lipase and milk fat

Fats, such as the fats in milk, need to be digested by your body. They are broken down into fatty acids and glycerol by an enzyme called lipase. You can do this in the lab if you mix lipase and fat together and measure the pH, or acidity, of the mixture. data logger with a pH sensor and electrode to take the readings for us.

One setup
Pour 5 cm3 of milk into each of two boiling tubes. The pH will be roughly neutral, add sodium carbonate solution until both milks are pH 8.5 – 9. We do this because the enzyme works better in alkali. Then add a drop of bile (washing up detergent – see picture) to one tube, add 5cm3 lipase to both tubes, stir and place the tubes in a water bath. Leave for 30 minutes

Here is a good result – collected by a couple of lucky beginners.

Join in the fun and answer these questions*:

a) why is one graph different from the other? Try to explain the shape of the graphs?

b) why are the starting points of the graphs similar?

c) why are the ending points of the graphs similar?

d) what can you say about the closeness of the starting points of the graphs?

e) what do you think about this idea: we used the software to subtract 0.2 pH units from every point on the red line. Here is the result, shown on a new brown line:

Result after applying twiddle factor

A variation of the experiment is to use different strengths of lipase suspension – in fact the chemicals illustrated above. Here is the result of measuring two tubes – 1% and 2% lipase. Which line is which below:

Another good result from St John’s in Leatherhead – the graph scales have been greatly magnified for the photograph hence the noisiness. The software shown is PASCO Data Studio used with a Science Workshop 500 interface.
If you have Excel or some data logging software you case analyse the data from our experiment. Here is the data