Dairy Laboratory Testing

Compositional Testing

Mid-infrared spectroscopy (FT-IR)

The workhorse of routine dairy compositional testing. Instruments (FOSS MilkoScan FT3 / FT1 / FT+; Bentley FTS, etc.) measure fat, protein, lactose, total solids, urea, freezing point and 20+ other parameters in seconds per sample via mid-infrared absorption.

• Throughput: 100–240 samples/hour
• Accuracy: typically ±0.02% for fat after calibration
• Calibration: needs regular calibration against reference methods (Mojonnier, Kjeldahl) using certified samples
• Best for: raw milk reception, in-process standardisation, finished product release

Mojonnier method (gravimetric fat)

The reference method for fat content in milk and dairy products. Sample is digested with ammonia and ethanol, fat extracted with ether and petroleum ether, ether evaporated, and the residue weighed.

• Time: ~2 hours per sample
• Accuracy: ±0.01% (highest precision)
• Standards: ISO 1211 / IDF 1; AOAC 989.05
• Best for: calibration reference, dispute resolution, certified results

Röse-Gottlieb method (gravimetric fat for cream and powders)

Similar to Mojonnier but adapted for higher-fat samples (cream, butter, powders). Sample is dispersed in dilute ammonia, extracted with ether and petroleum ether, weighed.

Kjeldahl method (nitrogen / protein)

The reference method for protein. Sample digested with H₂SO₄ and catalyst, nitrogen converted to ammonia, distilled and titrated. Protein = nitrogen × 6.38 (milk-specific factor).

• Time: ~3–4 hours per sample
• Accuracy: ±0.05% protein
• Standards: ISO 8968 / IDF 20; AOAC 991.20
• Note: now largely replaced by Dumas combustion in many labs (faster, no toxic reagents)

Dumas combustion (alternative nitrogen method)

Sample combusted at >900°C; resulting nitrogen gas measured by thermal conductivity. Faster (~5 min/sample) and safer than Kjeldahl. Increasingly the lab standard.

Karl Fischer (moisture in dry products)

Reference method for moisture in milk powders, butter, cheese. Iodine titration in non-aqueous solvent reacts stoichiometrically with water. Accuracy ±0.05% moisture.

Oven drying (gravimetric moisture)

Sample dried at 102°C or 105°C to constant weight. Cheaper than Karl Fischer but slower. Good for routine use; not suitable for samples containing volatile components.

Microbiological Testing

Total bacterial count (TBC)

Coliforms / E. coli

Hygiene indicators; presence suggests post-pasteurisation contamination or insufficient pasteurisation. Methods: ISO 4832 (pour plate with VRBA), Petrifilm coliform/E.coli, MPN methods. Regulatory limit for pasteurised milk: typically <1 coliform per mL.

Pathogens

Somatic cell count (SCC)

Direct microscopy (DMSCC; ISO 13366-1) or flow cytometry (FOSS Fossomatic). Used to assess raw milk udder health; UK regulatory limit 400,000/mL geometric mean.

Pasteurisation Verification

Alkaline phosphatase (ALP) test

The standard regulatory verification. ALP is destroyed by proper pasteurisation; a negative test confirms successful heat treatment. Modern instrument: fluorimetric (Charm, Eclipse) gives results in minutes; pass criterion ALP < 350 mU/L (EU). See milk pasteurisation.

Lactoperoxidase (LPO) test (for higher-heat pasteurisation)

LPO is destroyed at slightly higher temperatures than ALP (~75°C/15s). A negative LPO test verifies higher-heat pasteurisation regimes.

Sensory Evaluation

Dairy sensory testing combines product-quality release with consumer-acceptance research:

• Trained panel — 6–12 staff trained on dairy off-flavours (oxidised, lipolytic, cooked, bitter, malty, etc.); scored on standard scales
• Triangle test — ISO 4120; determines if two samples differ statistically
• Duo-trio test — ISO 10399; reference vs unknown
• Quantitative descriptive analysis (QDA) — comprehensive sensory profiles for product development
• Consumer panels — 60+ untrained consumers for acceptance testing

Modern dairy labs include a dedicated sensory facility — separated booths, controlled lighting, ventilation, palate cleansers, and a kitchen for sample preparation.

Modern Molecular Methods

PCR (polymerase chain reaction)

Increasingly used for rapid pathogen screening. PCR-based methods (BAX System, FoodChek, Hygiena BAX) detect target DNA sequences in 24–48 hours vs 3–5 days for traditional culture methods. Particularly valuable for:

• Listeria in finished cheese
• Cronobacter in infant formula environment
• Salmonella across all products
• Verification of starter culture identity and viability

Mass spectrometry (MALDI-TOF)

Identifies microbial species from a single colony in < 30 minutes. Replaces biochemical confirmation tests for isolate identification. Major capital investment but very useful in plants with frequent isolations.

Whole-genome sequencing (WGS)

Increasingly used in outbreak investigation and supply chain root-cause analysis. Allows precise tracking of a specific strain across plants, products and time. Not yet a routine method but emerging.

Method Validation and ISO 17025

For labs supplying regulatory or contractual results, ISO/IEC 17025 accreditation is the gold standard:

• Method validation — demonstrate accuracy, precision, recovery, linearity, limit of detection against reference methods
• Proficiency testing — participate in inter-laboratory schemes (e.g. LGC, FAPAS) for ongoing performance verification
• Equipment calibration — traceable to national/international standards (NPL, NIST)
• Documented procedures — SOPs for every method, every instrument
• Internal quality control — control samples and Westgard rules to detect drift
• Audit trail — LIMS-based or paper-based but complete

Common Lab Issues and Their Causes

Frequently Asked Questions

What is FT-IR and why is it the standard for milk testing?

FT-IR (Fourier Transform Infrared spectroscopy) measures the mid-infrared absorption of a milk sample at multiple wavelengths and uses calibrated models to predict fat, protein, lactose, urea, freezing point and other parameters — all in about 30 seconds per sample. It is the standard because of speed (100+ samples/hour), accuracy (±0.02% on fat after calibration), and non-destructive measurement. The main instruments are FOSS MilkoScan series and Bentley FTS.

What is the difference between Kjeldahl and Dumas for protein?

Both measure nitrogen, which is converted to protein via the conversion factor (6.38 for milk). Kjeldahl uses acid digestion + distillation + titration (3–4 hours; uses toxic H₂SO₄). Dumas uses high-temperature combustion + thermal conductivity detection (5 minutes; no toxic reagents). Dumas is increasingly the lab standard; Kjeldahl remains the reference method for ISO/AOAC.

How fast is rapid pathogen testing?

Traditional culture-based pathogen detection takes 3–7 days. PCR-based methods (BAX, FoodChek) deliver results in 24–48 hours including the enrichment step. Newer immunoassay and biosensor methods can detect in 4–24 hours. For infant formula and chilled RTE products where shelf life is short, rapid methods are essential.

What does the alkaline phosphatase test prove?

Alkaline phosphatase (ALP) is a milk enzyme that is destroyed by proper pasteurisation. A negative ALP test (activity below 350 mU/L for EU compliance) confirms pasteurisation has been effective. ALP has a thermal-death curve similar to Mycobacterium tuberculosis, making it a reliable proxy for pathogen kill.

Do I need ISO 17025 accreditation?

It depends on what your lab's results are used for. ISO 17025 accreditation is required if you provide testing results to regulators, official authorities, or as part of contractual obligations (e.g. supplying major retailers). For internal QC and process monitoring it is not strictly required but is good practice for credibility. Accreditation is significant investment in documentation, calibration and proficiency testing.

What is environmental monitoring (EMP)?

A structured swabbing programme that tracks pathogens in the production environment, not in the product itself. The plant is zoned (Zone 1 = food contact, Zone 4 = exterior), and swabs are taken at frequencies proportional to risk. Listeria is the priority for chilled RTE products; Cronobacter for infant formula; Salmonella across the board. Trend analysis identifies harbouring niches before they cause product contamination.

What instruments do I need for a basic dairy lab?

Minimum: FT-IR (FOSS MilkoScan or equivalent), microbiology incubators (30°C, 37°C, 42°C), autoclave, basic glassware, pH meter, viscometer, oven for moisture, balance. For full QC: add fluorimeter (ALP), centrifuge, microscope, PCR (rapid pathogen screening), sensory booths, LIMS. For ISO 17025: add proficiency testing programme, calibrated reference standards, audit-grade documentation.

Further reading: John Watson publishes articles on dairy industry topics on LinkedIn. Browse all articles by John Watson on LinkedIn →

See related: Dairy quality control, HACCP, Milk pasteurisation, Milk grading, Milk powder & infant formula, Cheese making, Dairy due diligence, all dairy science information, consultancy services.