Biology 11–12 · Year 11

Effect of temperature on lipase digesting milk fat

Module 1: Cells as the Basis of Life (Cell Function); also Module 2 (Nutrient and Gas Requirements)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Lipase releases fatty acids from fat, so the pH falls as digestion proceeds; the rate rises with temperature until the enzyme denatures.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

Hot water baths at 50 to 60 degrees Celsius: scalds
Phenolphthalein is dissolved in ethanol and is flammable
Sodium carbonate and lipase are mild irritants

Controls

Keep the indicator away from flames
Use tongs and eye protection at the hot baths
Rinse splashes of enzyme or carbonate solution promptly

Note

Phenolphthalein and sodium carbonate handling: NSW Department of Education Chemical Safety in Schools package (https://education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/1._Section_1_-_General_information_for_all_staff.pdf) and RiskAssess (https://www.riskassess.com.au/).

What you need

Full-cream milk, 5 mL per run
Lipase solution 5 percent, 1 mL per run
Sodium carbonate solution 0.05 mol/L, 7 mL per run
Phenolphthalein indicator in a dropper bottle
Optional emulsifier: washing-up liquid, 1 or 2 drops per 250 mL, as the source suggests
Water baths or beakers at 0 (ice), 20, 30, 40, 50 and 60 degrees Celsius; thermometer; test tubes; stop clock; syringes

How to do it

Add five drops of phenolphthalein to 5 mL milk in a test tube, then 7 mL sodium carbonate solution: the mixture turns pink (alkaline).
Stand the tube and a separate tube of lipase in the water bath until both reach the target temperature.
Add 1 mL lipase to the milk, stir, and start the clock.
Record the time for the pink colour to disappear (phenolphthalein becomes colourless below about pH 8.3 as fatty acids neutralise the alkali).
Repeat at each temperature, three trials each; calculate rate as 1 divided by time and plot against temperature.
Further run: repeat at 40 degrees Celsius with and without the emulsifier to show the effect of emulsification on the rate.

What you should see

At 0 degrees Celsius the pink colour lasts much longer than at 20 to 40 degrees Celsius; the time to decolourise falls as temperature rises through 20, 30 and 40 degrees Celsius, then lengthens again at 50 to 60 degrees Celsius as the lipase denatures. With emulsifier the time at 40 degrees Celsius is shorter than without. The learner knows it worked when the rate-against-temperature plot rises to a peak and falls, and the ice-bath tube is the slowest to lose its colour of the tubes from 0 to 40 degrees Celsius.

What changes

What you change: temperature (degrees Celsius)
What you measure: time for the indicator to decolourise (rate = 1/time)
What you keep the same: volumes and concentrations of milk, lipase and sodium carbonate
drops of indicator
stirring
milk brand

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Higher temperature always means a faster enzyme; above the tolerance limit the rate collapses.
The colour change shows the fat disappearing; it shows the fatty acids produced lowering the pH.
Emulsifiers digest fat; they increase the surface area on which lipase acts.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO11-8 BIO11-9 BIO11/12-3
Biology 11–12 Syllabus (2025), Year 11 focus area Cells as the basis of life; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-01
Biology 11–12 Syllabus (2025), Year 11 focus area Cells to systems; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-02
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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