Propionic acid (CAS 79-09-4) occupies a narrow but operationally significant position in the U.S. chemical and animal nutrition supply chain. The molecule is a saturated C3 monocarboxylic acid with a molecular weight of 74.08 g/mol, a density of approximately 0.993 g/cm³ at 20°C, a closed-cup flash point of 54°C, and a pKa of 4.88 at 25°C. In commercial practice, the compound moves in bulk as a liquid, but its regulatory identity changes according to the destination. A food-grade shipment may enter under 21 CFR 184.1081 as a chemical preservative, while a feed-grade shipment may be received under 21 CFR 582.3081 as an animal feed preservative. The U.S. market for propionic acid is concentrated in grain preservation, silage additives, finished feed antimicrobial treatment, and broiler litter management, with auxiliary demand in food processing and chemical intermediates. Because U.S. trade statistics do not routinely separate food-grade from feed-grade propionic acid, import compliance cannot be inferred from aggregate shipment volume alone.

At the receiving dock, a feed mill typically offloads propionic acid from a 22,700 L or 26,500 L tank truck. The liquid is transferred through a 75 mm PTFE-lined hose into a high-density polyethylene storage tank. Sampling follows ASTM E300 for industrial liquid chemicals. In most terminals, the release decision is based on a certificate of analysis that cites the Food Chemicals Codex monograph for propionic acid, not solely on a sales specification. The FCC compendial values are used in commercial overlap between food and feed supply chains. The feed mill should retain the lot number, the bill of lading, the assay, and the supplier’s regulatory statement for the period required under 21 CFR 507.208. If the material is imported, the importer is responsible for filing prior notice under 21 CFR 1.276 and for maintaining Foreign Supplier Verification Program records under 21 CFR 1 Subpart L. Two physical properties drive most receiving decisions: the acid assay and the water content. Commercial release often uses percent assay by gas chromatography rather than acid value alone. The boiling point of 141.1°C allows distillative purification from aqueous streams, but feed-grade material is normally not redistilled at the feed mill.

What FSMA preventive-control obligations apply to a tanker of propionic acid entering a feed mill?

Under 21 CFR 507, propionic acid is received as an incoming raw material that can function as a processing aid, a preservative, or both. The feed mill’s preventive control plan must contain a hazard analysis for the receiving, pumping, blending, and purge steps. The main hazards are not biological but chemical: a non-food-grade lot entering the feed system, cross-contamination from shared lines, and dissolved metal carryover from corroded carbon steel components. The plan should require supplier approval for each producer or trading company. In an audited facility, the supplier approval record includes the foreign manufacturer’s food safety plan, a copy of the current audit, and the lot-level certificate of analysis. During offloading, the mill should compare the assay on the certificate with the purchase specification; a 99.5% minimum assay is a typical commercial release threshold for feed-grade material. The feed safety plan must define a quarantine procedure for off-spec product, including return-to-shipper or segregation in a dedicated tank.

Imported material is subject to the Foreign Supplier Verification Program. The FSVP importer must verify that the foreign supplier is in compliance with the applicable preventive control requirements. In practice, this means the importer holds a written FSVP record for each product line, including the propionic acid product type and the hazard analysis review. The record must include the date of verification, method of verification, and any corrective actions. Because propionic acid can be supplied from a producer that also manufactures esters and other organic acids, the FSVP audit should specifically evaluate whether shared tanks and transfer lines are cleaned between incompatible products. A shared line that previously carried acrylic acid or ester monomer, for example, may require a documented cleaning procedure and a solvent flush before propionic acid service. Without this, a feed mill cannot conclude that the shipment meets feed-grade purity. The U.S. Food and Drug Administration has not issued a separate feed-grade specification for propionic acid beyond the GRAS listing; therefore, the burden of setting release limits falls on the feed mill’s own hazard analysis and the supplier’s compendial data. For USDA organic feed programs, synthetic propionic acid is generally not permitted in certified organic rations. A mill producing both conventional and organic feeds must segregate or flush lines because residues can jeopardize organic certification.

Tariff Classification Under HTSUS 2915.50 and Country-of-Origin Verification

Bulk propionic acid is generally classified under HTSUS 2915.50, which covers propionic acid, its salts, and esters. The importer of record must determine the correct subheading at the time of entry. The General Rules of Interpretation and the Chapter 29 explanatory notes require that a product be classified as a separate chemically defined organic compound only when it meets the applicable purity and trace impurity criteria. When propionic acid is blended with formic acid, acetic acid, or buffered ammonium propionate, the resulting feed additive may be classified outside 2915.50 as a mixture. Such mixtures are common in the feed industry, and a CBP binding ruling should be requested before switching to a new blend. For pure propionic acid, the invoice should state the correct HTSUS number and the country of origin. The duty rate is not static; importers must consult the current U.S. International Trade Commission HTS release and CBP CROSS rulings because additional Section 301 duties may apply to a covered country of origin. A product simply transshipped through a third country does not undergo a change in origin unless the transformation is substantial. Repackaging or dilution in a bonded warehouse is not enough to change origin for marking or trade remedy purposes.

At the port, the minimum document set includes the CBP Form 7501, the commercial invoice, the packing list, the bill of lading, and the manufacturer’s certificate of analysis. For road transport, the shipping paper should identify the proper shipping name and UN number for propionic acid; UN 3463 is used for the acid material. The physical entry may be reviewed for hazard communication, but FDA prior notice is separate. In a shared container or tanker, the entry package must distinguish the propionic acid weight from other co-loaded chemicals. Failure to provide an accurate net weight or country of origin can result in a hold at the terminal and demurrage costs. Importers often request a binding ruling for feed blends containing propionic acid and sodium propionate because the tariff classification of a mixed salt may shift to a different subheading even though the feed function remains the same. Such a shift can affect the total duty burden and must be evaluated in advance of purchase. A free trade agreement claim requires a valid certificate of origin and proof that the material meets the applicable rule of origin. If the starting stock is imported, the rule of origin may not be met even when the finished acid is packaged in the exporting country.

When Stored Grain Moisture Exceeds 22%, Persistence of the Undissociated Fraction Becomes the Controlling Variable

The efficacy of propionic acid in grain is determined by the undissociated fraction of the acid at the grain surface, not by total acid concentration. The pKa of propionic acid is 4.88 at 25°C. Using the Henderson-Hasselbalch relationship, the undissociated fraction is 1/(1+10^(pH−pKa)). At pH 4.5, the undissociated fraction is approximately 71%; at pH 4.88, exactly 50%; and at pH 5.5, approximately 19%. Many cereal grain surfaces have a pH near 5.5 or higher, meaning that most of the applied acid is ionized and less membrane-permeant. As moisture rises, mold-spore hydration improves and the dose-response curve shifts upward. The result is a nonlinear increase in treatment rate at moisture values above 22%. A single application rate cannot be transferred from one lot of corn to another without pH and moisture data.

In feed mills, propionic acid is applied at the auger or conveyor. A positive-displacement pump with 316L stainless steel wetted parts and PTFE seals delivers acid to spray nozzles placed across a grain curtain. Typical residence time before the grain reaches storage is 20–60 seconds, which is usually sufficient to coat kernels but not enough for uniform diffusion into the kernel. Poor spray distribution is a more frequent cause of mold breakthrough than an inaccurate total dosage. Regular nozzle pattern checks, pump calibration, and acid flow verification should be performed at least once per shift. In terminals, the acid flow is interlocked with the grain mass flow to maintain the set kg/tonne ratio. A loss-of-flow alarm should be included in the control system. The area around the application point is ventilated to control acid vapors.

These ranges are commercial guidance rather than regulatory maximums. The actual storage life also depends on grain temperature, initial mold burden, mechanical damage, and uniformity of mixture. Published data for grain moisture above 28% is limited; at such moisture, acid application alone is generally not sufficient and must be combined with drying, cooling, or aeration. Because propionic acid can reduce palatability at higher doses, feed mills should validate the maximum treatment rate for the intended species. In poultry and swine rations, voluntary intake of treated grain may decline if the acid odor is not managed by buffering or by allowing post-treatment aeration. This observation is an operational boundary rather than a regulatory limit.

Assay methodology, storage metallurgy, and incoming acceptance limits

Release testing for propionic acid in feed applications is typically performed by gas chromatography with flame ionization detection. The sample is diluted with water and injected onto a polar or acid-deactivated column. The Food Chemicals Codex monograph is frequently referenced for assay acceptance at 99.5%–100.5% on an anhydrous basis. Water content is determined by Karl Fischer titration. A water level above the supplier’s stated limit will lower the assay and may indicate improper storage or contamination. Residue on ignition and aldehyde limits are part of compendial acceptance for food-grade material. In feed-grade service, a mill may accept a slightly lower assay if the material is consistently documented and the feed safety plan addresses the impurity profile. However, this lower assay should not be used as a label basis for active ingredient unless the label guarantee reflects the actual acid content.

Bulk storage vessels are made of high-density polyethylene or fiberglass-reinforced polyester, and transfer piping is usually 316L stainless steel. Carbon steel is incompatible because propionic acid forms iron propionate and darkens the product, which may reduce feed acceptance. Copper, brass, and zinc are also incompatible. The closed-cup flash point is 54°C; storage should be maintained below 40°C and away from strong oxidizers. Transfer must be bonded and grounded, and the storage vent should be connected to a caustic scrubber or a closed system to reduce odor and volatile organic emissions. The acid is hygroscopic and can freeze at approximately -21°C; winterized blend formulations with ammonium propionate are used where low-temperature handling is required.

Analytical interferences arise from ester peaks if the supplier uses shared production equipment. A gas chromatographic run with a temperature ramp can separate propionic acid from acetic acid and propionaldehyde. If the supplier also produces methyl propionate or ethyl propionate, the receiving mill should include an ester screen in the release specification. Batch-to-batch variance in odor, color, or assay should be investigated before use. The feed safety plan should define a hold time for suspect shipments and require a retained sample. Retained samples are stored in airtight containers at room temperature for the same period as the lot record under 21 CFR 507.208.

State feed law enforcement adds a further compliance layer that is separate from FDA import clearance. Propionic acid is sold in many states as a feed ingredient or feed preservative, and the product label must follow the AAFCO Official Publication. The label should state the active ingredient content and the recommended use rate. A feed mill that resells treated grain or finished feed may need to register the product in the destination state and retain a copy of the propionic acid certificate of analysis. For a product received as food-grade under 21 CFR 184.1081, the feed mill still must meet any applicable state feed rule when the material is used in animal food. Because no single federal document covers all state feed requirements, distributors generally maintain a compliance package that includes the CBP entry summary, the FDA prior notice confirmation, the FSVP record, the FCC assay, and the AAFCO label statement. In all cases, the lot number is the operational link between the tariff entry and the feed safety record.