Result:
The dough has been expanded again. A golden brown crust is formed. The texture of the crust is hard and crispy and the interior is spongy and softer compared with crust.
Discussion:
The crust has turned brown because a browning reaction occurs. The reaction called maillard reaction. It is results from amino acid in protein, reducing sugar and heat. In our dough, we have sucrose as a reducing sugar, hard wheat flour to provide amino acid and also heat is provided during baking, so maillard reaction occurs. The sugar interacts with the amino acid in protein and undergoes some chemical reaction and brown nitrogenous polymers are formed.
There is another browning reaction called carmelization. It is different from maillard reaction. Carmelization is only the pyrolysis of sugar, only sugar and heat is involved in this reaction. So the browning reaction in bread is maillard reaction.
The texture of crust is crispy and the interior is spongy. There are difference in texture between crust and interior because the water at the surface is firstly evaporated at the high temperature. The loss of water in surface causes a dry crust. The interior is spongy because the water content at the centre can’t easily leave the bread and the water content change from water to steam. So there are air bubbles in the interior of bread, making it spongy.
The dough had expanded again during baking. It not only causes by expanded of carbon dioxide in dough. More important, it is due to the baking temperature is 200°C exceed the boiling point of water i.e. 100°C, so the water content in dough had changed from liquids to gaseous state(form steam). Gas occupy a large area than liquid causing the dough to rise again.
Expt 2 Browning reaction
Aim:
To investigate the ingredients that is necessary for a browning reaction.
Principle:
By mixing solution according to following table.
And place the test tube in a 120 C hot oven for at least 1 hour. Observe whether browning of solution occurs. The component need in the browning reaction can be determine. As browning will not occur if anyone of these is missing.
Result:
Test tube 1-3 remain colourless. Tube 4 changes from colorless to pale orange. Show both Glucose and Glycine are needed
Discussion:
Brown colour can be observed in tube 4. The browning reaction is a non-enzymatic browning, as no enzyme has been added for browning to occur. The reaction taking place is a maillard reaction because glucose, heat and amino acid (provided by glycine) are all needed for browning. Glucose alone i.e. tube 2 or glycine alone i.e. tube 3 can’t causes browning. Besides, carmelization has not take place as browning has not occur in tube 2.
Expt 3 flour and dough volume
Aim:
To compare the expansion of hard and soft flour and the effect of a chemical leavening agent.
Principle:
By prepare four sets of sample in a 50 ml beaker according to the following table
And then put into a 120 C oven to let dough rise. By measure and compared the original height and the final height of each sample, the effect of hard and soft flour and the effect of a chemical leavening agent can be investigate.
Result:
Hard flour sample has a elastic and harder texture while soft flour sample has a spongy and crumply texture.
In this part of experiment, it is difficult to measure the height of dough as it is in dome shaped after baking, also it is not easy to press down the content in break to form a total even surface. Thus, there an error in measuring the height and make the calculated percentage increase inaccurate.
Discussion:
In the same baking powder content, sample A rise more than sample B. showing that hard flour has a greater effect on expansion. With same flour used, the increase in content in baking powder, the sample will rise greater.
Hard flour has a higher gluten content than soft flour.The dough using hard flour can rise bigger because the gluten in the flour forms an elastic network which traps the carbon dioxide gas given off by the yeast more effectively. The more gluten, the greater the bread dough's tendency to rise.
The baking powder has a function of releasing carbon dioxide into dough. With more baking powder used more carbon dioxide is release . As the carbon dioxide will expand at high temperature, the dough will rise.
The chemical leavening agent used to increase the volume and lighten the texture. The agent works by releasing carbon dioxide into dough through and acid base reaction. Take a common baking powder(baking soda) as an example, its reaction is :
Chemical leavening agent is commonly used instead of yeast to generate carbon dioxide. It is because carbon dioxide is released in a faster rate through acid base reaction than fermentation.