Dairy Product Acidity Titration Test: Principles, Methods, Standards, and Applications

Dairy product acidity titration is a fundamental analytical test used to determine the titratable acidity of milk, yogurt, cheese, and other dairy products, serving as a reliable indicator of raw material freshness, fermentation progress, and regulatory compliance. This technical article explains the titration principle, applicable standards such as GB 5009.239 and ISO 6091, applicable sample types, test procedures, calculation methods, and practical applications of acidity determination in dairy quality control.

Technical Overview

Titratable acidity (TA) is defined as the amount of alkali required to neutralize the acidic components in a dairy sample, expressed as the volume of 0.1 mol/L sodium hydroxide consumed per 100 g of sample. In dairy chemistry, titratable acidity reflects the combined contribution of natural acidity (casein, whey proteins, phosphates, citrates, and dissolved carbon dioxide) and developed acidity (mainly lactic acid produced by microbial fermentation). Because lactic acid accumulates as bacteria metabolize lactose, the acidity value is one of the most sensitive and widely used indicators of milk freshness, hygienic quality, and fermentation control.

The dairy product acidity titration test is typically performed according to GB 5009.239 (National Food Safety Standard — Determination of Acidity in Foods), which specifies both a potentiometric titration method and a phenolphthalein indicator method. Internationally, ISO 6091 / IDF 86 (milk — determination of titratable acidity) and ISO/TS 11869 / IDF RM 150 (fermented milks — potentiometric titration) are commonly referenced. Results may be expressed in several conventional units, including Thorner degrees (°T), percent lactic acid equivalent, Dornic degrees (°D), and Soxhlet-Henkel degrees (°SH).

Typical reference values established by national food safety standards for dairy products include 12–18 °T for raw milk and pasteurized milk, and not less than 70 °T for fermented milks such as yogurt. Routine acidity testing is therefore an essential component of incoming raw material inspection, in-process fermentation monitoring, finished product release, and shelf-life evaluation in dairy production.

Testing Samples

Liquid Milk Products

Raw (bulk tank) milk, pasteurized milk, UHT and sterilized milk, modified milk, and flavored milk beverages can all be tested directly after gentle homogenization of the sample. For colored or turbid liquid products, the potentiometric method is preferred because visual endpoint detection of phenolphthalein becomes unreliable.

Fermented Dairy Products

Set-type and stirred-type yogurt, drinking yogurt, kefir, cultured buttermilk, sour cream, and other fermented milks are routinely tested to verify that fermentation has reached the target acidity. Because these products have high titratable acidity, a smaller sample mass or a larger burette capacity may be required to keep the titrant consumption within the calibrated range.

Concentrated and Powdered Dairy Products

Whole milk powder, skimmed milk powder, whey powder, evaporated milk, and sweetened condensed milk are reconstituted with carbon dioxide-free water according to the declared mixing ratio before titration. Reconstitution is usually performed with warm water (approximately 40 °C) to ensure complete dissolution, and the mixture is cooled to room temperature before analysis.

Cheese, Cream, and Butter

Natural cheese, processed cheese, cream, and butter require specific sample preparation such as grating, melting, or dispersion in warm carbon dioxide-free water to release the aqueous acidic phase. Due to the fat content, color, and turbidity of these matrices, potentiometric titration to a fixed endpoint pH is the recommended approach.

Testing Items

The core measurable parameters of the dairy product acidity titration test include:

• Titratable acidity expressed in Thorner degrees (°T), the number of milliliters of 0.1 mol/L sodium hydroxide required to neutralize 100 g of sample.
• Titratable acidity expressed as percent lactic acid equivalent (% lactic acid).
• Complementary pH measurement by potentiometry, used alongside titratable acidity to characterize the acid-base profile of the sample.
• Unit conversions for international reporting: 1 °T = 0.009% lactic acid; 1 °D (Dornic) = 0.01% lactic acid; 1 °SH (Soxhlet-Henkel) = 2.5 °T.
• Evaluation of developed acidity (microbial lactic acid) versus natural acidity for freshness assessment.

Testing Methods

Principle

A accurately weighed dairy sample is dispersed in carbon dioxide-free water and titrated with standardized sodium hydroxide solution. The endpoint is detected either visually using phenolphthalein indicator (appearance of a faint pink color persisting for approximately 30 seconds) or instrumentally with a calibrated pH electrode (endpoint pH 8.3). The volume of titrant consumed is proportional to the titratable acidity of the sample.

Reagents and Apparatus

• Sodium hydroxide standard titration solution, 0.1000 mol/L, standardized against potassium hydrogen phthalate.
• Phenolphthalein indicator solution, typically 10 g/L in 95% ethanol.
• Carbon dioxide-free distilled water, freshly boiled and cooled, confirmed neutral to phenolphthalein.
• Analytical balance with readability of 0.001 g or better.
• Class A burette (25 mL or 50 mL) or automatic potentiometric titrator equipped with a pH electrode.
• Conical flasks, pipettes, and a water bath for sample reconstitution where required.

Phenolphthalein Indicator Method

Approximately 10 g of the homogenized sample is weighed into a conical flask, and 20 mL of carbon dioxide-free water is added. Two to three drops of phenolphthalein indicator are introduced, and the flask is swirled continuously while 0.1000 mol/L sodium hydroxide is added dropwise from the burette. The titration is stopped when a faint pink color persists for about 30 seconds, and the titrant volume is recorded. A blank determination using the same volume of carbon dioxide-free water is performed to verify that neither the water nor the indicator contributes acidity.

Potentiometric Titration Method

The sample is prepared in the same manner and titrated under continuous stirring while the pH is monitored with a pH meter calibrated using standard buffer solutions. Sodium hydroxide solution is added until the endpoint pH of 8.3 is reached, and the corresponding titrant volume is recorded. This method eliminates subjective color judgment and is mandatory for colored, turbid, or high-fat samples such as cheese, cream, and flavored fermented products. Automatic titrators improve repeatability and are suitable for high-throughput laboratory workflows.

Calculation of Results

Titratable acidity in °T is calculated as X = (c × V × 100) / (0.1 × m), where c is the actual molarity of the sodium hydroxide solution in mol/L, V is the titrant volume in mL, and m is the sample mass in g. Percent lactic acid is obtained by multiplying the °T value by 0.009. Duplicate determinations are performed, and the arithmetic mean is reported, with the absolute difference between duplicates not exceeding the repeatability limit specified in the applicable standard.

Quality Control and Interference Control

• Use freshly prepared, carbon dioxide-free water to prevent underestimation of acidity from dissolved carbonic acid.
• Standardize the sodium hydroxide solution frequently, as it absorbs carbon dioxide and changes concentration over time.
• Equilibrate samples to approximately 20 °C before titration, since temperature affects dissociation and endpoint sharpness.
• Calibrate the pH meter with at least two buffer solutions bracketing the expected range before each potentiometric batch.
• Follow validated sample preparation procedures for high-fat or high-solids matrices to ensure complete release of acidic components.

Applications

Acidity titration data support a broad range of quality assurance and regulatory activities across the dairy industry:

• Raw milk reception control: verifying that incoming milk falls within the 12–18 °T range, screening for sour or adulterated milk, and supporting milk payment and grading schemes.
• Fermentation process control: determining the acidification endpoint of yogurt, kefir, and cheese starters to ensure consistent texture, flavor, and shelf stability.
• Finished product release: demonstrating conformity with national food safety standards for dairy products, including the minimum acidity requirement for fermented milks.
• Shelf-life and cold-chain monitoring: detecting abnormal acidity development that indicates microbial spoilage during storage and distribution.
• Research and product development: evaluating starter culture activity, heat treatment effects, and reformulation of dairy-based formulations.
• Regulatory inspection and trade: providing legally defensible test reports issued by accredited third-party laboratories for market surveillance, import and export verification, and certification purposes.

FAQ

What is titratable acidity in dairy products?

Titratable acidity is the total amount of acid in a dairy sample measured by neutralization titration with a standardized alkali. It includes both the inherent acidic constituents of milk and the lactic acid developed through bacterial fermentation, making it a combined indicator of composition and freshness.

What is the difference between pH and titratable acidity?

pH measures the hydrogen ion activity (intensity of acidity) at a single moment, whereas titratable acidity measures the total buffering capacity of all acidic and buffer components. Two yogurts can share the same pH but differ significantly in titratable acidity depending on their protein and buffer content, which is why both parameters are often reported together.

Which standards govern the dairy acidity titration test?

The primary reference is GB 5009.239 (Determination of Acidity in Foods), which covers potentiometric and indicator-based titration. International equivalents include ISO 6091 / IDF 86 for milk and ISO/TS 11869 / IDF RM 150 for fermented milks, all of which specify titration to approximately pH 8.3.

What is the normal acidity of fresh raw milk?

Fresh, normal raw milk typically shows a titratable acidity of 12–18 °T (approximately 0.11–0.16% lactic acid). Values below this range may indicate mastitis or possible adulteration with alkaline substances, while elevated values indicate lactic acid development from bacterial growth or improper storage.

Why must carbon dioxide-free water be used in the test?

Dissolved carbon dioxide forms carbonic acid that consumes sodium hydroxide and causes a delayed or masked phenolphthalein endpoint, leading to overestimation of sample acidity. Boiling and cooling the water before use removes carbon dioxide and ensures accurate, repeatable results.

How is °T converted to percent lactic acid?

Each milliliter of 0.1 mol/L sodium hydroxide corresponds to 0.009 g of lactic acid, so percent lactic acid equals the °T value multiplied by 0.009. For example, a milk sample with 16 °T corresponds to approximately 0.144% lactic acid.