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An investigation to determine the rate of bacterial growth in milk under different conditions.

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Introduction

An investigation to determine the rate of bacterial growth in milk under different conditions. Aim Fresh milk contains many microorganisms including the bacteria lactobacillus. Pasteurisation is the process where the milk is heated to a 72�c for 15 seconds, which kills most but not all of the bacteria. Under suitable conditions these bacteria reproduce rapidly. In anaerobic conditions i.e. when there is a lack of oxygen in the milk, the bacteria respire to produce lactic acid. The aim of the investigation is to determine the rate at which bacteria reproduce and therefore the rate at which they respire. This will enable us to determine how different sets of milk, react under different conditions. Although the rate of each bacterium does not change, as the density of bacteria increases through cell multiplication, the overall rate of oxygen uptake will increase. It is the rate of uptake that gives the colour changes in the rezararurin dye. The colour changes from blue to lilac to mauve to pink and finally it goes colourless. 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. ...read more.

Middle

Skimmed Semi-skimmed Full fat refrigerated Ultra heat treated 5 Lilac Lilac Pink Light blue 10 Lilac Lilac Light pink Light blue 15 Pink Light pink Cream Light blue 20 Pink Light pink Cream Light blue 25 Pink Light pink Cream Light blue 30 Pink Pink Cream Light blue Refrigerated Colour Time (Min) Skimmed Semi-skimmed Full fat refrigerated Ultra heat treated 5 Blue Light blue Light blue Light blue 10 Blue Light blue Light blue Light blue 15 Blue Light blue Light blue Light blue 20 Blue Light blue Light blue Light blue 25 Blue Light blue Light blue Light blue 30 Blue Light blue Light blue Light blue Room temperature Milk sample pH Skimmed 5.8 Semi skimmed 6.6 Full fat 6.0 UHT 6.4 Refrigerated Milk sample pH Skimmed 6.4 Semi skimmed 6.4 Full fat 6.4 UHT 6.2 Day 4 Room temperature Colour Time (Min) Skimmed Semi-skimmed Full fat refrigerated Ultra heat treated 5 Slightly turbid yellowish liquid White White Light blue 10 Slightly turbid yellowish liquid White White Light blue 15 Slightly turbid yellowish liquid White White Light blue 20 Slightly turbid yellowish liquid White White Light blue 25 Slightly turbid yellowish liquid White White Light blue 30 Slightly turbid yellowish liquid White White Light blue Refrigerated Colour Time (Min) Skimmed Semi-skimmed Full fat refrigerated Ultra heat treated 5 Blue Light blue Light blue Light blue 10 Blue Light blue Light blue Light blue 15 Blue Light blue Light blue Light blue 20 Blue Light blue Light blue Light blue 25 Blue Light blue Light blue Light blue 30 Blue Light ...read more.

Conclusion

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. ...read more.

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