Blue, lilac pink, colourless. These colours indicate the bacterial respiration. The faster the colour changes the more rapid the rate of bacterial respiration.
The faster the colour changes the more rapid the rate of bacterial respiration.
Record the pH using pH meter and record the pH using a different set of the same range of milk samples at the same stages as the samples above.
Use different samples of the refrigerated and the room temperature milk each day. Once the samples have been taken from the milk return the cartons of milk to the refrigerator and to the shelf (room temperature milk)
Results
Day 1
Day 2
Day 3
Day 4
Day 5- Saturday no results were obtained
Day 6- Sunday no results were obtained
Day 7
Discussion
The rate of growth of bacteria in each milk sample is different. Under certain conditions the bacteria reproduce and grow more quickly.
The rate of bacterial growth in the samples of milk is measured by the rate of microbial respiration, which is indicated by a fall in pH. A fall in pH is due the accumulation of lactic acid. The accumulation of lactic acid occurs when the density of bacteria increases through cell multiplication. As the density of bacteria increases the demand for oxygen increases, once all the oxygen is used up the conditions turn anaerobic, thus resulting in the production of lactic acid.
The rate of bacterial growth in pasteurised samples of milk is faster then that of UHT milk at room temperature.
At room temperature the bacteria in the different samples of pasteurised milk reproduce at different rates. This is shown by the colour changes of the rezasurin dye. Out of the different samples of pasteurised milk the full fat sample was fastest sample to change colour. At the start of day 2 the sample had immediately turned light blue after the required 30 mins the sample turned lilac. At the start of day 3 the milk was pink it then went white, it remained this way until day 7 where for the first 10 minutes it was orange. The rate of the changes in colour indicates that the full fat sample had the highest rate of bacterial respiration and therefore bacterial multiplication. The pH of the full fat milk fell from 6.0 on day 3 to 4.9 on day 4 to 4.4 on day 7, this fall in pH indicates a more acidic environment and therefore an accumulation of lactic acid.
Semi skimmed milk was the next fastest sample of pasteurised milk, at room temperature, to change colour. The rate of bacterial growth in the sample was slower then that of the full fat sample as the rezasurin dye did not change colour as quickly. This means that the density of the bacteria has increased within the sample i.e. the overall rate of oxygen uptake in the sample of milk has increased. The pH of the sample also fell the most dramatic fall occurring between day 4 and day 7, where the pH fell from 5.8 to 4.4. This fall was due to an increase the concentration of lactic acid that is produced in anaerobic respiration and results in the souring of milk.
Skimmed milk at room temperature was the slowest of the three pasteurised samples to change colour. This means that the rate of bacterial respiration and multiplication in this sample of pasteurised milk was the slowest. However the most significant fall in pH occurred between day 2 and day 3 where the pH fell from 7.0 to 5.8. This means that the rate of bacterial multiplication and hence respiration between these two day was the fastest for this sample.
The results obtained for these samples of milk can be justified by the relative amounts of bacteria left in each sample after they undergo pasteurisation. The full fat sample of milk has the most bacteria left in it as pasteurisation occurs over the shortest amount time of the three samples of milk. Therefore the rate of bacterial multiplication will be more rapid then the other two samples because initially there was more bacteria in the sample. Once the density of bacteria increases the demand by bacteria for oxygen will increase and this will increase the production of lactic acid, as the conditions will turn more anaerobic. The colour change of the rezasurin dye will also change the quickest. The skimmed milk sample is the most pasteurised sample, therefore it will have the least amount of bacteria in it initially. The rate of reproduction will be the same as the full fat sample however the density will be lower. This means that the demand for oxygen by the bacteria will not be as great as in the full fat sample ant equivalent times. Therefore the rate the colour of the dye will change will be slower.