Investigating the effect of temperature on decay

In a nutshell

When milk decays, the lactose sugar in the milk is converted to lactic acid. The fats in the milk are also broken down into fatty acids. The presence of the acid reduces the pH of the milk. Therefore, the effect of temperature on the rate of decay of fresh milk can be measured by measuring pH change.

Equipment list

The following equipment can be used to investigate the impact of temperature on the rate of decay of fresh milk.

Equipment	Use
Full fat milk	This is what will be decaying.
Sodium carbonate solution	To make the solution alkaline.
Lipase solution	To digest the fat in the milk.
$250\ cm^3$ beakers	To be used as water baths.
Test tubes	To hold the solutions.
Test tube rack	To hold the test tubes.
Marker pen	To write on the test tubes.
Thermometer	To measure the temperature of the water bath and the contents of the test tube.
Stopwatch	To measure the time taken for the indicator to turn yellow.
Cresol red or phenolphthalein indicator	A pH indicator.
Water	To create the water bath.
Electric kettle	To heat the water that will be used in the water bath.
Ice	To add to the water bath for lower temperature repeats.
Pipettes	To transfer solutions into the test tubes.
$10\ cm^3$ measuring cylinder	To measure the solutions.

Safety precautions

When carrying out experiments, it is very important to consider the safety precautions so nobody gets hurt.

Hazard	Risk	Safety measure
Sodium carbonate solution	At high concentrations this can be an irritant.	Be very careful when handling the sodium carbonate solution.
Enzymes	Allergen risk for some students.	Be very careful and do not ingest any of the milk. If you get it on your hands, wash them thoroughly and do not put your fingers in your mouth.

Experiment 1: Investigate the effect of temperature on the rate of decay of fresh milk.

Experimental variables

Every experiment will have an independent, dependent and constant variable. The independent variable is the variable that you change. The dependent variable is the variable that depends on the independent variable; therefore, it is the one you measure. The control variable is what is kept the same during the experiments.

Independent variable	$Temperature\space of\space the\space milk\space solution.$
Dependent variable	$The\space time\space taken\space for\space the\space solution\space to\space turn\space yellow.$
Control variables	$Volume\space of\space milk\space used,\space type\space of\space milk\space used, amount\space of\space lipase\space solution\space added\space\\ and\space the\space volume\space of\space sodium\space carbonate\space solution\space added.$

Method

The following method is used to investigate the effect of temperature on the rate of decay of fresh milk.

1.	Boil a kettle and use the hot water to create a water bath by half filling a $250 \, cm^3$ beaker. Note: Place a thermometer in the beaker so you can monitor the temperature. The water shouldn’t exceed $60\degree C$ .
2.	Label a clean test tube with an 'L' (for lipase).
3.	Using a measuring cylinder, transfer $5\, cm^3$ of lipase solution to the test tube labelled ‘L’.
4.	Label another clean test tube with an ‘M’ (for milk).
5.	Using a pipette, put five drops of phenolphthalein solution into the tube labelled ‘M’. *Note:* You can use Cresol red instead of phenolphthalein.
6.	Using another measuring cylinder, transfer $5 \, cm^3$ of milk to the tube labelled ‘M’.
7.	Using another measuring cylinder, transfer $7 \, cm^3$ of sodium carbonate solution to the test tube labelled ‘M’. *Note:* The solution in the test tube should be pink. The sodium carbonate makes the solution alkaline so it will be pink/purple.
8.	Place a thermometer into the test tube labelled ‘M’.
9.	Place both test tubes (‘L’ and ‘M’) into the water bath.
10.	Monitor the temperature of the test tube and wait until the temperature reaches the same temperature as the water bath.
11.	Using another pipette, transfer $1\, cm^3$ of lipase from tube ‘L’ into the tube labelled ‘M’.
12.	Start the stopwatch immediately.
13.	Stir the test tube until the solution becomes a yellow colour.
14.	Record the time taken for the contents of the tube to turn yellow.

Repeat this experiment for different water bath temperatures in a range of $20\degree C$ to $60\degree C$ . The temperature can be lowered by placing ice in the water bath. Ensure you carry out the experiment three times (or use class data).

Analysis

This is how you would use your data to form conclusions. At this stage you should have a table containing your data like the one shown below.

	Time taken for the solution to turn yellow (seconds)
Temperature ( $\degree C$ )	Your results	Class repeat $1$	Class repeat $2$	Mean
$20$
$30$
$40$
$50$
$60$

1.	Using your results, calculate the mean time taken for the pink solution to turn yellow at each temperature. $mean = \frac{time \space 1 + time\space 2 + time\space 3}{3}$
2.	Use this data to plot a graph. The dependent variable is the time taken for the solution to turn yellow. This should be plotted on the $y$ -axis. The independent variable is the temperature of the milk solution. This should be plotted on the $x$ -axis.
3.	You can use the mean time to work out the rate of decay for each temperature. $rate=\frac{1000}{time}$ Note: The units will be $s^-{^1}$

Conclusion

At lower temperatures, the rate of decay will be lower as the lipase enzymes will have less energy to digest the fats in the milk. As the temperature increases, the enzymes will work quicker so the rate of decay will increase. However, the optimum temperature of lipase enzymes is $37\degree C$ . Therefore, at temperatures higher than this, the rate of decay will start to decrease.

Evaluation

After completing the experiment, you must comment on the quality of your data. You should also think about how your method could be improved. A common source of human error is to mislabel the test tubes or to pipette the wrong solutions into the different tubes. This will lead to incorrect results. There may also be errors in adding the phenolphthalein as it is a colourless solution. It is important to ensure you have added the indicator correctly.

The method says to measure the time it takes for the solution to turn yellow. However, there may be varying shades of yellow observed at different time points. Therefore, it may be useful to compare the colour of the test tube to a standardised colour chart to ensure you are noting the correct time for the correct colour change.