Thermal Processing: Pasteurisation & Sterilisation

Using heat to make food safe. Learn microbial death kinetics (D-value, z-value, log reductions), thermal death time, the 12D botulinum cook, F-value and F₀ lethality, the general (Bigelow) method from heat-penetration data, cold spot and heat penetration in cans, pasteurisation (HTST, LTLT, UHT milk), blanching, retorts, aseptic processing, and quality retention (C-value, HTST principle) — with Python for D-values at different temperatures and F₀ from a time–temperature profile.

Heat is the oldest and most widely used food preservation method. Pasteurisation kills vegetative pathogens (milk, juices); sterilisation in cans kills heat-resistant spores so low-acid foods can be stored at room temperature for years.

Thermal process engineers calculate exactly how much heat is needed — too little is dangerous, too much destroys flavour, colour and nutrients.

An analogy

Killing microbes with heat is like popping balloons with random darts: in each minute a fixed fraction bursts, so the count falls logarithmically. The D-value is how long it takes to pop 90% of them; hotter conditions throw darts much faster.

Pasteurisation vs sterilisation

Pasteurisation (mild heat, e.g. milk 72 °C/15 s) kills vegetative pathogens and reduces spoilage organisms; products still need refrigeration. Commercial sterilisation (e.g. 121 °C) destroys spores of C. botulinum in low-acid foods (pH > 4.6) so products are shelf-stable. Acid foods (pH < 4.6, e.g. fruit juices, pickles) need only pasteurisation-level heat.

Kinetics: D and z

At constant temperature microbial death is first-order: each D minutes kills 90%. z tells how sensitive D is to temperature (bacterial spores ~10 °C, vegetative cells 4–7 °C). Quality factors (vitamins, colour) have larger z (25–40 °C), so higher-temperature, shorter-time (HTST, UHT) processes kill microbes while preserving quality.

Heat penetration and F₀

In a can, the slowest-heating point (cold spot — geometric centre for conduction foods like pastes; lower for convection foods) determines safety. Thermocouples record its temperature during retorting; the general method integrates lethal rates to give F₀. Ball's formula method uses heating-curve parameters (f_h, j) to predict process times for different conditions.

Equipment and modern methods

Retorts (batch still, rotary, water spray, steam–air for pouches), continuous hydrostatic sterilisers, plate and tubular heat exchangers for HTST/UHT, direct steam injection/infusion, and aseptic processing (sterilise product and package separately, fill in sterile zone — Tetra Pak). Emerging alternatives: ohmic heating, microwave sterilisation, high-pressure processing (non-thermal). Process validation uses biological indicators and temperature distribution tests.

Formulas

Worked example: Log reduction

Given: D = 0.5 min, t = 3 min

Find: Logs killed.

Answer: 6 log (99.9999%)

Worked example: D at another temperature

Given: D₁₂₁ 0.21, z 10, T 111 °C

Find: D₁₁₁.

Answer: 2.14 min

Worked example: Lethal rate

Given: T = 116.1 °C

Find: L.

Answer: 0.316 (1 min counts as 0.32 min at 121.1 °C)

D-values and F₀ from a can's heat-penetration data

# Thermal processing: D-value, log reduction and F0 from a time-temperature profile
D121, z = 0.21, 10.0                       # C. botulinum D at 121.1 C (min), z (C)
print(f"12D process time at 121.1 C = {12*D121:.2f} min")
# D at other temperatures
for T in (115.0, 121.1, 125.0):
    D = D121 * 10 ** ((121.1 - T) / z)
    print(f"D at {T:.1f} C = {D:.3f} min")
# F0 by general method from can centre temperatures every 1 min
temps = [100, 106, 111, 115, 118, 120, 121, 121, 121, 121, 120, 116, 110, 102]
F0 = sum(10 ** ((T - 121.1) / z) * 1.0 for T in temps)
print(f"F0 = {F0:.2f} min -> log reductions of C. botulinum = {F0/D121:.1f}")
# Pasteurisation of milk (HTST 72 C / 15 s): equivalent time at 63 C with z = 5.5 C
print(f"LTLT equivalent at 63 C: {15*10**((72-63)/5.5)/60:.1f} min")
# 12D process time at 121.1 C = 2.52 min
# D at 115.0 C = 0.855 min
# D at 121.1 C = 0.210 min
# D at 125.0 C = 0.086 min
# F0 = 6.73 min -> log reductions of C. botulinum = 32.1
# LTLT equivalent at 63 C: 10.8 min

The can centre only spends a few minutes near 121 °C, but summing lethal rates over the whole heating and cooling curve gives F₀ = 6.7 min — well above the 12D minimum (2.52 min), as commercial processes add margins for spoilage organisms. The HTST calculation shows 72 °C/15 s is as lethal as ~11 min at 63 °C for the target organism; regulations specify 30 min LTLT for a safety margin.

Common mistakes

Applications

Key takeaways

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