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ISO 27871:2011 — Cheese and Processed Cheese: Nitrogenous Fractions Determination
Analytical Methods for Protein and Nitrogen Characterization in Dairy Products
Introduction to Nitrogenous Fraction Analysis in Cheese
ISO 27871:2011 (IDF 224:2011) specifies methods for the determination of nitrogenous fractions in cheese and processed cheese products. This standard is essential for characterizing the protein composition throughout the cheese-making process from milk coagulation through ripening. The nitrogenous fractions — total nitrogen (TN), non-casein nitrogen (NCN), non-protein nitrogen (NPN), and casein nitrogen — provide critical indicators of cheese maturity, quality, and nutritional value.
The nitrogen-to-protein conversion factor for dairy products (6.38) differs from the general food factor (6.25) due to higher nitrogen content in casein and whey proteins. Always use 6.38 for cheese calculations.
The fractionation scheme defined in this standard enables precise tracking of proteolysis during cheese ripening. As cheese ages, casein is broken down into peptides and amino acids, shifting nitrogen from the casein fraction to the NPN fraction. This proteolysis index (NPN/TN ratio) is a key quality parameter for aged cheeses such as Cheddar, Parmesan, and Gouda.
Analytical Procedures and Fractionation Protocol
| Fraction | Definition | Separation Method | Typical % of TN |
|---|---|---|---|
| Total Nitrogen (TN) | All nitrogen compounds | Kjeldahl or Dumas | 100% |
| Non-Casein N (NCN) | Whey proteins + NPN | pH 4.6 precipitation | 15-25% |
| Non-Protein N (NPN) | Peptides, amino acids, urea | TCA 12% precipitation | 1-15% |
| Casein Nitrogen | TN – NCN | By difference | 75-85% |
| Whey Protein N | NCN – NPN | By difference | 10-20% |
The standard specifies two precipitation methods. Method A uses isoelectric precipitation at pH 4.6 (the isoelectric point of casein) to separate casein from whey proteins. Method B uses 12% trichloroacetic acid (TCA) to precipitate all proteins, leaving only low-molecular-weight nitrogen compounds in solution. Nitrogen content of each fraction is determined by the Kjeldahl method (ISO 8968-1) or Dumas combustion (ISO 14891).
Sample preparation is critical: cheese samples must be finely grated or homogenized. Processed cheese containing emulsifying salts requires particular attention as these can affect protein precipitation efficiency.
Engineering Applications and Quality Control
Ripening Monitoring and Process Optimization
The NPN/TN ratio increases from approximately 1-3% in fresh curd to 15-30% in fully ripened hard cheeses. Monitoring this ratio enables precise control over ripening time and conditions. For processed cheese manufacture, the casein-to-whey protein ratio determines melt characteristics, with higher casein content producing firmer, less flowable products.
The proteolysis index (NPN/TN) is the single most informative parameter for cheese maturity assessment. Below 5% indicates fresh cheese, 5-15% semi-ripened, and above 15% fully ripened.
Frequently Asked Questions
Q: Why is the nitrogen-to-protein factor 6.38 instead of 6.25 for dairy?
Dairy proteins contain approximately 15.67% nitrogen versus 16% for most food proteins. The factor 6.38 = 100/15.67 reflects this difference.
Dairy proteins contain approximately 15.67% nitrogen versus 16% for most food proteins. The factor 6.38 = 100/15.67 reflects this difference.
Q: Can ISO 27871 distinguish between added and native milk proteins?
No, the standard determines total nitrogenous fractions without distinguishing sources. Added proteins will be included in the appropriate fraction based on solubility.
No, the standard determines total nitrogenous fractions without distinguishing sources. Added proteins will be included in the appropriate fraction based on solubility.
Q: What is the minimum sample size required?
The standard recommends a minimum of 2-5 g for accurate fractionation, with homogenization of at least 50 g before subsampling.
The standard recommends a minimum of 2-5 g for accurate fractionation, with homogenization of at least 50 g before subsampling.
