Last reviewed 1 Oct 2026 · 8 min read
Heat preservation — overview
Heat destroys micro-organisms and inactivates enzymes so that food can be stored. The severity of the treatment depends on the target microbe and the food's acidity.
| Treatment | Temperature–time | Purpose |
|---|---|---|
| Blanching | About 85–100 °C, 1–5 min | Inactivate enzymes; not a preservative in itself |
| Pasteurisation | Below 100 °C | Kill vegetative pathogens and spoilage organisms; shelf life extended by days to months (with refrigeration or low pH) |
| Commercial sterilisation (canning) | About 110–125 °C (retort) for minutes | Destroy spores of Clostridium botulinum and spoilage spores; shelf-stable |
| UHT (ultra-high temperature) | About 135–150 °C for 2–5 s | Sterilise liquids (milk, juice) followed by aseptic filling |
Pasteurisation
| Method | Conditions (milk) |
|---|---|
| Low-temperature long-time (LTLT, batch/holder) | 63 °C for 30 min |
| High-temperature short-time (HTST) | 72 °C for 15 s |
| UHT | 135–150 °C for 2–4 s |
| Fruit juices and acid foods | About 85–95 °C for 15–60 s (hot-fill or flash) |
Pasteurisation is verified by the alkaline phosphatase test (negative after proper pasteurisation of milk) and for juices by colony counts/enzyme inactivation (pectinesterase).
Acidity classes and the pH 4.6 boundary
- Low-acid foods (pH > 4.6; e.g., vegetables, meat, fish, milk) — they can support the growth of Clostridium botulinum spores, whose toxin causes botulism, so they require sterilisation at above 100 °C (pressure retort).
- Acid foods (pH ≤ 4.6; e.g., most fruits, tomato (borderline), pickles) — spores of botulinum cannot grow; milder pasteurisation (about 85–100 °C) is enough, aimed at moulds, yeasts and acid-tolerant spoilage bacteria.
- Some classifications use four classes: low-acid (pH > 5.3), medium-acid (4.5–5.3), acid (3.7–4.5), high-acid (< 3.7).
Microbial heat resistance
- D value — the time at a given temperature to reduce the microbial population by 90 % (one log cycle). , so the process time for a reduction from to is .
- z value — the temperature rise that reduces D by a factor of 10: .
- F value — equivalent time at a reference temperature giving the same lethality: .
- F₀ value — F at °C (250 °F) with °C — the standard for low-acid foods (sterilising value in minutes).
The botulinum cook (12-D concept)
- C. botulinum has –0.25 min and °C.
- A 12-log (12-D) reduction is the minimum process for low-acid canned foods: –3.0 min. Commercial processes use F₀ of about 3–6 min for safety against more resistant spoilage bacteria (e.g., Bacillus stearothermophilus requires F₀ of about 5–20 min for some products).
A product needs min at 121.1 °C ( °C). What is the equivalent holding time at 111.1 °C and at 131.1 °C?
- At 111.1 °C: .
- At 131.1 °C: .
A 10 °C rise in temperature cuts the required time ten-fold — the principle behind high-temperature short-time (HTST and UHT) processing, which kills microbes with less damage to colour, flavour and vitamins (the quality factors have larger z values: about 20–40 °C, so they degrade less quickly than spores die).
Spores of an organism with min are heated at 121 °C for 6 min. Number of log cycles — a 12-log reduction (a starting load of spores per can becomes spores per can, i.e., one survivor in a billion cans).