Titratable Acidity Analysis of Dairy Products: Principles, Methods, and Quality Applications

Titratable acidity analysis is a cornerstone quality-control test in the dairy industry, used to evaluate freshness, monitor fermentation, and verify compliance with product specifications. This technical article explains the titration principles, applicable standards such as ISO 6091 and GB 5009.239, sample preparation and calculation procedures, and practical applications across liquid milk, fermented milks, cheese, and milk powders. It is intended for laboratory analysts, dairy technologists, and quality assurance professionals seeking a clear, standards-based reference for dairy acidity determination.

Technical Overview

Titratable acidity (TA) is defined as the amount of alkali, typically sodium hydroxide of accurately known concentration, required to titrate a dairy sample to a defined endpoint, conventionally pH 8.3, which corresponds to the color transition of phenolphthalein. In dairy matrices, the measured acidity reflects not only lactic acid produced by bacterial fermentation of lactose, but also the natural buffering components of milk, including casein and whey proteins, phosphates, citrates, and dissolved carbon dioxide.

It is important to distinguish titratable acidity from pH. While pH measures the instantaneous hydrogen ion activity, titratable acidity quantifies the total acid and buffer capacity that must be neutralized, which correlates more directly with sensory sourness, protein stability, and process behavior such as heat coagulation and curd formation.

Results are expressed in several traditional units, depending on the region and standard applied:

• Degrees Thorner, written as T, defined as the milliliters of 0.1 mol/L sodium hydroxide consumed per 100 g of sample; this is the unit used in Chinese national standards.
• Percent lactic acid, the international trade convention, where one degree Thorner equals 0.009 percent lactic acid.
• Degrees Soxhlet-Henkel, written as SH, defined by titration with 0.25 mol/L sodium hydroxide per 100 mL of sample and commonly used in Central Europe.
• Degrees Dornic, written as D, used primarily in France, based on a 0.111 mol/L alkali titration of a 10 mL sample.

Fresh raw milk normally shows a titratable acidity of approximately 14 to 18 T, equivalent to roughly 0.13 to 0.17 percent lactic acid. Values above this range indicate developed acidity from microbial growth, while unusually low values may suggest added water or physiological abnormalities such as mastitis.

Testing Samples

Liquid Milk Products

Raw milk at farm or dairy reception, pasteurized milk, ultra-high temperature sterilized milk, reconstituted milk, flavored milk beverages, and cream can all be analyzed. Samples are tempered to approximately 20 °C and mixed gently but thoroughly to homogenize the fat phase without incorporating air.

Fermented Dairy Products

Set yogurt, stirred yogurt, drinking yogurt, kefir, cultured buttermilk, and other fermented milks are the most acidic category, with titratable acidity frequently reaching 70 to 110 T during and after incubation. These matrices are dense and viscous, requiring careful homogenization before subsampling.

Milk Powders and Concentrates

Skimmed and whole milk powders, whey powders, and infant formula bases are reconstituted with carbon dioxide-free water, commonly at a ratio of about 10 g of powder to 100 mL of water, before titration. The result is reported for the reconstituted product and, where required, recalculated to the powder as-is basis.

Cheese and Whey Products

Soft, semi-hard, and hard cheeses are comminuted or grated to a uniform particle size and weighed directly for titration, with results expressed per unit mass. Whey, whey permeate, and dairy process streams may also be monitored for acidification control.

Testing Items

Typical parameters determined within a dairy acidity testing program include:

• Titratable acidity in degrees Thorner, degrees Soxhlet-Henkel, or degrees Dornic.
• Titratable acidity expressed as percent lactic acid equivalent.
• Sample pH, measured as a complementary indicator alongside TA.

Natural acidity versus developed acidity, evaluated by comparing the acidity of fresh secretion against the increase caused by bacterial lactic acid production.

• Fermentation endpoint confirmation for cultured products.
• Acidity stability over shelf life in storage trials.

Testing Methods

Reference Standards

The principal standardized methods are ISO 6091 and IDF 86 for the determination of titratable acidity of milk by potentiometric titration, ISO/TS 11869 and IDF/RM 150 for the potentiometric determination of titratable acidity of fermented milks, and GB 5009.239, the national food safety standard for the determination of acidity in foods, which provides both a potentiometric method and a phenolphthalein indicator method. Sampling guidance follows ISO 707 and IDF 50. Product-level limits are established in standards such as GB 19301 for raw milk and GB 19302 for fermented milk. Accredited third-party laboratories and in-plant quality laboratories routinely perform these methods under ISO/IEC 17025 management systems.

Sample Preparation

Liquid samples are brought to 20 ± 2 °C and inverted or stirred to homogeneity. Powder samples are reconstituted with freshly boiled and cooled, carbon dioxide-free distilled water and allowed to hydrate fully. Cheese samples are grated and mixed thoroughly. All water used for dilution must be free of carbon dioxide, because dissolved carbon dioxide consumes alkali and biases the result downward.

Potentiometric Titration Procedure

• Calibrate the pH meter at 20 °C using buffer solutions at pH 4.00 and pH 9.00, or pH 6.86 and pH 9.18, verifying slope and offset.
• Pipette or weigh the specified test portion, commonly 10 g, into a titration beaker and dilute with carbon dioxide-free water as required by the method.
• Immerse the electrode, stir at a constant moderate rate, and titrate with standardized 0.1000 mol/L sodium hydroxide to the endpoint of pH 8.3.
• Record the volume of titrant consumed and perform a duplicate determination.

Visual Indicator Titration Procedure

• Prepare the test portion as above and add 2 to 3 drops of phenolphthalein indicator solution.
• Titrate with 0.1000 mol/L sodium hydroxide while swirling until a faint pink color persists for approximately 30 seconds.
• Run a blank on the dilution water and indicator to correct the titrant volume.
• This variant is suitable for white, uncolored matrices; colored or highly turbid samples should be analyzed potentiometrically.

Calculation and Expression of Results

Titratable acidity in degrees Thorner is calculated as the volume of 0.1000 mol/L sodium hydroxide consumed, normalized to 100 g of sample. Percent lactic acid is obtained by multiplying the degrees Thorner by 0.009, or directly from the formula relating titrant concentration, titrant volume, and sample mass. Conversion between customary units is straightforward: one degree Soxhlet-Henkel equals 2.5 degrees Thorner, and one degree Thorner equals 0.009 percent lactic acid.

Quality Control and Interference Management

• Standardize the sodium hydroxide titrant regularly against a primary standard such as potassium hydrogen phthalate.
• Use class A volumetric glassware or a calibrated automatic titrator, and verify performance with a certified reference material or control milk.
• Control the titration temperature, since indicator transition and electrode response shift with temperature.
• Minimize carbon dioxide pickup by protecting dilution water and titrant from atmosphere.
• Clean electrodes regularly to remove protein and fat films that cause sluggish or drifting endpoints.
• Accept results only when duplicate determinations agree within the repeatability limit, typically 0.5 T or better for liquid milk.

Applications

Titratable acidity data support decisions across the entire dairy value chain:

• Raw milk reception screening, where elevated acidity signals bacterial growth, potential heat instability, and shortened shelf life, supporting acceptance or rejection decisions.
• Process control in pasteurization and sterilization, since high-acid milk fouls heat exchangers and risks casein coagulation.
• Fermentation monitoring for yogurt and cultured products, where the incubation is terminated at a target acidity, commonly 70 to 90 T, to balance flavor, texture, and whey-holding capacity.
• Cheese making, where the acidification profile governs curd syneresis, moisture control, and final texture.
• Regulatory compliance verification against national and international compositional standards.
• Milk powder specification testing and grading for trade and procurement.
• Shelf-life studies, stability protocols, and research on starter cultures and buffering systems.

FAQ

What is the difference between titratable acidity and pH?

pH measures the free hydrogen ion concentration at a moment in time, while titratable acidity measures the total amount of alkali needed to neutralize all titratable acid and buffering substances. Two milks can share the same pH yet differ in titratable acidity because of different protein and phosphate buffering capacities.

What acidity level indicates fresh raw milk?

Fresh raw milk typically titrates between 14 and 18 T, approximately 0.13 to 0.17 percent lactic acid. Values above 18 T generally indicate developed acidity from microbial activity, and milk approaching 25 T is considered unsuitable for heat processing.

Why is pH 8.3 used as the endpoint?

The pH 8.3 endpoint corresponds to the midpoint of the phenolphthalein transition range and ensures complete neutralization of lactic acid and acid phosphates. Modern potentiometric detection reproduces this endpoint instrumentally, improving precision and eliminating operator subjectivity.

How are fermented products like yogurt tested?

Fermented milks are homogenized, and a weighed portion is titrated potentiometrically according to ISO/TS 11869 and IDF/RM 150 or GB 5009.239. National standards such as GB 19302 require fermented milk acidity to be at least 70 T, confirming adequate fermentation.

What common errors affect dairy acidity titration?

Frequent error sources include carbon dioxide in dilution water, unstandardized titrant, incorrect sample temperature, fat coating on electrodes, over-titration past the endpoint, and non-homogeneous samples. Each is controlled through the preparation and quality assurance practices described above.

Can testing be outsourced?

Yes. Accredited third-party laboratories perform titratable acidity analysis to ISO, IDF, and GB methods, and issue results suitable for regulatory submission, supplier qualification, trade documentation, and internal benchmarking against in-plant quality control data.