Dairy Product Acidity Testing: Titratable Acidity and pH Determination

Dairy product acidity testing is a fundamental quality control analysis that measures titratable acidity and pH to evaluate freshness, fermentation progress, and regulatory compliance in milk, yogurt, cheese, milk powder, and other dairy products. Accredited third-party laboratories perform these tests using recognized standard methods such as GB 5009.239, ISO/TS 11869, and ISO 6091 to support manufacturers, suppliers, and regulatory bodies worldwide.

Technical Overview

Acidity is one of the most important chemical indicators of quality, freshness, and process control in dairy products. In milk and dairy-based foods, acidity originates from two sources: natural acidity and developed acidity. Natural acidity comes from inherent components such as casein proteins, phosphates, citrates, and dissolved carbon dioxide, while developed acidity results from lactic acid produced by bacterial fermentation of lactose. As microbial activity increases, developed acidity rises, making acidity measurement a sensitive early indicator of spoilage or improper storage.

Two distinct parameters are evaluated in dairy acidity testing:

• Titratable acidity: the total amount of acid (both dissociated and undissociated) neutralized by titration with a standardized sodium hydroxide solution, expressed in degrees Thorner (°T), degrees Soxhlet-Henkel (°SH), degrees Dornic (°D), or as a percentage of lactic acid equivalent.
• pH value: the negative logarithm of hydrogen ion activity, reflecting the active (free) hydrogen ion concentration and directly influencing protein stability, texture, and microbial growth.

Because titratable acidity and pH respond differently to buffering compounds in milk, both parameters are typically required for a complete assessment. National and international standards, including GB 5009.239, GB 19301, GB 19302, ISO/TS 11869, and ISO 6091/IDF 86, define the accepted test methods and specification limits for various dairy categories.

Testing Samples

Liquid Milk Products

Raw milk, pasteurized milk, UHT-treated milk, sterilized milk, flavored milk drinks, and reconstituted milk are tested to verify freshness and conformity with acidity limits. Fresh raw milk typically exhibits a titratable acidity of 12–18 °T, and values above this range indicate microbial souring or mastitis-related abnormalities.

Fermented Milk and Yogurt

Set-type, stirred-type, and drinking yogurt, as well as kefir and other cultured dairy beverages, require acidity testing to confirm fermentation completion. Fermented milk products generally must reach a titratable acidity of at least 70 °T to comply with product standard requirements for viable lactic acid bacteria content and characteristic flavor.

Cheese and Processed Cheese

Acidity measurement in cheese production, often expressed as pH or as a percentage of lactic acid in whey or curd, is used to monitor starter culture activity, coagulation control, syneresis, and maturation progression.

Milk Powder and Whey Powder

Skimmed milk powder, whole milk powder, whey powder, and whey protein concentrates are reconstituted with water before testing. Acidity of the reconstituted product reflects the quality of the original raw material and the thermal processing history of the powder.

Cream, Butter, and Concentrated Milk

Whipping cream, butter, sweetened condensed milk, and evaporated milk are tested to detect excessive lactic acid development that indicates microbiological deterioration during storage or distribution.

Frozen Dairy Desserts

Ice cream and frozen yogurt mixes are analyzed for acidity to verify mix quality, stabilize overrun and texture, and confirm that cultured ingredients meet declared specifications.

Testing Items

Typical acidity-related testing items for dairy products include:

• Titratable acidity expressed in °T (degrees Thorner), °SH (degrees Soxhlet-Henkel), or °D (degrees Dornic)
• Titratable acidity expressed as percent lactic acid equivalent (% LA)
• pH value at a defined reference temperature (typically 20 °C or 25 °C)
• Acidity of reconstituted milk powder at a standardized solids concentration
• Acidity development curves for fermentation process monitoring
• Free fatty acid value for high-fat products such as cream and butter, where lipolytic rancidity is a concern

Common conversion relationships include 1 °T corresponding to 0.009% lactic acid equivalent, and results can be reported interchangeably among °T, °SH, and % lactic acid depending on market or regulatory requirements.

Testing Methods

Phenolphthalein Indicator Titration Method

In the classic visual endpoint method described in GB 5009.239, a measured sample (typically 10 g or 10 mL for liquid products, or a reconstituted slurry for powders) is diluted with carbon-dioxide-free water. A phenolphthalein indicator is added, and the mixture is titrated with standardized 0.1000 mol/L sodium hydroxide solution to a persistent faint pink color lasting approximately 30 seconds. The titratable acidity in °T is calculated as the volume of sodium hydroxide consumed multiplied by the appropriate factor per 100 g or 100 mL of sample. This method is simple, economical, and widely used for routine quality control.

Potentiometric Titration Method

The instrumental approach uses a calibrated pH electrode to detect the titration endpoint at pH 8.2–8.3, eliminating subjective color judgment. This method is specified in international references such as ISO/TS 11869 for fermented milk, ISO 6091/IDF 86 for dried milk, and AOAC official methods for milk and milk products. Potentiometric detection offers improved precision, repeatability, and suitability for colored or turbid samples such as flavored fermented milk and cheese dispersions. Automated titration systems provide traceable results with full data integrity for laboratory accreditation purposes.

Direct pH Measurement

pH is measured using a calibrated glass electrode combined pH meter inserted directly into the homogenized sample, or into a diluted suspension for semi-solid products such as yogurt and soft cheese. Two-point or three-point calibration with standard buffer solutions is performed before each analytical session, and temperature compensation is applied to ensure accuracy.

Sample Preparation Considerations

Proper sample handling is critical for reliable acidity results:

• Liquid samples are mixed thoroughly and brought to approximately 20 °C before analysis.
• Milk powders are reconstituted with boiled and cooled distilled water, typically to 10% total solids, and allowed to hydrate fully before titration.
• Fermented products are gently homogenized without incorporating excessive air.
• Carbon-dioxide-free water must be used for dilution to avoid neutralization errors.
• Sodium hydroxide titrant concentration must be verified daily against a primary standard.

Applications

Dairy acidity testing supports multiple stages of the product lifecycle:

• Incoming raw material inspection: rapid acidity screening at milk collection centers identifies sour or adulterated raw milk before it enters the processing line.
• Fermentation process control: online and offline acidity monitoring determines the optimal fermentation end point for yogurt and cultured products, ensuring consistent texture, flavor, and probiotic viability.
• Shelf-life evaluation and stability studies: acidity trending during accelerated and real-time storage trials predicts product behavior across the distribution chain.
• Regulatory compliance and product certification: acidity results demonstrate conformity with national food safety standards, product standards, and import-export requirements.
• Cheese and casein production optimization: precise acidity and pH control governs rennet action, curd handling, moisture loss, and maturation characteristics.
• Failure investigation and complaint resolution: abnormal acidity data help diagnose heat abuse, refrigeration failure, post-pasteurization contamination, or raw milk quality defects.

FAQ

What is the difference between titratable acidity and pH?

Titratable acidity measures the total acid content neutralizable by alkali, including buffered acids bound to proteins and salts, while pH measures only the free hydrogen ion activity. Two milks can share the same pH but differ in titratable acidity due to differing buffering capacities, which is why both parameters are routinely reported.

What are the typical acidity values for common dairy products?

Fresh raw milk normally ranges from 12 to 18 °T, pasteurized and UHT milk should not fall below 12 °T, and fermented milk should reach at least 70 °T. Milk powder reconstituted at standard dilution typically measures 12–18 °T, depending on the original milk quality and heat treatment intensity.

Which standards govern dairy acidity testing?

Widely applied references include GB 5009.239 for the determination of food acidity, GB 19301 for raw milk, GB 19302 for fermented milk, ISO/TS 11869 for yogurt titratable acidity, and ISO 6091/IDF 86 for dried milk acidity. Method selection depends on the product type and the destination market.

Why did a visually normal milk sample fail the acidity test?

Elevated titratable acidity can develop before off-odors or curdling become apparent. Lactic acid accumulation from psychrotrophic or mesophilic bacteria during extended refrigerated storage frequently causes borderline or failing results even when the sample appears acceptable.

How should samples be submitted for acidity testing?

Samples should be collected in clean, sealed, food-grade containers, kept refrigerated at 0–4 °C, and delivered to the laboratory promptly, ideally within 24 hours of sampling. Fermented products should be shipped cold-chained to prevent continued acidification by active cultures. Accredited third-party laboratories typically require 100–500 g or mL per sample and provide results within three to seven working days.