Overview
ISO/TS 2963:2006 (IDF/RM 34:2006) is a joint ISO–IDF Technical Specification that specifies an enzymatic method for determining citric acid content in cheese and processed cheese products. Published in 2006, it replaces earlier editions and provides a validated laboratory procedure for quantitative measurement of citric acid expressed as a mass fraction (percent).
Key topics and technical requirements
- Analytical principle: Citric acid is enzymatically converted by citrate lyase (CL) to oxaloacetate and acetate. Oxaloacetate is subsequently reduced via malate dehydrogenase (MDH) and lactate dehydrogenase (LDH) in the presence of NADH; the reaction consumes NADH. The decrease in NADH concentration is monitored spectrophotometrically at 340 nm and is proportional to citric acid content.
- Reagents and preparations: Detailed reagent recipes and storage guidance are provided, including:
- Buffer (glycylglycine) at pH 7.8 with zinc chloride
- NADH solution and citric acid standard
- Enzyme suspensions: MDH/LDH mixture (~600 U MDH/ml and ~1 400 U LDH/ml) and citrate lyase solution (40 U/ml)
- Trichloroacetic acid and sodium hydroxide solutions at specified concentrations
- Apparatus: Standard analytical equipment including analytical balance, pH meter, macerator, pipettes, and a spectrometer for 340 nm measurements with 1 cm pathlength cells.
- Sample handling: Instructions for representative sampling, homogenization, and separation of inclusions (e.g., fruit, ham) when analysing processed cheese components. ISO 707|IDF 50 is referenced for recommended sampling methods.
- Procedure and calculations: Steps cover deproteination, reagent blank tests, incubation, measurement and calculation of mass fraction. The document includes formulae for expressing results.
- Quality and performance: Precision data, repeatability and reproducibility considerations, and an Annex A with Good Laboratory Practice (GLP) rules for enzymatic analyses.
Applications and users
This standard is intended for:
- Dairy testing laboratories and quality control teams
- Regulatory and compliance bodies verifying product composition and labeling
- Food scientists and R&D groups developing cheese and processed cheese formulations
- Contract testing and certification labs performing routine or dispute analyses
Typical uses include product quality control, nutritional labeling verification, process monitoring, benchmarking of raw materials, and research into formulation impacts on citric acid levels.
Related standards
- ISO 707 | IDF 50 - sampling of milk and milk products (recommended)
- ISO 5725 series - methods for accuracy (precision, repeatability, reproducibility) referenced in the document
Keywords: ISO/TS 2963, citric acid determination, enzymatic method, cheese analysis, processed cheese, citrate lyase, MDH, LDH, NADH, spectrophotometer 340 nm, dairy laboratory, GLP.
