is possible to quantify the difference between the product core temperature and the temperature in the oven. In some cases this difference can be high.
- The bigger the difference between external and internal temperature, the larger the temperature gradient will be for heat transfer to bake the product.
- A data logger may be used to check the temperature profile of the oven and identify variations from set temperatures and other problems.2
- Variations in oven temperature need to be addressed properly, as these can be caused by unbalanced heat transfer generated by the incorrect location of energy sources across the width of conveying bands or decks. This imbalance can cause unevenness in coloring and final moisture content distribution in products.
How is oven temperature controlled?
Specific temperatures are set inside the baking chamber to achieve the required baking profile of a given product. Controlling the heat input from the energy sources (e.g., burners, electric resistances) is then vital for maintaining the set baking temperatures. Control of oven temperature can be achieved by two means:
- A temperature sensor (thermocouple probe) senses, measures, and transmits the temperature (controlled variable) of the air inside the baking chamber.
- As the demand for hot air increases or decreases (e.g., in moments when the load of the oven increases, oven temperature goes down; fuel combustion must then increase to return oven temperature to its set point).
- A change in oven temperature is sensed and converted to an electrical signal, amplified, and sent to a controller that evaluates the signal and sends a correction signal to an actuator.4
- The actuator (gas valve) opens or closes to adjust the flow rate of the air and fuel (carbureted mixture) in the burner (manipulated variables) to keep flame intensity such that it can consistently deliver the power required. In this way, the temperature of the baking chamber is returned to its predetermined value.4
Block diagram of the oven temperature control system.
- Dough pieces are loaded into the oven, where heat from the energy sources is used to bring the products to the required temperature in order for them to cook and dry.
- A thermometer is used to measure the temperature of the product (the measured variable). The temperature is observed by an operator who adjusts the flow of air and gas in the burner (the manipulated variables) to keep the baking chamber at the constant set temperature.4
- Manley, D. “Biscuit Baking.” Manley’s Technology of Biscuits, Crackers and Cookies, 4th ed., Woodhead Publishing Limited, 2011, pp. 477–500.
- Davidson, I. “Baking Process.” Biscuit Baking Technology: Processing and Engineering Manual, 2nd ed., Elsevier Inc. , 2016, pp. 35–48.
- Tucker, G. “Process Optimization and Control.” Bakery Products Science and Technology, 2nd ed., John Wiley & Sons, Ltd, 2014, p. 386.
- Dunn, W.C. “Introduction and Review.” Fundamentals of Industrial Instrumentation and Process Control, The McGraw-Hill Companies, Inc., 2005, pp. 1–5.