Abstract
Chemical composition viz., fatty acids, amino acids and minerals of yellowfin tuna (Thunnus albacares) eye were analyzed for better utilization of fish processing discards. Analysis of fatty acids composition by gas chromatography–mass spectrometry revealed the presence of polyunsaturated fatty acids namely docosahexaenoic acid, eicosapentaenoic acid, arachidonic acid and linoleic acid at the levels of 37.8, 7.1, 3.6 and 1.4%, respectively. The major available monounsaturated fatty acids, palmitoleic acid and oleic acid were present at the level of 17.9 and 18.3% respectively. Myristic acid was the dominant saturated fatty acid and occupied 2% of the total fatty acids. Amino acid analysis by high-performance liquid chromatography showed that the tuna eye has a high concentration of glycine (19.24 mg/100 g) followed by glutamic acid (16 mg/100 g) and aspartic acid (12.76 mg/100 g). Analysis of mineral content of tuna eye showed the presence of higher sodium and lower copper content. The results revealed that yellowfin tuna eye could be used as a potential source of ω-3 fatty acids and essential amino acids.
Keywords: Yellowfin tuna eye, Polyunsaturated fatty acid, Amino acid, Mineral
Introduction
Fish is realized as an essential source of nutrients and fish processing industries are the major economic force for many countries in the world. Seafood processing generates more than 60% of the raw material as processing discards which include head, skin, trimmings, fins, frames, viscera and roes (Dekkers et al. 2011). Fish processing discards in developing countries are disposed as waste or converted into low value products such as animal feed, fish meal and fertilizer (Kristinsson and Rasco 2000). Since the disposal of processing discards is under strict regulations due to environmental issues, the processing industries are advised to eliminate the discards in a proper way. This process increases the operational cost of seafood industries. So the utilization of fish processing discards in an effective manner to produce high value by-products is gaining importance in several countries.
In the way ahead, quality evaluation and biochemical characterization with reference to specific discards (head, eye, fin, tail) is essential to develop appropriate technology for better utilization. Biochemical characterization will help to understand/identify major biomolecules with specific properties. Fish processing discards are good source of high value compounds such as ω-3 polyunsaturated fatty acids (PUFA), bioactive peptides, polysaccharides, minerals, vitamins, antioxidants and enzymes (Kim and Wijesekara 2010).
During the year 2014–2015, marine products export from India reached 10,51,243 tonnes and there are 465 seafood freezing plants along the Indian coast, with a built in capacity of 20,256 MT (MPEDA 2015). In Indian Exclusive Economic Zone, tuna potential is estimated as 2,78,000 tonnes (Pillai and Mallia 2007). The average production of tuna from 2006 to 2010 in India were 78,400 tonnes (Pillai and Satheeshkumar 2013) and the Lakshadweep group of Islands are the major productive area for skipjack tuna followed by yellowfin tuna.
With the growing market for sashimi grade tuna and tuna loins/steaks, the use of yellowfin tuna is increasing with an annual worldwide production of 3,400,000 tonnes (Woo et al. 2008). While processing, the tuna heads are discarded as waste and used for feed and fertilizer preparation. Tuna eyes, rich in DHA, occupies a considerable portion in the head and not utilized properly. Hence this study intended to characterize the yellowfin tuna eye (YFTE) for the bio chemical constituents such as fatty acids, amino acids, minerals and trace metals for the realization of potentiality and better utilization.
Materials and methods
Yellowfin tuna (Thunnus albacares) heads were procured from the local tuna processing factory, Aroor, Cochin, Kerala in the month of March, 2015. The average weight of yellowfin tuna head was in the range of 7–8 kg (n = 20). The samples were brought to the laboratory in iced condition at the ratio of 1:1 (tuna head to ice w/w). The eyes were removed from the head by exercising the optic nerve and the skin fold surrounded the eyes. The eye balls were weighed individually and the average weight was recorded (120 ± 3 g n = 20). To achieve the homogeneity for the analysis minimum of five eyes were homogenized in a blender. Each analysis were carried out in triplicates.
Proximate composition of YFTE was estimated according to the methods described in AOAC (2000). The values are expressed in percentage on wet weight basis. The fatty acid composition of YFTE was analyzed using GC–MS (GCMS-QP2010, Shimadzu, Columbia, USA). Fatty acid methyl esters (FAMEs) were prepared by the method described by Metcalfe et al. (1966). Amino acid composition was profiled according to the method of Ishida et al. (1981) using HPLC (LC 10 AS, FLOM, Tokyo, Japan). Minerals were examined according to the method of AOAC (2000) using flame photometer (BWB-XP, BWB Technologies, Newbury, England). Trace elements and heavy metals were quantified using atomic absorption spectrophotometer (Varian SpectrAA-220, Varian, Melbourne, Australia) according to the method of AOAC (2000).
Results and discussion
Proximate composition of yellowfin tuna eye
The proximate composition of YFTE revealed the presence of fat and protein at 12.04 and 10.17% respectively. Earlier study by Stoknes et al. (2004) on the proximate composition of eyes from saithe (Pollachius virens), cod (Gadus morhua), red fish (Sebastes marinus), gulper shark (Centrophorus squamosus) and salmon (Salmo salar) showed the fat content value of 1, 0.7, 2.3, 0.3 and 35.9% respectively. In the present investigation, the moisture content of YFTE was found to be 71%. The reported values on moisture content of different fish eyes were in the range of 42.20–90.30% (Stoknes et al. 2004). The ash content of YFTE was 2.09%. Results indicate that the YFTE can serve as a secondary raw material for the recovery high value protein and lipid based compounds.
Fatty acid profile of yellowfin tuna eye
The fatty acid composition of YFTE is presented in Table 1. The sum of polyenes (ω-3 and ω-6 fatty acid) was 50.01% of the total fatty acids. The most abundant fatty acid in YFTE was the docosahexaenoic acid (DHA) and accounts about 36.72% of total fatty acids. This amount is twice the DHA content of yellowfin tuna muscle (16.91%) and crude oil from skip jack tuna head (18.8%) (Peng et al. 2013; Chantachum et al. 2000). Other ω-3 fatty acids found in YFTE such as eicosapentaenoic acid (EPA), arachidonic acid and linoleic acid accounted 7, 3.6 and 1.3% to the total fatty acids, respectively. The DHA/EPA ratio was 5.25 and n-3/n-6 ratio was 9.17. Higher n-3/n-6 ratio of fatty acids have the influence on the fat content (Stoknes et al. 2004). Results obtained in the present study are in agreement with the study conducted by Bell and Ghioni (1993) in which the EPA and DHA were the most abundant fatty acids from the eyes of American and European lobsters (EPA:24.3, 18.3; DHA = 12.2,11.6). Another study showed the fatty acid analyses of eye from teleost’s revealed that eye contained high ratio of DHA to EPA and high ratios of n-3 fatty acids to n-6 fatty acids. The DHA/EPA ratio and n-3/n-6 ratio of eyes of teleost was ranging from 1.4 to 3.8 and 2.6 to 15.3 (Stoknes et al. 2004). The palmitoleic acid and elaidic acid were the predominant mono unsaturated fatty acids and accounted for 17 and 1.3% of total fatty acids, respectively. The major saturated fatty acid was myristic acid (2%). The PUFA/SAF ratio obtained in this study was 4.01 revealing that YFTE was the good source of PUFAs.
Table 1.
Fatty acid profile of yellowfin tuna eye
| Fatty acids | Composition (%) | |
|---|---|---|
| C14:0 | Myristic acid | 2.50 ± 0.02 |
| C16:0 | Palmitic acid | 3.02 ± 0.05 |
| C18:0 | Stearic acid | 3.42 ± 0.51 |
| C20:0 | Arachidic acid | 3.20 ± 0.05 |
| Total SFA | 12.14 ± 0.63 | |
| C16:1 | Palmitoleic acid | 17.12 ± 0.74 |
| C18:1 | Oleic acid | 18.11 ± 0.18 |
| C20:1 | Gadoleic acid | 2.85 ± 0.11 |
| Total MUFA | 38.08 ± 1.03 | |
| C18:2 | Linoleic acid | 1.25 ± 0.14 |
| C18:3n3 | α-Linolenic acid | 0.14 ± 0.01 |
| C18:3n6 | λ-Linolenic acid | 0.02 ± 0.04 |
| C20:2n6 | Eicosadienoic acid | 0.03 ± 0.06 |
| C20:3n3 | Eicosatrienoic acid | 0.01 ± 0.02 |
| C20:4 | Arachidonic acid | 3.51 ± 0.12 |
| C20:5 | Eicosapentaenoic acid | 7.07 ± 0.09 |
| C22:6 | Docosahexaenoic acid | 36.72 ± 1.11 |
| Total PUFA | 48.75 ± 1.59 | |
| PUFA/SFA | 4.01 | |
| n-3 | 43.94 | |
| n-6 | 4.79 | |
| n-3/n-6 | 9.17 | |
| DHA/EPA | 5.2 |
Values are reported as mean ± standard deviation for n = 3
Amino acid profile of yellowfin tuna eye
The amino acid composition of YFTE is depicted in Table 2. Amino acid profile of YFTE revealed that the highest content of glycine (19.24 mg/100 g) followed by glutamic acid (16 mg/100 g) to the total amino acids. The higher content of glycine in the YFTE could be sourced to collagen protein. The collagen content in YFTE would vary with the age of species. However it is well established that glycine is a major amino acid in collagen protein (Wang et al. 2013) According to Kalloniatis et al. (2013) glutamate is a major excitatory neurotransmitter in the retina and found to be most abundant amino acid in the eye of vertebrates. Other major amino acids are aspartic acid, alanine and leucine. Imino acids viz., proline and hydroxyproline content were 0.29 and 1.32 mg/100 g, respectively. Essential amino acids accounted for 30.4% of total amino acids and hydrophobic amino acids proportion was 58.07% to total amino acid composition. YFTE contained 28.76% acidic amino acids. Very low amount of cysteine, methionine and arginine were detected. The amino acid composition is expected to vary with proteins and the parts of the fish and also the life stage. Falco et al. (2016) has proposed the amino acid composition of eye as a powerful tool to differentiate between species of different life style and inhabiting environment.
Table 2.
Amino acid profile of yellowfin tuna eye
| Amino acid | mg/100 g |
|---|---|
| Serine | 0.12 ± 0.01 |
| Taurine | 2.56 ± 0.05 |
| Hydroxyproline | 1.32 ± 0.02 |
| Proline | 0.29 ± 0.01 |
| Aspartic acid | 12.76 ± 0.12 |
| Threonine | 1.27 ± 0.01 |
| Serine | 3.64 ± 0.01 |
| Glutamic acid | 16 ± 0.08 |
| Glycine | 19.24 ± 0.08 |
| Alanine | 11.83 ± 0.05 |
| Valine | 5.31 ± 0.02 |
| Cysteine | 0.05 ± 0.01 |
| Methionine | 0.3 ± 0.02 |
| Isoleucine | 2.23 ± 0.03 |
| Leucine | 9.52 ± 0.03 |
| Tyrosine | 1.3 ± 0.01 |
| Phenylalanine | 3.26 ± 0.02 |
| Histidine | 6.04 ± 0.11 |
| Lysine | 2.05 ± 0.10 |
| Arginine | 0.42 ± 0.02 |
Values are reported as mean ± standard deviation for n = 3
Minerals, trace elements and heavy metals of yellowfin tuna eye
Minerals, trace elements and heavy metals composition of YFTE is presented in Table 3. Sodium was the most abundant mineral (677 mg/kg) followed by calcium (379 mg/kg) and potassium (266 mg/kg). Sodium in vertebrate eye playing a major role in maintaining the cells either in depolarized or hyper polarized depends on the day light and dark condition. The adequate potassium and sodium is necessary for proper functioning of nervous system (Morgano et al. 2011). It should be mentioned that studies on mineral composition of fish eye are scarce. For better utilization of fish processing waste, particularly fish eye is essentially to be characterized for the presence of basic chemical elements which would help to develop suitable technology. Magnesium is the major constituent in YFTE followed by iron. Cobalt was the least element with the value of 0.3 mg/kg of sample. Trace elements such as magnesium (Mg), manganese (Mn), copper (Cu), zinc (Zn), iron (Fe) and selenium (Sn) are involved in metabolism as cofactors of several enzymatic reactions and iron present in hemoglobin used for oxygen transport (Flemming 1989). Interestingly no heavy metals such as cadmium and lead were detected in YFTE.
Table 3.
Minerals, trace elements and heavy metals of yellowfin tuna eye
| Mineral | mg/kg |
|---|---|
| Na | 677 ± 0.92 |
| Ca | 379 ± 1.02 |
| K | 266 ± 1.31 |
| Mg | 46 ± 0.18 |
| Fe | 33 ± 0.92 |
| Zn | 10 ± 0.64 |
| Cu | 7 ± 0.14 |
| Mn | 0.9 ± 0.01 |
| Co | 0.3 ± 0.01 |
| Cd | Nil |
| Pb | Nil |
Values are reported as mean ± standard deviation for n = 3
Conclusion
Fish processing waste has gained a greater attention in the last decade as a valuable secondary raw material for the high end products. In order to develop a proper technology and utilization of fish waste, the basic knowledge on various biochemical constituents present in the specific parts is essential. In the present investigation YFTE was chosen as a study material and it accounts 1.7% of the total head mass. The average weight of YFTE was found to be 120 g. High fat and protein content of YFTE revealed it could be a good source for by-product utilization. Tuna fish eye found to be rich source of PUFA (49%) with the dominant position occupying by DHA. YFTE was rich in glycine and glutamic acid. The major minerals presented in the YFTE were sodium, calcium and potassium. Interestingly no heavy metal were detected in YFTE. The outcome of the present study would help to enhance our understanding and technology development in turn pave the way for better utilization.
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