Heat preservation and blanching
Heat preserves food by killing microbes and inactivating enzymes. Blanching is a short heat treatment in hot water or steam before freezing, drying or canning.
Levels of heat treatment
| Treatment | Temperature | What it does | Example |
|---|---|---|---|
| Blanching | About 95-100 °C, a few minutes | Inactivates enzymes; reduces surface microbes | Peas, beans, carrots before freezing or drying |
| Pasteurisation | Below 100 °C (milk: 63 °C for 30 min or 72 °C for 15 s) | Kills disease germs and most spoilage organisms | Milk, fruit juices, squash |
| Boiling | 100 °C | Kills most vegetative cells, not all spores | Home cooking, boiling milk |
| Sterilisation | Usually 121 °C under pressure | Kills all microbes including spores | Canned peas, bottled sterilised milk |
| UHT | 135-150 °C for a few seconds | Sterilises with little cooked flavour | Carton milk |
Factors that affect the heat resistance of microbes
- Number of microbes: the more there are, the longer the heating needed.
- Type and form: spores (e.g. Clostridium botulinum) are far more resistant than growing cells, yeasts or moulds.
- pH: acid lowers heat resistance, so acid fruits need only 100 °C while low-acid vegetables need 121 °C.
- Food composition: sugar, fat and protein protect microbes from heat.
- Moisture: moist heat (steam, water) kills far better than dry heat.
- Time and temperature: a higher temperature kills in a shorter time.
How to blanch vegetables
- 1Sort, wash, peel and cut into even pieceseven size heats evenly
- 2Bring plenty of water to a rolling boil, or set up a steamerwater about 95-100 °C; steam 100 °C
- 3★Dip the vegetables in a wire basket for the set timea few minutes, by vegetable and size
- 4★Cool at once in cold or iced waterstops cooking; keeps texture and colour
- 5Drain and check a sample with the peroxidase testno brown colour = enough blanching
- 6Freeze, dry or can without delay
★ one-way, controls the pace
Water blanching vs steam blanching
Under-blanching is worse than none: it breaks cells and can speed enzyme action. Over-blanching softens texture and dulls colour.
Purposes of blanching
- Inactivates enzymes (peroxidase, catalase, lipoxygenase, polyphenol oxidase) that cause off-flavours and colour and texture changes in storage.
- Reduces surface microbes and cleans the produce.
- Removes trapped air from tissues, which helps form a vacuum in cans and cuts oxidation.
- Softens and wilts tissue so more fits into a can or pack.
- Sets and brightens green colour and removes raw taste.
Blanching keeps the colour and vitamins of frozen or dried vegetables over months, but it also leaches vitamin C and B vitamins into the water. Steam, short times and quick cooling save nutrients; the same rule applies when a hospital kitchen par-boils vegetables in advance.
2 marks: “What is blanching? Why is it done?” A brief heat treatment of vegetables in hot water or steam, followed by quick cooling, mainly to inactivate enzymes before freezing, drying or canning.
5 marks: “Explain the purposes and methods of blanching.” Define; purposes (enzymes, microbes, air, softening, colour); water vs steam blanching; peroxidase test; quick cooling; nutrient loss.
10 marks: “Explain the principles of preservation by heat. Add a note on blanching.” Heat kills microbes and enzymes; levels of heat treatment; factors affecting heat resistance; blanching purposes, methods, test, merits and limits.
Blanching PEACS: Peroxidase test, Enzymes stopped, Air removed, Colour set, Softened for packing.