Bread browning reaction

Effect of the Maillard Browning Reaction on the Nutritive Value of Breads and Pizza Crusts... 

In another study by Tsen et al (14), a substantial PER difference was observed between diets with bread crust (0.36) and bread crumb (1.35). Palamidis and Markakis (12) also found that the PERs of their bread and its crumb were 0.46 and 0.91, respectively, while the crust showed a negative PER, -0.23. Hansen, et al.(18) reported that bread crumb and crust had respective PERs of"T.36 and 0.62. The difference in PERs for bread crumb or crust reported by the three groups of investigators is largely due to different raw materials and processing conditions for the breads. Nevertheless, the marked difference in PER between bread crumb and crust indicates clearly that the browning reaction can reduce the nutritive value of bread (Table IV). 

E. Guerra-Hernandez, A. Ramirez-Jimenez, and B. Garcia-Villanova, Glucosylisomaltol, a new indicator of browning reaction in baby cereals and bread, J. Agric. Food Chem.,... 

These nonenzymatic reactions are responsible for numerous changes on food properties and may impair food safety. Although these reactions are of great importance in the production of aroma, taste and color, they are often accompanied by a reduction of the nutritive value of different foods and by the formation of toxic compounds harmful for human health (Ledl and Schleicher, 1990). Results of nonenzymatic browning can be either desirable or undesirable. The brown crust formation on bread is desirable the brown discoloration of evaporated and sterilized milk is undesirable. For products in which the browning reaction is favorable, the resulting color and flavor characteristics are generally experienced as pleasant. In other products, color and flavor may become quite unpleasant. 

Starch degrades to dextrins, mono- and disaccharides at the relatively high temperatures to which the outer part of the dough is exposed. Caramelization and nonenzymatic browning reactions also occur, providing the sweetness and color of the crust. The thickness of the crust is dependent on temperature and baking time (Table 15.51) and type of baked product (Table 15.54). The composition of some types of bread is presented in Table 15.55. 

This is bread that has been baked to the point where the crumb has set and oven spring has occurred but before the Maillard reaction causes the crust to brown. The bread is then packed. 

Describe the reaction engineering of a kitchen toaster. The reactions are thermal drying (which alone makes stale bread) and thermal decomposition and oxidation of starch, which requires temperatures of -300°C. [Recall that the bread appears white until it is nearly done, and then it browns quickly and blackens if left in too long. Recall also that good toast has a brown layer < 1 mm thick while the interior is still white and soft. The control of temperature profiles and heat transfer aspects are essential in producing good toast.]... 

Browning of breads, as induced by baking, is a complex process consisting mainly of the Maillard reaction between flour starch or other carbohydrates and flour protein or other protein-rich additives, and the caramelization of sugar and other carbohydrates. 

The experiments have been completed by additional reaction of xylose, fructose and some characteristic sugar degradation products like cyclotene, Furaneol and diacetyl and by thermal decomposition of Ama-dori rearrangement products. It is well knwon that sugars can react with suitable amino compounds very easily. In the course of these reactions sugars are mostly decomposed and brown melanoidins are formed. By-products of these melanoidins are many volatile compounds of characteristic aroma properties. They are also responsible for the well known aromas of heated food like meat, coffee and bread. 

During baking there is an evaporation of water from the loaf, this is particularly marked near the surface of the loaf, and this evaporation plus the occurrence of the Maillard reaction cause a characteristic dark brown crust to be formed on the exterior of the loaf. The Maillard reaction is a complex set of chemical reactions in which the amino acids in proteins react with reducing sugars such as glucose and fructose, and which are very important to our perception of flavour in baked bread. 

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