Heat flux and humidity affect oven performance
August 1, 2016
by Stephen St. Clair-Thompson
The humidity on new ovens is usually controlled by variable-speed extraction fans.
A variety of physical and chemical changes that take place during cookie and bread baking, and these are time-dependent as well as temperature-dependent.
One of the most important chemical changes during baking is the Maillard reaction, which gives the baked product its flavor and much of its color. This reaction combines certain amino acids from the proteins with sugars from the carbohydrates (Moreth 1987a, 1987b). In the context of baking, the Maillard reaction produces its attractive reddish-brown hues at about 150 to 160°C (300 to 320°F) (Manley 2000). Taken to excess, however, the Maillard reaction can lead to the development of acrylamides, reported to be carcinogenic (Mottram et al. 2002). Prudence dictates that the heat flux to the product surface should be limited, allowing the Maillard reaction to take place slowly and avoid over-browning.
In addition to the roles of heat flux and temperature, the other important parameter during baking is bake chamber humidity. The presence of water vapor, or humidity, in the oven’s atmosphere comes mainly from the liberation of water from products as they bake. The humidity in existing ovens is usually controlled by fixed-speed extraction fans and extraction dampers; the humidity on new ovens is usually controlled by variable-speed extraction fans.
When considering humidity in the context of baking ovens, it is best to think in terms of the “dew point” — the temperature at which water in the oven atmosphere would condense. If the oven is less humid, the dew point is lower; if it is more humid, the dew point is higher. A dew point temperature of 100°C (212°F) would correspond to an oven atmosphere that is all water vapor. The Internet offers numerous humidity calculators.
When dough pieces enter the oven, they are soft, elastic and moist. Usually, dough pieces must retain their elasticity so they can develop into the required shape. The best way to keep them elastic is to keep them moist. Maintaining high humidity in the oven is the best way to keep dough surfaces moist, thus a high humidity is necessary until the final shape of the product is set. In other words, a high humidity is important in the early part of the baking process.
A lack of humidity in the early part of a cookie or cracker oven will cause the product to “case harden,” which stops the ability of a cracker to rise, thus limiting its stack height, an important parameter that affects the weight of product in a package of a fixed length. For cookies, case-hardening inhibits the proper escape of internal moisture, leading to differential moisture levels within the cookie. This difference, in turn, causes “checking,” the spontaneous cracking of the cookie (Johnson and Walker 2003).
An extreme case could be in an oven baking ginger snaps. This style and similar cookies have high sugar content. Keeping the humidity high in the early part of the oven promotes extensive cracking in the surface, which is a desirable feature for these items.
Lack of humidity in a cracker oven causes development of excessive blisters, which burn easily, causing undesirable dark spots. The blisters are also easily broken (Johnson and Walker 2003).
Once the surface temperature of the dough piece exceeds the dew point temperature relevant to the oven humidity, it will start to dry out. After that point, the humidity in the bake chamber is of little importance to the baking. This point is currently not understood by bakers who are keen to reduce baking times. During the second half of the baking process, bakers will often open the extraction dampers excessively in the mistaken belief that having a drier oven will improve the drying process and reduce the bake time. It does not: The only effect is to increase the baker’s fuel bill.
Humidity in a bread oven is a special case. Here, the development of the product has already taken place in the proofer. Well-established crusts are important to these products, unlike cookies or crackers. In particular, if the baker is seeking a glossy crust, then bake chamber humidity early in the bake will be vital. The glossy crust is caused by gelatinization of the starch on the dough surface, promoted by a reaction between the water and starch. The higher the dew point temperature, the shorter the time the high dew point must be maintained. For example, optimum gloss formation can be achieved with a dew point temperature of 93°C (200°F) for 40 seconds, or 82°C (180°F) for 3 minutes (Dersch 1989).
Fortunately for those who need to keep track of the humidity within ovens, humidity loggers (similar in concept to the heat flux loggers described above) are now available and can be used to good effect.
Dersch, J. A. 1989. The use of steam in bread ovens. Am. Soc. Bakery Engrs. Tech. Bull. 218.
Johnson, A. M. and Walker, C. E. 2003. Humidity inside ovens. AIB Tech. Bull. 25 (6).
Manley, D.J.R. 2000. Technology of Biscuits, Crackers and Cookies. Woodhead Publishing Ltd.: Cambridge, UK.
Moreth, N.W. 1987a. Cookie and cracker ovens. I and II. AIB Tech. Bull. 9 (6).
Moreth, N.W. 1987b. Cookie and cracker ovens. I and II. AIB Tech. Bull. 9 (7).
Mottram, D.S., Wedzicha, B. L. and Dodson, T.A. 2002. Food chemistry: Acrylamide is formed in the Maillard reaction. Nature 419: 448.
Mowbray, W.R. 1981. Technology of the ‘hot box.’ Food Manufacture (Oct.)
More on this topic can be found in “Baking Science & Technology, 4th ed., Vol. II,” Page 495, by E.J. Pyler and L.A. Gorton. Details are in our store.