A mineral acid vs organic acid preservation: total cost comparison for stored grain, silage, and aqueous feed matrices must account for acid equivalent mass, antimicrobial activity at target pH, corrosion rate in dosing and storage circuits, vapour control, and disposal of residual anions. Propionic acid (CAS 79-09-4, CH₃CH₂COOH, molecular weight 74.08 g/mol, pKa 4.88) functions as both a pH depressant and a cell-permeable antimycotic. Its undissociated fraction at preservation pH 5.0 is approximately 43%, whereas sulphuric acid, hydrochloric acid, and phosphoric acid are almost fully dissociated at the same pH and contribute protons without the neutral lipophilic acid reservoir that crosses fungal membranes.

What Does Undissociated Acid Fraction Mean for Preservation Cost?

Organic acid efficacy is governed by the Henderson-Hasselbalch partition between protonated and dissociated species. For propionic acid at pH 5.0, the dissociated:undissociated ratio is 1.3:1, leaving 43% as the membrane-permeable neutral acid. At pH 4.0, the undissociated fraction rises above 88%. The neutral acid diffuses into the fungal cytosol, dissociates at intracellular pH near 7.2, releases protons, and accumulates propionate anions that interfere with acetyl-CoA metabolism. Mineral acids cannot provide this anion-specific inhibition; their preservation effect depends solely on bulk pH reduction. Because many storage fungi tolerate pH values down to 2.0–3.0, a mineral acid programme must depress the substrate pH far lower than an organic acid programme for equivalent mould suppression, increasing proton loading and corrosion burden on carbon steel and 304L stainless contact surfaces.

In high-moisture maize storage at 25% moisture, propionic acid-based preservatives are metered into a continuous grain auger at 0.5–1.0% w/w. The acid is sprayed through flat-fan nozzles with polypropylene or 316L stainless steel wetted parts; a positive-displacement diaphragm pump with mass flow feedback controls dosing. Mineral acid programmes for the same grain moisture require pre-dilution to 10–20% w/w to prevent starch hydrolysis and local heating. This dilution adds water and can increase grain moisture by 1–3 percentage points depending on throughput. The extra water reduces storage stability independently of pH and must be removed by aeration or blending with dry grain. Propionic acid has a density of 0.993 g/cm³ at 20 °C and a viscosity near 1.0 mPa·s, allowing direct injection without pre-dilution. Published data for specific dosing accuracy on production grain augers is limited; however, manufacturer pump selection curves place propionic acid within the operating envelope of standard diaphragm and peristaltic heads at feed rates below 2,000 L/h.

Corrosion Coupons in 304L and 316L Circuits Show Mineral Acid Handling Penalties.

In mineral acid service, material engineers use ASTM G31-72 immersion tests to define safe concentration-temperature envelopes. Supplier isocorrosion diagrams place 316L stainless steel outside the acceptable range for sulphuric acid above 5% at 25 °C; 304L is more severely attacked. Hydrochloric acid promotes chloride pitting of 304L and 316L at chloride levels as low as 20 mg/L when oxygen is present. Propionic acid at 25 °C is handled in 316L stainless steel, high-density polyethylene, or polypropylene; carbon steel is limited to short transfer lines with water content below 0.1% and temperature below 30 °C. The mineral acid route forces capital into acid-brick-lined concrete bunds, fluoropolymer-lined pipe, and scrubber systems, while propionic acid can use standard food-grade 316L tanks with closed-loop vents. Maintenance intervals on mineral acid dosing sets in feed mills are commonly shortened to 6–12 weeks for seal replacement; propionic acid diaphragm pumps on grain preservatives typically operate for a full season without diaphragm rupture if EPDM or Kalrez elastomers are specified. A single acid brick lining failure in a sulphuric acid sump creates concrete spalling, iron sulphate fouling, and pH neutralisation costs that do not occur with propionic acid storage.

The proton equivalent cost of propionic acid is 74.08 g/mol; sulphuric acid supplies 2 equivalents at 98.08 g/mol, yielding 49.04 g/equivalent. Hydrochloric acid yields 36.46 g/equivalent, and phosphoric acid has 97.99 g/mol but its first pKa of 2.15 means that only the first proton is fully available at preservation pH values near 4.5–5.5. Price per acid equivalent at point of delivery favours mineral acids. However, antimycotic action in propionic acid does not scale with proton equivalents alone. At pH 5.0, propionic acid retains 43% undissociated species; the same pH is achieved by a mineral acid with no neutral lipophilic reservoir. Because many spoilage moulds remain viable down to pH 2.0–3.0, preserving with mineral acid requires a lower target pH and a nonlinear increase in acid requirement as the grain buffering capacity is overcome. Bulk acid spot prices are regionally variable and contract-linked to feedstock benchmarks. If a mineral acid price per tonne is set at 100 index units, propionic acid is frequently 4–8 times higher per metric ton of product. The parity calculation changes at the point of use because the active preservation dose of propionic acid is lower and metal loss, liner replacement, and waste neutralisation costs are lower. Total cost of ownership analyses from chemical delivery distributors often use 5-year net present value, a discount rate of 8–10%, and maintenance cost multipliers of 2–3 for mineral acid service relative to organic acid service.

Vapour Pressure, Odour, and Exhaust Gas Scrubber Costs

Propionic acid has a boiling point of 141.1 °C and a vapour pressure near 0.32 kPa at 20 °C. Bulk storage tanks are equipped with pressure/vacuum vents and weak alkaline scrubbers to manage the pungent headspace. Hydrochloric acid at 37% concentration evolves hydrogen chloride gas, requiring mist eliminators and packed-bed acid scrubbers; its vapour pressure is orders of magnitude higher than propionic acid at equal temperature. Sulphuric acid has low vapour pressure at ambient temperature, but its hygroscopicity forms dilute acid films that corrode tank roofs and requires nitrogen blanketing in closed systems. The vent system for a 25,000 L hydrochloric acid day tank typically includes a scrubbing column with caustic circulation pump and continuous pH monitoring; the equivalent propionic acid tank can use a closed-loop return line to the delivery vehicle with a simple carbon scrubber. Odour complaints around grain storage facilities are more severe for propionic acid than for mineral acids, but this is offset by the absence of strong acid mist and chlorine gas exposure risk. Industrial hygiene monitoring under 8-hour TWA exposure limits requires ventilation of the transfer station; hydrochloric acid additionally requires dedicated local exhaust at the scrubber discharge because of the lower vapour density of HCl.

When Phosphate and Sulphate Anions Create By-Product Disposal Costs

Where mineral acids are used for preservation, the anionic load remains in the product or wastewater after neutralisation. Sulphuric acid reacts with lime to form calcium sulphate dihydrate, which deposits in pH adjustment sumps and spray nozzles; the solubility of gypsum is approximately 2.4 g/L at 25 °C, so pH-neutralised batch tanks can exceed saturation and require mechanical descaling. Phosphoric acid leaves orthophosphate residue and may exceed discharge permits for total phosphorus, especially in watersheds with sensitive nutrient limits. Propionic acid is metabolised through the citric acid cycle; its chemical oxygen demand is 1.51 g COD/g acid, which is treatable in conventional biological wastewater plants. The disposal burden of a mineral acid preservation programme includes tank clean-out wastewater with high dissolved solids, neutralisation sludge, and permitted discharge monitoring; an organic acid programme generates BOD load but no inorganic scale or phosphatic sludge. For a plant preserving 10,000 t of high-moisture grain, mineral acid neutralisation sludge from a sulphuric acid programme can require vacuum filtration and off-site disposal, while propionic acid residues are compatible with standard mill hygiene wash-down and biotreatment.

The following data are used to establish the acid-equivalent and undissociated-species input parameters for preservation cost models. Undissociated fractions are calculated from the Henderson-Hasselbalch equation at pH 5.0 and 25 °C.

Regulatory Recognition and Residual Limits in Feed and Food Preservation

Propionic acid is listed as a generally recognised as safe substance in 21 CFR 184.1081 for food use when used as a chemical preservative under GMP. In the European Union, propionic acid and propionates are authorised as feed additives under Regulation (EC) No 1831/2003 in the technological category of preservatives without a maximum residue limit for the acid itself. Mineral acids are not authorised as feed preservatives in the same manner; sulphuric acid and hydrochloric acid are processing aids with no preservative claim and their residues must be removed or neutralised before feed sale. Phosphoric acid is used as an acidifier in feed, but its role is pH reduction rather than antimycotic activity and it contributes to phosphorus intake. The regulatory difference adds cost to mineral acid programmes because they may not be labelled as mould-inhibiting preservation treatments in finished feed unless combined with an organic acid or dry propionate salt.

In silage preservation, propionic acid is applied at 0.2–0.5% w/w on wilted forage at the harvester through variable-speed peristaltic pumps with EPDM or polyurethane flexible elements. The organic acid reduces aerobic spoilage after silo opening by penetrating yeast cells in the aerobic zone. Mineral acid injection into silage is uncommon because strong acid reacts with plant fibre and accelerates protein hydrolysis, and chloride or sulphate residues depress voluntary dry matter intake. Total cost for silage therefore includes the opportunity cost of dry matter loss and re-heating at the feed bunk; organic acid preservation reduces mould and heating, while mineral acid pH depression does not provide the same yeast inhibition at safe inclusion rates.

In a feed mill preserving high-moisture maize, the total cost comparison typically includes the initial acid purchase, the amortised cost of storage and dosing hardware, the maintenance interval for elastomers and nozzles, and the waste treatment load. Propionic acid at 0.5–1.0% w/w on a 10,000 t lot consumes 50–100 t acid; a mineral acid programme targeting the same visible absence of mould may require pH depression below 3.0, incurring secondary costs in palatability, corrosion, and neutralisation. Published data for full-scale mineral acid grain preservation trials is limited because mineral acids are not registered for that use in many jurisdictions. For aqueous feed biosecurity, propionic acid is fed at 0.1–0.3% w/w of liquid feed; lactic acid and formic acid are blended to lower pH and improve palatability. The operational boundary for propionic acid is pre-drying of grain to 20–25% moisture before treatment; above 30% moisture the acid demand becomes nonlinear and storage life drops even with treatment. Propionic acid should not be combined with hypochlorite oxidants or strong bases in the same dosing line, because exothermic neutralisation and chloroform generation may occur. Dosing lines should be flushed with low-moisture grain after each batch to avoid acid condensing in dead legs and attacking carbon steel spools.