Thermal processing of foods: canning, sterilisation and F-value

In short

Thermal processing is the controlled application of heat to a food, usually after it is sealed in a hermetic container, to destroy micro-organisms and enzymes so the product keeps at ambient temperature without refrigeration. The target is commercial sterility, not absolute sterility: the process is designed so that the probability of a surviving spore of *Clostridium botulinum* in a low-acid canned food is about one in a million million (10⁻¹²). The arithmetic of this design — D value, z value, F₀ and heat penetration at the cold point — is the single most calculation-heavy part of food engineering operations and is examined every year in GATE (Food Technology, section on thermal operations/food process engineering), ICAR-JRF/SRF, and university papers. In industry it is the core competence of process authorities and QA engineers at ITC, Nestlé India, Hindustan Unilever, Mother Dairy, Amul (GCMMF), Dabur, Capital Foods, MTR/Orkla, Lijjat, Hatsun and the hundreds of fruit-and-vegetable canneries and MRE/ready-to-eat plants supplying DRDO and the Army.

Explained

Why heat works, and what "commercial sterility" means

• Heat denatures microbial proteins, nucleic acids and ribosomes; vegetative cells die easily, bacterial spores do not. Spores of *C. botulinum*, *Bacillus stearothermophilus* (now *Geobacillus stearothermophilus*), *C. sporogenes* (PA 3679) and *B. coagulans* set the process.
• Commercial sterility: the food contains no micro-organism capable of growing under normal non-refrigerated storage, and no viable pathogen. Thermophilic spores may survive but cannot germinate below about 43 °C.
• Target organism depends on pH. Low-acid foods (pH > 4.6) → *C. botulinum* type A/B (toxin is lethal; spores survive 100 °C) → process above 115 °C. Acid/acidified foods (pH ≤ 4.6) → spoilage yeasts, moulds, lactobacilli, *Alicyclobacillus* → 90–100 °C is enough.
• Water activity also matters: at a_w below about 0.93 *C. botulinum* cannot grow, which is why cured, salted and sugar-preserved products need a milder process.
• Enzyme inactivation can be the limiting requirement: peroxidase, polyphenol oxidase, pectin methylesterase (cloud loss in citrus juice), lipoxygenase and milk plasmin. Spores die faster than some enzymes are inactivated in acid foods and in UHT milk.

Classification of thermal processes

The canning operation, step by step

1. Raw-material selection and grading — maturity, no bruising; spore load on raw material (soil-borne) directly raises the required process.
2. Washing, peeling, cutting — lye, steam or abrasive peeling; washing reduces initial load N₀.
3. Blanching — inactivates enzymes, expels intercellular gas (less strain on the can, better vacuum), fixes colour, shrinks tissue for filling.
4. Filling — solids to a declared drained weight, then brine/syrup/sauce at 80–90 °C. Headspace of about 6–10 mm is left: too little strains the seam, too much leaves residual oxygen.
5. Exhausting / de-aeration — steam flow, hot filling or mechanical/steam-flow closing creates a partial vacuum (about 250–400 mm Hg) so the can ends stay concave, oxidation and internal corrosion are reduced, and the can does not buckle in the retort.
6. Seaming — the double seam interlocks the five thicknesses of body hook and end hook with a sealing compound. Routinely checked for seam thickness, height, body-hook and end-hook butting, overlap (≥ 45 %), tightness rating and countersink depth. A seam failure, not under-processing, causes most real-world spoilage.
7. Retorting — still or agitating retort; steam, steam/air or water immersion with overpressure.
8. Cooling — to about 38–40 °C so the can dries (no rusting) and thermophiles cannot grow; cooling water must be chlorinated (free residual chlorine typically 1–2 ppm at the outlet) because the cooling vacuum can suck water through a micro-leak.
9. Coding, labelling, incubation and warehouse hold — incubation tests at 37 °C and 55 °C on samples; records of every retort cycle kept.

Retorts and containers

• Still retorts: vertical or horizontal batch, saturated steam. Venting is compulsory — residual air gives pockets of lower-temperature air-steam mixture; the come-up time and vent schedule must be validated.
• Agitating retorts: end-over-end or axial (Steritort, FMC) rotation induces forced convection in the headspace bubble, cutting process time sharply for viscous liquids — used for evaporated milk, soups, pet food.
• Hydrostatic steriliser: continuous, water legs balance the steam-chamber pressure; high throughput, low floor-space-to-output ratio.
• Overpressure processing (steam/air or water immersion) is mandatory for retort pouches, glass jars and plastic tubs, where internal pressure would otherwise burst the seal.
• Containers: three-piece tinplate cans (ETP, lacquer-lined — sulphur-resistant lacquer for peas/fish, acid-resistant for fruit), two-piece DRD/DWI cans, aluminium cans, glass jars with lug caps, retort pouches (PET/aluminium foil/CPP laminate — thin profile, so a much shorter process and better quality: the basis of Army MRE and most Indian RTE curries), and aseptic cartons/bag-in-box for UHT products.
• Corrosion defects to know: sulphide staining (harmless black FeS/SnS), hydrogen swell (acid attack on tin, slow), springer, flipper, buckled can, and hard swell (microbial or hydrogen; always treated as suspect).

Microbial death kinetics — D, z and F

• Thermal destruction of spores at constant temperature is first order: log N falls linearly with time.
• D value (decimal reduction time) = time at a constant temperature to destroy 90 % of the population (one log cycle), in minutes. Smaller D = less heat-resistant.
• z value = the temperature change, in °C, that changes D by a factor of 10 — the slope index of the thermal death time curve. For *C. botulinum* spores z ≈ 10 °C; for enzymes and quality factors z is much larger (25–45 °C), which is the whole reason HTST/UHT works.
• F value = equivalent heating time at a reference temperature with a stated z. F₀ is the special case: reference 121.1 °C (250 °F), z = 10 °C.
• Botulinum cook / 12D concept: D₁₂₁ of *C. botulinum* ≈ 0.21 min, so 12D = 12 × 0.21 ≈ 2.52 min, rounded to a minimum F₀ = 3 min for low-acid canned foods. This takes a spore load of 10¹² down to 1 — i.e. one surviving spore in 10¹² cans if each started with one spore.
• For spoilage organisms a 5D reduction is normal commercial practice; for thermophilic *G. stearothermophilus* (D₁₂₁ ≈ 4–5 min) products sold to hot climates may need F₀ of 12–18.
• Arrhenius alternative: k = A·exp(−Ea/RT); z and Ea are interconvertible. Ea ≈ 250–350 kJ/mol for spores, 50–150 kJ/mol for vitamins and colour.
• Relative-rate / quality criteria for UHT: B\* (bacteriological effect, reference 135 °C, z = 10.5 °C; B\* = 1 means a 9D spore reduction) and C\* (chemical effect, reference 135 °C, z = 31.4 °C; C\* = 1 means 3 % thiamine loss). Good UHT milk aims at B\* > 1 with C\* < 1.

Heat penetration and process calculation

• Cold point (slowest heating point): in a conduction-heated pack (solid pack, purée) it is the geometric centre; in a convection-heated pack (thin liquids) it is on the axis about one-third of the height from the base; in agitating retorts it moves towards the centre. The thermocouple goes at the cold point — the process is designed for the worst-heated particle, not the average.
• Heating curve: plot log(T_R − T) against time; a straight line gives a simple heating curve with slope index f_h (time for the unaccomplished temperature difference to fall by one log cycle) and intercept factor j_h. A broken heating curve (starch gelatinisation, a change from convection to conduction) needs two slopes, f₁ and f₂, with the break at x_b.
• General (Bigelow) method: from the measured cold-point time–temperature history, compute the lethal rate L = 10^{(T − 121.1)/z} at each instant and integrate — graphically (area under the L–t curve) or numerically (trapezoidal/Simpson). It is the reference method and handles any curve shape.
• Formula (Ball) method: uses f_h, j_h, the initial temperature I = T_R − T_i, and tables of f_h/U against g (the difference T_R − T at the end of heating) to get the process time B. Ball credits 42 % of the come-up time as effective process time; operator's process time = B − 0.42·t_cut.
• Stumbo's method refines Ball by accounting for the whole container volume (mass-average lethality, F_s) and for different j values; Pham's and numerical finite-difference/CFD methods are now standard in industry.
• Integrated lethality must be the sum over heating and cooling — ignoring the cooling lethality over-processes the product.
• Factors raising the required process time: larger container, viscous/conduction-heated product, lower initial temperature, lower retort temperature, high solids or starch, low fill temperature, higher initial spore load, low acidity.

Indian regulation and standards

• Food Safety and Standards Act, 2006 with the FSS (Food Products Standards and Food Additives) Regulations, 2011 govern canned and thermally processed foods; FSSAI licensing is mandatory for every manufacturer.
• The FSS (Licensing and Registration of Food Businesses) Regulations, 2011 — Schedule 4 requires a documented process schedule, retort records and a trained person responsible for thermal processing. Food businesses must implement an FSMS/HACCP plan; Codex *Recommended International Code of Hygienic Practice for Low-Acid and Acidified Low-Acid Canned Foods* (CAC/RCP 23) is the reference practice.
• FSSAI thermally processed fruit and vegetable product standards cover fill, drained weight, total soluble solids and metal contaminants; tin in canned food is limited by the FSS (Contaminants, Toxins and Residues) Regulations, 2011 — check the current limit before quoting it, as the Schedules are amended frequently.
• BIS: *IS 2168* (canned fish), *IS 2860* (canned pineapple), *IS 1040* (canned peas), *IS 13428*-type standards for packaged water, and tinplate container specifications; several older IS numbers have been withdrawn or revised, so quote the current edition from the BIS catalogue rather than from memory.
• Milk: FSSAI requires pasteurised milk to pass the alkaline phosphatase test (negative) and show a positive peroxidase test for ordinary pasteurisation; UHT milk must be commercially sterile and is sold under the FSSAI standard for UHT/sterilised milk.
• Pressure vessels: a retort is a pressure vessel. The Boilers Act, 2025 with the Indian Boiler Regulations, 2026 (in force October 2026) govern the steam boiler and its mountings; unfired pressure vessels are usually designed to IS 2825 / ASME VIII and inspected under state factory-type rules, now administered under the OSH&WC Code, 2020 (in force from 21 November 2025, replacing the Factories Act, 1948).
• Export consignments need an FSSAI/EIC (Export Inspection Council) certificate; US-bound low-acid canned food requires an FDA-filed scheduled process signed by a recognised process authority, and the EU requires an approved establishment number for animal products.

Key points

• Commercial sterility – no organism able to grow in normal ambient storage; not the same as absolute sterility.
• pH 4.6 – the dividing line: above it, *C. botulinum* can grow and a 121 °C process is needed; at or below it, 100 °C suffices.
• D value – minutes at a fixed temperature for a 90 % (1-log) kill; D₁₂₁ of *C. botulinum* ≈ 0.21 min.
• z value – °C change that alters D tenfold; ≈ 10 °C for spores, 25–45 °C for vitamins, colour and most enzymes.
• F₀ – equivalent minutes at 121.1 °C with z = 10 °C; the universal yardstick of a sterilising process.
• 12D / botulinum cook – 2.52 min at 121.1 °C; minimum design value F₀ = 3 for low-acid canned food.
• Lethal rate L – 10^{(T − 121.1)/10}; F₀ = ∫L dt over heating *and* cooling.
• Cold point – centre for conduction packs, about one-third up the axis for convection packs; thermocouple goes there.
• f_h and j_h – slope index and lag factor from the semi-log heat-penetration plot; inputs to Ball's formula method.
• Come-up time credit – Ball's method credits 42 % of the come-up time as process time.
• Venting – removes air from the retort; unvented air pockets give cold spots and an invalid process.
• Overpressure – compulsory for pouches, glass and plastic containers to stop seal failure and panelling.
• Chlorinated cooling water – prevents post-process leaker spoilage when the cooling vacuum forms.
• Double seam – the main point of integrity failure; checked by tear-down for overlap, hooks and tightness.
• HTST/UHT advantage – because quality factors have a larger z than spores, a short hot process gives the same lethality with far less quality loss.
• Agitation – end-over-end rotation induces convection in viscous packs and can cut process time by half or more.
• Target organisms by product – *C. botulinum* (low-acid), *G. stearothermophilus* (flat sour, hot climates), *A. acidoterrestris* (juices), yeasts/moulds (acid foods).
• Cook value C – lethality-style integral with reference 100 °C and z = 33 °C, used to quantify quality damage.

Formulas and examples

Example 1 — D and z from two thermal death times. A spore suspension needs 36 min at 110 °C and 2.4 min at 125 °C for a 6-log reduction.

• D₁₁₀ = 36/6 = 6.0 min; D₁₂₅ = 2.4/6 = 0.40 min.
• z = (125 − 110)/(log 6.0 − log 0.40) = 15/(0.7782 − (−0.3979)) = 15/1.1761 = 12.75 °C.
• D at 121.1 °C = 6.0 × 10^{(110 − 121.1)/12.75} = 6.0 × 10^−0.8706 = 6.0 × 0.1347 = 0.808 min.

Example 2 — the 12D botulinum cook and an equivalent process at 115 °C. D₁₂₁.₁ = 0.21 min, z = 10 °C.

• F₀ required for 12 decimal reductions = 12 × 0.21 = 2.52 min (design minimum 3 min).
• Time needed at 115 °C: D₁₁₅ = 0.21 × 10^{(121.1 − 115)/10} = 0.21 × 10^0.61 = 0.21 × 4.074 = 0.855 min.
• Process time = 12 × 0.855 = 10.3 min at 115 °C — i.e. L at 115 °C is 10^−0.61 = 0.245, and 10.3 × 0.245 = 2.52 min of F₀. Correct.
• At 110 °C: D = 0.21 × 10^1.11 = 0.21 × 12.88 = 2.705 min → 12 × 2.705 = 32.5 min.

Example 3 — F₀ by the general method (trapezoidal integration). Cold-point temperatures recorded every 5 min in a 121.1 °C retort, z = 10 °C:

• Trapezoid areas (Δt = 5 min), using ½(L₁+L₂)×5:

0–5: 0.0061; 5–10: 0.200; 10–15: 1.167; 15–20: 2.913; 20–25: 4.384; 25–30: 4.886; 30–35: 4.886; 35–40: 2.504; 40–45: 0.0614.

• F₀ = 21.01 min (of which about 2.57 min comes from the cooling phase).
• Decimal reductions of *C. botulinum* = F₀/D₁₂₁ = 21.01/0.21 ≈ 100 — far above 12D, so this schedule is badly over-processed for safety alone; it would only be justified if *G. stearothermophilus* (D₁₂₁ ≈ 4 min, so 21.01/4 ≈ 5.3D) were the design organism.

Example 4 — Ball formula method. A conduction-heated pack: f_h = 50 min, j_h = 1.8, retort T_R = 121.1 °C, initial product T_i = 71.1 °C, come-up time t_cut = 10 min, required F₀ = 6 min. For f_h/U with U = F₀ × 10^{(121.1 − T_R)/z} = 6 min, f_h/U = 50/6 = 8.33; from Ball's tables for z = 10 °C this corresponds to g ≈ 1.6 °C.

• I = 121.1 − 71.1 = 50 °C.
• B = f_h × [log(j_h × I) − log g] = 50 × [log(1.8 × 50) − log 1.6] = 50 × [log 90 − log 1.6] = 50 × [1.9542 − 0.2041] = 50 × 1.7501 = 87.5 min.
• Operator's process time = 87.5 − 0.42 × 10 = 83.3 min holding at 121.1 °C after the retort reaches temperature.
• (The g value must be read from Ball's/Stumbo's tables; in an exam the g or f_h/U table value is supplied.)

Example 5 — why UHT beats in-can sterilisation on quality. Compare F₀ = 6 min delivered (a) at 121 °C and (b) at 140 °C, with a quality factor (thiamine) having z_q = 31 °C and reference 121 °C.

• (a) Time at 121 °C ≈ 6 min (L ≈ 1). Quality lethality = 6 × 10^{(121−121)/31} = 6 min equivalent.
• (b) L at 140 °C = 10^{(140−121.1)/10} = 10^1.89 = 77.6, so holding time = 6/77.6 = 0.0773 min = 4.6 s.
• Quality damage = 0.0773 × 10^{(140−121)/31} = 0.0773 × 10^0.613 = 0.0773 × 4.10 = 0.317 min equivalent — about 19 times less quality loss for the same microbial kill. This ratio is the entire engineering case for HTST, UHT and thin retort pouches.

Where it is used

• Fruit and vegetable canneries — canned pineapple and mango pulp (Nashik, Chittoor, Krishnagiri), peas, mushrooms, tomato products; aseptically packed mango and guava pulp is a major Indian export.
• Dairy — UHT toned milk and flavoured milk in Tetra Pak (Amul, Mother Dairy, Nandini, Hatsun, Heritage); HTST plate pasteurisers in every city dairy; sterilised flavoured milk in bottles and retort pouches.
• Ready-to-eat and retort-pouch plants — ITC Master Chef, MTR, Haldiram's, Gits, Tasty Bite, and DRDO-DFRL-developed ration packs for the Armed Forces and the ISRO/Himalayan deployments.
• Fish and meat canning — tuna, sardine and prawn canning in Kerala, Tamil Nadu, Goa and Andhra; sulphur-resistant lacquered cans and strict seam control; MPEDA-registered export units.
• Beverages — hot-fill-hold mango drinks, nimbu paani and tomato juice; in-pack pasteurisation (tunnel pasteurisers) for beer and carbonated acid drinks.
• Job roles — process authority / thermal process specialist, retort supervisor, QA and FSMS/HACCP coordinator, heat-penetration study engineer, equipment design and validation engineer with retort and UHT suppliers (Tetra Pak, GEA, JBT, SPX, Alfa Laval and Indian makers).

Interview questions

• Q: Why is pH 4.6 the dividing line in thermal processing? A: *Clostridium botulinum* cannot grow or produce toxin below pH 4.6, so foods at or below that pH need only a mild 90–100 °C process, while low-acid foods above it need a full 121 °C botulinum cook.
• Q: Define F₀ and state what reference conditions it implies. A: F₀ is the number of minutes of heating at 121.1 °C (250 °F) with z = 10 °C that would give the same lethality as the actual time–temperature history at the cold point.
• Q: Why is the minimum F₀ for low-acid canned food taken as 3 minutes? A: The 12D botulinum cook is 12 × D₁₂₁ (0.21 min) = 2.52 min; this is rounded up to 3 min as a safety margin, giving a survival probability of about 10⁻¹² per container.
• Q: What is the difference between D value and F value? A: D is the time at one constant temperature for a single 90 % reduction; F is the total equivalent time at a reference temperature for the whole required reduction, so F = D × number of log cycles.
• Q: Where is the cold point in a can, and does it move? A: At the geometric centre for conduction-heated products and about one-third of the height up the axis for convection-heated liquids; it shifts towards the centre with agitation and can move during a process if the product thickens (broken heating curve).
• Q: Why must a retort be vented before timing starts? A: Air left in the retort forms a steam–air mixture whose temperature is below the saturation temperature for that pressure, giving cold pockets so that the gauge pressure no longer guarantees the process temperature.
• Q: Why is overpressure used for retort pouches and glass jars? A: Internal pressure from steam and expanded headspace gas would burst the seal or break the glass, so air or water overpressure is applied to balance it during the hold and the cooling stage.
• Q: Why must cooling water be chlorinated? A: As the can cools, the internal vacuum can draw water through a micro-leak in the seam; chlorinated water (typically 1–2 ppm free residual) prevents post-process contamination, called leaker spoilage.
• Q: Why does UHT give better quality than in-can sterilisation for the same lethality? A: Spores have a small z (about 10 °C) and quality factors a large z (25–45 °C), so raising temperature speeds up spore destruction far more than it speeds up quality loss; the holding time can then be seconds instead of minutes.
• Q: A can is found swollen. How do you decide whether it is microbial or chemical? A: Incubate and plate samples — microbial swells give gas and pH/odour changes with viable organisms, while a hydrogen swell is odourless, occurs in acid foods after long storage, and shows tin and iron pick-up with internal corrosion.
• Q: What is a broken heating curve and how is it handled? A: Heat penetration that changes slope part-way through, usually when starch gelatinises and convection becomes conduction; it is treated with two slope indices f₁ and f₂ and the Ball calculation applied piecewise.
• Q: Why is the general method preferred over the formula method in validation? A: The general method integrates the measured lethal rate directly over the actual curve with no assumption about its shape, including the cooling phase, so it is the reference against which formula methods are checked.
• Q: What does a negative alkaline phosphatase test prove? A: That the milk received at least the minimum legal pasteurisation (72 °C/15 s or equivalent), since the enzyme's heat resistance is slightly greater than that of the target pathogens.

Common mistakes

• Treating commercial sterility as absolute sterility, or claiming the process kills all spores.
• Confusing D and z — writing z in minutes or D in °C — and mixing up which one appears in the exponent.
• Forgetting that the exponent in the lethal rate uses 121.1 °C, not 121 °C, and using the wrong z for a quality factor.
• Ignoring the lethality accumulated during come-up and cooling, which can be 10–30 % of F₀, so the schedule comes out over-processed.
• Placing the thermocouple at the can's centre for a thin liquid (convection) pack instead of one-third up the axis, and so measuring the wrong cold point.
• Assuming a measured process on a 400 g can transfers to a 3 kg can or a pouch — process time scales with container geometry and must be re-validated.
• Timing the process from the moment steam is turned on rather than from when the retort reaches the scheduled temperature (with the allowed come-up credit).
• Quoting pasteurisation as a sterilisation process, or applying a 100 °C process to a low-acid food such as plain peas, corn or mushrooms.

Related topics

Food microbiology and spoilage · Heat transfer: conduction, convection and unsteady-state heating · Evaporation · Food freezing and refrigeration · Novel food processing: extrusion, high-pressure processing and pulsed electric fields · Food packaging and shelf-life · FSSAI standards and HACCP.