Agriculture and Food 7–12 · Years 9–10

Blanching and enzymes: a catalase test on raw and blanched vegetables

Food Service and Catering

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

Enzymes in raw vegetables keep working after harvest and in the freezer, and blanching in boiling water denatures them, which a catalase test with hydrogen peroxide makes visible.

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

scalds from boiling water
hydrogen peroxide irritates eyes and skin (stronger stock solutions are hazardous)
Bunsen flame and glowing splints

Controls

teacher or trained learner blanches with a slotted spoon
technician dilutes the peroxide
eye protection throughout
samples are not eaten

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

What you need

carrots sliced 5 mm thick and potato cut into 1 cm cubes
saucepan of boiling water, slotted spoon, bowl of iced water, stop clock
hydrogen peroxide solution, '5 volume' (diluted by the technician), 25 mL per test
100 mL conical flasks or 100 mL measuring cylinders, 25 mL measuring cylinder
wooden splints and a Bunsen burner for the glowing splint test
balance reading to 0.1 g, eye protection

How to do it

Blanch one set of carrot slices for 2 minutes (the National Center for Home Food Preservation time for sliced carrots) and a second set for 30 seconds; cool both in iced water and drain. Keep a third set raw. Treat the potato cubes the same way, but blanch the full set for 3 minutes, the shortest time the same table gives for potatoes (new potatoes, 3 to 5 minutes).
Weigh 5.0 g of each sample.
Add each sample to 25 mL of hydrogen peroxide solution at the same moment and start the clock.
Record the height of the foam after 1 minute (in a measuring cylinder) or the time taken for a glowing splint held in the flask neck to relight.
Cut a thick blanched piece in half and test the centre separately.
Put all used samples in the bin provided, not the sink.

What you should see

Raw carrot and potato make steady bubbles of oxygen that relight a glowing splint. Carrot slices blanched for 2 minutes and potato cubes blanched for 3 minutes give little or no bubbling, and 30-second samples are expected to fall in between. The centre of a thick blanched piece may still bubble because heat had not reached it, which is why the blanching table sets times by piece size. The National Center for Home Food Preservation explains that blanching slows or stops the enzymes that cause loss of flavour, colour and texture in frozen vegetables. The learner knows it worked when the raw samples bubble clearly and the fully blanched ones do not.

What changes

What you change: blanching time (0, 30 seconds, and 2 minutes for carrot or 3 minutes for potato)
What you measure: foam height after 1 minute (mm) or time for the splint to relight (s)
What you keep the same: vegetable and piece size
5.0 g sample mass
peroxide volume and strength
temperature of the peroxide

Common misconceptions

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

Freezing kills the enzymes in vegetables.
Blanching is done to cook the vegetables.
Bubbles on a vegetable in peroxide mean it is contaminated with bacteria.

Curriculum references

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

Food Technology Years 7–10 Syllabus (2019)FT5-4 FT5-5
Food Technology 7–10 Syllabus (2025), new syllabus, implementation from 2028FT5-FPP-01
Australian Curriculum v9AC9TDE10K05AC9S10U07

Sources

The pages the author read to write this activity.

All Concept Studio activities