Physical and chemical changes during baking of bread


Most important step in bread making is baking. The purpose of baking is to transform unpalatable dough into a light porous and readily digestible flavoured product. The duration of baking, humidity and temperature of the oven influences the quality of product. The baking time depends on baking temperature, size and weight of product being baked. There are two types of changes that take place during baking-physical and chemical



Physical changes 

  • Film formation: when the dough enters the oven, the observable effect produced by the heat formation of thin elastic film. The elasticity is a direct function of the moisture content of the oven atmosphere.
  • Oven rise (gas expansion): the next important reactions the sudden expansion of the dough, volume by 1/3 of its original size called ‘oven spring’. As the temperature rises, the solubility of entrapped gasses decreases, and the pressure of gas increases. The next visible effect is the expansion of the dough. The release of dissolved gases takes place when the internal is around 50ºC.
  • Alcohol evaporation: The third physical effect is to change the liquids with low boiling points into vapour. Low boiling point alcohol formed by the yeast action distils when the temperature is about 80ºC. the alcohol vapours further increase the internal pressure which helps in further over rise.

       Thus, there is steady increase in the pressure during early baking period. Then occurs, a marked drop in pressure at the temperature when starch swelling takes place which coincides with the oven spring. Some of pressure drop is due to the coalescence of minute gas cells into fewer larger cells. Weak flours tend to coalesce to a greater extend with the result bread will have an uneven gain and larger air cells. Strong flours or flour treated with oxidizing agents withstand the rising pressure.   



Chemical changes



  • Yeast activity: Activity of yeast depends on the temperature. Hence, until the thermal death point is reached at 60ºC, the yeast in the dough generates more CO2 and alcohol which contributes further to expansion of the dough. Yeast activity is aided at the elevated temperature by the increased amylolytic activity.
  • Starch gelatinization: Oven spring is further favoured by pronounced softening of the gluten which sets in at the initial baking stage, this process is, however, counteracted by the starch gelatinization which begins at about 55ºC. The degree of swelling is restricted by limiting availability of water.
  • Gluten coagulation: Starch gelatinization is associated with absorption of water while gluten denaturation is associated with the removal of water. Coagulation sets in when the temperature is around 74ºC, and continues till the end of baking. In this process, gluten matrix surrounding the individual cells is transformed into a semi-rigid structure. Thus a major change that takes place during the oven process in the redistribution of water from the gluten phase to the starch phase.
  • Enzyme activity: the action of amylase on starch increases with temperature approximately doubling with every 10ºC rise. At the same time, heat inactivation of the enzymes also commences. β-amylase denature at lower temperature (57 to 71 ºC) as compared alpha-amylase which denatures at temperatures ranging from 65 to 95ºC.
  • Sugar caramelization: When sugar is heated to around 170 ºC the molecules polymerizes to form coloured substances called caramels. This reaction known as caramelization, can only take place in the crust because of the internal loaf temperature never exceeds 100 ºC. The caramelized impart colour and flavour to the baked products.
  • Browning reaction: the browning reaction starts at around 160 ºC. it is the result of heating that reducing sugars reacts with protein and other nitrogen containing substance to form coloured compounds, known as melanoidins. This reaction also imparts colour and flavour to the bread. Steaming during baking has 3 important reasons. 
    • Produces glossy crust
    • Prevents splits in crust
    • Agitation of oven atmosphere

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Comment by Er. Usha Kiran on December 4, 2011 at 1:13pm

how to increase the self life of bread more than 6 days.....

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