Cold storage for food testing labs serves two distinct functions that each carry independent compliance obligations: preserving the physical and chemical integrity of test samples from receipt through analysis, and maintaining reference microbial cultures in a state of verified, stable identity for use as quality controls. Conflating these two functions — storing working cultures alongside food matrices, or applying a single temperature protocol to sample types with different stability requirements — is a recurring source of nonconformances under ISO/IEC 17025 accreditation audits. Getting both right requires understanding the temperature, segregation, and documentation requirements that apply to each.

Temperature requirements for cold storage of food samples in testing labs

Food testing laboratories receive samples across a wide range of matrices and stability profiles, and the storage conditions applied from the moment of receipt determine whether those samples remain fit for the purpose of analysis. ISO/IEC 17025:2017, the international standard governing laboratory competence, requires laboratories to establish procedures for receiving, handling, protecting, storing, retaining, and disposing of test items in a manner that prevents deterioration, contamination, or loss of identity. For food samples, this requirement has direct temperature implications defined by sample type.

The 0°C to 4°C range for perishable samples reflects AOAC International guidelines for laboratories performing microbiological and chemical analyses of food, which specify that perishable foods should be held under refrigeration and tested as quickly as practical after receipt. Holding temperature excursions — a sample arriving refrigerated but warming during transit or during extended check-in — must be documented and evaluated for impact on result validity. Samples that cannot be confirmed to have remained within specification from collection to receipt should be flagged in the sample receipt record before analysis proceeds.

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Frozen sample storage at −18°C or below is consistent with FDA validation requirements, which specify that frozen food matrices used in method validation studies should be aged for a minimum of two weeks at −20°C. Laboratories using frozen matrices as representative challenge materials must verify that the storage unit maintaining those matrices holds at or below −18°C at all times, including during defrost cycles on units equipped with automatic defrost — a feature that can introduce brief but repeated excursions if the defrost cycle is not validated.

Reference culture cold storage: stock systems, passage limits, and temperature tiers

Reference microbial cultures in food testing laboratories — including strains obtained from the American Type Culture Collection (ATCC) or National Collection of Type Cultures (NCTC) — require a tiered cold storage approach that is categorically different from sample storage. The purpose of reference culture storage is not simply preservation but maintenance of verified genetic and phenotypic identity. Passage through successive growth cycles introduces risk of genotypic drift, phenotypic change, and loss of the strain characteristics that make a reference culture valid for use as a quality control organism.

Standard practice, consistent with ATCC guidance and ISO 11133:2014 on the preparation, production, storage, and performance testing of culture media, organizes reference cultures into three levels:

• Primary stock: The original culture received from the reference collection, stored long-term at −130°C or below in liquid nitrogen vapor, or at −70°C to −80°C for short-term interim use only — ATCC specifies that viability at this temperature range is adequate for 1 to 5 days but will decline with extended storage above −130°C. The primary stock should never be used directly for testing — it exists solely to regenerate working stocks.
• Working stock: Prepared from one thaw of the primary stock with the minimum number of passages required to produce sufficient material. No more than five transfers should be made from the original ATCC or NCTC source culture; beyond this threshold, the risk of accumulated phenotypic drift renders the culture unreliable as a quality control reference. Working stocks are typically stored in glycerol broth at −50°C to −70°C.
• Daily use culture: Prepared fresh from the working stock for each testing day or testing batch. Daily cultures should not be retained beyond the scope of the work for which they were prepared.

Each tier must be stored in dedicated, clearly labeled vessels in a temperature-monitored unit, with access restricted to trained personnel. Mixing reference culture vials with food sample matrices on the same freezer shelf or in the same drawer creates contamination risk in both directions and is a direct non-conformance under ISO/IEC 17025 requirements for preventing cross-contamination and maintaining item identity.

Physical segregation and contamination controls in food testing lab cold storage

The risk of cross-contamination in a food testing laboratory cold storage environment is not limited to microbiological transfer. Chemical carryover — volatile compounds from highly concentrated extracts contaminating adjacent samples, or residual cleaning agent from a freshly washed container affecting a sensitive matrix — can compromise results without any microbiological event occurring. Physical segregation within cold storage units is the primary control for both categories of risk.

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Practical segregation requirements for food testing lab cold storage include:

• Separate refrigerator or dedicated shelving for reference cultures, segregated from food matrices at all times
• Dedicated storage for samples awaiting analysis, samples under analysis, and retain samples, with clear labeling distinguishing each status
• Reagents and standards stored separately from test items, not on shared shelves where spills or volatilization could affect adjacent samples
• Incoming samples held in a designated quarantine zone within the refrigerator until receipt paperwork is complete and condition is confirmed — never placed directly into active sample storage on arrival
• Retain samples stored in sealed, clearly labeled containers for the retention period specified in the laboratory's quality management system or the client's contract

Temperature excursions in sample storage refrigerators must be recorded and investigated as deviations. Automated monitoring systems with documented alert thresholds and escalation protocols provide the continuous record needed to demonstrate that samples remained within specification throughout the retention period — a requirement that paper-based temperature logs checked once per shift cannot reliably satisfy.

Cold storage documentation and chain-of-custody requirements under ISO/IEC 17025

Sample integrity in food testing labs is not only a physical question — it is a documentation question. ISO/IEC 17025:2017 requires that every test item be uniquely identified, and that its condition at receipt be recorded along with any deviations from specified storage or handling conditions. This chain-of-custody record must follow the sample from arrival through disposal, and must be sufficient to allow reconstruction of the sample's storage history in the event of a disputed result or regulatory inquiry.

For cold storage specifically, the documentation requirements extend to the storage unit itself. Temperature logs must be retained for a period consistent with the laboratory's record retention policy and any contractual or regulatory requirements that apply to the work performed. Where clients or regulators specify minimum retention periods — common in food safety testing performed under the Food Safety Modernization Act, for instance — the laboratory must ensure its monitoring and record-keeping infrastructure meets those timelines.

Reference culture documentation carries additional requirements. Each reference strain in the laboratory's collection should have a corresponding record capturing its source, lot number, passage history, storage location, storage condition, and the date and result of each performance test conducted to verify its identity and viability before use. This traceability record is examined during ISO/IEC 17025 accreditation assessments and is often the first document requested when an auditor questions whether a quality control organism used in a method was fit for purpose. Understanding which freezer design and temperature class best supports long-term culture viability is a practical starting point for labs building out or upgrading their reference culture cold storage infrastructure.

Conclusion: cold storage for food testing labs

Cold storage for food testing labs must satisfy two parallel sets of requirements simultaneously — one governing the integrity of test samples from receipt to result, and one governing the stability and traceability of reference cultures used as quality controls throughout the testing process. ISO/IEC 17025:2017 provides the framework, but the practical controls — temperature tiers matched to sample type, physical segregation between sample classes, passage discipline for reference cultures, and continuous temperature monitoring with documented chain of custody — are the mechanisms that make compliance achievable and defensible under audit. Laboratories that treat cold storage as logistics rather than a quality function find that gap during accreditation assessments.

References

1. AOAC International. AOAC Laboratory Accreditation Guidelines for Laboratories Performing Microbiological and Chemical Analyses of Food, Dietary Supplements, Pharmaceuticals, and Cannabis (ALACC). 7th ed. AOAC International, 2024. https://www.aoac.org/aoac-accreditation-guidelines-for-laboratories-alacc/
2. American Type Culture Collection. Bacteriology Culture Guide. ATCC. https://www.atcc.org/resources/culture-guides/bacteriology-culture-guide
3. U.S. Food and Drug Administration. Laboratory Manual of Quality Policies: ISO 17025 Requirements. FDA Office of Regulatory Affairs. https://www.fda.gov/science-research/field-science-laboratory-manual/laboratory-manual-quality-policies-iso-17025-requirements

This article was created with the assistance of Generative AI and has undergone editorial review before publishing.