Overview
ISO/TS 11869:2012 (IDF/RM 150:2012) specifies a potentiometric method for determining the titratable acidity of fermented milk products. It covers natural, flavoured and fruit yoghurts, drinking yoghurts, fresh cheese (with/without fruit), buttermilk and similar cultured dairy products. The method titrates a prepared test portion against 0.1 mol/L sodium hydroxide potentiometrically to a fixed endpoint of pH 8.30 ± 0.01 and expresses acidity in mmol NaOH per 100 g (or optionally as g lactic acid/100 g).
Key topics and technical requirements
- Principle: Suspension of a test portion in water is titrated potentiometrically to pH 8.30 ± 0.01 using c(NaOH) = 0.1 mol/L ± 0.002 mol/L.
- Test portion and preparation: Approximately 10 g sample, homogenized (spoon/spatula or homogenizer such as Ultra‑Turrax) at 22 °C ± 2 °C; fruit-containing samples require thorough homogenization and may be heated to 38 °C ± 1 °C to eliminate fat separation.
- Instrumentation:
- pH meter calibrated in the pH 7–10 range.
- Analytical balance readable to 0.01 g.
- Burette (25 or 50 ml, ISO 385 class A) or automatic burette/titrator (ISO 8655‑3 compatible).
- Water bath for temperature control.
- Reagents and handling: Use carbonate‑free NaOH and protect the titrant from CO2 absorption (closed system or CO2 trap). Use distilled/deionized water boiled 10 min to remove dissolved CO2.
- Calculation: Titratable acidity I (mmol/100 g) = V × 10 / m, where V = mL NaOH used, m = g sample. Results expressed to two decimal places. Annex A gives conversion to g lactic acid/100 g: w = V × 0.9 / m.
- Precision: Interlaboratory trial across six labs; repeatability limit r = 0.20 mmol/100 g (difference >0.20 in not more than 5% of cases).
- Reporting: Include sample identification, sampling method, reference to ISO/TS 11869:2012, method details and any deviations.
Applications and users
ISO/TS 11869:2012 is intended for:
- Dairy quality‑control laboratories monitoring fermentation and product consistency.
- Product development and R&D assessing acidity profiles in yoghurts, buttermilk and fresh cheeses.
- Regulatory and conformity testing where standardized acidity measurement is required.
- Accredited testing labs that require a repeatable, traceable potentiometric titration method.
Keywords: ISO/TS 11869:2012, titratable acidity, potentiometric method, fermented milks, yoghurt acidity testing, NaOH titration, pH 8.30.
Related standards
- ISO 707 | IDF 50 - Sampling guidance for milk and milk products
- ISO 385 - Laboratory glassware - Burettes
- ISO 8655-3 - Piston-operated volumetric apparatus - Piston burettes
- ISO 5725-1 - Accuracy (trueness and precision) of measurement methods
