Agriculture and Food 7–12 · Years 11–12

Preserving apple by drying: mass loss and pretreatment

Food Manufacture: preservation

Practical, model not builtLow risk

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The idea

Drying preserves food by removing the water that microbes and enzymes need, and weighing slices as they dry shows how much of fresh fruit is water.

Safety card

Low riskAn adult supervises

Hazards

cuts from knives and mandolines
hot oven and trays
ascorbic acid dust irritates eyes

Controls

mandoline hand guard; cut on a board
oven mitts
eye protection when mixing the dip

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. Heat and ascorbic acid: 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

firm apples, about 1 kg
ascorbic acid, 34 g dissolved in 1 litre of cold water (the Colorado State University pretreatment)
knife or guarded mandoline, cutting board
food dehydrator or fan-forced oven at 60 °C, with plastic or stainless steel mesh trays (not galvanised)
balance reading to 0.1 g, airtight glass jars

How to do it

Peel and core the apples and weigh the fruit before and after, to record preparation losses.
Cut rings or slices 3 to 6 mm thick; dip half in the ascorbic acid solution for 10 minutes and drain, and leave half untreated.
Spread the slices in single layers, weigh each loaded tray, and dry at 60 °C.
Weigh the trays every hour and plot mass remaining against time; separately dry a weighed 20 g subsample until two weighings an hour apart agree, to estimate the solids fraction.
Test for dryness: cooled pieces should be leathery and pliable with no moisture in the centre, and should spring apart after squeezing.
Condition the cooled fruit in a loosely covered jar for 4 to 10 days, shaking daily, before sealing, and compare the colour of pretreated and untreated slices.

What you should see

Mass falls quickly at first and then more slowly as the slices dry; thin apple slices take about 6 hours at 60 °C (6 to 24 hours depending on thickness, according to Colorado State University Extension). Pretreated slices stay lighter in colour than untreated ones. The same source reports 12 lb of apples as bought yielding 1¼ lb of dried apple, 10.4% of the bought mass after peel, core and water losses. With the model, 200.0 g of prepared slices with a measured solids fraction of 0.15, dried to 20% moisture, should finish at 37.5 g. The learner knows it worked when the dried pieces pass the squeeze test and the mass curve has levelled.

What changes

What you change: pretreatment (ascorbic acid dip or none)
What you measure: mass remaining over time (g); drying time; colour
What you keep the same: apple variety
slice thickness
drying temperature 60 °C
tray loading

Common misconceptions

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

Dried fruit has more kilojoules than the fresh fruit it was made from.
Dried fruit keeps for ever in any container.
The darker the dried fruit, the better it has been dried.

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 Stage 6 Syllabus (2013)H1.1 H4.2
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities