Guidelines for Storing and Transporting Milk Samples

Every milk sample collected at a farm or processing facility carries one critical expectation: it must reach the laboratory in exactly the same condition as when it left the source. This sounds straightforward, but in practice, milk is a biologically active matrix – it contains living microorganisms, active enzymes, and chemically reactive components that begin to change the moment sampling is complete. Poor storage or careless transportation does not just degrade the sample; it can produce results that misrepresent the original product entirely, leading to wrong decisions about milk safety, quality, and compliance. Following established guidelines for storing and transporting milk samples is, therefore, a non-negotiable part of any quality assurance programme.

Why milk samples change after collection 🔗

Milk is not a stable substance sitting quietly in a bottle. From the moment it is collected, several processes compete to alter its composition. Understanding these processes is the first step to controlling them.

Microbial activity 🔗

Bacteria present in raw milk multiply rapidly at room temperature. According to the FAO/WHO Codex Code of Hygienic Practice for Milk, the milking procedure and subsequent storage of milk carry real risks of microbial contamination and growth of inherent pathogens. Even without additional contamination, the native bacterial population in a collected sample can shift dramatically within a few hours if temperature is not controlled. A sample tested hours late may show a bacterial count that is exponentially higher than was present at the time of collection – giving a completely false picture of initial milk quality.

Enzymatic changes 🔗

Milk contains endogenous enzymes such as lipase and protease that continue to act on fat and protein fractions after collection. These enzymatic reactions can alter fat content readings, degrade protein fractions, and change the apparent somatic cell count. The problem is compounded by temperature: warmth accelerates enzyme activity, meaning even a short delay at ambient temperature can introduce measurable changes in composition.

Evaporation and moisture absorption 🔗

Milk samples are primarily water-based, which makes them vulnerable in two directions. Exposure to heat or dry air causes evaporation of the aqueous phase, concentrating solids like fat, protein, and minerals – and inflating their measured values. Conversely, in humid environments, hygroscopic samples such as dry milk powders absorb moisture from the air, altering their weight and apparent moisture content. Both effects produce results that do not reflect the original product.

Oxidation 🔗

Exposure to oxygen triggers oxidative reactions that affect fat-soluble vitamins, lipid fractions, and certain flavour compounds. While oxidation is less immediately dramatic than microbial growth, it can meaningfully distort the chemical profile of samples intended for compositional analysis, particularly for fat quality assessments.

Core principles of milk sample storage 🔗

The overarching goal of storage is to keep the sample as close as possible to its state at the time of collection. ISO 707 | IDF 50, the internationally recognised standard for sampling of milk and milk products, provides the foundation for these protocols – and its central message is that samples must be protected from conditions that cause any of the changes described above.

Temperature control 🔗

Temperature is the most powerful lever available. Refrigeration slows both microbial multiplication and enzymatic reactions significantly. For liquid milk samples, storage at 1-4°C is standard practice. Collected raw milk should be maintained below 4°C in insulated conditions to prevent spoilage, and this principle applies equally to laboratory samples. Where refrigeration is not immediately available, the sample must be transported to cold storage as quickly as possible. Freezing at −20°C is used for longer-term storage, though it must be applied carefully – freezing can affect the viability of certain microorganisms and alter some compositional parameters if freeze-thaw cycles are not controlled.

Airtight, inert containers 🔗

Containers must be sealed to prevent evaporation and to block environmental moisture and oxygen. They must also be chemically inert – equipment and containers used for milk storage must be made of smooth, nonabsorbent, corrosion-resistant, and nontoxic materials that will neither absorb components from the sample nor leach substances into it. Glass is frequently preferred for chemical analysis samples; certain food-grade plastics are also acceptable when their compatibility with the analysis method has been confirmed.

Sample labelling and documentation 🔗

Each sample must be clearly labelled at the point of collection with identifying information, collection time, and the intended analysis type. This is not merely administrative – it determines which storage protocol applies. A bacteriological sample and a chemical analysis sample from the same batch may require different conditions, and mislabelling or delayed labelling creates the risk of applying the wrong protocol. Procedures for test sample preservation, transport, and storage should be documented, strictly standardised, and controlled in accordance with quality management frameworks such as ISO/IEC 17025.

Storage requirements by sample type 🔗

Not all milk samples have the same storage needs. The type of analysis planned directly determines what conditions are required.

Bacteriological samples 🔗

These samples require the strictest temperature control because microbial populations are inherently unstable. Even under refrigeration, bacterial communities shift over time. Research has shown that without preservatives, unrefrigerated samples experience a drastic decrease in microbial diversity and a significant shift in community structure within just five days. The practical recommendation is to analyse bacteriological samples as soon as possible after collection – ideally within 24 hours. When this is not achievable, cold storage at 1-4°C is mandatory, and the delay must be minimised.

Chemical analysis samples 🔗

Samples intended for compositional analysis – fat, protein, lactose, total solids – are primarily threatened by enzymatic changes and evaporation rather than microbial shifts. Refrigeration is required, though the temperature tolerance is slightly broader than for bacteriological work. Chemical preservatives are frequently added to inhibit microbial growth and stabilise composition during transit. The most commonly used preservatives for this purpose include potassium dichromate, sodium azide, and bronopol, each acting through different mechanisms to slow microbial and chemical degradation. Critically, preservative concentration must be carefully controlled – studies have shown that preservative concentration has a significant effect on the results of laboratory analyses, meaning both under- and over-dosing can produce erroneous results.

Dry milk and powdered samples 🔗

Dry milk, skimmed milk powder, whey powder, and similar products have a unique vulnerability: moisture. These samples must be stored in airtight, sealed containers in low-humidity environments to prevent moisture absorption. Even small amounts of absorbed water can alter the apparent moisture content, affect flowability, and change the activity of any residual enzymes. Unlike liquid samples, temperature elevation is less immediately damaging for dry samples – but humidity control is non-negotiable.

Preservatives: extending sample stability 🔗

Raw milk undergoes rapid microbial and enzymatic changes after collection, and delayed transportation or testing can compromise laboratory analyses. Chemical preservatives are therefore added to extend holding time under refrigeration. The two most widely used are azidiol and bronopol.

Azidiol is a combination of sodium azide and chloramphenicol. It exerts bacteriostatic activity across a broad spectrum and has been widely validated for milk compositional work. However, azidiol carries recognised toxicological risks – sodium azide is a potent mitochondrial toxin, and chloramphenicol is associated with idiosyncratic aplastic anemia – which has prompted research into safer alternatives.

Bronopol (2-bromo-2-nitropropane-1,3-diol) works through oxidative stress on bacteria. It is gaining popularity in milk preservation due to its antimicrobial activity against a broad range of microorganisms and its comparatively lower toxicological profile. It is particularly useful for preserving somatic cells before analysis.

The key principle governing preservative use is that the chosen preservative must not interfere with the analysis itself. The minimum requirement for a chemical milk preservative is that it must ensure the testability of the sample – meaning the sample must maintain its original composition from milking to analysis, and the preservative must not affect the test outcome.

Transportation guidelines 🔗

Transportation introduces risks that static storage does not. Temperature fluctuations, physical agitation, vibration, and delays in transit can all accelerate the deterioration processes described above. Effective sample transport requires that the cold chain be maintained without interruption from the point of collection to receipt in the laboratory.

Cold chain maintenance 🔗

Dairy samples must be refrigerated or kept cold throughout the entire handling process. Insulated transport boxes with ice packs or dry ice are used for short-distance transfers; refrigerated vehicles are required for longer routes. The transport container should maintain samples at 1-4°C for liquid milk and bacteriological samples. Temperature monitoring during transport – using data loggers or indicators – is good practice for any situation where transit time exceeds two hours.

Physical protection 🔗

Containers must be secured against spillage and physical shock. Agitation can disrupt the fat phase of liquid milk, potentially affecting subsequent compositional readings. For samples in glass containers, padding and secondary containment are required. All containers must remain upright and sealed throughout transit.

Time in transit 🔗

Even under optimal cold-chain conditions, time is a limiting factor. It is advisable to store milk samples with a preservative at 4°C and to carry out analyses within the first 48 hours of sampling. For bacteriological samples, the standard recommendation is analysis within 24 hours of collection. Longer transit times require stronger justification and enhanced preservation measures.

The principle of representativeness 🔗

All storage and transportation protocols exist to serve a single underlying objective: the sample that arrives at the laboratory must still be representative of the batch from which it was drawn. This concept is central to ISO 707 | IDF 50, which emphasises that sampling is an operation that requires most careful attention, and that the necessity of obtaining a properly representative sample cannot be overstated. Any departure from recommended storage or transport conditions risks making the sample unrepresentative – and an unrepresentative sample produces results that cannot be used to make valid decisions about product quality, safety, or compliance.

This is why even seemingly minor lapses – a few hours at ambient temperature, an inadequately sealed container, a single freeze-thaw cycle without documentation – can invalidate the entire analytical effort. The FDA’s Grade A Pasteurized Milk Ordinance covers production, transportation, processing, handling, sampling, and examination of milk and milk products as a unified chain, precisely because no single link can be treated as independent from the others.

When immediate analysis is not possible 🔗

In many real-world settings – particularly in regions where farms are distant from testing facilities – immediate analysis is not feasible. In these situations, the protocol must shift from ideal to best-achievable. This means adding an appropriate preservative at the time of collection, sealing the container correctly, placing it in cold storage without delay, and documenting the time elapsed between collection and analysis. Any deviation from the standard conditions should be noted and considered when interpreting the results.

Freezing at −20°C can extend the holding time for chemical analysis samples, but it must be applied consistently. Freezing milk samples may affect the viability of some coliforms, psychrotrophic microorganisms, or bacteria belonging to the genus Mycoplasma, which makes frozen storage unsuitable for bacteriological enumeration without careful validation. For compositional analysis, however, frozen storage with a validated preservative can maintain sample integrity for several weeks when necessary.

What do you think? Given that bacteriological and chemical analysis samples require different storage conditions even when collected from the same batch, how should dairy laboratories communicate these requirements to farm-level sampling staff who may handle both types simultaneously? And in regions where refrigerated transport is limited, which combination of preservative and storage approach do you think offers the best balance between practicality and sample integrity?

References

1. https://www.fao.org/fileadmin/user_upload/livestockgov/documents/CXP_057e.pdf
2. https://www.iso.org/standard/37882.html
3. https://wiseias.com/raw-milk-collection-transportation/
4. https://law.lis.virginia.gov/admincode/title2/agency5/chapter531/section70/
5. https://cdn.standards.iteh.ai/samples/72825/7cfc4b2daab24e54ab968b82ea4691a9/ISO-21187-2021.pdf
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC7142718/
7. https://www.sciencedirect.com/science/article/abs/pii/S0958694616301790
8. https://www.sciencedirect.com/science/article/abs/pii/S0958694625003565
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC6542876/
10. https://www.drink-milk.com/wp-content/uploads/2025/01/Dairy-Handling-Guide-for-Food-Banks-2024.pdf
11. https://pubmed.ncbi.nlm.nih.gov/24103551/
12. https://cdn.standards.iteh.ai/samples/37882/0bfcc2f1e15f49e49bdd18e9d641651d/ISO-707-2008.pdf
13. https://www.ncbi.nlm.nih.gov/books/NBK221549/