Determination of Physicochemical Indicators in Dairy Products: Principles, Methods, and Standards

Physicochemical indicator testing forms the analytical backbone of dairy quality control, verifying compositional parameters such as fat, protein, moisture, ash, acidity, freezing point, and density against national and international standards. This technical article summarizes the sample types covered, the key indicators measured, the reference methods specified in ISO, GB, EN, and ASTM standards, and the practical applications of dairy physicochemical analysis in manufacturing, regulation, and international trade.

Technical Overview

Dairy products are complex oil-in-water systems whose nutritional value, processing behavior, and legal conformity are defined by a specific set of physicochemical indicators. These indicators—principally fat, protein, lactose, moisture and total solids, ash (total mineral residue), titratable acidity, pH, freezing point, and relative density—are used to verify compliance with product specifications, nutritional labeling declarations, and the compositional requirements laid down in national food safety standards such as the GB 19301 (raw milk), GB 25190 (sterilized and UHT milk), GB 19302 (fermented milk), and GB 19644 (milk powder) families, as well as internationally harmonized ISO and EN method standards.

Because milk composition varies with breed, season, feeding regime, and herd health, physicochemical measurement is applied at every stage of the value chain: raw milk reception, standardization and evaporation, fermentation control, spray-drying, and final product release. In-plant quality departments and accredited third-party laboratories rely on a hierarchy of reference methods (gravimetric, titrimetric) and rapid routine techniques (butyrometric, infrared), with routine instruments calibrated against and traceable to the reference procedures.

The core indicator categories covered by dairy physicochemical testing are:

• Macronutrient composition: total fat, crude protein, lactose and other carbohydrates, moisture and total solids, solids-not-fat (SNF)
• Mineral fraction: ash and individual minerals (calcium, phosphorus, sodium, potassium, magnesium, zinc, iron)
• Acid–base status: titratable acidity (°T or % lactic acid) and pH
• Colligative and physical properties: freezing point, relative density
• Authenticity markers: freezing point (added-water screening) and nitrogen fractions (non-protein nitrogen, casein nitrogen)

Testing Samples

Liquid Milk Products

Raw farm and tanker milk, pasteurized milk, UHT/sterilized milk, and modified or flavored milk drinks. Representative parameters include fat, protein, solids-not-fat, freezing point, titratable acidity, and relative density.

Fermented Milk Products

Set and stirred yogurt, drinking yogurt, kefir, and cultured buttermilk. Titratable acidity (commonly required to reach or exceed 70 °T), pH, protein, and fat define the fermentation endpoint and product identity.

Milk Powders and Whey Powders

Whole and skimmed milk powder, infant-formula base powders, whey powder, and whey protein concentrates. Moisture (typically specified at no more than 5%), fat, protein, ash, lactose, and nitrogen fractions dominate the specification sheet.

Cheese and Processed Cheese

Natural rennet and acid cheeses as well as processed cheese products. Moisture-in-product and fat-in-dry-matter (FDM) are used to classify cheese type, while salt, pH, and nitrogen fractions monitor ripening and formulation.

Butter, Cream, and Concentrated Milks

Butter (fat, moisture, and salt), cream and whipped products (fat and acidity), and sweetened condensed or evaporated milk (total solids, sucrose, fat, and protein).

Testing Items

A comprehensive physicochemical panel for dairy materials typically includes the following determinations:

• Total fat content (% m/m)
• Crude protein (total nitrogen multiplied by the dairy factor 6.38)
• Moisture and total solids
• Solids-not-fat (SNF)
• Ash (total mineral residue after incineration)
• Lactose and total sugars
• Titratable acidity and pH
• Freezing point (added-water screening)
• Relative density at 20 °C
• Individual minerals: Ca, P, Na, K, Mg, Zn, Fe
• Nitrogen fractions: non-protein nitrogen (NPN), casein nitrogen, whey protein nitrogen
• Cheese classification parameters: fat-in-dry-matter and moisture content

Testing Methods

Sampling and Sample Preparation

Sampling follows ISO 707 and the general provisions of the GB 5009 series. Liquid samples are warmed to approximately 35–40 °C to redisperse the milk fat, mixed gently by inversion or stirring, and equilibrated to 20 °C before aliquoting. Cheese is grated and comminuted, butter is softened and homogenized, and powders are blended and protected from moisture uptake with desiccant storage. Careful preparation is essential because fat separation and moisture exchange are the largest sources of analytical bias in dairy testing.

Fat Determination

• Gravimetric Röse–Gottlieb extraction (ISO 1211; GB 5009.6): the reference method for liquid milk. Proteins are dissolved with ammonia, the aqueous phase is dehydrated with ethanol, fat is extracted with diethyl ether and petroleum ether, and the extracted residue is weighed after solvent evaporation.
• Weibull–Berntrup gravimetric extraction (ISO 8262): applicable to milk-based foods, processed cheese, and products containing sugars or stabilizers that interfere with direct solvent extraction.
• Gerber butyrometric method (ISO 488): fat is released with sulfuric acid and amyl alcohol, separated in a centrifuge, and read volumetrically in a calibrated butyrometer—a rapid, inexpensive routine technique.
• Van Gulik butyrometric method (ISO 1735) for cheese; ISO 3433 covers fat determination in dried milk.
• Mid-infrared instrumental measurement (ISO 9622): high-throughput determination calibrated against the gravimetric reference, widely used for raw-milk grading and payment testing.

Protein Determination

• Kjeldahl method (ISO 8968-1; GB 5009.5): digestion with concentrated sulfuric acid and a catalyst converts organic nitrogen to ammonium sulfate; alkaline distillation liberates ammonia, which is titrated with standardized acid. Crude protein equals total nitrogen multiplied by 6.38.
• Dumas combustion method (ISO 14891; GB 5009.5): combustion at approximately 950 °C followed by catalytic reduction and thermal-conductivity detection of nitrogen—a reagent-free, rapid alternative for routine control.
• Nitrogen fractions: non-protein nitrogen after precipitation with trichloroacetic acid (ISO 8968-4); casein nitrogen by precipitation techniques (ISO 17997).

Moisture, Total Solids, and Ash

• Total solids in milk and cream: drying to constant mass at 102 ± 2 °C (ISO 6731), with pre-drying on a water bath or sand bed for high-solids liquids.
• Moisture in dried milk: reference vacuum-oven procedure (ISO 5537) and loss-on-drying under GB 5009.3.
• Moisture in cheese and processed cheese: drying of a grated sample at 102 ± 2 °C (ISO 5534).
• Ash: incineration of the dried residue in a muffle furnace at 550 ± 25 °C to constant mass (GB 5009.4; ISO 5545 for caseins and caseinates).

Acidity, pH, Freezing Point, and Density

• Titratable acidity: titration with standardized sodium hydroxide to the phenolphthalein endpoint, expressed in degrees Thorner (°T) or as percent lactic acid (GB 5413.34; ISO 6091 for reconstituted dried milk).
• pH: direct potentiometric measurement with a calibrated combination glass electrode at 20–25 °C.
• Freezing point: thermistor cryoscope method (ISO 5764). Genuine bovine milk freezes at approximately −0.512 to −0.550 °C; elevated values indicate the presence of added water.
• Relative density: lactometer, calibrated hydrometer, or oscillating U-tube densitometer at 20 °C; typical whole milk values fall within 1.027–1.033.

Lactose, Minerals, and Rapid Instrumental Screening

• Lactose: high-performance liquid chromatography (ISO 22662), enzymatic procedures, or polarimetry where no other optically active substances are present.
• Minerals: microwave digestion followed by flame atomic absorption spectrometry or inductively coupled plasma optical emission spectrometry in accordance with the relevant parts of the GB 5009 series.
• Routine screening: mid-infrared (ISO 9622) and near-infrared analyzers provide simultaneous fat, protein, lactose, and total-solids results within minutes; such instruments must be validated and periodically recalibrated against the reference methods.

Applications

Physicochemical indicator data supports decision-making across the entire dairy supply chain:

• Inbound raw-milk grading and payment testing based on fat, protein, freezing point, and acidity
• Process control: cream and skim standardization, evaporation endpoint, and fermentation endpoint monitored through titratable acidity
• Batch release against product standards, including GB 19301, GB 25190, GB 19302, GB 19644, and GB 10765 for infant formula
• Verification of nutrition labeling declarations for energy, protein, fat, and carbohydrates
• Authenticity and adulteration screening, including added-water detection and abnormal non-protein nitrogen levels
• Shelf-life and stability studies for UHT, powdered, and fermented products
• Import and export conformity testing performed by accredited third-party laboratories

FAQ

Why is the protein conversion factor 6.38 used for dairy?

Milk proteins contain a higher average nitrogen fraction (approximately 15.7%) than the general food protein assumption behind the 6.25 factor. Dividing 100 by 15.7 yields 6.38, which gives a more accurate estimate of true milk protein mass and is the factor specified in ISO 8968 and the dairy application of GB 5009.5.

What is the difference between the Gerber and Röse–Gottlieb methods?

The Röse–Gottlieb procedure is the gravimetric reference method with the lowest measurement uncertainty, suitable for validation and dispute resolution. The Gerber method is a volumetric butyrometric routine technique that is faster and less expensive but slightly less precise, making it ideal for daily in-plant quality control.

How is added water in milk detected?

Through freezing-point determination under ISO 5764. The dissolved lactose and minerals keep genuine milk freezing within a narrow range near −0.52 °C; added water shifts the freezing point toward 0 °C, providing a sensitive and well-established screening indicator for dilution.

How much sample is required and how long does testing take?

A composite submission of approximately 200–500 mL for liquids or 200–500 g for solids generally covers the full indicator panel. Reference gravimetric and titrimetric methods typically require 3–7 working days, while infrared screening results can be reported the same day.

Which standards govern infant-formula physicochemical testing?

Compositional limits are defined by GB 10765 and related national food safety standards, with individual determinations carried out using the corresponding parts of the GB 5009 series; internationally harmonized ISO and EN method standards support conformity assessment for global markets.