Nowadays, aquaculture is probably the fastest growing food production sector [1
]. In 2016, global aquaculture production accounted for almost 50% of the world’s fish products destined for food, including 80.0 million tonnes of food fish and 30.1 million tonnes of aquatic plants. Regarding food fish outputs, 54.1 million tonnes were from finfish, 17.1 million tonnes were from molluscs, 7.9 million tonnes were from crustaceans, and 938,500 tonnes were from other aquatic animals [2
]. China is by far the largest world producer at 15.3 million tonnes, followed by other Asiatic countries such as Indonesia, India, Japan, and Viet Nam. In Europe, Norway and Spain top the list at 2.03 and 0.91 million tonnes of production, respectively.
However, this large production volume also involves the generation of by-products that are not intended for human consumption. In 2016 approximately 37,900 tonnes of non-food products were obtained from fish processing industries [2
]. These aquaculture by-products include backbones, belly flaps, fish fins, gills, heads, liver, roe, skin, viscera, and meat adhered to the bones [3
]. The ratio between food fish and by-product varies by fishing fish size and species, season, and zone [4
]. Thus, bones can account for percentages of 9%–15% of whole fish, while the head comprises 9%–12%, viscera 12%–18%, skin 1%–3%, and scales 5% [5
On the other hand, fish one of the most nutritious animal-derived foods [6
]. It contains high-quality proteins, balanced essential amino acids, high levels of fat-soluble vitamins (A and D), and essential macro and micro minerals (iodine, magnesium, phosphorus, and selenium) [7
]. Fish nutrient composition depends on fish species, age, gender, health, nutritional status, and time of the year [8
]. It is characterized by 15%–30% proteins, 0%–25% lipids, and 50%–80% moisture. Thus, depending on their fat content there are lean fish such as cod and hake with compositions of 20% protein, 80% water, and rather low lipid levels (0.5%–3%); fatty fish, such as mackerel and salmon, have the same protein contents but higher fat content (10%–18%) and therefore lower water contents (62%–70%) [9
]; and gilthead sea bream, sea bass, and trout are classified as semi-fatty fish.
The healthy compounds present in fish composition are also part of its by-products. Therefore, it is necessary to analyze these by-products, especially regarding their nutritional properties. This will allow us to characterize more in detail these wastes and the active substances which could be present, and thus determine the value of these products [10
]. The bioactive compounds, which can be extracted from foods or foods by-products, are nutritional elements that are usually present in small quantities, and are able to regulate metabolic functions, leading to beneficial effects [11
]. Therefore, the extraction of bioactive compounds is one of the most feasible options for utilization of by-products [12
]. In fact, a great number of bioactive compounds can be obtained from aquaculture by-products [3
], allowing an efficient use of fish by-products, transforming them into ingredients that can be incorporated into feed, food, or other high-value products [15
]. The use of fish by-products in animal feeds (flours and oils) is common today [17
]. Moreover, these phytochemicals, especially polyunsaturated fatty acids (PUFAs), minerals, proteins, and peptides, have several nutritional benefits, which together with their biological activities could give rise to a beneficial effect on human health [19
]. Among their fatty acids, high levels of long chain n
-3 fatty acids stand out, associated with reduction of the risk of cardiovascular diseases in humans [22
Gilthead sea bream (Sparus aurata
) is one of the most important marine finfish species reared in Europe, and is mainly produced in the Mediterranean and Northern European countries [24
], where it represents 4.1% relative production of the primary aquaculture species. Aquaculture allows for the production of gilthead sea bream at a low cost [26
]. Traditionally, it was reared extensively in coastal lagoons and saltwater ponds in almost all Mediterranean countries. Its characteristics related to a high survival rate and feeding habits makes it suitable for extensive aquaculture in the Mediterranean. In the 1980s, the intensive systems that have been evolving to this day began to develop, successfully achieving the artificial reproduction of this species whereby the production of juveniles increases, with intensive captive breeding both in ponds and in floating cages [27
]. The rapid development of the production in floating cages of culture in the sea has allowed the development of the sector, increasing the production of this species.
Little information exists about the chemical composition of by-products from Mediterranean species. Therefore, the aim of this study was characterized the nutritional value of several by-products (fishbone, gills, guts, head, liver, and skin) obtained from the processing of gilthead sea bream, comparing their composition with those presented in muscle, the portion most commonly marketed.
4. Materials and Methods
4.1. Experimental Design and Fish Sampling
A total of 10 pieces from male sea bream aged under two years were used to carry out the study. The animals, collected in a local market, were captured in the east of Spain. The sampling was done for five weeks, acquiring two pieces every week. Then, the samples were transported to the laboratories of the Centro Tecnológico de la Carne de Galicia, where the pieces were weighed and dissected. The weight and length of fish were determined. After that, fish portions (fins, fishbone, gills, guts, head, heart, liver, muscle, skin, and others) were obtained as shown in Figure 1
. Once the different fish parts were obtained, we proceeded to the nutritional characterization of the selected portions (fishbone, gills, guts, head, liver, muscle, and skin). The samples were homogenized and frozen at −80 °C until their analysis. The proximate composition, fatty acid and amino acid profile, and mineral content were determined.
4.2. Chemical Composition
The chemical analysis (moisture, protein, and ash content) was assessed according to the ISO recommended standards 1442:1997 [74
], 937:1978 [75
], and 936:1998 [76
]. Fat was extracted in an Ankom XT10 extractor (ANKOM Technology Corp., Macedon, NY, USA) following the A.O.C.S. Official Procedure Am 5-04 [77
4.3. Fatty Acid Profile
For fatty acid analysis, total fat was extracted from 10 g of sample [78
]. The fatty acids were transesterified according to the procedure of Domínguez et al. [79
] with modifications: For the fatty acid transesterification, 20 milligrams of extracted fat dissolved in 1 mL of toluene were mixed with 2 mL of a sodium methoxide (0.5 N) solution, vortexed during 10 s, and allowed to stand for 15 min at room temperature. Then, 4 mL of a H2
solution (10% of H2
in methanol) were added, vortexed for a few seconds, and vortexed again before adding 2 mL of saturated sodium bicarbonate solution. For the extraction of fatty acid methyl esters, 1 mL of hexane was added to the samples and vortexed for 10 seconds, and the organic phase was then transferred to an appropriate GC vial.
Separation and quantification of fatty acid methyl esters (FAMEs) were carried out using a gas chromatograph (GC-Agilent 7890B, Agilent Technologies, Santa Clara, CA, USA) equipped with a flame ionization detector (FID) and PAL RTC-120 autosampler. One microliter of sample was injected in split mode (1:50). The injector was maintained at 250 °C and 64.2 mL/min of total flow. For the separation of FAMEs, a DB-23 fused silica capillary column (60 m, 0.25-mm i.d., 0.25-μm film thickness; Agilent Technologies, Santa Clara, CA, USA) was used. Chromatographic conditions were as follows: initial oven temperature of 50 °C (held for 1 min), first ramp at 25 °C/min to 175 °C, second ramp at 4 °C/min to 230 °C (held for 5 min), and third ramp at 4 °C/min to a final temperature of 240 °C (held for 2.75 min). Helium was used as carrier gas at a constant flow-rate of 1.2 mL/min, with the column head pressure set at 22.9 psi. The FID detector was maintained at 280 °C, while the operational flows were set as 40 mL/min of H2, 450 mL/min of air, and 30 mL/min of makeup flow. The total time for chromatographic analysis was 30 minutes. Data acquisition and equipment control was carried out using the software MassHunter GC/MS Acquisition B.07.05.2479 (Agilent Technologies, Santa Clara, CA, USA), while the data analysis was carried out with the software MassHunter Quantitative Analysis B.07.01. Individual FAMEs were identified by comparing their retention times with those of authenticated standards (FAME Mix-37 components; docosapentaenoic acid, trans-vaccenic acid, cis-vaccenic acid, and conjugated linoleic acid - CLA) and the results were expressed as g/100 g of total fatty acids identified.
The total saturated fatty acid (SFA), monounsaturated fatty acid (MUFA), and polyunsaturated fatty acid (PUFA) contents were calculated. To assess the nutritional properties of fat the DHA/EPA, n-6/n-3, and PUFA/SFA ratios were determined.
4.4. Amino Acid Profile
The hydrolysis of the protein, derivatization and identification of amino acids was carried out following the procedure and using the equipment described by Domínguez et al. [80
]. Completion of hydrolysis of any tyrosine phenol modification was accelerated by heating for 10 min at 55 °C. Moreover, tryptophan determination was not possible because acidic hydrolysis transforms it into ammonium. Data regarding amino acid composition were expressed in mg/100 g of tissue.
4.5. Protein Quality: Chemical Score of Amino Acids
Once the amount of amino acids in the different muscles was determined, the chemical score of the essential amino acids (CS) was calculated in relation to the reference on pattern protein proposed by FAO/WHO/UNU [28
] applying the following equation:
The essential amino acid index (IEAA) value was also calculated applying the following equation described by Shahidi and Synowiecki [81
refer to the content of histidine, isoleucine, leucine, lysine, methionine, phenylalanine, tyrosine, threonine, and valine in each sample, respectively; ap
, …, jp
are the content of histidine, isoleucine, leucine, lysine, methionine, phenylalanine, tyrosine, threonine, and valine in protein standard [28
], respectively; n
is the number of amino acids used.
4.6. Mineral Content
The procedure previously described by Lorenzo et al. [82
] was used for mineral determination (Ca, K, Mg, Na, P, Fe, Mn, Zn and Cu). For that purpose, a Thermo-Fisher ICAP 6000 plasma emission spectrometer (Thermo-Fisher, Cambridge, UK), equipped with a radio frequency source of 27.12 MHz, a peristaltic pump, a spraying chamber, and a concentric spray nebulizer, totally controlled with a ICP software using 99.996% liquid argon plasma gas (Praxair, Madrid, Spain), was utilized. An external standard for setting the calibration curve was used in order to determine the mineral contents, being the results expressed as mg/100 g.
4.7. Statistical Analysis
A total of 140 samples (7 by-products × 2 replicates of each sample × 10 fish pieces) were used to analyse the statistical significance of nutritional differences depending on the fish by-products. Statistical analyses were performed using IBM SPSS Statistics 21 software (IBM Corp., Armonk, NY, USA). Normal distribution and homogeneity of variance were previously tested (Shapiro-Wilk). Data were submitted for analysis of variance (ANOVA) and Duncan test, when ANOVA had a significant effect (p < 0.05). The values were given in terms of mean values and standard error of the mean (SEM).
Research is being carried out to explore the possible use of different fish processing by-products in order to allow their valorisation through the extraction of bioactive compounds, particularly lipids and proteins. More than half of the total wet weight of processed gilthead sea results in processing by-products. These wastes are rich in protein (10%–25%) and fat (17%–35%). Profiles of amino acids, fatty acids, and minerals showed that these discards had high levels of essential amino acids (41%–49% of total amino acids), omega-3 fatty acids (about 13% of total fatty acids), and macro and micro elements, especially calcium and phosphorous in head, fishbone, and gills, and iron and zinc in liver. The results obtained improve the understanding and the characterization of processing by-products from gilthead sea bream, with the aim of ongoing development of value-added products from these wastes. Moreover, the health properties associated with these bioactive compounds give value to these products, resulting in more attractive products for consumers and at the same time avoiding environmental problems linked with these materials. However, more studies are needed on the microbiological evaluation and the safety of these products before they are incorporated as nutraceuticals. Therefore, this study demonstrated the great potential of gilthead sea bream by-products as a promising source of valuable bioactive compounds for the food industry, enabling their use as ingredients in novel foods.