The Lock & Key Theory
Emil Fischer originated the lock and key theory in 1894. It states that it takes the correct key to open a lock. It takes a correct enzyme to bond to the substrate and catalyse its reaction. An active site of an enzyme is made up of a binding site and a catalytic site.
This theory had some bad points and was not correct so in 1958, Koshland said that the shape of an active site of an enzyme does not have to be the same type of the substrate. This was called the Induced Fit Theory.
The Collision Theory
In order for a reaction to take place, the reacting substances must collide and energy, called the activation energy, must be reached. If the collision between the particles can produce a lot of energy, then a reaction can take place. For the collision to take place, the particles must collide fast enough and in the right direction. The higher the number of collisions, the faster the rate of reaction. Increasing the temperature of the substrate can increase the number of collisions.
Kinetic Theory
The kinetic theory is a theory, which emphasises on pressure, volume and temperature. It states that gases contain particles that are always moving. Gases then exert pressure because their particles are colliding with the wall of the container. If the volume of the gas is decreased, then the particles would hit the wall more often and the pressure would increase.
When a lump of dough, which contains air and carbon dioxide, is heated in the oven, the volumes of the gases increase, resulting in the bread rising. This process can be shown by this word equation:
Increase in temperature Volume increases (Gas expands)
This is because as the temperature of the gas increases, the particles get more energy. This causes the particles to move faster and collide with each other and the walls of the container. If the gas cannot expand, the pressure would increase. If the pressure is kept constant while the temperature is increased, then the gas will expand.
Glucose Ethanol + Carbon Dioxide +Energy
C6H1206 CO2 + C2H2OH +Energy
In my preliminary experiment I measured the rise very inaccurately I simply marked the position on a boiling tube and measured the distance between the original mark and the new level of the dough.
Here are the results from this experiment:
(Line of best fit to be drawn on by hand)
From the graph I can see that I need to take a wider range of results and especially the results from below room temperature (22°c).
To take more accurate results I thought about using something with a thinner internal diameter to make the rise more evident also I thought about using a measuring cylinder as this has a graduated scale on it and would be fairly accurate to measure the rise in the dough. I will also use a range of temperatures ranging from five to eighty degrees centigrade going up in fifteen or twenty degree increments. I will achieve the constant temps in either a water bath or iced water with a thermometer in it; this range will give me more results to analyse. If I mixed a dough using 10 ml of yeast solution to every 5grams of flower and 1 gram of sugar I think I would get a noticeable rise. If I put the solution into a 25 ml glass measuring cylinder and note the level of the solution in the cylinder. Then place the cylinder into either a water bath or iced water for ten minutes. After ten minutes remove the cylinder and read off the new level on the cylinder. Now if you take the original level and subtract it from the new level you will get the amount the solution has rose in centimetres cubed. From my preliminary I discovered that the thinner the yeast solution was the more it rose.
Apparatus
Water bath
Thermometer
Ice
25cc measuring cylinder
Flour
Sugar
Yeast solution
Stirring rod
Balance
Method
Measure out ten cc of yeast and mix it in a twenty five cc measuring cylinder with 5 grams of flour and one gram of sugar. Note the level of the solution on the cylinder then place the cylinder in a beaker with ice and water add enough ice to the water to bring the temperature down to 10°c leave for ten minutes then remove from the water and read of the new level then subtract the original level from this and this result is the amount in cc that the solution has rose.
Results
These are the results which I gathered from my experiment.
(Graph drawn by hand)
And here are the class results:
As you can see from the results and the graph the amount that yeast rises is very heavily influenced by the temperature. As I have said in my introduction yeast is an enzyme and therefore requires energy( in the form of heat) however if too much heat is applied the enzyme will de-nature and cease to function correctly, resulting in the dough not rising as much. From my graph’s and tables of results from both my data and the class results I can see clearly that the optimum temperature for yeast to function in is forty degrees centigrade. I can see this as it has risen dramatically more at this temperature. From the graph’s you can also see that the reaction worked better at twenty degrees centigrade than at sixty degrees so by the time it reaches sixty degrees many of the yeast enzymes must start to denature. As I have explained in my introduction. The lock and key theory states that enzymes will denature past a certain temperature. When this happens the enzymes effected will cease to function.
Conclusion
In my prediction, I stated that as the temperature is increased, the dough would rise quicker. This was correct because if you take the volume of the mixture at 0ºc and at 100ºc, you would see that the volume of the mixture increased more at 100ºc than at 0ºc. I also predicted that at 40ºc, the reaction would be greatest. My results do not prove this but my graph shows that if more time was give, the volume of the mixture at 80ºc would stop, due to the enzymes denaturing and the yeast being killed. The volume of the mixture at 40ºc would overtake the volume of the mixture at 80ºc and therefore be the optimum temperature. My other prediction was that by doubling the concentration of yeast would double the reaction. I could not prove this because the concentration of the yeast was constant.
Evaluation
I believe that the experiment went fairly well. I measured the volume of the mixture after every one minute for twenty minutes. I then repeated the experiment until I had three results for every minute at each temperature. By increasing the temperature by 20ºc resulted in me having a good range of results.
I think that the result of the volume of the mixture at 80ºc was inaccurate because when I did the experiment at that specific temperature, I left the mixture for a few minutes before starting to measure the volume of the mixture. This let the temperature of the mixture cool down which had a vast effect on my results. The other reason could be that the concentration of the yeast added was different to those at the other temperatures. I think that if I had done the experiment for another twenty minutes, I would have seen that the optimum temperature for this experiment would have been 40ºc.
To make this experiment even more accurate, I think that the volume of the mixture should be measured for forty minutes. Due to a lack of time, I could not do this but in the future, I would be glad to do this experiment again and measure the volume of the mixture for forty minutes. Also I could have measured the volume of the mixture to three or maybe four decimal places. To do this, you need the correct apparatus, which I did not have. In the future, I would like to measure the pH of the mixture because enzymes work best at a pH of six. Alternatively, I could vary the concentration of the sugar, flour or yeast to see what effect this has on the rate of reaction. Including vast amounts of oxygen, would have increased the pressure on the particles and move them closer together. This would lead to more collisions and an increase in the rate of reaction.
