Agriculture and Food 7–12 · Years 7–8

Why cut apple turns brown, and how acid dips and blanching stop it

Food and agricultural practices

PracticalLow risk

The idea

Cutting fruit lets an enzyme (polyphenol oxidase) use oxygen to turn natural phenolic compounds into brown pigments, so acid and antioxidant dips, cold, or heat that denatures the enzyme slow or stop the browning.

Safety card

Low riskAn adult supervises

Hazards

knife cuts
scalds from boiling water and steam
ascorbic and citric acid powders irritate eyes

Controls

cut on a board with fingers tucked
teacher or trained learner lifts the steamer lid away from the face
eye protection while dissolving the acids
samples are not eaten if the practical is run in a science laboratory

Note

Food handling follows the FSANZ food safety standards (Standard 3.2.2) as explained on the FSANZ food temperature and thermometers page; check class allergy and anaphylaxis records before any food is prepared; the Food Technology Years 7–10 Syllabus reminds teachers that food allergies can cause anaphylaxis. Acids and heat: 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

2 apples of one pale-fleshed variety
ascorbic acid solution, 3.4 g in 100 mL cold water (the 34 g per litre dip in the Colorado State University Extension guide)
citric acid solution, 0.5 g in 100 mL cold water (5 g per litre)
lemon juice and cold water mixed 1:1, 100 mL
cold tap water, 100 mL
saucepan of boiling water with a steamer basket and lid, and a bowl of iced water
knife, cutting board, 7 labelled white plates
phone camera and a white card for photographs under the same light

How to do it

Cut the apples into 14 slices of equal thickness (about 5 mm), keeping the peel on.
Put two slices straight onto the untreated plate, and two more onto a plate that goes into the refrigerator.
Dip two slices in each of water, ascorbic acid, citric acid and lemon juice for 10 minutes, then drain and plate them.
Steam-blanch two slices in the steamer basket over boiling water for 2 minutes (the National Center for Home Food Preservation gives 1½ to 2 minutes for apple slices), cool them in iced water, drain and plate them.
Photograph every plate against the white card at 0, 15, 30 and 60 minutes and score the cut surface from 0 (no browning) to 4 (dark brown).
Rank the treatments by their 60-minute score and explain each result by what the treatment does to the enzyme, the pH or the oxygen supply.

What you should see

By 60 minutes the untreated room-temperature slices have the darkest cut surfaces. Slices dipped in ascorbic acid, citric acid or lemon juice stay closest to the starting colour; these are the anti-darkening pretreatments Colorado State University Extension recommends. Blanched slices do not brown because heat denatures the enzyme, but they are softer and look cooked. Refrigerated untreated slices brown more slowly than the room-temperature ones. The learner knows it worked when untreated and acid-dipped slices from the same apple are clearly different in the 60-minute photographs.

What changes

What you change: treatment (none, water, ascorbic acid, citric acid, lemon juice, blanching, refrigeration)
What you measure: browning score (0 to 4) at 15, 30 and 60 minutes
What you keep the same: apple variety
slice thickness
10 minute dip time
photograph lighting
room temperature except for the refrigerated plate

Common misconceptions

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

Cut apple turns brown because it is starting to rot.
Bacteria landing on the cut surface cause the browning.
Lemon juice only hides the brown colour with its own colour and flavour.

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)FT4-4 FT4-5
Technology 7–8 Syllabus (2023), focus area Food and agricultural practicesTE4-SAF-01
Australian Curriculum v9AC9TDE8K05AC9S8U07

Sources

The pages the author read to write this activity.

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