Non-fat milk solids (SNF) determination is a fundamental analytical test for evaluating the compositional quality, authenticity, and regulatory compliance of milk and dairy products. This technical article explains the definition and significance of solids-not-fat, the reference and routine testing methods—including gravimetric oven drying, Rose-Gottlieb fat analysis, and rapid instrumental techniques—together with applicable standards such as ISO 6731, ISO 1211, ISO 488, and GB 5009.239, as well as typical sample types and industrial applications.
Technical Overview
Non-fat milk solids (SNF, also known as solids-not-fat) represent the total solid constituents of milk remaining after the fat fraction is removed. SNF principally comprises lactose, milk proteins (caseins and whey proteins), minerals (ash), water-soluble vitamins, and trace non-protein nitrogen compounds. In routine dairy analysis, non-fat milk solids are not measured directly but are obtained by difference:
- SNF (%) = Total Solids (%) − Fat Content (%)
Normal bovine milk typically contains 8.3–9.0% SNF, and most national and international specifications set a minimum threshold of 8.1% for fluid milk. Because the SNF value is strongly correlated with the nutritional density of milk, it serves as a key indicator for raw milk payment systems, process standardization, and the detection of economically motivated adulteration, particularly the illegal addition of water. The principal reference standards include GB 5009.239 (determination of non-fat milk solids in foods), ISO 6731/IDF 21 (total solids, reference method), ISO 1211/IDF 105 (fat content, gravimetric reference method), ISO 488 (fat, Gerber butyrometric method), and ISO 8968 (nitrogen content for protein verification).
Testing Samples
Liquid Milk Products
Raw (farm) milk, pasteurized milk, UHT and sterilized milk, reconstituted milk, and milk-based beverages. Liquid samples are homogenized and tempered to approximately 20 °C before subsampling to ensure representative results.
Fermented Dairy Products
Set and stirred yogurt, drinking yogurt, kefir, and other cultured products. For fermented samples, the analysis is typically performed on the whole product, with attention to complete dispersion of the coagulum prior to weighing.
Concentrated and Dried Dairy Products
Evaporated milk, sweetened condensed milk, whole and skimmed milk powder, and whey powder. Dried products are reconstituted with distilled water to a defined solids level before analysis, while concentrated products may require dilution to ensure accurate weighing and drying behavior.
High-Fat and Composite Products
Cream, butter (SNF of the serum phase), ice cream mixes, and formulated dairy-containing foods. These matrices often demand modified drying conditions or supporting determinations (protein, lactose, ash) to interpret the SNF value correctly.
Testing Items
A complete non-fat milk solids determination involves the following measurable parameters:
- Total solids content (gravimetric drying or instrumental equivalent)
- Fat content (Rose-Gottlieb gravimetry, Gerber butyrometry, Babcock, or instrumental infrared analysis)
- Non-fat milk solids, calculated as total solids minus fat
- Supporting parameters when required: protein (Kjeldahl or Dumas, ISO 8968), lactose, ash, titratable acidity, density, and freezing point depression
Testing Methods
Reference Method for Total Solids (Oven Drying)
A weighed test portion (typically 1–2 g for milk, or 0.5–1 g for concentrated products) is mixed with a drying aid such as washed and ignited sea sand in a pre-dried, pre-weighed dish. The dish is dried in a ventilated oven at 102 ± 2 °C (or under reduced pressure at lower temperature for heat-sensitive products) until constant mass is achieved, then cooled in a desiccator and reweighed. Total solids are calculated from the mass loss. This procedure is specified in ISO 6731/IDF 21 and adopted in GB 5009.239 as the-level (referee) approach.
Reference and Routine Methods for Fat
- Rose-Gottlieb gravimetric method (ISO 1211/IDF 105): the sample is treated with ammonia and ethanol, the fat is extracted with diethyl ether and petroleum ether, the solvent is evaporated, and the residue is weighed. This is the internationally recognized reference method.
- Gerber butyrometric method (ISO 488): the sample is digested with sulfuric acid and amyl alcohol in a calibrated butyrometer, centrifuged, and the fat column is read directly. Suitable for rapid routine control of milk and cream.
- Babcock method: a related volumetric approach used in several regulatory environments.
- Instrumental infrared (mid-IR/FTIR) milk analyzers: validated rapid alternatives for liquid milk, requiring periodic calibration against reference chemistry.
Calculation and Expression of Results
The non-fat milk solids content is reported as mass fraction in percent:
- SNF (g/100 g) = Total Solids (g/100 g) − Fat (g/100 g)
Results are conventionally reported to two decimal places. For reconstituted or diluted products, results are corrected back to the original product basis, and any added sucrose or other non-milk solids must be accounted for and stated separately, as they are not legitimately part of the milk SNF fraction.
Rapid Instrumental and Field Methods
Thermogravimetric and microwave dryers can determine total solids in minutes and, when paired with an instrumental fat result, yield SNF rapidly for process control. Lactometric estimation using the Babcock/Richmond relationship (SNF ≈ 0.25 × lactometer reading + 0.20 × fat % + 0.14) remains a field-screening tool for raw milk, but it is an approximation and must be verified against reference methods before use in compliance testing.
Applications
- Regulatory compliance: verifying that fluid milk and fermented products meet minimum SNF requirements (for example, ≥8.1% non-fat milk solids stipulated for raw and pasteurized milk under GB 19301 and GB 25190, and corresponding Codex Alimentarius specifications).
- Raw milk quality grading and payment schemes: SNF is a core parameter in dairy processor incentive programs, together with fat, protein, and somatic cell count.
- Adulteration screening: added water depresses both SNF and freezing point; a low SNF value triggers confirmatory testing for extraneous water.
- Process standardization: controlling fat and SNF ratios during standardization, evaporation, and spray drying to meet milk powder specifications such as skimmed milk powder minimum milk-solids requirements.
- Label verification and product development: confirming declared compositional claims for reconstituted products, milk beverages, and formulated foods containing milk solids.
- Trade and certification: accredited third-party laboratories perform SNF determinations for import/export inspection, batch release, and third-party certification against ISO and GB methods.
FAQ
What are non-fat milk solids composed of?
Non-fat milk solids consist mainly of lactose (approximately 4.6–5.0% in bovine milk), proteins (approximately 3.0–3.6%, chiefly caseins), and minerals/ash (approximately 0.7%), together with minor amounts of water-soluble vitamins, citrate, and non-protein nitrogen compounds.
Why is SNF determined by difference rather than measured directly?
The SNF fraction is a composite of many chemically diverse constituents with no single direct analytical target. Measuring total solids and fat by well-defined reference methods and subtracting fat is the most accurate and internationally accepted approach, and it is the basis of GB 5009.239 and the ISO/IDF method framework.
What is a typical SNF value for cow milk?
Healthy bovine milk generally contains 8.3–9.0% non-fat milk solids. Values below approximately 8.1% may indicate added water, nutritional deficiency, mastitis-related changes, or breed and seasonal effects, and warrant further investigation.
Which standards govern the determination?
GB 5009.239 specifies the determination of non-fat milk solids in foods; ISO 6731/IDF 21 covers the gravimetric determination of total solids; ISO 1211/IDF 105 covers the Rose-Gottlieb fat reference method; and ISO 488 covers Gerber butyrometry. Supporting protein verification can be performed according to ISO 8968.
Can rapid analyzers replace the reference method?
Rapid thermogravimetric, infrared, and ultrasonic instruments are widely used for in-process and routine control, provided they are calibrated and periodically verified against the gravimetric reference methods. For dispute resolution, regulatory compliance, and certification, the reference methods remain the criteria of decision.
What precautions improve accuracy?
Thorough homogenization of the sample, accurate weighing to 0.1 mg, use of pre-dried dishes and desiccant, avoidance of moisture pick-up during cooling and weighing, complete fat extraction, and replicate determinations with agreement checks are all essential for reliable SNF results.