decay of milk

Question 1

what happens when milk decays?

Answer 1

as milk decays its pH reduces. This is because bacteria present in milk carry out a chemical process to provide them with energy. This process converts lactose sugar into lactic acid, and producing this acid reduces the pH of the milk.

Question 2

How to investigate the effect of temperature on the rate of decay of fresh milk by measuring the pH change.

Answer 2

• transfer 5cm^3 of lipase into a test tube labelled ‘lipse’ using a pipette
• add 5 drops of the indicator cresol red to a test tube labelled ‘milk’ as well as 7cm^3 of sodium carbonate and 5cm^3 of milk
• place a thermometer in the test tube labelled ‘milk’ and place both test tubes into a beaker filled with water at our first chose temperature ( 20 degrees )
• wait till the solutions in the test tubes are the same temperature as the water
• use a pipette to transfer 1cm^3 of lipase solution to the test tube labelled milk and stir, at the same time starting a timer
• the enzyme will start to break down the fat molecules in the milk which releases fatty acids and causes the pH of the milk to reduce
• in acidic conditions the indicator cresol red turns yellow, so once the solution turns yellow, we stop timing and record the results
• repeat the experiment at a range of different temperatures

Question 3

what are the dependent, independent and control variables in the milk decay practical?

Answer 3

independent = temperature 
dependent = time taken for solution to turn yellow 
control = volume of solutions

Question 4

Things to keep in mind for the milk decay practical

Answer 4

• use clean test tubes for the milk after each experiment as the traces of lipase in the test tube could cause a reaction earlier than wanted
• it is hard to pinpoint the exact time of colour change so to get more accurate results we can repeat the experiment multiple times at each temperature to calculate a mean result
• enzymes work slower at lower temperatures