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

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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. Rate of microbial respiration in milk can be measured by measuring the rate of the colour change of rezazurin dye at 37°c and by the accumulation of lactic acid which is indicated by a fall in pH

 

 

Apparatus

Pasteurised milk i.e. samples of:

-Full fat

-Skimmed and

-Semi-skimmed

Sterilised milk i.e. samples of:

-Ultra Heat Treated milk

Rezazurin dye

Water bath

Test tubes

Pipette

Measuring cylinder

Thermometer

Rubber stopper

Timer

pH meter

Method

Measure 10 cm of each sample of milk, both the refrigerate samples and the room temperature samples, in a measuring cylinder. Pour the 10cm of each sample into separate test tubes. Mark on each of the test tubes, which samples are refrigerated and which are at room temperature. Also differentiate between the different types of milk used by marking each test tube. Add the 1 cm of the rezazurin dye into each of the test tubes and insert the rubber stopper in the tube and invert the tube 2-3 times. Place all the test tubes in the water bath at 37°c and start the timer. Record the milk sample colours from 5 minutes at 5 minute intervals for 30 minutes. Use the following colour descriptions to record each change

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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 ...

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