Techniques - Proximates Analysis (including Nitrogen Factors)

Meat of Fish Content of Food ("QUID") using Nitrogen Factors

The determination of nitrogen as a quantitative marker for meat/fish/seafood fat-free protein, and the calculation of meat/fish content of compound foods began almost 100 years ago. It is the established official chemical method to enforce labelling and reserved description declarations of meat/fish content in food products, and also widely used by food processors to check the specification and added water of their meat/seafood raw materials.

In the case of meat, a “nitrogen factor” is the average nitrogen content of a specific cut or whole animal on a fat free basis. Meat animals are normally slaughtered and prepared without the use of water. However, poultry is killed and prepared using some wet processes (defeathering, rinsing after evisceration, cooling). The European Poultrymeat Marketing Standards Regulation (EC) 543/2008 regulates the amount of extraneous water poultry and poultry parts are allowed to pick up during preparation in poultry plants without any declaration on the product label. This extraneous water has to be considered when poultry contents of preparations and products are determined.

In the case of fish/seafood, a “nitrogen factor” is the average nitrogen content of seafood tissues on a fat free basis, unless the fat content is low e.g., white fish. Most seafood is prepared using wet processes as part of good hygienic practice (GHP). Therefore, in establishing nitrogen factors for seafood, keeping water uptake to a minimum and using good manufacturing practice (GMP) has to be considered.

Analyses undertaken – nitrogen, moisture, ash and if necessary, fat

Determination of nitrogen content is according to internationally accepted methodology (British Standard, BS 4401:1980 ISO 937:1978).

Determination of ash is according to British Standards Institution, Analytical Methods for Meat and Meat Products, Part 1 1970 (1993).

Determination of moisture is according to BSI, Analytical Methods for Meat and Meat Products, Part 3 1970 (1997).

Determination of total fat is according to BSI, Analytical Methods for Meat and Meat Products, Part 4 1970 (1993).

The nitrogen factor can only be used in the calculation of meat/seafood content using the  meat/seafood nitrogen content. If there are other nitrogen contributing ingredients present in the food, these have to subtracted from the total nitrogen of the food.

Meat Content Calculation

 % fat free meat =        (total meat nitrogen / nitrogen factor) x 100             

weight of calculated total meat =   fat free meat + fat

% meat content =  (weight of calculated total meat / weight of meat sample) x 100

Added Water Calculation

Added water = 100 – (calculated total meat + ash + [carbohydrate])

If carbohydrate is present as an ingredient, it is calculated as:

Carbohydrate = 100 – (protein + fat + ash + moisture)     where protein is calculated as total nitrogen x 6.25.

In the case of coated fish/seafood, the calculation has to take into account  the nitrogen of the coating. The nitrogen determination is made on the whole product (not just  the core) because when the product is flash fried before freezing, there is soluble protein migration from the core to the coating.

% fish/seafood    =  (% total nitrogen - % non-fish nitrogen)  X 100

                                                       N factor*

* appropriate N (nitrogen) factor

The non-fish nitrogen is calculated as:              % non-fish nitrogen = % carbohydrate  X  0.02

 Where the % carbohydrate is calculated by difference     =   100 – (%water + %fat + %protein + %ash).

Other calculations

Related analyses and calculations include undeclared excess fat (in sausages, for example) and non-connective tissues at above legally-declarable values.

Using nitrogen factors is one of the few ways to determine added water in meat or seafood products or raw materials. Hence it has been the basis of UK enforcement of food standards and labelling legislation of meat and seafood products for many years.

For identifying and quantifying the presence of meat, the method is no worse than many other quantitative approaches such as real-time PCR or proteomics, and the methods involved are internationally tested and relatively cheap to undertake

Expert Interpretation

Calculations can be complex, and care must be taken that results are compared with legal definitions on a like-for-like basis.  It can take considerable expertise to correctly interpret results, and this expertise is more common within the Public Analyst network than in commercial laboratories.

Comparability of Nitrogen Analytical Definitions

Most laboratories in the UK use the rapid Dumas method for nitrogen determination. This measures the non-protein nitrogen, as well as the protein nitrogen, and hence gives higher results than the Kjeldahl method. The difference is small, and Dumas is higher by a factor of 1.014, which is more important for seafood because of its higher non-protein nitrogen content than meat. Therefore, it is usual to quote the nitrogen results for both methods of analysis as Kjeldahl is still widely used in laboratories worldwide.

Sample results are compared with historical reference tables of typical nitrogen factors for specific breeds or cuts of meat.  Determining these reference nitrogen factors normally involves extensive studies to cover all the variables which affect the raw materials being used in manufacturing of meat and seafood products. In the case of meat animals, this will include breed, age, gender, possibly season and location e.g., in the case of pork , samples were obtained in the UK, the Netherlands and Denmark as pork from these 3 countries are extensively used to prepare meat products for the UK market.

The study for seafood nitrogen factors is usually more challenging than for meat or poultry because there are many more species of seafood, the production or catch areas are often not in the UK, and seafood absorbs water more easily than meat, so GMP has to be considered as the baseline value.  

Hence a nitrogen factor considers natural variation, which means that the uncertainty in the calculation of meat/seafood content can have a range as high as +/- 10%.