Revisiting the nitrogen factor for skinless chicken breast

Abstract

The Analytical Methods Committee has received and approved the following report from the Nitrogen Factors Sub-Committee.

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Summary

Skinless chicken breast is the most used portion in the manufacture of chicken products and in catering. There is a large trade in chicken breast preparations (injected and tumbled with water and water retaining agents) for the catering trade, which are produced in the Netherlands, Poland and some in the UK.

The last AMC study to determine nitrogen factors for chicken (whole chicken, breast, leg etc.) was funded by MAFF and published in 2000. The nitrogen factor for skinless chicken breast was central to uncovering misdescription and mislabelling in the chicken preparation market.

The European Commission (DG-AGRI) funded a recent project (project 30-CE-0460798/00-25) to re-assess the physiological water content of skinless chicken breast and skin-on legs. Samples of skinless chicken breast and skin-on legs from Ross and Cobb breeds, male and female, light and heavy broilers were collected from each of the seven main poultry producing countries (UK, Germany, France, Italy, Spain, the Netherlands and Poland). The new breeds of Ross (308) and Cobb (500) have shorter rearing times to reach killing weights than the breeds used in the 2000 AMC study. As the birds now used by industry are younger, the nitrogen factor may have also changed.

Eighty four samples of homogenised chicken breast from the European Commission study were selected from the 144 UK, Polish and Dutch breast samples, as these three countries were considered the most important for the UK market. The samples were analysed for their ash, fat, and hydroxyproline contents, with the moisture and nitrogen results taken from the European Commission study. The results indicated that the average fat content was slightly lower than in the 2000 study (1.5 g/100 g instead of 2.0 g/100 g), as was the hydroxyproline content (0.05 g/100 g instead of 0.08 g/100 g).

As the fat and ash contents were low, it was decided that the nitrogen factor could be calculated using the nitrogen results of all 144 breast samples from the UK, Poland and the Netherlands determined in the European Commission study. The results indicated that the main differences in nitrogen content from the 2000 study were with the chicken breasts from the heavy male birds, which were significantly lower than the previous study. Also in this study, chicken breasts from the heavy male birds were also significantly lower in nitrogen than those from lighter males or the light or heavy female birds.

The average nitrogen content of all broilers is 3.70 g/100 g with a standard error of the mean of 0.02. Where the raw material of a product is known to be skinless chicken breast, a fat-free nitrogen factor for skinless chicken breast of 3.75 is recommended. This value has been calculated from an average fat content of 1.5 g/100 g, rounded to the nearest 0.05 and compares to the recommended fat-free factor in the 2000 study for skinless chicken breast of 3.85, which represents a 3% difference.

Report

The membership of the Nitrogen Factors Sub-Committee responsible for the preparation of this report was: Dr Mark Woolfe, (Chairman), Dr Roger Wood (Secretary), Mr Andrew Caines, Ms Bhavna Parmar, Mrs Theresa Ekong, (until 11 Oct 2013), Mr Charles Boardman, Mr David Keeble, Mr Duncan Campbell, Mr Jeremy Hall, Prof. Michael Thompson, Mr Robbie Beattie, Mrs Selvarani Elahi, Mr Steve Lamming, Mr Steve Moore, Ms Henrietta Tambo, Mr Andrew Furmage, Ms Liz Moran. Participation in the study was by Dr Mark Woolfe, Mrs Selvarani Elahi, Dr Joanna Topping and Mr Stephen Ellison, who was responsible for the statistical evaluation.

Introduction

The UK market for chicken is worth over £2 billion per year. Skinless chicken breast is the most used portion in the manufacture of chicken products and in catering. The UK cannot produce sufficient chicken breast to meet demand, and significant quantities of chicken breast are imported from third countries, mainly Brazil and Thailand, to be used for further processing and in catering. In addition, there is a large trade (over £200 million per year) in chicken breast preparations (injected and tumbled with water and water retaining agents) for the catering trade produced in the Netherlands, Poland and, to a lesser extent, the UK. The raw material for injected and tumbled chicken preparations was previously Brazilian chicken breast with 1.2% salt, which was imported into the EU with a lower tariff than unsalted chicken breast. However, the customs classification of salted chicken breast was changed in 2006, and the provisions in the EU Poultrymeat Standard Regulation (part of the EU Marketing Regulation 1234/2007¹) were amended in 2009² to include chicken preparations. One of the consequences has been halting the use of Brazilian salted frozen chicken breast as the raw material for tumbled and injected chicken preparations. The raw material for these preparations now is predominantly the more expensive frozen but unsalted chicken breast from Brazil.

The last AMC study to determine nitrogen factors for chicken (whole chicken, breast, leg etc.) was funded by MAFF and published in 2000.³ In that study, 120 birds (comprising of Ross and Cobb breeds, male and female, light and heavy broilers) were used. The light birds were reared to 35 days, and the heavy broilers to 49–56 days. The methodology to measure chicken content and added water in chicken products and preparations uses an accurate, agreed nitrogen factor, and was instrumental in exposing misleading labelling of injected chicken breast investigated by the Food Standards Agency⁴ and enforcement authorities. The methodologies for analysing chicken preparations and the chicken breast N-factor also have been published in the OJ.⁵ These were used in 2005 during an EC Coordinated Control Programme analysing injected chicken breast.

The European Commission (DG-AGRI) funded a project⁶ (project 30-CE-0460798/00-25) with LGC and other European National Reference Laboratories (NRLs) to re-assess the physiological water content of skinless chicken breast and skin-on legs. This was to facilitate enforcement of the Poultrymeat Marketing Standards Regulation (EC) 543/2008,⁷ which 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. In this project, forty eight birds (Ross and Cobb breeds, male and female, light and heavy broilers) were collected from each of the seven main poultry producing European countries (UK, Germany, France, Italy, Spain, the Netherlands and Poland). Samples of skinless breast and skin-on legs were prepared and homogenised. The new chicken breeds of Ross (308) and Cobb (500) have shorter rearing times to reach killing weights than the breeds used in the 2000 AMC study i.e. light weight birds are reared only to 30–32 days and heavy birds only to 48–52 days. As the birds now used by industry are younger, it is possible that the nitrogen factor could have changed (more water and less protein). Given changes in rearing breed and rearing practice, an up to date and accurate nitrogen factor for chicken breast is needed to ensure there is no unfair competition to UK processors and manufacturers, and that there is no misleading labelling of chicken breast preparations in the retail and catering sector. As samples of Ross and Cobb skinless chicken breast had already been collected and analysed for water and protein content as part of the EC study, it was considered extremely cost effective to do the necessary additional analyses (fat, ash and hydroxyproline) to enable the nitrogen factor for chicken breast produced today to be compared to the factor derived in the 2000 study.

Experimental

1. Chicken breast samples

Collection of samples in the DG-AGRI project was under supervision by LGC staff, and according to an SOP developed by LGC to ensure as far as possible that the samples were collected from the 7 countries in the same way and without contact with water. Selected birds were taken off the production line after killing and bleeding, but before entering the scald tank. The skin (with feathers attached) around the breast was removed and the whole breast cut away. The breast muscles were removed from the breast bone and trimmed to remove adhering fat or connective tissue and then double bagged and coded. The samples were frozen and sent to LGC where they were homogenised according to European Regulation No. 543/2008 Annex VIII. Each homogenised sample was divided into two aliquots and stored frozen.

The project budget was sufficient for the selection and analysis of 84 chicken breast samples from the samples collected for the DG-AGRI project. The samples were chosen on the basis that they should be representative of those found on the UK market. It was decided to use samples from UK birds or those with a similar weight to the British chickens, and which were likely to be exported to the UK market. A stratified sampling plan was devised by LGC statisticians to cover a selection of equal numbers of light (around 1.8 kg) and heavy birds (around 2.4 kg) from the UK, Poland and the Netherlands. The sampling was planned to balance breed and weight as these were considered the most important factors affecting nitrogen content. Cobb was included as a breed likely to reach the UK even though Ross currently is predominant in UK production. The resulting samples contained equal numbers of each breed, equal numbers at each weight, and retained a reasonably even ratio of male to female birds (see Table 1).

Table 1 Selection of 84 chicken breast samples

Country	No. of samples	Samples by breed	Samples by gender	Samples by size
UK	28	14 Ross + 14 Cobb	16 males + 12 females	14 light + 14 heavy
Poland	28	14 Ross + 14 Cobb	13 males + 15 females	14 light + 14 heavy
The Netherlands	28	14 Ross + 14 Cobb	14 males + 14 females	14 light + 14 heavy

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2. Methods of analysis

The moisture and nitrogen content of the 84 samples selected in Table 1 were obtained from the DG-AGRI project. The samples had been distributed equally to eight European NRLs, who analysed each sample for moisture and nitrogen. The methods used were those laid down in Commission Regulation (EC) 543/2008, which are the same ISO (BSI) standard methods used routinely by Official Control Laboratories in the UK:

• Analytical methods for meat and meat products: part 2 1980 (1993) determination of nitrogen or equivalent method;

• Part 3 1970 (1997) determination of moisture. Each sample was analysed in triplicate and a certified reference material (ERM®-BB501a) included in each batch.

Agreement between the triplicate results was to be less than 0.4 g/100 g for moisture and less than 0.16 g/100 g for nitrogen.

The 84 breast samples were analysed in duplicate for ash, fat and hydroxyproline by LGC using the following BSI validated methods or their equivalents:

• Part 1 1970 (1993) determination of ash.

• Part 4 1970 (1993) determination of total fat.

• Part 11 1995 determination of L-(−)-hydroxyproline.

The detailed requirements for the analyses included:

(a) A standard reference material (e.g. FAPAS meat sample or LGC reference material 7152, processed pork) was to be run in duplicate with each batch of chicken samples.

(b) Two replicates of the three analyses were required per sample and the total content of moisture, ash, fat and protein (nitrogen × 6.25) should add up to 100% ± 2%.

(c) The duplicates were to be randomised within each batch. Analytical agreement was required between the duplicates – the repeatability limits were 0.5 g/100 g (fat) and 0.1 g/100 g (ash and hydroxyproline).

Results

1. Results of ash, fat and hydroxyproline analyses

Table 2 gives the means of the fat, ash, hydroxyproline, moisture and nitrogen contents for all 84 analyses, and compares them with the 2000 study³i.e. the mean of the same analytes for skinless breast from light and heavy broilers.

Table 2 Ash, fat, hydroxyproline, moisture and nitrogen contents for 84 chicken breast samples

Analyte (g/100 g)	Mean 84 samples	Mean of 2000 study
Ash	1.24	1.12
Fat	1.51	2.0
Hydroxyproline	0.05	0.08
Moisture	74.7	74.2
Nitrogen	3.70	3.78

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Tables 3–5 show data for the mean composition for fat, ash and hydroxyproline contents respectively. All values have been calculated for breed, size and gender for comparison with those generated in the 2000 study. The means shown are least square means (LSMean), that is, means calculated from a linear model, corrected for imbalance. The hydroxyproline values showed no anomalous results. Statistically significant differences were found for the effect of gender on hydroxyproline content. As expected, the size of the chicken breast had a significant effect on values for ash content. The breed of chicken was not a statistically significant variable at the 95% level of confidence for any analyte.

Table 3 Fat (%) by breed, size and gender

Breed	Size	Gender	LSMean	SEM
Cobb	L	F	1.41	0.13
Ross	L	F	1.41	0.13
Cobb	H	F	1.34	0.13
Ross	H	F	1.51	0.13
Cobb	L	M	1.48	0.13
Ross	L	M	1.31	0.13
Cobb	H	M	1.80	0.13
Ross	H	M	1.81	0.13

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Table 4 Ash (%) by breed, size and gender

Breed	Size	Gender	LSMean	SEM
Cobb	L	F	1.26	0.016
Ross	L	F	1.29	0.016
Cobb	H	F	1.25	0.016
Ross	H	F	1.22	0.016
Cobb	L	M	1.24	0.016
Ross	L	M	1.27	0.016
Cobb	H	M	1.22	0.016
Ross	H	M	1.20	0.016

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Table 5 Hydroxyproline (g/100 g) by breed, size and gender

Breed	Size	Gender	LSMean	SEM
Cobb	L	F	0.045	0.0020
Ross	L	F	0.046	0.0020
Cobb	H	F	0.043	0.0020
Ross	H	F	0.045	0.0020
Cobb	L	M	0.050	0.0020
Ross	L	M	0.050	0.0021
Cobb	H	M	0.050	0.0020
Ross	H	M	0.051	0.0020

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Examining the overall means from the 84 samples, the data in Table 2 shows that there has been a small reduction in ash, fat, hydroxyproline and nitrogen contents when compared to the results found in the 2000 study. The main contributor to the reduction in nitrogen content is from heavy male birds, where it would be expected that there would be a higher nitrogen content than with younger birds.

As the mean fat and ash contents are very low, and moisture and protein (nitrogen) contents contribute more than 97.8% of the chicken breast by weight, it was decided to increase the sample size from 84 to 144. This was done by utilising the moisture and nitrogen results from all the chicken breast samples from the UK, Poland and the Netherlands (48 from each country) used in the DG-AGRI study.

2. Results from analyses of 144 samples for nitrogen and moisture

The data in Tables 6–9 show the estimated mean contents for moisture and nitrogen for different categories of sample. The means shown are least square means (LSMean). The data in Tables 6 and 7 show that gender is strongly significant (p < 0.01) for moisture, and very strongly significant for nitrogen (p < 0.001). In addition, size is very strongly significant (p < 0.001) for nitrogen and significant for moisture. Breed was statistically significant at the 95% level of confidence for nitrogen and country significant at the 95% level for both nitrogen and moisture. Noting the modest significance of country effects, means by country of origin were calculated, and are shown in Table 8. The differences are all comparatively small: not more than 0.06 g/100 g for nitrogen and under 0.4 g/100 g for moisture.

Table 6 Moisture (%) by breed, size and gender of 144 samples

Breed	Size	Gender	LSMean	SEM
Cobb	H	F	74.19	0.14
Ross	H	F	75.01	0.14
Cobb	L	F	74.56	0.14
Ross	L	F	74.67	0.14
Cobb	H	M	74.99	0.14
Ross	H	M	75.25	0.14
Cobb	L	M	74.92	0.14
Ross	L	M	74.46	0.14

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Table 7 Nitrogen (%) by breed, size and gender of 144 samples

Breed	Size	Gender	LSMean	SEM
Cobb	H	F	3.77	0.03
Ross	H	F	3.66	0.03
Cobb	L	F	3.76	0.03
Ross	L	F	3.74	0.03
Cobb	H	M	3.65	0.03
Ross	H	M	3.57	0.03
Cobb	L	M	3.71	0.03
Ross	L	M	3.72	0.03

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Table 8 Mean moisture and nitrogen (%) by country of 144 samples

Country	Nitrogen	Moisture
The Netherlands	3.73	74.65
Poland	3.69	74.97
UK	3.67	74.65

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Table 9 Nitrogen (%) averaged over breed and country of 144 samples

Size	Gender	LSMean	SEM
^a,b Statistically significant differences (p < 0.01) were found between means a and b.
H	F	3.71^a	0.02
L	F	3.75^a	0.02
H	M	3.61^b	0.02
L	M	3.72^a	0.02

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Table 9 shows data for the nitrogen content by gender and size averaged across breed and country of origin. Again, as with the original 84 samples, heavy male birds differed significantly (p < 0.001) in nitrogen content from the remaining three subsets. Comparing the averages shown in Table 9 with averages for breed in the 2000 study, values for females and light males are not significantly different from those for broilers in the 2000 study. However for heavy males, the values found here are appreciably lower (by 0.15 to 0.2 g/100 g) than previously found either for broilers or for heavy broilers.

Discussion and conclusions

The UK market for chicken breast portions is obtained from both light and heavy broilers of both Ross and Cobb breeds. Although the Ross breed predominates in the UK market at the moment, this could change in the near future. It now is common practice not to separate male and female birds destined for the broiler market after hatching. Hence in arriving at mean values representative of the market, equal numbers of light, heavy, Ross and Cobb samples were taken. In addition, spent layer hen meat, which was incorporated into the recommended breast factor in the 2000 study, is no longer used in the manufacture of UK chicken products.

The mean results of the chemical analyses on the population of 84 samples indicate that there has been a small reduction in fat content (1.5 g/100 g instead of 2.0 g/100 g) and hydroxyproline content (0.05 g/100 g instead of 0.08 g/100 g) when compared to the 2000 study. The average nitrogen content for all broilers is also slightly down from the 2000 study as well (3.70 g/100 g instead of 3.78 g/100 g). The observation that the nitrogen content of breasts from heavy male broilers was significantly lower than the nitrogen contents from lighter birds and heavy females was unexpected.

In order to check that the above result was not anomalous, all of the nitrogen and moisture results obtained in the DG-AGRI project from all breast samples collected from the UK, Poland and the Netherlands (144 samples) were analysed statistically. Again, nitrogen values for breast samples from heavy male birds are significantly lower than those from light birds or heavy females. Taking equal numbers of heavy and light birds, the average nitrogen content calculated from Table 9 is 3.70 g/100 g.

In terms of the new values from the analyses of chicken breast samples, the average fat content of lean skinless chicken breast is 1.5 g/100 g, and the hydroxyproline content is 0.05 g/100 g.

The average nitrogen content of lean skinless breast from all broilers is 3.70 g/100 g with a standard error of the mean of 0.02 (n = 144). Where the raw material of a product is known to be skinless chicken breast, a fat-free nitrogen factor for skinless chicken breast of 3.75 is recommended. This value has been calculated from an average fat content of 1.5 g/100 g, rounded to the nearest 0.05, and compares to the recommended fat-free factor in the 2000 study for skinless chicken breast of 3.85, which represents a 3% difference. In any future study on chicken preparations, the recommended nitrogen factor of 3.75 should be used in conjunction with the methodology described in Commission Recommendation 175/2005.⁵

Acknowledgements

This study was undertaken as part of the programme of work of the Royal Society of Chemistry, Analytical Methods Committee – Nitrogen Factors Sub-Committee. The project was part-funded by Defra under its Food Authenticity Programme, and partly by the Analytical Methods Trust. The European Commission – DG-AGRI is thanked for giving permission to use the samples and results from its project 30-CE-0460798/00-25. LGC is also acknowledged for storing and providing the samples.

References

1. Council Regulation (EC) No. 1234/2007 of 22 October 2007 establishing a common organisation of agricultural markets and on specific provisions for certain agricultural products (Single CMO Regulation), OJ L299, 16.11.2007, pp. 1–242.
2. Council Regulation (EC) No. 1047/2009 of 19 October 2009 amending Regulation (EC) No. 1234/2007 establishing a common organisation of agricultural markets as regards the marketing standards for poultrymeat, OJ L 290, 6.11.2009, pp. 1–3.
3. Nitrogen Factors for Chicken Meat, Analyst, 2000, 125, 1359–1366.
4. Food Standards Agency Survey of Meat Content, Added Water and Hydrolysed Protein In Catering Chicken Breasts (Number 20/01), http://tna.europarchive.org/20120530191353/http://www.food.gov.uk/science/surveillance/fsis2001/20chick.
5. Commission Recommendation of 1 March 2005 concerning a Coordinated Programme for the Official Control of Foodstuffs for 2005, ANNEX V, OJ L59 05.03.2005, pp. 27–39.
6. Study of the Physiological Water Content of Poultry Reared in the EU, Final Report 2012, http://ec.europa.eu/agriculture/external-studies/index_en.htm.
7. Commission Regulation (EC) No. 543/2008 of 16 June 2008 laying down detailed rules for the application of Council Regulation (EC) No. 1234/2007 as regards the marketing standards for poultrymeat, OJ L157, 17.6.2008, pp. 46–87.

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Footnote

† Correspondence should be addressed to Dr M. Woolfe, e-mail: mjwoolfe@gmail.com.

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