Retort Food Formulation: pH, Salt, Texture and Heat Penetration for Soups and Ready Meals

Retort food formulation: what makes a recipe retort-stable?

A retort-stable recipe comes through sterilisation with its safety, texture and flavour intact. Four recipe variables decide whether it will: acidity, salt, viscosity and the size and number of solid pieces. Each one changes how heat travels through the pack, and the speed of that heat travel sets the process time.

Good retort food formulation therefore treats the recipe, the pack and the process as one system, not three separate decisions. The law treats them the same way. In the US, consistency, headspace, fill weight and water activity are named as critical factors that must be written into the scheduled process for a low-acid food^[1]. In Great Britain, any heat treatment of food in hermetically sealed containers must raise every part of the product to a given temperature for a given time, and the process should follow an internationally recognised standard^[3].

This guide covers the formulation decisions. It is educational and does not replace validation by a qualified thermal process authority.

pH and acidity: low-acid vs acidified products

pH is the first design decision. A low-acid food has a finished equilibrium pH above 4.6 and a water activity above 0.85^[1]. Above that pH, spores of Clostridium botulinum can germinate and produce toxin in sealed, oxygen-free packs unless the process destroys them. Most soups, broths, curries and ready meals made with meat, vegetables, pulses or dairy are low-acid.

Why pH 4.6 matters

An acidified food is a low-acid food brought to a finished equilibrium pH of 4.6 or below by adding acid or acid ingredients^[2]. Because C. botulinum does not grow at or below that pH, acidified products can be given a milder heat treatment than full sterilisation. Acidifying a soup is possible, often with citric or lactic acid, but it changes the flavour and the pH has to be controlled and recorded batch by batch. The international Codex definition looks at every component, so large particulates must be acidified to the centre, not only the sauce^[11]. Most ready meals stay low-acid and rely on a full sterilising process.

Read our citric acid Q&A for what the acid does in food and how it is regulated.

Salt and sodium: flavour, labelling and UK targets

In a fully sterilised recipe, salt is there for flavour, not safety: the heat process, not the salt, makes the product shelf-stable. That makes sodium reduction feasible, but the flavour has to be rebuilt with acidity, herbs and spices, umami sources such as tomato, mushroom or yeast extract, or partial replacement with potassium salts. Taste the reduced-salt version after retorting, because heat can flatten or sharpen flavours.

For the UK market, salt on the nutrition declaration is calculated as sodium × 2.5^[4]. Public Health England’s voluntary salt reduction targets for 2024 set a sales-weighted average of 0.50 g salt per 100 g for soups (including canned and ambient soups, as consumed), with a maximum of 0.59 g, and an average of 0.60 g per 100 g for ready meals, with a maximum of 0.90 g^[5]. Retail buyers often use these figures in specifications, so it is worth formulating to them from the first prototype.

Texture: starches, hydrocolloids and vegetables after sterilisation

Heat, pressure and time break some thickeners down. Native starch can thin out or separate during a long cycle, while modified (cross-linked) starches and some hydrocolloids are chosen because they hold viscosity through the process. If your brand needs a clean label, see our guide to replacing modified starch and gums. Vegetables and pulses keep softening in the retort, so piece size and blanching must be judged on the retorted product, not the bench sample.

Viscosity and its effect on heat penetration

Viscosity and heat transfer are linked. A thin soup circulates inside the pack and heats mainly by convection, while a thick purée or a dense curry heats by conduction from the wall inwards, which is much slower. Raising viscosity can therefore slow the slowest-heating point enough to need a longer, harsher process. That is why US rules treat consistency as a critical factor of the scheduled process^[1], and why thickener choice is a safety decision as well as a texture one.

Heat penetration and F₀ lethality

F₀ expresses the lethality of a process as the equivalent number of minutes at 121.1 °C, using a z-value of 10 °C for C. botulinum spores^[6]. The minimum public-health process for low-acid foods is usually taken as F₀ 3 minutes, which corresponds to a 12-log (12D) reduction of proteolytic C. botulinum^[6][7]. That figure covers the pathogen only. The process authority also allows for spoilage organisms, including heat-resistant thermophiles^[6], so the F₀ in a scheduled process is set product by product and is often higher.

Heat penetration testing measures it. Thermocouples are placed in packs at the slowest-heating point, temperature is logged through come-up, hold and cooling, and lethality is calculated from that curve. US rules require scheduled processes for low-acid foods to be set by qualified persons with expert knowledge of thermal processing, using methods such as heat penetration data, thermal death time data and inoculated packs^[1].

Finding the slowest-heating point

The slowest-heating point, or cold spot, is where sterilisation has to be proven. In a can or jar heated in a stationary retort it is usually taken to be at the geometric centre for conduction-heating products and about one-third up from the base for convection-heating products^[8]. Particulates, headspace and pack shape can move it, so it is located by test, not assumed. Always probe the worst-case pack, not an average one.

Table 1. Methods used in retort food formulation.

Lethality vs quality: why process design matters

Heat destroys spores, but it also damages colour, flavour, texture and some nutrients. The z-value is the temperature change that alters a destruction rate tenfold. For C. botulinum spores it is about 10 °C^[6], and most quality changes are less sensitive to temperature than that. A hotter, shorter cycle that delivers the same F₀ can therefore protect quality, but only where heat reaches the centre quickly: in thin packs, liquids and agitated processes. In a thick, conduction-heating pack the outer layers overcook while the centre catches up, and there is an optimum process temperature that depends on the product’s thermal diffusivity, the pack dimensions and the target F₀^[10].

Particulates, fill weight and headspace

Large particulates slow the heat reaching the centre of each piece, so the largest piece, not the average one, sets the process. Headspace and fill weight matter as well: headspace affects agitation and pack behaviour, and overfilling slows heating. Minimum headspace and maximum fill or drained weight are named critical factors in the US rules^[1], and in pouches residual air must be controlled as a critical factor^[9]. Fix these early, because changing them later restarts the validation.

Common retort formulation failures and fixes

Illustrative example (not client data). A starch-thickened vegetable soup separates after sterilisation and heats slowly at the centre of the pouch. The fix is usually two-part: lower the starting viscosity with a more heat-stable thickener, then re-run heat penetration to confirm that the slowest-heating point still reaches the target lethality.

Table 2. Common failures, causes and fixes.

For a worked pilot trial on soups and broths, see our pilot retort trial worked example.

Pack format and retort type: how they change the recipe

The pack sets how far heat must travel. A retort pouch has a thin profile, so it heats faster than a can of similar volume. That allows a shorter process for the same lethality and less overcooking, with better colour, firmer texture and less nutrient loss^[9]. Pouches bring their own control points: thickness and residual air must be controlled, and pouches should be restrained during processing^[9]. Our guide to retort pouch materials covers the laminate layers and barrier properties.

Retort type matters as well. Agitating or rotating retorts move the packs during the cycle, which speeds heat transfer in liquid and particulate products, while static retorts rely on the pack’s own heat flow.

Table 3. How pack and retort choices affect the recipe.

UK and export rules that shape the recipe

In Great Britain, food businesses must run food safety procedures based on HACCP principles^[3], and operators heat-treating food in hermetically sealed containers must regularly check the main parameters, particularly temperature, pressure, sealing and microbiology^[3]. There is no UK equivalent of the US process-filing system, so the process is usually built to an internationally recognised code such as Codex CXC 23-1979 for low-acid and acidified low-acid canned foods^[11]. Labelling follows retained Regulation 1169/2011, enforced in England through the Food Information Regulations 2014^[4][12].

If the same recipe will also be sold in the US, the low-acid and acidified food rules in 21 CFR Parts 113 and 114 apply^[1][2]. Registration, labelling and HFSS rules for UK ready meals are covered in our guide to shelf-stable ready meal regulations in the UK.

From bench recipe to pilot retort trial

Before a pilot run, confirm pH, water activity and viscosity, fix particle size and fill, and choose the pack. A pilot retort trial then proves the process with real heat penetration data on the pack you will sell. This is where retort food formulation becomes a validated process, and where problems show up while they are still cheap to fix. If you are moving from a kitchen recipe, our recipe scale-up worked example explains how batch size changes the process, and our pilot retort trial batch guide explains what a trial batch involves.

Formulator’s note. Probe the worst-case pack. A trial that only logs the easiest-heating pouch can give a process that looks safe on paper but is not.

After the trial, a real-time or accelerated shelf-life study confirms that texture, flavour and pack integrity hold for the claimed life.

Key scientific takeaways

• Decide low-acid or acidified first: pH 4.6 and water activity 0.85 are the boundaries^[1][2].
• Viscosity controls heat transfer, so it is a safety variable as well as a texture one.
• Find the slowest-heating point with probes and set the process from measured lethality; F₀ 3 minutes is the public-health minimum, not the target^[6][7].
• A thin pack and a recipe built for fast heat transfer allow a shorter process that protects quality^[9].
• For the UK, formulate salt to the 2024 targets from the first prototype^[5].
• Test the recipe after retorting, not before.

Conclusion

Retort food formulation works when the recipe, pack and process are designed together. Decide low-acid or acidified first, control viscosity and piece size so heat reaches the slowest-heating point, and prove the result with logged heat penetration data before scale-up. Done well, a soup or ready meal keeps its safety, texture and flavour through its shelf life, and the brand avoids failed trials and costly reformulation.

For brands & formulators

Food Research Lab develops retort recipes and runs pilot retort trials at our pilot retort unit in Chennai, India, for brands selling into the UK and other markets. We build the formulation, run heat penetration and F₀ validation on pilot batches and hand over the process data for scale-up. Microbiological and incubation testing is run in-house or arranged through accredited partner laboratories.

See our retort soups and broths and ready-to-eat meals programmes, or our pilot and contract food manufacturing service. You can also contact us with your recipe.

In this series

• Are canned foods healthy?: what the evidence says on nutrients, sodium and BPA for UK shoppers.
• Retort pouch materials: laminate layers, barrier, microwave use and recyclability.
• Retort food formulation: pH, salt, texture and heat penetration for soups and ready meals (you are here).
• Shelf-stable ready meal regulations in the UK: registration, process controls and labelling.
• Pilot retort trial for soups and broths: a worked example from recipe to validated batch.
• Is citric acid bad for you?: what E330 does in food and how it is regulated.
• Pilot and contract food manufacturing: take your recipe to pilot and supply.

1. U.S. Food and Drug Administration. (2025). 21 CFR Part 113: Thermally processed low-acid foods packaged in hermetically sealed containers (§113.3 definitions; §113.83 establishing scheduled processes). Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-113 (cited for the low-acid definition, pH above 4.6 and a_w above 0.85; qualified persons; heat penetration data; critical factors including headspace, consistency and fill weight)
2. U.S. Food and Drug Administration. (2025). 21 CFR Part 114: Acidified foods (§114.3 definitions). Electronic Code of Federal Regulations. https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-114 (cited for the acidified-food definition, finished equilibrium pH 4.6 or below)
3. European Parliament and Council. (2004). Regulation (EC) No 852/2004 on the hygiene of foodstuffs, Article 5 and Annex II Chapter XI (assimilated law, Great Britain). legislation.gov.uk. https://www.legislation.gov.uk/eur/2004/852/annex/II/chapter/XI (cited for HACCP-based procedures and the heat-treatment requirements for food in hermetically sealed containers)
4. European Parliament and Council. (2011). Regulation (EU) No 1169/2011 on the provision of food information to consumers, Annex I point 11 (assimilated law, Great Britain). legislation.gov.uk. https://www.legislation.gov.uk/eur/2011/1169/annex/I (cited for salt = sodium × 2.5)
5. Public Health England. (2020). Salt reduction targets for 2024 (PHE publications gateway GW-1435). GOV.UK. https://www.gov.uk/government/publications/salt-reduction-targets-for-2024 (cited for the voluntary 2024 targets: soups 0.50 g average and 0.59 g maximum salt per 100 g; ready meals 0.60 g average and 0.90 g maximum)
6. Pflug, I. J. (1996). Discussion of the z-value to use in calculating the F₀-value for high-temperature sterilization processes. PDA Journal of Pharmaceutical Science and Technology, 50(1), 51–54. https://pubmed.ncbi.nlm.nih.gov/8846059/ (cited for z = 10 °C for C. botulinum, F₀ 3.0 minutes as the minimum public-health process, and thermophiles)
7. Food Standards Australia New Zealand. (2016). Safe Food Australia (3rd ed.), Appendix 3: Limits for food processes. https://www.foodstandards.gov.au/sites/default/files/publications/SiteAssets/Pages/safefoodaustralia3rd16/Appendix%203%20-%20Limits%20for%20food%20processes.pdf (cited for canning low-acid food at pH above 4.6: 121 °C for 3 minutes or equivalent, a 12D process for proteolytic C. botulinum)
8. Etzel, M. R., Willmore, P., & Ingham, B. H. (2015). Heat penetration and thermocouple location in home canning. Food Science & Nutrition, 3(1), 25–31. https://doi.org/10.1002/fsn3.185 (cited for the assumed cold-spot location: geometric centre for conduction heating, one-third from the base for convection heating)
9. Canadian Food Inspection Agency. (n.d.). Flexible retort pouch defects manual: Identification and classification. https://inspection.canada.ca/preventive-controls/flexible-retort-pouch/eng/1510257590976/1510257653803 (cited for the thin profile reducing heating time and overcooking, residual air as a critical factor, pouch thickness, and restraint during processing)
10. Hendrickx, M., Silva, C., Oliveira, F., & Tobback, P. (1992). Optimizing thermal processes of conduction heated foods: Generalized equations for optimal processing temperatures. Institution of Chemical Engineers Symposium Series, 126, 271–276. https://ciencia.ucp.pt/en/publications/optimizing-thermal-processes-of-conduction-heated-foods-generaliz/ (cited for optimal processing temperatures in conduction-heated foods depending on thermal diffusivity, z-value, geometry and target F₀)
11. Codex Alimentarius Commission. (1993). Code of hygienic practice for low and acidified low acid canned foods (CAC/RCP 23-1979, Rev. 2-1993). FAO/WHO. https://www.fao.org/input/download/standards/24/CXP_023e.pdf (cited for the international definition of a low-acid food, any component with pH above 4.6 and a_w above 0.85, and as the internationally recognised code for canned low-acid and acidified low-acid foods)
12. UK Government. (2014). The Food Information Regulations 2014 (SI 2014/1855). legislation.gov.uk. https://www.legislation.gov.uk/uksi/2014/1855/contents (cited for enforcement of Regulation 1169/2011 in England)