This study on SA lamb was long overdue, since all the nutritional tables which were used in articles and discussions and industry up to now has been based on research undertaken in the USA. Thus this is the first SA study conducted to determine the nutrient content of lamb and mutton analyzed according to International guidelines.

This report, The Nutrient content of South African lamb, has been compiled by Ina (SM) van Heerden and Hettie (prof HC) Schonfeldt at the Meat industry Centre, the ARC, in Irene.  

ANALYSES OF SOUTH AFRICAN LAMB

RAW AND COOKED

Never before, has it been attempted to determine the nutrient composition of South African lamb. This is the first up-to-date determination of the exact nutrient content of lamb (A2 class), as purchased by the South African consumer. The Meat and Livestock Australia (MLA) /the Red Meat Producers’ Organization (RPO) and the National Emerging Red Meat Producers’ Organization (NERPO) commissioned and sponsored this study to the amount of R 260 000-00 to enable all South Africans to have more reliable nutrient composition data. The study was performed at the Agricultural Research Council in collaboration with the University of Pretoria.

SAMPLING FOR NUTRIENT CONTENT

Correct sampling requires careful attention. It is important that analytical food data is accurate, representative and that careful analysis and appropriate sample storage and preparation procedures are followed (Greenfield & Southgate, 2003). The sampling of South African lamb was designed in collaboration with the meat industry, according to market share. Dorper and Merino breeds were selected from two different abattoirs that slaughter carcasses for the three production areas in South Africa namely the Karoo, Kalahari and Ermelo districts (Table 1).

Table 1: Experimental design for nutrient analysis of South African lamb (A2 Class)

18 A age class, fat class 2 lamb carcasses
Ermelo		Kalahari		Karoo
3 Mutton Merino 3 Dorper		3 Mutton Merino 3 Dorper		3 Mutton Merino 3 Dorper
6 Right sides Raw	6 Left side Cooked	6 Right sides Raw	6 Left side Cooked	6 Right sides Raw	6 Left side Cooked
Composite sample	Composite sample	Composite sample	Composite sample	Composite sample	Composite sample
Macronutrient analysis on meat & fat for 7 cuts Micronutrient analysis on meat & fat for 3 cuts	Macro- & micro nutrient analysis on meat & fat for 3 cuts	Macronutrient analysis on meat & fat for 7 cuts Micronutrient analysis on meat & fat for 3 cuts	Macro- & micro nutrient analysis on meat & fat for 3 cuts	Macronutrient analysis on meat & fat for 7 cuts Micronutrient analysis on meat & fat for 3 cuts	Macro- & micro nutrient analysis on meat & fat for 3 cuts

Sample preparation for carcass composition

The lambs were slaughtered and dressed, using standard commercial procedures during four consecutive weeks. After selection, the carcasses were transported in a refrigerated truck (4 - 6 °C) to the Meat Industry Centre of the ARC - ANPI, Irene. Upon arrival all the carcasses were weighed, covered with plastic wrap to prevent moisture loss, chilled at 4 °C overnight, and dissected the following day. The lamb carcasses consisted of the skinned, eviscerated body from which the head and feet were removed.

 Figure 1: Dissection Diagram (Casey, 1982)

Carcasses were sectioned down the vertebral column with a band saw and then subdivided into the following primal cuts: neck, thick rib, flank, shoulder, breast, loin, leg and shin (Figure 1). A trained de boning team was responsible for the physical dissection of the 18 lamb carcasses that had been weighed prior to being sub divided into primal cuts. The right sides of the carcasses were used to determine the physical cut carcass composition as well as raw nutrient analysis. The three wholesale cuts of matching side (left) were analyzed for cooked nutrient content (proximate, vitamins, minerals fatty acid and cholesterol).

Sample preparation

In order to comply with the new Draft Regulations relating to the labeling and advertising of foodstuffs, as part of the Foodstuffs, Cosmetics and Disinfectants Act, Act 54 of 1972, (http://www.doh.gov.za/department/dir_foodcontr.html), it was decided that a composite of three carcasses would be used as a basis of the study. The use of composite samples for analysis rather than individual samples is justified through budgeting constraints and is an accepted approach in food composition studies (Greenfield & Southgate, 2003). The raw nutrient contents for macro- and some micronutrients for each cut were determined. The raw and cooked nutrient data of the three cuts were compared based on the assumption (Kirton, Barton & Rae, 1962), that the composition of the two sides is similar.

Preparation of the raw and cooked composite samples

The shoulder, leg and loin cuts of the right sides were used for the raw micronutrient analysis. These cuts were dissected by knife into three portions namely meat (muscle + intramuscular fat i.e. fat within the muscle), bone and subcutaneous fat, in an environmentally controlled (10 °C) de-boning room. Raw meat and fat samples were taken from the wholesale cuts of the right side for the proximate analysis (macronutrients).

All the wholesale cuts from the left sides were vacuum packed and frozen for approximately two months until it was used for the cooked nutrient analysis. The frozen shoulder, loin and leg cuts from the left sides were thawed, weighed and cooked (as a whole cut) according to standardized cooking methods (leg - combination dry and moist heat cooking method; loin - dry heat cooking method and shoulder - moist heat cooking method) in identical Mielé ovens at 160 °C, to an internal temperature of 70 °C, measured in the geometrical centre of the cut (American Meat Science Association, 1995). The cuts were cooled and dissected into three portions namely meat (muscle inter- + intramuscular fat), bone and subcutaneous fat, in an environmentally controlled (10 °C) de-boning room by a team of experienced dissectors.

All physical dissected raw and cooked meat and fat were cubed, thoroughly mixed and then minced first through a 5 mm and then through a 3 mm mesh plate. After mincing, samples of 300 g meat and separable fat were homogenized with an Ultra Turrax T25 homogenizer and put into aluminium trays. After covering the samples with a vacuum bag, the meat were freeze-dried and sent off to the ARC analytical laboratory at Irene for proximate analysis (macronutrients analyzed).

The analytical procedures (Table 2) for the nutrient content of the lamb samples were done on a double blind basis in laboratories (ISO/IEC:17025:2005) that form part of the South African National Accreditation Services (SANAS). Control samples form part of the daily routine in these laboratories to assure the quality of results. On the operational level, all sampling was representative and handled with utmost accuracy. A proper sampling plan was followed with representative samples from each area and sufficient replications of each sample were used to ensure statistically reliable and valid data.

Table 2: Methods used for the nutrient analyses of raw and cooked lamb

Analysis	Method
Moisture	Official Method 950.46 AOAC (2005)
Ash	Official Method 820.153 AOAC (2005)
Protein (N)	International Dairy Foundation (IDF) Standard 20B (1993)
Fat	International Dairy Foundation (IDF) Standard 20B (1996)
Energy	Calculated
Minerals	Ion Chromatography (IC) (Dionex System 20001, 1988 & 1991)
Water-soluble vitamins	High Performance Liquid Chromatography (HPLC) (Fellman et al., 1992)
Fatty acid profile	Gas Chromatography (GC) (Christopherson & Glass, 1969)
Cholesterol	Gas Chromatography (GC) (Smuts et al., 1992)

Proximate analysis

Proximate analysis was carried out to determine the percentages of total moisture, fat, nitrogen (N x 6,25 = protein) and ash. A 150 g fresh (±50 g freeze-dried) sample was used for these analyses. For the determination of the protein content, the standard Kjeldahl procedure for nitrogen (IDF-Standard 20B, 1993) was used. A conversion factor of 6,25 was used in the calculation of the protein content. Duplicate samples were analyzed.

For determination of moisture content the weight loss of a 5 g sample was measured in triplicate (AOAC, 2005).

For determination of fat content a 2 g freeze-dried sample was used to ensure that all the moisture had escaped. The Tecator Soxtec System 1034 extraction unit was used. The reagent petroleum ether (40-60 °C) was used for the extraction (IDF Standard 20B, 1996)

Food energy content

The energy content was calculated from the percentage protein and fat by using the following factors:
% Protein x (4,184 x 4,27) = A
% Fat x (4,184 x 9,02) = B
Energy (kJ) = (A + B)
where 4,27 = energy content of protein in kcal and
9,02 = energy content of fat in kcal

Fatty acid profile

The liquid fractions of weighed duplicate samples were extracted with a chloroform-methanol (2:1) mixture. The glycerides and free fatty acids were esterified (BF3-methanol) for gas chromatographic analyses (Gas Chromatography (GC) (Christopherson & Glass, 1969)). It was analyzed by Foodtek of the CSIR, using the accredited analytical method AM 055, revision A, 20/08/1996; for fatty acid composition by GC.

Total cholesterol

The liquid fractions of weighed, duplicate samples were extracted with a chloroform-methanol (2:1) mixture. This mixed solvent extraction is followed by saponification of the isolated lipid to remove glyceride fatty acids. The cholesterol (non-saponifiable) was directly chromatographed on a capillary column (Smuts et al., 1992) by the Foodtek of the CSIR using the accredited analytical method AM 083, revision A, 12/08/1996; for determination of cholesterol in foods.

Water soluble vitamins

Thiamin (Vit B1), riboflavin (Vit B2) niacin (Vit B3), pyridoxine (Vit B6) and cyanocobalamin (Vit B12) were determined according to an Analytical Method AM 027 by HPLC (Fellman et al., 1992). All analyses were performed in duplicate.

Minerals

Freeze-dried samples were ashed, absorbed with hydrochloric acid and analysed with an Ion Chromatography (IC) (Dionex System 20001, 1988 & 1991). The following minerals were determined: sodium, potassium, iron, magnesium and zinc.

RESULTS

Table 3: Mean values of the nutrient composition of lean meat only for raw and cooked 100 g portion of South African A2 lamb

Nutrients analyzed	Unit	Raw	Cooked
PROXIMATE ANALYSIS:
Moisture	g	71.5	65.4
Protein (Nx6.25)	g	18.3	25.1
Fat	g	9.01	8.44
Ash	g	2.88	1.07
Food energy (calculated)	kJ	644	745
MINERALS:
Magnesium (Mg)	mg	20.1	21.7
Potassium (K)	mg	291	298
Sodium (Na)	mg	83.4	71.3
Zinc (Zn)	mg	2.25	1.72
Iron (Fe)	mg	0.96	0.63
VITAMINS:
Thiamin (B1)	mg	0.10	0.04
Riboflavin (B2)	mg	0.09	0.05
Niacin (B3)	mg	1.47	1.42
Pyridoxine (B6)	mg	0.40	0.12
Cyanocobalamin (B12)	µg	3.54	0.93
LIPIDS: Saturated fatty acids (SFA)
14:0	g	0.57	0.50
16:0	g	2.22	1.99
18:0	g	1.46	1.07
20:0	g	0.02	0.02
Monounsaturated fatty acids (MUFA)
16:1	g	0.19	0.19
18:1n9t	g	0.31	0.21
18:1n9c	g	3.12	2.65
Polyunsaturated fatty acids (PUFA)
18:2n6t	g	0.02	0.02
18:2n6c	g	0.25	0.22
Cholesterol	mg	62.8	87.7

Table 4: Mean values of the nutrient composition of lean meat only for three raw and three cooked 100 g portions of South African A2 lamb

Nutrients analyzed	Unit	Raw cuts (n = 12)			Cooked cuts (n = 12)
		Shoulder	Loin	Leg	Shoulder	Loin	Leg
PROXIMATE ANALYSIS:
Moisture	g	70.8	70.1	73.7	66.8	63.5	66.0
Protein (Nx6.25)	g	18.0	17.8	19.0	23.1	27.8	24.5
Fat	g	9.63	11.3	6.15	9.86	7.80	7.67
Ash	g	2.93	2.65	3.06	0.95	1.20	1.05
Food energy (calculated)	kJ	662	718	552	757	755	722
MINERALS:
Magnesium (Mg)	mg	13.9	22.7	23.8	17.7	24.2	22.9
Potassium (K)	mg	201	323	351	261	331	303
Sodium (Na)	mg	68.0	101.0	82.0	68.9	83.3	61.9
Zinc (Zn)	mg	1.99	2.05	2.71	1.24	2.20	1.71
Iron (Fe)	mg	0.75	0.99	1.14	-	0.62	1.20
VITAMINS:
Thiamin (B1)	mg	0.11	0.09	0.10	0.03	0.04	0.06
Riboflavin (B2)	mg	0.09	0.08	0.09	0.04	0.07	0.05
Niacin (B3)	mg	1.70	1.02	1.71	1.37	1.25	1.63
Pyridoxine (B6)	mg	0.26	0.32	0.64	0.12	0.13	0.10
Cyanocobalamin (B12)	µg	4.02	3.61	3.00	0.80	0.90	1.10
LIPIDS: Saturated fatty acids (SFA)
14:0	g	0.62	0.69	0.39	0.50	0.47	0.42
16:0	g	2.50	2.67	1.50	2.06	1.97	1.86
18:0	g	1.64	1.86	0.88	1.10	1.08	1.30
20:0	g	0.03	0.03	0.01	0.02	0.01	0.02
Monounsaturated fatty acids (MUFA)
16:1	g	0.21	0.21	0.14	0.20	0.18	0.16
18:1n9t	g	0.33	0.42	0.19	0.23	0.24	0.21
18:1n9c	g	3.53	3.70	2.14	2.88	2.63	2.71
Polyunsaturated fatty acids (PUFA)
18:2n6t	g	0.03	0.02	0.02	0.02	0.02	0.02
18:2n6c	g	0.28	0.29	0.17	0.24	0.22	0.24
Cholesterol	mg	64.0	61.8	62.7	85.0	86.3	91.7

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ACKNOWLEDGEMENTS

Thank you to the personnel of the Sensory Analysis and Human Nutrition Unit, ARC - Irene Analytical Services and the abattoir team at the ARC - ANPI, Irene, for their technical assistance that is greatly appreciated. The Meat and Livestock Australia (MLA), Red Meat Producers Organisation (RPO), National Emerging Red Meat Producers Organisation (NERPO) and the Agricultural Research Council (ARC – ANPI), Irene, who provided valuable funding for the project.