Nutritional quality of African catfish Clarias gariepinus (Burchell 1822): a positive criterion for the future development of the European production of Siluroidei

Original article

Nutritional quality of African catfish Clarias gariepinus (Burchell

1822): a positive criterion for the future development of the

European production of Siluroidei

Rui Rosa,1* Narcisa M. Bandarra & Maria Leonor Nunes

Departamento de Inovacao Tecnologica e Valorizacao dos Produtos da Pesca, IPIMAR, Avenida de Brasılia, 1449-006 Lisboa, Portugal

(Received 21 October 2005; Accepted in revised form 23 February 2006)

Summary The aims of this study were to evaluate the nutritional quality (proximate composition, amino and fatty acid

profiles, cholesterol and mineral contents) of African catfish (Clarias gariepinus) and determine the effect of

different cooking methods (grilling, boiling and frying) on biochemical composition. Frying produced the

highest water loss and fat gain (P < 0.05). Frying also affected the fatty acid composition significantly

(P < 0.05), increasing oleic (18:1n-9) and linoleic (18:2n-6) acid contents. The major essential amino acids

were arginine and lysine, and the limiting was the sulphur amino acid methionine. The changes in the

cholesterol and mineral contents (K > P > Na > Mg > Ca > Zn > Fe > Cu > Mn) and nutrient-

recommended dietary intakes are discussed and several indices (chemical score, amino acid score, essential

amino acid index, biological value, nutritional index, retention values, atherogenic index, thrombogenic

index and hypercholesterolaemic potential) are presented. The valorisation of these products may stimulate

the development of aquaculture production and consumers’ interest in Europe.

Keywords African catfish, biochemical composition, cooking effects, nutritional value.

Introduction

In Europe, as in many other parts of the world, theincreasing demand for fishery products continues tostimulate the further development and growth of bothmarine and freshwater aquaculture (FAO, 2005). How-ever, catfish (Siluroidei) farming in Europe is not aswidespread as in other regions, because of the weakconsumer response, which limits the effectiveness ofpromotional campaigns and continued producer interest(Varadi et al., 2001). An opposite situation occurs in theUnited States, where this industry is the leading sector offreshwater aquaculture and it is the fifth most consumedfish (0.36 kg per capita edible weight) (USDA, 2005).The catfish species have been of interest to producersbecause of their fast growth rates and efficient feedconversion (Hecht et al., 1996; Hargreaves & Tucker,

2003; Ali & Jauncey, 2005). Nevertheless, their growthpotential depends on environmental factors such asoptimum temperature, water quality or nutrients (Mat-ter et al., 2004).The fast growing African catfish Clarias gariepinus

(Burchell 1822) is propagated in Africa (mainly SouthAfrica and Nigeria) and in Europe and has beenpromoted in Netherlands, Germany and Belgium. Dur-ing the last decade, the intensive culture of C. gariepinushas also propagated in Southeast Asian countries,including the production of many interspecific hybrids(Khan et al., 2002).African catfish processing by filleting (90% of pro-

duction) gives added values to the fish and facilitatesmarketing (Proteau et al., 1996). As the nutritional dataare still incomplete for the African catfish C. gariepinus,the present study gives a comprehensive characterisationof the biochemical composition (proximate chemicalcomposition, energy, amino acids, fatty acids, choles-terol and mineral contents) of the fillets of this siluridspecies. During cooking, chemical and physical reactionstake place, which improve or prejudice the food nutri-tional value (e.g. digestibility is increased as a result ofprotein denaturation in food but the content of thermo-labile compounds, fat-soluble vitamins or polyunsatu-rated fatty acids is often reduced). Therefore, the present

*Correspondent: Fax: +(401) 8744256;

e-mail: [email protected]

This article was orally presented at the 35th Annual Meeting of

WEFTA, Antwerp, Belgium, 19–22 September 2005.

1Present address: Biological Sciences Center, University of Rhode

Island, 100 Flagg Road, Kingston, RI 02881, USA.

International Journal of Food Science and Technology 2007, 42, 342–351 342

doi:10.1111/j.1365-2621.2006.01256.x

� 2007 The Authors. Journal compilation � 2007 Institute of Food Science and Technology Trust Fund

study also evaluates the effect of several cooking meth-ods (boiling, frying and grilling) on the nutritional valueof catfish. Nutrient-recommended dietary intakes arediscussed and several indices [chemical score (CS), aminoacid score (AAS), essential amino acid index (EAAI),biological value (BV), retention values (RVs), athero-genic index (AI), thrombogenic index (TI) and hyper-cholesterolaemic potential] are presented.

Material and methods

Sample preparation and cooking

The specimens of C. gariepinus were reared in catfishfarms and purchased from a retailer market atIjmuiden, Holland. A total of ten fish with a meanweight of 2052 ± 294 g was provided. Fish werebeheaded, eviscerated and the bones and skinremoved. The fillets were randomly divided into fivegroups (each group with two fillets). As the filletcomposition differs in the various parts of the fillets,the fillets of each group were divided into smallerparts, which randomly received the four differenttreatments (raw, boiling, frying and grilling). Prior tocooking, the products were washed, drained and saltedin the following way: (1) boiled fish – addition of1.5% of salt to the cooking tap water (100 �C during7 min; relation fish/water – 1:2); (2) fried or grilledfish – spiked with 1.5% salt, and after 15 min the saltwas partially removed. Regarding frying, fillets werepreviously coated in wheat flour and fried in vegetableoil [mainly consist of linoleic acid (LA), 18:2n-6, andoleic acid, 18:1n-9] during 5 min with an initialtemperature of 180 �C. Grilling was done in anelectrical grill for 5 min with the thermostat tempera-ture set at 350 �C. After cooking, the products werehomogenised and the biochemical analyses were donein the five pooled groups (quintuplicate).

Proximate chemical composition and energy content

Moisture, protein, fat and ash content were determinedaccording to AOAC procedures (1998). Moisture con-tent was determined by constant weight drying in anoven at 100 �C, protein levels by a modified Kjeldahlmethod, using the value 6.25 as a conversion factor oftotal nitrogen content to protein, fat content using theSoxhlet extraction method with ethyl ether and ashdetermination was performed in a muffle furnace at550 �C to constant weight.The energy content was estimated according to FAO

(1987) report and calculated as proteins – 4.27 kcal g)1

wet weight; lipids – 9.02 kcal g)1 wet weight; carbohy-drates – 4.11 kcal g)1 wet weight (1 kcal ¼ 4.184 kJ). Inthis study, the carbohydrate fraction was subestimatedas it only quantified the glycogen content.

Amino acid analysis

In order to determine the total amino acid (TAA)profile, proteins were hydrolysed with 6 N hydrochloricacid (containing 0.1% phenol) in an MLS-1200 MegaMicrowave System (Milestone, Bergamo, Italy), at800 W, 160 �C for 10 min. The hydrolysis was per-formed under inert and anaerobic conditions to preventoxidative degradation of amino acids. The hydrolysateswere filtered and dissolved in sodium citrate buffer, pH2.2. It is noteworthy that with this hydrolysis proceduretryptophan was not determined. Amino acids wereseparated by ion exchange liquid chromatography in anautomatic analyser, Biochrom 20 (Amersham Bioscienc-es, Little Chalfont, UK), equipped with a column filledwith a polysulphonated resin (250 · 4.6 mm), usingthree sodium citrate buffers – pH 3.20, 4.25 and 6.45(Amersham Biosciences, Little Chalfont, UK) – andthree different temperatures (50, 58 and 95 �C). Thedetection of amino acids was done at 440 and 570 nmafter reaction with ninhydrin (Amersham Biosciences,Little Chalfont, UK). Amino acids were identified bycomparison of their retention times with those of specificstandards (Sigma, Sintra, Portugal) and quantified withthe software EZChromTM Chromatography DataSystem, vers. 6.7. (Scientific Software Inc., Cambridge,UK) using norleucine (Sigma) as internal standard.The CS and AAS were determined using the Block &

Mitchell (1946) method and FAO/WHO/UNU (1985)scoring pattern, respectively. The EAAI for the sum ofessential amino acids (EAAs) was determined using Oser(1951) method.The BV was calculated using the formula of Oser

(1959): BV ¼ 1.09 (EAAI) ) 11.7, and the nutritionalindex (NI) was calculated with the formula of Crisan &Sands (1978): NI ¼ (EAAI · % protein)/100.

Fatty acid analysis

The determination of fatty acid profile was based on theexperimental procedure of Lepage & Roy (1986)modified by Cohen et al. (1988). The fatty acid methylesters were analysed in a CP 3800 Varian gas Chroma-tograph (Varian Inc., Palo Alto, CA, USA), equippedwith an autosampler and a flame ionisation detector.The separation was carried out with helium as carriergas in a DB-Wax Polyethylene Glycol Column (AgilentTechnologies, S.L., Madrid, Spain) (30 m · 0.25 mmi.d.) programmed to start at 180 �C for 5 min, heating at4 �C min)1 for 10 min and hold up at 220 �C for25 min, with a detector at 250 �C. A split injector(100:1) at 250 �C was used. Fatty acid methyl esterswere identified by comparison of their retention timewith those of chromatographic Sigma standards.The AI and TI were calculated according to Ulbricht

& Southgate (1991) equations: AI ¼ (12:0 + 4 ·

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14:0 + 16:0)/[P

MUFA +P

PUFA (n-6) and (n-3)],where MUFA are the monounsaturated fatty acids andPUFA the polyunsaturated fatty acids. The 12:0 was notdetected in the samples and therefore not taken intoaccount for the calculations; TI ¼ (14:0 + 16:0 +18:0)/[0.5 ·

PMUFA + 0.5 ·

PPUFA (n-6) + 3 ·P

PUFA (n-3) + (n-3)/(n-6)].

Cholesterol analysis

The quantification of cholesterol content was based onthe experimental procedure of Naemmi et al. (1995)modified by Oehlenschlager (2000). It was analysed in aHewlett Packard 5890 Gas Chromatograph (Palo Alto,CA, USA) and the separation was carried out withhelium as carrier gas in a column HP5 (30 m · 0.5 mmi.d.). The temperatures of the oven, injector and detectorwere 280, 285 and 300 �C, respectively. Cholesterol wasidentified and quantified by comparison with a purestandard (Sigma) from which a calibration curve wasprepared. The cholesterol-saturated fat index (CSI) wasdetermined according to Connor et al. (1986) as follows:CSI ¼ (1.01 · g of SFA 100 g)1 of wet weight) +(0.05 · mg of cholesterol 100 g)1 of wet weight), whereSFA are the saturated fatty acids. The cholesterol index(CI) was calculated according to Zilversmit (1979)equation: 1.01 · (g of SFA 100 g)1 of wetweight )0.5 · g of PUFA 100 g)1 of wet weight) +(0.06 · mg of cholesterol 100 g)1 of wet weight).

Mineral analyses

Phosphorus was analysed by UV–visible spectropho-tometry according to ISO/TC 34/SC6 N371 (1991). Allthe other minerals were measured by flame atomicabsorption spectrometry (in a Varian Spectr AA 55B,Varian Inc., Palo Alto, CA, USA) after dry ashing,according to Official Methods of Analysis (AOAC,1995) and Jorhem (2000).

Nutrient retention values

True RVs were calculated using the following formula(USDA, 2002): RV (%) ¼ [(nutrient content per g

of cooked food · g of food after cooking)/nutrientcontent per g of raw food · g of food beforecooking)] · 100.

Statistical analysis

Biochemical data were analysed using an anova whencomparing multiple groups (k > 3). Normality andhomogeneity of variances were verified by Kolmogorov–Smirnov and Bartlett tests, respectively. Havingdemonstrated significant differences among the groupswith anova, Tukey test was used to establish what thosedifferences were (Zar, 1996).

Results and discussion

Proximate chemical composition and energy

The proximate chemical composition of the catfish fillets(in raw and cooked forms) is showed in Table 1. Allcooking methods resulted in a significant water loss(P < 0.05), being the highest obtained with frying.Fried fish had the highest fat gain (P < 0.05), which isdue to the oil penetration after water is partially lost byevaporation. Consequently, frying provided a signifi-cantly higher RV of fat (Table 3; P < 0.05), whichresulted in a higher energy retention (P < 0.05). It isalso noteworthy that by using wheat flour to cover friedfish there is an addition of carbohydrates to the cookedfillet, which was reflected in the total sum of theproximate composition (�97%, contrary to 99% forthe rest of the treatments).Protein and ash contents also varied significantly

between treatments (P < 0.05), with the highest valuesobserved in the grilled product. These differences alsoresulted from water loss and, consequently, the energycontent of the cooked forms was higher (P < 0.05). Theprotein RVs for boiling, frying and grilling had a valuearound 100% (P > 0.05) confirming that this nutrient isnot susceptible to cooking losses but only to a concen-tration effect (caused by moisture loss). The water lossand higher organic content in the cooked fish in relationto the raw fish are in accordance with the findingsof other studies in fish products (Gall et al., 1983;

Table 1 Proximate chemical composition

(% wet weight), energy (kJ 100 g-1 wet

weight) and cholesterol content (mg 100 g wet

weight) of raw and cooked forms of catfish,

Clarias gariepinus

Raw Boiled Fried Grilled

Moisture 75.68 ± 0.58a 71.08 ± 0.59b 63.32 ± 2.16c 65.76 ± 2.04c

Protein 16.80 ± 0.44a 21.14 ± 0.51b 21.82 ± 1.64b 24.28 ± 1.99c

Fat 5.70 ± 0.16a 5.90 ± 0.60a 9.30 ± 1.75b 6.88 ± 1.11a

Ash 1.00 ± 0.00a 1.20 ± 0.00b 2.30 ± 0.21c 2.62 ± 0.23d

Energy 457.90 ± 26.57a 600.34 ± 20.04b 740.81 ± 70.95c 693.43 ± 43.87c

Cholesterol 11.04 ± 0.43a 14.23 ± 1.59b 13.82 ± 2.43ab 19.84 ± 1.01c

Values are the mean ± SD. Different superscript letters within rows represent significant differences

(P < 0.05).

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Castrillon et al., 1997; Wu & Lillard, 1998; Garcıa-Ariaset al., 2003a, 2004; Gokoglu et al., 2004; Kalogeropo-ulos et al., 2004).

Amino acids

Table 2 shows the amino acid profile of raw and cookedcatfish. In all products, the quantitatively most import-ant EAA were, by order of decreasing magnitude, lysine,leucine and arginine; in relation to non-EAAs (NEAA),the major were glutamic acid, aspartic acid, alanine,glycine and proline. TAA, EAA and NEAA contentsvaried significantly between treatments (P < 0.05) andthe highest values were observed in grilling, followed byfrying. The amino acid RVs were also higher in thesecooking processes (Table 3). The ratio of EAA andNEAA varied between 0.97 and 0.99, which reveals thatthis species is well balanced with respect to EAA andmay be considered as a food source of high-qualityprotein. The CS, AAS, EAAI, BV and NI are showed inTable 4. These values are similar to those found in theliterature (if the same standards were considered)(Iwasaki & Harada, 1985; Garcıa-Arias et al., 2003b).The CS suggested that the first limiting amino acid wasmethionine and the second isoleucine. In fact, thesulphur-containing amino acids are generally found tobe primarily limiting in shellfish and fish meal (Acton &

Rudd, 1987). It is worth noting that the cooked formshad slightly higher values for the other calculated indices(EAAI, BV, NI) than the raw product. Therefore, thesecooking processes do not seem to induce losses in theprotein nutritional quality.

Fatty acids

The fatty acid profile of raw and cooked catfish isshowed in Table 5. There were no significant differencesin the SFA fraction between treatments (P > 0.05),neither in the most predominant fatty acids, e.g. palmiticacid (16:0) (P > 0.05). Nonetheless, the second majorSFA, stearic acid (18:0), revealed a significant increase infrying (P < 0.05), which significantly affected its RV(Table 6).With respect to the MUFA fraction, it varied signifi-

cantly between the raw and cooked products (P < 0.05),because of the significant increase of the most commonMUFA – oleic acid, 18:1n-9 (P < 0.05). Consequently,the RVs of total MUFA and 18:1n-9 were alsosignificantly higher (P < 0.05) in fried fish (Table 6).

Table 3 True nutrient (proximate chemical composition, cholesterol

and amino acids) retention values (%) for cooked catfish, Clarias

gariepinus

Boiled Fried Grilled

Moisture 74.74 ± 0.63a 64.29 ± 2.40b 64.01 ± 2.33b

Protein 100.17 ± 2.65 99.78 ± 6.86 106.37 ± 7.09

Fat 82.45 ± 9.10a 125.25 ± 22.90b 88.73 ± 12.74a

Ash 95.50 ± 0.00a 176.71 ± 16.30b 192.96 ± 16.79b

Energy 104.49 ± 4.03a 124.58 ± 13.51b 111.92 ± 10.58a

Cholesterol 107.57 ± 5.18a 98.60 ± 15.16a 132.46 ± 6.14b

Essential amino acids (EAA)

Threonine 108.00 ± 3.72a 118.34 ± 6.54ab 120.48 ± 4.69b

Methionine 108.58 ± 3.73 106.78 ± 10.04 109.34 ± 6.79

Isoleucine 108.85 ± 4.17 120.22 ± 5.45 119.47 ± 5.22

Leucine 108.08 ± 3.79a 120.95 ± 6.14b 120.51 ± 3.71b

Phenylalanine 109.09 ± 4.24 118.79 ± 7.50 117.48 ± 2.98

Valine 110.92 ± 4.51 123.01 ± 4.79 122.38 ± 6.43

Lysine 104.89 ± 3.28a 118.58 ± 4.86b 118.46 ± 4.99b

Histidine 108.84 ± 5.24 120.25 ± 9.71 117.64 ± 7.03

Arginine 110.24 ± 3.90a 122.50 ± 5.05b 121.34 ± 5.90b

PEAA 108.23 ± 3.76a 119.65 ± 5.93b 119.30 ± 4.81b

Non-essential amino acids (NEAA)

Aspartic acid 109.56 ± 5.85a 120.94 ± 4.82b 122.29 ± 4.83b

Serine 108.69 ± 3.92 118.55 ± 6.75 121.70 ± 5.07

Glutamic acid 106.62 ± 4.17a 122.08 ± 4.95b 120.41 ± 4.61b

Glycine 113.76 ± 12.15 128.71 ± 9.16 127.76 ± 13.61

Alanine 115.50 ± 9.20 128.68 ± 7.18 126.93 ± 7.30

Tyrosine 108.18 ± 2.95 116.39 ± 8.26 118.61 ± 3.68

Proline 110.85 ± 11.51 103.69 ± 24.64 125.08 ± 22.25P

NEAA 109.60 ± 4.45a 120.54 ± 2.06b 122.60 ± 6.56b

PTAA 108.92 ± 4.08a 120.09 ± 3.93b 120.95 ± 5.63b

Values are the mean ± SD. Different superscript letters within rows

represent significant differences (P < 0.05).

Table 2 Total amino acid profile (% wet weight) raw and cooked

forms of catfish, Clarias gariepinus

Amino acids Raw Boiled Fried Grilled

Essential (EAA)

Threonine 0.63 ± 0.06a 0.86 ± 0.09b 0.97 ± 0.12b 1.03 ± 0.11b

Methionine 0.35 ± 0.04a 0.48 ± 0.05b 0.49 ± 0.08b 0.52 ± 0.06b

Isoleucine 0.64 ± 0.06a 0.88 ± 0.09b 1.01 ± 0.11b 1.04 ± 0.12b

Leucine 1.27 ± 0.13a 1.72 ± 0.16b 2.00 ± 0.23b 2.08 ± 0.24b

Phenylalanine 0.68 ± 0.07a 0.93 ± 0.09b 1.05 ± 0.12b 1.08 ± 0.12b

Valine 0.79 ± 0.09a 1.09 ± 0.11b 1.26 ± 0.14b 1.31 ± 0.17b

Lysine 1.49 ± 0.12a 1.96 ± 0.18b 2.29 ± 0.23b 2.39 ± 0.27b

Histidine 0.39 ± 0.04a 0.53 ± 0.07b 0.60 ± 0.06b 0.62 ± 0.08b

Arginine 0.94 ± 0.08a 1.30 ± 0.13b 1.50 ± 0.15c 1.55 ± 0.19c

PEAA 7.17 ± 0.69a 9.75 ± 0.95bc 11.17 ± 1.26c 11.62 ± 1.35c

Non-essential (NEAA)

Aspartic acid 1.51 ± 0.17a 2.07 ± 0.19b 2.38 ± 0.29bc 2.50 ± 0.28c

Serine 0.56 ± 0.06a 0.77 ± 0.07b 0.87 ± 0.11b 0.93 ± 0.11b

Glutamic acid 2.22 ± 0.22a 2.97 ± 0.27b 3.53 ± 0.37c 3.64 ± 0.44c

Glycine 0.73 ± 0.01a 1.04 ± 0.12b 1.22 ± 0.11b 1.26 ± 0.15b

Alanine 0.90 ± 0.13a 1.29 ± 0.13b 1.49 ± 0.15b 1.54 ± 0.21b

Tyrosine 0.56 ± 0.04a 0.76 ± 0.07b 0.85 ± 0.10b 0.90 ± 0.09b

Proline 0.75 ± 0.03a 1.04 ± 0.10b 1.00 ± 0.20b 1.27 ± 0.19b

PNEAA 7.22 ± 0.66a 9.95 ± 0.96b 11.34 ± 1.34bc 12.04 ± 1.47bc

EAA/NEAA 0.99 ± 0.01 0.98 ± 0.00 0.98 ± 0.03 0.97 ± 0.01P

TAA 14.39 ± 1.36a 19.69 ± 1.91bc 22.51 ± 2.60c 23.66 ± 2.82c





The linoleic acid (LA) (18:2n-6), EPA (20:5n-3) anddocosahexaenoic acid (DHA) (22:6n-3) were the dom-inant PUFA. However, only the 18:2n-6 increasedsignificantly with frying (LA: P < 0.05; EPA/DHA:P > 0.05). LA gain was reflected in the PUFA, n-6 andtotal FA contents (P < 0.05) and their respective RVs(Table 6). The consequent decrease in n-3/n-6 ratio(P < 0.05) is in agreement with previous reports (San-chez-Muniz et al., 1992; Echarte et al., 2001).Fish long-chain n-3 PUFA, especially EPA and DHA,

are hypotriglyceridemic and are important in theprevention of human cardiovascular and inflammatorydiseases (Lands, 1986). In order to measure the propen-sity of the catfish diet to influence the incidence ofcoronary heart disease, the AI and TI were calculated(Table 7). The AI varied significantly between thetreatments (P < 0.05) and found lower in the friedsamples (because of oil absorption), but the TI did not(P > 0.05). The AI values obtained in the catfish werelower than those in other animal foods, such as lamb(Salvatori et al., 2004), rabbit (Dal Bosco et al., 2001),beef, pork, cod, sardine (Paul et al., 1980; Perez-Llamaset al., 1998), anchovy, mackerel, mussel (Kalogeropou-los et al., 2004), sea bass (Krajnovic-Ozretic et al., 1994)and gilthead sea bream (Trigari et al., 1997), similar tochicken (Paul et al., 1980) and other finfish foods,namely red porgy (Rueda et al., 1997), sharpsnout seabream (Rueda et al., 2001) and tuna (Paul et al., 1980),and higher than squid (Kalogeropoulos et al., 2004),penaeid shrimps and Norway lobster (Rosa & Nunes,2004). Values of TI were lower than those reported byPaul et al. (1980), Perez-Llamas et al. (1998), Kaloge-ropoulos et al. (2004), similar to those from Rueda et al.

(2001) and higher than those obtained by Rosa & Nunes(2004).With respect to dietary intakes for n-3 fatty acids and

their consumption in Western countries, numerousofficial committees of nutrition and scientific societiessuch as the International Society for the Study of Fattyacids and Lipids have proposed recommendations forn-3 PUFA intake to cover human requirementsthroughout life (Galli, 2000; Simopoulos et al., 2000).For example, for the linolenic acid (a-LNA; 18:3n-3),the precursor of the n-3 series, the recommended dailyintake is 1.6 and 2.2 g day)1 for adult males andfemales, respectively, and for DHA is 100–200 and120–240 mg day)1 . The consumption of 100 g of catfishproducts may contribute 3–4% and 5–7% of therecommended daily a-LNA intake for females andmales, respectively, and 240–290% and 290–350% ofthe recommended DHA for females and males, respect-ively.It is worth noting that the estimated daily intake of

n-3 PUFA in Western countries varies largely, but isoften under the recommended intakes (Alessandri et al.,2004). The mean a-LNA intake rarely reaches therecommended values (Meyer et al., 2003) and the meanDHA intake mainly depends on fish consumption,which can differ greatly between countries (Welch et al.,2002). The latter may well cover recommended values incountries with fish-eating habits, like Portugal.

Cholesterol

Cholesterol content varied significantly (P < 0.05)between raw and cooked products (Table 1); the highest

Table 4 Chemical score (CS), amino acid score (AAS), essential amino acid index (EAAI), biological value (BV) and nutritional index (NI) of

catfish, Clarias gariepinus

Amino acid

Egg patterna FAO/WHO/UNU 1991b Egg ratioc (log10)

Raw Boiled Fried Grilled Raw Boiled Fried Grilled Raw Boiled Fried Grilled

Threonine 73.40 79.12 87.15 83.07 110.53 119.14 131.24 125.09 1.87 1.90 1.94 1.92

Methionine 36.29 39.32 38.94 37.20 84.05 91.08 90.19 86.16 1.56 1.59 1.59 1.57

Isoleucine 60.75 66.03 73.25 68.23 136.48 148.33 164.55 153.27 1.78 1.82 1.86 1.83

Leucine 85.66 92.32 103.82 97.02 114.48 123.37 138.74 129.65 1.93 1.97 2.02 1.99

Phenylalanine + tyrosine 74.51 80.86 87.94 82.68 116.97 126.94 138.05 129.79 1.87 1.91 1.94 1.92

Valine 68.24 75.50 84.12 78.50 133.56 147.77 164.64 153.64 1.83 1.88 1.92 1.89

Lysine 126.72 132.74 150.66 141.20 152.50 159.75 181.32 169.93 2.10 2.12 2.18 2.15

CS AAS EAAI (anti logP

/7)

36.29 39.32 38.94 37.20 84.05 91.08 90.19 86.16 70.82 76.53 83.69 78.73

BVd 65.49 71.71 79.53 74.12

NIe 11.00 15.16 17.35 18.00

aAccording to method described by Block & Mitchell (1946).bScoring pattern of FAO/WHO/UNU (1985).cAccording to method described by Oser (1951).dAccording to method described by Oser (1959).eAccording to method described by Crisan & Sands (1978).



values were observed in the grilled product. Conse-quently, cholesterol RV for grilling was rather high(P < 0.05), a phenomenon for which there is no readyexplanation (Table 3).According to the American Heart Association, at

least two servings of fish per week are recommendedto confer cardioprotective effects (Krauss et al., 2000).As the hypercholesterolaemic–atherogenic potential ofthe food lies in its cholesterol and fatty acid compo-sition, the CSI and CI were determined (Table 7). Thefirst index may be used to compare different foods(Connor et al., 1986), while CI expresses the relativeeffect of individual portions of foods on the serumcholesterol of an average individual (Zilversmit, 1979).

In the present study, the CSI and CI varied signifi-cantly between treatments (P < 0.05) and the highestvalues were attained in the grilled products. Althoughthe grilling values were unexpected, frying has beenreported to increase the cholesterol levels in fish food(Ewaida, 1993; Echarte et al., 2001). The CSI and CIresults are comparable with those referenced in theliterature (Connor et al., 1986; Sanchez-Muniz et al.,1992; Kalogeropoulos et al., 2004). As the dietarycholesterol intake should be limited at £300 mg perday (AHA – American Heart Association, 2005),the consumption of 100 g of catfish products maycontribute 42–60% of the recommended maximumcholesterol intake.

Table 5 Fatty acid composition (mg 100 g

wet weight) of raw and cooked forms of

catfish, Clarias gariepinus

Fatty acids Raw Boiled Fried Grilled

14:0 202.75 ± 12.83 218.62 ± 25.68 209.34 ± 49.74 255.55 ± 50.79

15:0 21.01 ± 1.30 22.60 ± 2.63 21.60 ± 4.70 26.06 ± 5.02

16:0 1069.59 ± 32.58 1121.11 ± 135.46 1495.15 ± 664.03 1314.46 ± 275.41

17:0 16.25 ± 0.38 16.43 ± 2.15 16.94 ± 3.48 19.71 ± 3.37

18:0 283.67 ± 24.00a 292.41 ± 51.90ab 400.85 ± 86.36b 341.30 ± 66.87ab

19:0 9.00 ± 0.86 10.38 ± 0.89 10.28 ± 2.40 12.72 ± 3.49

20:0 9.41 ± 0.19 9.41 ± 0.72 15.48 ± 3.00 10.25 ± 1.44P

Saturated 1611.67 ± 43.40 1690.97 ± 207.79 2169.64 ± 790.19 1980.05 ± 401.94

Iso 14:0 7.55 ± 0.64 7.91 ± 1.02 7.37 ± 1.62 8.89 ± 1.44

Iso 16:0 2.79 ± 0.24 3.23 ± 0.60 2.90 ± 0.71 4.57 ± 2.38

Anteiso 16:0 5.18 ± 0.15 5.47 ± 0.68 5.13 ± 1.24 5.83 ± 1.56P

Branched 15.52 ± 0.87 16.62 ± 2.18 15.40 ± 3.44 19.30 ± 3.69

16:1n-7 252.08 ± 9.91 264.07 ± 30.22 255.92 ± 60.71 313.10 ± 63.72

17:1n-8 5.18 ± 0.15 5.33 ± 0.52 4.81 ± 1.16 6.15 ± 1.10

18:1n-9 842.39 ± 52.64a 853.35 ± 131.29a 1707.79 ± 313.91b 947.98 ± 202.25a

18:1n-7 134.77 ± 5.44 140.04 ± 14.20 158.60 ± 32.79 165.76 ± 26.36

20:1n-9 156.72 ± 6.15 152.93 ± 13.94 150.36 ± 32.57 174.70 ± 27.22

20:1n-7 9.73 ± 0.63 9.43 ± 1.08 9.78 ± 2.60 10.75 ± 1.61

22:1n-11 107.53 ± 5.23 97.46 ± 7.54 93.36 ± 23.21 107.26 ± 13.38

22:1n-9 13.04 ± 0.58 6.85 ± 6.27 13.06 ± 3.13 7.37 ± 6.81P

Monounsaturated 1521.45 ± 54.08a 1529.46 ± 178.23a 2393.67 ± 458.42b 1733.08 ± 311.73a

16:3n-3 10.05 ± 0.55 10.08 ± 1.22 10.31 ± 2.42 11.69 ± 2.09

16:4n-3 20.59 ± 0.54 23.08 ± 3.02 22.80 ± 5.96 28.40 ± 4.93

18:2n-6 541.32 ± 17.25a 612.12 ± 71.97a 2582.39 ± 413.88b 679.46 ± 107.99a

18:3n-6 10.36 ± 0.73 10.17 ± 1.69 10.83 ± 2.64 10.93 ± 1.07

18:3n-3 83.83 ± 2.42 87.39 ± 9.62 89.51 ± 20.71 104.95 ± 18.65

18:4n-3 65.09 ± 3.39 68.04 ± 5.98 67.09 ± 15.26 83.26 ± 14.41

20:4n-6 24.63 ± 1.55 26.93 ± 2.33 27.63 ± 5.45 32.48 ± 5.46

20:3n-3 7.87 ± 0.43 4.74 ± 4.34 3.27 ± 4.61 1.83 ± 4.10

20:4n-3 42.96 ± 1.47 43.33 ± 4.80 43.09 ± 9.83 51.01 ± 8.04

20:5n-3 295.74 ± 15.53 310.64 ± 30.08 315.78 ± 67.32 379.03 ± 56.69

22:4n-6 17.08 ± 1.02 16.87 ± 2.41 16.90 ± 4.55 20.05 ± 2.13

22:5n-6 6.83 ± 3.86 5.17 ± 4.76 3.55 ± 4.46 5.50 ± 5.02

22:5n-3 88.95 ± 4.89 88.69 ± 7.25 94.21 ± 22.67 106.08 ± 15.47

22:6n-3 572.79 ± 29.78 587.12 ± 46.89 612.02 ± 136.82 697.84 ± 81.11P

Polyunsaturated 1788.07 ± 69.81a 1894.37 ± 149.62a 3899.38 ± 691.53b 2212.51 ± 306.08a

P(n-3) 1187.86 ± 51.71 1223.11 ± 104.89 1258.07 ± 280.32 1464.10 ± 196.78

P(n-6) 600.21 ± 18.19a 671.26 ± 74.32a 2641.31 ± 424.86b 748.42 ± 111.93a

(n-3)/(n-6) 102.40 ± 3.74ab 98.35 ± 13.91b 40.78 ± 10.38c 122.80 ± 18.74a

Total 4936.70 ± 136.99a 5131.41 ± 531.83a 8478.09 ± 1834.15b 5944.94 ± 1004.71a

Values are the mean ± SD. Different superscript letters within rows represent significant differences

(P < 0.05).



Minerals

In Table 8, the mineral profiles of raw and cookedcatfish are presented. The mineral concentration indescending order was K > P> Na> Mg > Ca>Zn> Fe> Cu> Mn. In fact, as in other fish andshellfish products (Sidwell et al., 1978; Oehlenschlager,1997), potassium was the major mineral and signifi-cant differences between treatments were detected(P < 0.05). The highest concentration of this elementand consequently the highest RV (P < 0.05) wereobtained in the grilled products ) 5063.53 mg kg)1 wetweight (Table 9). It is worth noting that the recommen-ded dietary allowance (RDA) of this mineral is 3500 mg(HWN, 2005). Recent studies have found that anincrease in the intake of potassium with calcium andmagnesium decreased the blood pressure, thereforereducing the risk of hypertension and stroke (Reddy &Katan, 2004). The mechanism is not, as yet, established,although a suggested possibility for the potassium’shypotensive effect lies in the rennin–angiotensin–aldosterone system (Suter, 1999; Fang, 2000). Thismineral may also be associated with the prevention ofosteoporosis by maintaining the bone mineral density(New et al., 2000).The next quantitatively most important minerals were

phosphorus and sodium. Their levels were also signifi-cantly affected by cooking methods (P < 0.05). Thehighest RVs (P < 0.05) were observed in grilled andfried products (Table 9). In relation to the former,according to Oehlenschlager (1997), there is a relationbetween fat and phosphorous contents. Lean fish speciescontain usually lower values than fatty species. TheRDA is 1000 mg (HWN, 2005), and regarding itspossible therapeutic uses it has been recommended forthe treatment of arthritis, stress and tooth/gum disor-ders (Fang, 2000). The daily intake of sodium should belower than 2400 mg (HWN, 2005). Typically, anexcessively high intake of sodium is much more likelythan deficiency. This element is the main cation inextracellular fluid and acts in the body’s acid–basebalance and in the transmission of nerve impulses. TheNa/K ratio was always below 1.0, which is interestingfrom the point of view of nutrition, as ratios higher than1.5 have been related to the possible incidence ofhypertension (Nutinf, 2005).The fourth mineral most abundant in fish muscle was

magnesium and its content also varied between cookingmethods (highest values in the grilled products;P < 0.05). The RDA is 400 mg (HWN, 2005). In thehuman body, this mineral is found mostly in the bone.On the contrary, it is a component of more than 300enzymatic reactions, necessary for neuromuscular trans-mission and in addition is required for cell metabolism,protein and nucleic acid synthesis (Weisinger & Bellorin,1998; Saris et al., 2000).

Table 6 True fatty acid retention values (%) for cooked catfish, Clarias

gariepinus

Fatty acids Boiled Fried Grilled

14:0 86.08 ± 11.52 79.95 ± 21.64 93.24 ± 19.83

15:0 86.02 ± 12.64 79.56 ± 19.81 91.60 ± 17.91

16:0 83.56 ± 11.32 107.84 ± 49.95 90.21 ± 16.62

17:0 80.57 ± 11.44 80.09 ± 16.32 89.29 ± 14.65

18:0 81.75 ± 9.64a 108.05 ± 18.56b 88.27 ± 13.57a

19:0 92.94 ± 15.78 89.52 ± 27.47 103.81 ± 23.26

20:0 79.68 ± 7.54a 126.46 ± 25.24 80.14 ± 10.67P

Saturated 83.58 ± 10.98 103.51 ± 38.49 90.23 ± 16.44

Iso 14:0 83.61 ± 10.24 75.85 ± 20.15 87.24 ± 15.82

Iso 16:0 92.38 ± 17.32 80.21 ± 18.69 119.71 ± 54.23

Anteiso 16:0 84.26 ± 10.99 76.16 ± 18.56 82.81 ± 20.88P

Branched 85.38 ± 11.56 76.51 ± 17.82 91.79 ± 17.71

16:1n-7 83.61 ± 11.60 78.29 ± 20.11 91.46 ± 17.86

17:1n-8 82.14 ± 9.72 71.32 ± 17.21 87.38 ± 13.77

18:1n-9 80.52 ± 10.06a 155.76 ± 27.31b 82.59 ± 14.55a

18:1n-7 82.86 ± 9.89 90.41 ± 18.52 90.50 ± 13.04

20:1n-9 77.83 ± 8.67 73.60 ± 15.29 81.97 ± 10.93

20:1n-7 77.33 ± 10.01 76.73 ± 18.47 81.40 ± 10.91

22:1n-11 72.26 ± 6.45 66.46 ± 14.77 73.47 ± 8.37

22:1n-9 40.91 ± 37.63 77.24 ± 19.33 42.24 ± 39.06P

Monounsaturated 80.05 ± 9.48a 120.81 ± 22.65b 83.68 ± 12.87a

16:3n-3 80.10 ± 11.02 78.98 ± 19.15 85.44 ± 12.18

16:4n-3 89.26 ± 12.00 84.98 ± 21.92 101.73 ± 18.61

18:2n-6 90.17 ± 12.14a 366.08 ± 54.64b 92.53 ± 15.20a

18:3n-6 77.87 ± 9.63 80.61 ± 20.88 77.66 ± 4.30

18:3n-3 83.13 ± 10.69 82.09 ± 19.38 92.12 ± 15.30

18:4n-3 83.50 ± 10.38 79.39 ± 18.69 94.35 ± 16.40

20:4n-6 87.21 ± 8.36 86.35 ± 16.82 97.54 ± 18.57

20:3n-3 46.82 ± 43.09 32.11 ± 45.43 17.71 ± 39.59

20:4n-3 80.38 ± 9.73 76.95 ± 17.23 87.26 ± 11.45

20:5n-3 83.69 ± 8.33 82.00 ± 16.69 94.83 ± 17.07

22:4n-6 78.43 ± 8.64 75.32 ± 16.80 86.52 ± 8.43

22:5n-6 29.53 ± 40.45 16.26 ± 32.12 32.99 ± 45.38

22:5n-3 79.51 ± 7.40 81.08 ± 18.52 87.61 ± 9.26

22:6n-3 81.68 ± 7.08 81.89 ± 16.74 89.95 ± 11.65P

Polyunsaturated 84.44 ± 7.98a 167.33 ± 27.78b 91.26 ± 13.32a

P(n-3) 82.04 ± 7.77 81.24 ± 17.17 90.94 ± 13.10

P(n-6) 89.15 ± 11.23a 337.75 ± 51.16b 91.90 ± 14.05a

Total 82.80 ± 9.29a 131.87 ± 28.41b 88.59 ± 13.91a



Table 7 Atherogenic (AI), thrombogenic (TI), cholesterol-saturated

fat index (CSI) and cholesterol (CI) indices of the raw and cooked

forms of catfish, Clarias gariepinus


AI 0.57 ± 0.02a 0.58 ± 0.02a 0.36 ± 0.08b 0.59 ± 0.04a

TI 0.33 ± 0.01 0.33 ± 0.02 0.33 ± 0.08 0.33 ± 0.02

CSI 2.18 ± 0.06a 2.42 ± 0.16ab 2.88 ± 0.81ab 2.99 ± 0.44b

CI 1.39 ± 0.05a 1.61 ± 0.09b 1.05 ± 0.55ab 2.07 ± 0.30b





Regarding calcium, the highest values were attained inthe fried products (110.96 mg kg)1; P < 0.05). TheRDA for this element is 1200 mg (HWN, 2005). Likecatfish, other fish products are not usually a good sourceof Ca with the exception of canned fish, which enablessoft bones to be eaten safely. A deficiency in calcium andmagnesium has been associated with an increased risk ofhypertension (Paolisso & Barbagallo, 1997; Nutinf,2005).Concerning the other minerals analysed, only iron and

zinc did not vary significantly with the cooking proce-dures (P > 0.05). The RDA for these elements are15 mg and 15–19 mg, respectively. Such values indicatethat catfish is a good source of these elements. Iron is anessential nutrient that carries oxygen and forms part ofthe oxygen-carrying proteins and zinc is an essentialconstituent in a number of enzymes, including RNA andDNA polymerases.Relatively to copper, the highest value was attained in

the grilled products (0.50 mg kg)1; P < 0.05). Thisessential trace element is required as a component ofseveral enzymes and the RDA is between 1.5 and 3 mg(HWN, 2005). These values indicate that catfish is also agood source of this element. Manganese content alsovaried significantly between cooking procedures

(P < 0.05). This mineral plays an essential part ofproper bone and cartilage formation and in glucosemetabolism, and the RDA is 2–5 mg (HWN, 2005).

Conclusions

The results of this study reveal the high nutritionalquality of catfish products and, consequently, they arevaluable protein, lipid and mineral sources to humandiet. Concomitantly, the present study also revealed thatthe different cooking procedures have a significant effecton their biochemical composition, primarily frying. Theuse of this species in diets on regular basis should bevaluable, especially if it replaces foods of animal originwith high fats. Dietary fats associated with an increasedrisk of cardiovascular diseases include trans fats andsaturated fats, while polyunsaturated fats are known tobe protective. The favourable Na/K ratio (below 1.0) isalso very important in terms of human health, as dietarysodium is associated with elevation of blood pressure,while dietary potassium lowers the risk of hypertensionand stroke. Additionally, the nutritional quality ofcatfish should be a positive criterion for the futuredevelopment of the European production of Siluroidei.

Acknowledgments

This study was supported by the project ‘ConsumerDriven Development of Innovative Tailor-Made Sea-food Products (with Functional Components from Plantor Marine Origin) to Improve Health of Consumers(CONPROD)’ under the Integrated Project SEA-FOODplus, granted by European Union (Contract no.506359–2003).

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Table 8 Mineral content (in wet weight) of the

raw and cooked forms of catfish, Clarias

gariepinus


P (g per 100 g) 0.18 ± 0.01a 0.19 ± 0.00a 0.25 ± 0.02b 0.26 ± 0.02b

K (mg kg)1) 3014.43 ± 187.19a 3570.38 ± 280.12a 4371.12 ± 314.69b 5063.53 ± 340.45c

Na (mg kg)1) 406.34 ± 97.24a 1656.06 ± 316.39b 3843.81 ± 566.87c 4583.4 ± 322.13d

Mg (mg kg)1) 237.85 ± 16.01a 303.30 ± 30.07b 324.76 ± 22.50bc 357.22 ± 28.49c

Ca (mg kg)1) 46.65 ± 11.68a 67.68 ± 26.11a 110.96 ± 19.55b 66.48 ± 17.41a

Fe (mg kg)1) 5.42 ± 0.70 6.42 ± 0.87 9.08 ± 4.98 8.08 ± 2.03

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Cu (mg kg)1) 0.36 ± 0.02a 0.43 ± 0.03ab 0.46 ± 0.06b 0.50 ± 0.07b

Mn (mg kg)1) 0.11 ± 0.01a 0.16 ± 0.02ac 0.41 ± 0.07b 0.18 ± 0.03c

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Table 9 True mineral retention values (%) for cooked catfish, Clarias

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Documents

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