Average Differences
Analysing Results
I have little confidence in the measurements based on weight, because the masses before decay should have incremented by 10 each time, as the water was increased each time by 10ml. Also, I expected the mass after decay should have all been smaller than the mass before decay, as the mould has respired on the bread to give out Carbon Dioxide and Water, making the overall mass lighter. So therefore, with small amounts of water, such as 0 or 10ml, decay was slow, however with medium amounts of water, the decay grew quite fast, and with large amounts of water, such as 40ml, the decay went down again because there was too much water to too little oxygen, so the bacteria could not reproduce as fast, as the conditions were too anaerobic when the bread was waterlogged. If there is no oxygen, the organisms causing the decay cannot grow. However we didn’t have the equipment in the lab to test this.
The end results however were correct, but I think the main reason that they had lost so much mass was evaporation. The Petri dish was designed so that water vapour could escape, gasses could exchange, but no bacteria could get in.
Conclusion
The amount of mould went up as the water was increased to 20ml, before dropping with 30 and 40ml of water.
Evaluation
The two sets of results obtained were quite different, for example on the first experiment, the reading for after the decay was 31.6g(with 30ml of water), and while on the second experiment it was 84.5g. Such different readings could have been obtained for a number of reasons.
Firstly, there was leakage in some of the Petri dishes, because they were not taped down strongly enough. Another problem could have been that not all the pieces of bread were the same shape, so this may have affected the rate of decay. There was some water spilt on some of the balances, causing them to weigh too heavy. Also, the temperature of each Petri dish could have effected the results, as one was nearer a window and one was nearer a heater.
There were problems with the method as well. There was a time constraint; the bacteria and fungi were only left to grow for four days, which was not enough for all the decay to take place that could have. Another problem with the method was that there was only one size of Petri dish, so no more experiments could be done using both more water and more oxygen. The masses before decay were inaccurate because the measurements of the volumes of water were not accurate enough, and the two sets of balances weighed differently, and both were used. Also, if there was a draft, the rate of evaporation would have been speeded up, which could have explained decrease in mass, such as from 60.2g to 43.0g, in the first experiment, on the 40ml reading.
Although the overall trend was the same in both experiments, that the decay initially larger as more water was added, then decay decreases again as water is increased further, the results were significantly different. This could be because of factors out of our control, such as the amount of oxygen present; it was impossible to measure, as we did not have the equipment to do this. Finally, it had to be assumed that the density of every piece of bread used was the same. This was probably not true, as some pieces of bread were taken from different slices, but within the same packet.
For further work, I would investigate using a different method to asses the decay, such as find out the surface area of the bread, and finding out the amount infected, to give a percentage cover. Using transparent mm2 graph paper could do this.
