MOLECULAR AND QUALITY MEAT CHARACTERISTICS OF NEW FISH SPECIES INTRODUCED TO THE EU MARKET
DOI:10.30825/5.EJPAU.172.2019.22.2

Remigiusz Panicz¹, Piotr Eljasik¹, Małgorzata Sobczak¹, Joanna Sadowska², Sławomir Keszka^3
1 Department of Meat Technology, Faculty of Food Technology and Fisheries,
West Pomeranian University of Technology, Szczecin, Poland
² 2 Division of Human Nutrition Physiology, Faculty of Food Science and Fisheries, West Pomeranian University of Technology, Szczecin, Poland
³ Division of Aquaculture, Faculty of Food Science and Fisheries, West Pomeranian University of Technology, Szczecin, Poland

ABSTRACT

Global consumption steadily increases due to development of aquaculture but also more efficient fisheries sector. Therefore, fish processing plants and ultimately consumers face products from new fish species which significantly differ in case of their nutritional values, technological or culinary properties. The aim of the study was to randomly collect specimens of bigeye sea perch, Helicolenus barathri; New Zealand sole, Peltorhamphus novaezeelandiae; white bass, Morone chrysops; thicklip grey mullet, Chelon labrosus and ridge scaled rattail, Macrourus carinatus and provide multidimensional characteristics i.e. species authentication, proximate assessment, structure and texture analysis as well sensory evaluation. Study confirmed authenticity of all fish samples and new DNA barcodes were introduced into database of fish profiles. Chemical composition of fillets differed significantly among species, and the unfavourable nutritional values had ridge scaled rattail, bigeye sea perch and New Zealand sole which had unfavorable fatty acid profile, high atherogenic (AI) and thrombogenic (TI) indexes, and low hypocholesterolemic to hypercholesterolemic acids ratio. On the other hand, fillets of freshwater white bass had the best PUFA:SFA ratio,  and favourable TI and AI indexes from the consumer’s health point of view. Moreover, fillet of the white bass was selected by panellists as the most desirable. Structural analysis also revealed different degree of undesirable changes in fillets observed as breakdown of myofibrils and connective tissue. Multidimensional analysis characterized randomly selected sample and provided a set of useful information both for customers and fish processing sector.

Key words: fish trade, DNA barcoding, chemical composition, nutritional values, fillet structure and texture.

INTRODUCTION

Over the past few years, number of new fish species which are traded globally has increased. Observed tendency result from growing interest of consumers in fish and fishery products, development of more efficient fishing gears, and combination of overexploitation of existing resources and exploitation of less desirable species [16]. However, to introduce new fish species on the market a correct labelling must be meet and fish name (scientific and common in native language) listed on the official List of the Commercial Designations of Fish. Both names are of utmost importance to identify imported seafood products and to allow Veterinary Inspectorates run risk-dependent analyses (microbiological, genetic or toxicological) and Curstomer Offices to calculate taxes and customs for goods. Numerous issues arises when imported fish products, mainly sold as fillets, were deliberately or unintentionally substituted for another, in most cases cheeper or simply available [24]. To reveal and exclude those practices a range molecular markers and tests for fish authentication were developed [20, 25, 38]. However, molecular markers used in identification of fish and fish products except of their numerous advantages (short analysis time, low complexity, affordable costs) have one major limitation, require a reliable database to compare. GenBank database, despite the fact that provide numerous molecular records for fish species, has also a high level of redundancy and in some cases lack additional information about sampling. To overcome these constraints specyfic DNA barcode repositories (e.g. FISH-BOL) were created, and filled with sampling information (site, number of samples, date) and sample (weight, sex, photo, biometric parameters, DNA barcode). Usage of such diverse information in seafood monitoring increase probability that consumers purchase a desired product, rather than counterfeited.

Except of traceability issues seafood industry has also ensure that fish and fisheries products unchange quality (nutritional, general appearance) along whole cold chain, i.e form catch to retail store). Moreover information on nutritional value provided on the seafood labels should assist diverse groups of consumers with different feeding needs to purchase energy-balanced and nutrient-rich products. It is of vital importance for children, pregnant women and elderly people who require healthy diet with increased levels of n-3 long-chain polyunsaturated fatty acids (n-3 PUFA), iodine, leucine or selenium [29, 45]. Except of correct and detailed labelling fisheries sector must also profile texture, structure and sensory properties of new seafood products. It is especially important for those consumers who have physiological difficulties with chewing, swallowing, those who avoid fishy smell or flavour, chefs (restaurants) or consumers who care about quality of seafood procucts [9]. However, abovementioned paramenters to the greatest extent depend on quality of imported raw fish material which should be tested always when frozen fish (fillets) shipement enter destination harbours or processing plants [33].

Disparity between the growing demand and shrinking supply of fish today lead to increase of new species offered for sale at world markets. Usually those species have low value for processing sector due to their low economic value, limited or unpredictable availability and undeveloped comerical processing lines. However, modern processing plants should be enough fexible and quick to smoothly switch their lines to efficiently process new fish resources. Therefore, the aim of the study was to evaluate quality of raw matrial of five species which could be potentially utilized by fish processing industry in the very near future.

MATERIALS AND METHODS

Fish species
The study material consisted of deheaded and degutted frozen 5 fish species. Fish specimens were selected randomly and intentionally (n=7), to reflect situation when processing plant or consumer receive particular raw material. Bigeye sea perch, Helicolenus barathri (Hector, 1875) and New Zealand sole, Peltorhamphus novaezeelandiae (Günther, 1862) were caught in Southwest Pacific (FAO 81), frozen by immersion method and obtained at Border Veterinary Inspectorate (BVI) located in the Port of Szczecin in 2014. White bass, Morone chrysops (Rafinesque, 1820) was fished in the Great Lakes (FAO 02), frozen individually in the IQF (Individually Quick Frozen) method and collected at BVI in June 2016. Fourth specimen, a thicklip grey mullet, Chelon labrosus (Risso, 1827) was caught using fishing nets in Piast Canal (FAO 04) in 2015 and frozen using a traditional method to -21°C. Last sample, a ridge scaled rattail, Macrourus carinatus (Günther, 1878) was fished in Southeast Pacific (FAO 87), immediately processed into a carcass, frozen by contact method and obtained BVI in 2016.

DNA Barcoding
From each specimen, a piece of muscle tissue was collected and total DNA extracted using the Quick-DNA TM Universal Kit (Zymo Research). Quality and quantity of DNA isolates were assessed by separation in 1.5% agarose gel and spectrophotometric measurements using a NanoDrop 2000 (Thermo Scientific). Species-specific DNA barcodes were analysed based on amplification of fragment of the first cytochrome oxidase subunit (COI) according to method described by Ivanova et al. [21]. PCR products were assessed by 2% gel electrophoresis and bidirectionally sequenced by Genomed company (Warsaw). Raw sequences were assembled using BioEdit and submitted to BLAST to retrieve the records from the nucleotide GenBank database [4, 19].

Fillets basic chemical composition
Proximate composition of samples was determined in 100 g of homogenate prepared from collected fillets in 3 replicates. Dry matter content was determined by drying samples in SUP-4M dryer (Warmed, Warszawa) at 100ºC for 6 hours until reached a constant weight; total ash was determined by incinerating in a muffle furnace (FCF7SHM, Czylok) at 550ºC for 6 hours; total nitrogen content, converted into protein amount (nitrogen × 6.25), was determined by Kjeldahl method (Foss Tecator Kjeltec 2100); fat content was assayed by solvent extraction method, using a light petroleum ether as solvent in Foss Tecator Soxtec System HT 1043 [5].

Fatty acid profile
Fatty acid profile in specimen homogenates (in 3 raplicates) was assessed by Polcargo (Szczecin), using gas chromatography in accordance with PN-EN ISO 12966-1:2015-01 [36]. Based on obtained profiles specific indicators were calculated as follows [17, 40, 47]:

• SFA:MUFA:PUFA ratio,
• MUFA+PUFA to SFA ratio,
• n-6/n-3 fatty acids ratio,
• hypocholesterolemic (h) and hypercholesterolemic (H) fatty acid content proportions,
  h/H = (C18:1+C18:2+C20:4+C18:3+C20:5+C22:5+C22:6)/(C14:0+C16:0),
• DFA (desirable fatty acid) content: DFA= (UFA + C18:0),
• atherogenic index:
  AI = (C12:0+4xC14:0+C16:0) / (?MUFA+?PUFA n-6+?PUFA n-3),
• thrombogenic index:
  TI = (C14:0+C16:0+C18:0) / (0.5x?MUFA+0.5x?PUFA n-6+3x?PUFA n-3+?PUFA n-3/?PUFA n-6),
• EPA and DHA fatty acid content in 100g fillet.

Moreover, minimal weight of fillets required to cover daily level (≥ 250 mg) of intake of EPA + DHA was established for each species [15].

Structural characteristics of fillets
The twenty muscle samples (5x5x10mm) from fillets were cut, fixed for 12 hours in Sannomiya solution, dehydrated using alcohol and benzene, embedded in paraffin blocks and sectioned with the Rotary Microtome MPS-2 (Opta-Tech) into 10±1 μm slices. The sections were mounted onto clear glass slides, contrast-stained with haematoxylin and eosin, and sealed with Canada balsam [11]. MultiScanBase v.13.01. (Computer Scanning System Ltd., Warszawa) a computer image analysis software was used to measure muscle fibre cross-sectional area (CSA) as well thickness of connective tissue (perimysium and endomysium). For each excised sample from 8 to 10 slides were prepared and analysed. Approximately 150 muscle fibres and up to 100 perimysium and endomysium were measured in each slide. Values obtained for cross-sectional areas were used to calculate diameters of fibres (D) using following equation  [2]. Next, values of D were divided into classes (<20 μm; 20–40 μm; 40–60 μm; 60–80 μm; 80–100 μm; >100 μm) and displayed on the graph as percentage share in each graph [49].

Texture profile of fillets
In the next step fillets were heat treated in evaporator until temperature reached 68±1ºC in the thickest part of fillet and next cooled to 20±1ºC. Texture of fillets was measured with Instron 1140 machine (Instron, Norwood, MA, USA), applying a Kramer test [10]. The 10-bladed head speed was 50 mm / min. Samples of fillets were formed into balls (5±0.01g) and placed into shear cell. The force-deformation curve was used to calculate maximum force (N) and work (J) parameters. The Kramer test were applied minimum five times for balls prepared from each specimen.

Sensory analysis
Sensory evaluation of steamed fillets (in 3 raplicates) was conducted by a trained team, consisting of four members, reaching sensory minimum. Texture characteristics (hardness, cohesiveness, springiness, juiciness, stringiness, chewiness), fish odour and taste intensity were evaluated. Each feature intensity was rated using a 5-point scale, where 1 point corresponded to the lowest and 5 points – the highest feature intensity [37].

Data analysis
Statistical analyses were performed using Microsoft Excel spreadsheets and STATISTICA 12 software (StatSoft, Inc.). Differences between each species average parameters were determined using Kruskal-Wallis test, followed by Tukey HSD post-hoc test. Differences were statistically significant at p ≤ 0.05.

RESULTS AND DISCUSSIONS

Molecular authentication of fillets
Sequencing and raw sequence processing steps yelded a 652 bp sequence of COI for each specimen. Analyses performed using GenBank sequence database confirmed identity of the analysed specimens, i.e. New Zealand sole (GenBank Acc. Nr. EU513694.1), white bass (GenBank Acc. Nr. EU524140.1), thicklip grey mullet (GenBank Acc. Nr. KJ552931.1) and ridge scaled rattail (GenBank Acc. Nr. EU074450.1), since obtained COI sequences had their counterparts in GenBank. Only bigeye sea perch COI sequence was submitted into GenBank database (GenBank Acc. Nr. MF135477) since no identical DNA barcode was found in the repository. Identification of COI barcodes for analysed allowed us to update our internal database which consists of fish DNA profiles and aim to thwart seafood counterfeiting. Cawthorn et al. [12] revealed that one of the analysed species (H. barathri) was sold on the South African market as half moon rockcod (Epinephelus rivulatus). Genus Helicolenus (Sebastidae) includes also eight other species which are commercially fished and might be potentally counterfeited, posing a serious health problems to consumers as evidenced for venomous blackbelly rosefish (Helicolenus dactylopterus) [18]. Misidentification and and next mislabelling are also frequently reported for flatfishes when captured as part of a mixed bag trawl fishery. Additionally, illegal practices include fillet trimming to resemble cuts which are typical for other flatfish species caugt legally within valid quotas (Keszka pers. commun.), or even distinct species such as cod species [50]. Analysed P. novaezeelandiae, is one of theeight commercial flatfish species, and as indicated by Rijnsdorp et al. [39] sustainable fisheries should rely on catch statistics supported by scientific monitoring. Recommendation is also important in relation to numerous fish species formerly regarded as by-catch but now due to increasing global demand for seafood commonly found on the market. M. carinatus one of Grenadiers is taken as bycatch in fisheries targeting hake, squid and patagonian toothfish [14], and as evidenced by Xiong et al. [50] sometimes fileted and mislabelled as cod. Processing (cutting, reforming, mincing) opens a possibility of fraudulent substitutions of expensive species by those of lesser value, e.g. grouper (Epinephelus marginatus) substituted with striped mullet (Mugil cephalus) [6]. Therefore, C. labrosus and other species from diverse Mugilidae family (17 genera, 72 species) may also be utilized as source of frauds, even in case of roe products (botarga) produced only from selected species [30]. Except of fisheries products, aquaculture species as analysed in our study M. chrysops (white bass), might also be mislabelled and sold on the market under various names, to name just a few; striped bass, red snapper [48] or perch eventually identified as a hybrid of striped bass (M. saxatilis) with white bass [8].

Table 1. Chemical composition of analysed fish fillets

	Bigeye sea perch	New Zealand sole	White bass	Thicklip grey mullet	Ridge scaled rattail
Dry matter [%]	20.54a ± 0.10	21.19a ± 0.03	23.45b ± 0.69	28.55c ± 0.20	18.11d ± 0.22
Crude ash [%]	1.14a ± 0.01	1.32b ± 0.02	1.11c ± 0.09	1.33b ± 0.02	1.16a ± 0.00
Crude fat [%]	0.16a ± 0.04	0.40a ± 0.09	3.84b ± 0.36	3.53b ± 0.05	0.17a ± 0.03
Crude protein [%]	20.01a ± 0.07	19.98a ± 0.18	19.82a ± 0.02	24.01b ± 0.21	17.90c ± 0.18
a – values in rows with the same index do not differ significantly (p > 0.05) between species

Table 2. Profile of selected fatty acids in fish fillets (g/100 g of fat)

	Bigeye sea perch	New Zealand sole	White bass	Thicklip grey mullet	Ridged scale rattail
Palmitic acid	34.9a ± 0.68	32.0b ± 0.41	22.2c ± 0.07	29.0d ± 0.13	39.1e ± 0.16
Palmitoleic acid	3.4a ± 0.10	2.8b ± 0.02	10.3c ± 0.05	20.3d ± 0.09	2.0e ± 0.03
Stearic acid	13.7a ± 0.31	13.3a ± 0.25	3.6b ± 0.02	2.8c ± 0.02	12.5d ± 0.17
Oleic acid	17.9a ± 0.22	17.3b ± 0.25	26.4c ± 0.09	13.2d ± 0.12	16.7e ± 0.02
Linoleic acid	0.9a ± 0.10	1.2b ± 0.07	2.9c ± 0.03	0.8a ± 0.03	0.5d ± 0.04
Gamma-linolenic acid	0.7a ± 0.03	0.6b ± 0.02	0.1c ± 0.02	0.2d ± 0.02	0.2d ± 0.04
Alpha-lonolenic acid	0.1a ± 0.02	0.2a ± 0.03	1.9b ± 0.02	0.5c ± 0.03	0.1a ± 0.02
Cis-11-eicosanoic acid	1.8a ± 0.04	1.1b ± 0.02	1.9c ± 0.01	0.5d ± 0.02	3.8e ± 0.03
Arachidonic acid	0.5a ± 0.12	0.9b ± 0.16	3.1c ± 0.02	0.7ab ± 0.02	0.6a ± 0.09
Eicosapentaenoic acid	0.2a ± 0.08	1.8b ± 0.38	3,1c ± 0.00	1.9b ± 0.08	1.1d ± 0.26
Docosahexaenoic acid	1.1a ± 0.47	2.6b ± 0.61	4.1c ± 0.01	0.9a ± 0.06	3.1bc ± 0.75
Nervonic acid	2.8a ± 0.04	1.6b ± 0.03	0.6c ± 0.02	0.2d ± 0.02	3.8e ± 0.01
?SFA	53.5a ± 1.07	53.8a ± 0.78	30.2b ± 0.10	43.2c ± 0.07	55.4d ± 0.06
?MUFA	32.5a ± 0.31	27.6b ± 0.31	44.1c ± 0.17	39.4d ±0 .26	30.5e ± 0.02
?PUFA	14.0a ± 0.90	18.6b ± 1.53	25.7c ± 0.27	17.4b ± 0.22	14.1a ± 1.02
?PUFA n-3	1.6a ± 0.59	5.1b ± 1.09	10.0c ± 0.03	3.7b ± 0.14	4.4b ± 1.05
?PUFA n-6	2.3a ± 0.17	2.8b ± 0.23	6.3c ± 0.02	2.0a ±0.03	1.4d ± 0.11
a – values in rows with the same index do not differ significantly (p > 0.05) between species

Nutritional and technological properties of fillets
Results of our study showed that protein, fat, dry matter and ash levels differed significantly between meat of analysed fish species (Tab. 1). Meat of ridge scaled rattail, bigeye sea perch and New Zeland sole had the lowest level of dry matter and fat in comparison to meat of white bass and thicklip grey mullet. Among all tested fish species thicklip grey mullet had also the highest level of dry matter and protein, whereas the lowest level of protein was observed for ridge scaled retail. Particular attention should be given to thicklip grey mullet meat, which contained considerably higher content of protein (24%) in comparison to mammalian species [34] and most fish species [41]. Based on measured fat content, bigeye sea perch, New Zealand sole and ridge scaled rattail fillets were classified as lean fish (< 2%), while white bass and thicklip grey mullet as fish with low fat content (2–4%) [31]. Thicklip grey mullet (3.53%) and white bass (3.84%) had intermediate level of fat in fillets comparing to carp (2.81%) and trout (4.39%), comparable to Baltic herring (3.70%), noticeably lower than North Sea herring (7%), mackerel (11.60%) or salmon (11.57%), [7, 28, 48]. Fat content in the fillets of bigeye sea perch (0.16%) and ridge scaled rattail (0.17%) was comparable to those determined by Usydus et al. [50] for cod (0.08%) and pollock (0.09%), while level of fat in New Zealand sole fillet (0.4%) was similar to obtained for common sole (0.5%). Further differences between analysed fillets showed significant differences in the content of 12 fatty acids, especially stearic acid which had several times higher concentarion in fillets of marine fish species than freshwater (Tab. 2, S1). Observed differences might be explained by entirely different habitat parameters, i.e. feeding sources and trophic position, which affected analysed species during their lifespan [13]. In general fillets of white bass had the lowest level of SFA and the highest content of MUFA and PUFA (both n-3 and n-6) what resulted in the most favourable SFA:MUFA:PUFA ratio (Tab. 3). Moreover, meat of this species was characterised by the lowest AI and TI indexes, the highest content of DFA (unsaturated + C18:0) and the highest hypocholesterolemic to hypercholesterolemic ratio. Contrarily, the most unfavourable SFA:MUFA:PUFA ratio, and additionally high level of AI, increased level of TI, and low hypocholesterolemic to hypercholesterolemic acids ratio were found in the fat of ridge scaled rattail. It is widely accepted that ratios of PUFA:SFA above 0.45 and of n-6/n-3 below 4.0 exert health-enhancing effects through reduction of cardiovascular diseases and cancers [42]. Our study showed that PUFA:SFA ratio was more than two-fold higher (0.851) in fillets of white bass as compared against recommended level. Whereas the most unfavourable ratio was observed for ridge scaled rattail (0.254) and bigeye sea perch (0.261). According to Ulbricht and Southgate [47] atherogenic (AI) and thrombogenic (TI) indexes more precisely than ratio of PUFA:SFA describe atherosclerosis and prothrombotic risks of diet. Calculations of AI and TI consider various effects of fatty acids on increased incidences of atherosclerosis and (or) thrombosis. Products with AI < 0.6 are desirable in diet. In our study, only fillets of white bass had AI (0.5) was lower than abovementioned treshold. According to the results of numerous studies, it can be concluded that fat of white bass fillets has better health benefits than, for example, fat of tilapia fillets [46]. Moreover, our study showed that 90 g of white bass fillets cover daily recommended intake for EPA and DHA (250 mg). Whereas in case of ridge scaled rattail or bigeye sea perch daily intake level is over 3.5 kg and 12 kg of fillets, respectively.

Table 3. Selected health values of fat extracted from analysed fish fillets

	Bigeye sea perch	New Zealand sole	White bass	Thicklip grey mullet	Ridge scaled rattail
SFA:MUFA:PUFA	1:0.61:0.26ad	1:0.51:0.34a	1:1.46:0.85b	1:0.91:0.40c	1:0.55:0.25d
AI	0.88a ± 0.049	1.02b ± 0.057	0.50c ± 0.003	1.03b ± 0.004	1.01b ± 0.045
TI	2.21a ± 0.260	1.54bc ± 0.510	0.51d ± 0.001	1.16b ± 0.021	1.65c ± 0.190
PUFA:SFA	0.261a ± 0.022	0.346b ± 0.034	0.851c ± 0.011	0.403d ± 0.006	0.254a ± 0.022
MUFA:SFA	0.613a ± 0.018	0.513b ± 0.002	1.464c ± 0.001	0.912d ± 0.005	0.555e ± 0.001
UFA:SFA	0.873a ± 0.040	0.859a ± 0.035	2.314b ± 0.010	1.315c ± 0.001	0.810a  ± 0.022
n-6:n-3	1.50:1a	0.55:1b	0.63:1b	0.54:1b	0.32:1b
DFA	60.4a ± 0.9	59.5ab ± 1.0	73.5c ± 0.2	59.6a ± 0.1	57.4b ± 1.4
h:H	0.876a ± 0.047	1.000b ± 0.049	2.050c ± 0.012	0.843ad ± 0.006	0.758d ± 0.032
EPA+DHA [mg/100 g fillet]	2.08a ± 0.87	17.60b ± 3.94	276.00c ± 0.22	98.80d ± 4.47	7.14a ± 1.72
Fillet RDI weight [g]	12018a ± 2714.3	1420b ± 336.6	90c ± 0.1	253c ± 11.8	3501c ± 476.1
Explanations: SFA – saturated, MUFA – monounsaturated, PUFA – polyunsaturated, UFA – unsaturated, DFA – desirable (UFA+18:0), AI – atherogenic index, TI – thrombogenic index, h – hypocholesterolemic acids, H – hypercholesterolemic acids (C18:1+PUFA)/(C14+C16:0), fillet RDI weight – fillet weight that covers daily EPA     and DHA requirements (250 mg) a – values in rows with the same index do not differ significantly (p > 0.05) between species

Table 4. Kramer test parameters and sensory properties (pt.) of analysed fish fillets

	Bigeye sea perch	New Zealand sole	White bass	Thicklip grey mullet	Ridge scaled rattail
Maximal force [N]	178.34 a±11.50	108.18 b±20.93	158.51 a±26.09	228.25 c±11.82	243.34 c±24.28
Work [J]	1.29 ab±0.24	1.02 b±0.24	1.07 b±0.32	1.85 c±0.16	1.71ac±0.19
Hardness	3.00 a±0.41	2.00 a±0.82	3.13 a±0.48	2.88 a±0.63	2.75 a±0.50
Cohesiveness	2.50 ab±0.35	2.25 a±0.50	3.00 ab±0.71	3.50 b±0.71	2.67 ab±0.58
Springiness	3.00 a±0.71	2.25 a±0.96	3.00 a±0.71	3.13 a±1.03	3.25 a±0.50
Juiciness	3.00 a±0.58	2.25 a±1.26	3.25 a±0.50	2.63 a±0.75	3.63 a±0.48
Stringiness	3.25 a±0.65	1.63 b±0.48	2.88 ab±1.03	2.75 ab±1.19	4.00 c±0.05
Chewiness	2.63 a±0.75	2.25 a±1.19	3.00 a±0.71	2.88 a±1.03	2.88 a±1.03
Fish taste intensity	2.25 a±0.29	2.50 ab±0.71	2.88 b±0.25	2.88 b±1.03	2.63 ab±1.11
Fish odour intensity	2.63 ab±1.11	3.13 ab±1.11	3.75 b±0.50	2.75 ab±0.96	2.63 a±0.48
a – values in rows with the same index do not differ significantly (p > 0.05) between species

Table 5. Characteristics of muscle fibre and connective tissue of analysed fish samples

	Bigeye sea perch	New Zealand sole	White bass	Thicklip grey mullet	Ridge scaled rattail
Muscle fibres:
CSA [µm2]	6554a±3186.90	3480a±1528.25	2741a±1133.47	4396a±1108.81	3945a±1263.28
Connective tissue [myocommata]:
Perimysium thickness [µm]	10.96a±2.59	10.05a±2.79	9.47a±2.42	11.04a±4.13	9.82a±2.50
Endomysium thickness [µm]	2.75a±0.95	2.83a±0.68	2.95a±0.72	3.80a±1.13	2.67a±0.73
Explanations: CSA – cross-section area a – values in rows with the same index do not differ significantly (p > 0.05) between species

Mechanical and sensory properties as well as nutritional values are important in consumer assessment of meat quality. These quality parameters determine the acceptance of product and its selection in future. Meat texture is a result its histological structure (muscle fibres size and shape, fibres density, connective tissue quantity and quality, and abundance of intramuscular fat) as well as interrelationships between those elements. Generally higher firmness and hardness of cooked fish meat increase with number of smaller fibres, higher fibres density [26], higher amount of connective tissue and especially amount of insoluble collagen [29], and lower amount of fat [40]. The results of our study indicated that texture (measured by the senses as well using analyser) and tastiness differ between analysed fillets (Tab. 4). Study did not find signifficant differences in CSA of fibres and thickness of connective tissue between analysed fish fillets (Tab. 5), however the percentage share of individual fibre size classes depended on fish species (Fig. 1).

Fig. 1. Distribution of muscle fiber size class among fillets of the analysed species

Filets of ridge scaled rattail, that had the highest hardness as assessed by analyser and the highest springness, juciness and stringiness had also the thinnest endomysium and the highest number of large fibres. Further analysis of fibres distribution for this species revealed that percentage share of large fibres (> 80 µm), medium (60–80 µm) and small (< 60 µm) was comparable among classes (≈1:1:1). Fillets of the thicklip grey mullet had the highest cohesiveness and the thickest connective tissue (as for peri and endomysium). According to Lazo et al. [27] cohesiveness of meat depends on the strengh of protein binding, which indicates on the existence of cross-links between collagen molecues. In the least hard, cohessive, juicy, spring and easy to chew fillets of sole, low content of fat, relatively thin connective tissue and small fibres (44.31% of fibres with size < 60 µm) were observed. Fillets of white bass were rated by panellists with the highest scores for hardness and chewiness due to high number of the smalles fibres < 60µm wich accounted for more than 50%. Higher level of small fibres in muscles increases fibre density, cellularity and amount of connective tissue in muscles. According to Johnston et al. [23] and Periago et al. [35] muscle fibre density showed positive correlation with collagen and textural parameters. Filets of bigeye sea perch had a low cohession due to thin endomysium. Despite the fact that fillets of this species had the biggest fibers and the highest (43.13%) share of fibers with size > 80 µm, meat had intermediate hardness. Differet share of muscle fibre class in each of the analysed species resulted from diverse habitat conditions, mainly quality and quantity of available nutients, environmental conditions which affected growth rate. Bigeye sea perch a bathydemersal species is an active predator and swimmer in comparison to white bass that occupy freshwater environment of the Great Lakes and mainly feed on invertebrates and fish when adult. Thus, higher number of the biggest fibrers in the first species was determined by the effect of swimming which was largely attributable to muscle hypertrophy. According to the results of other studies growth of fish muscle [26] and fish muscle cellularity [35] are the combined effect of contribution of hyperplasia and hypertrophy. However, muscle growth depend [1, 22].

Histological analysis showed various levels of damage and alteration to the structure of muscle fibres and connective tissue (Fig. 2). The most profound changes of the muscle structure were observed for bigeye sea perch and ridge scaled rattail samples as myofibre-myofibre and myofibre-myocommata detachments and myofibre and myocommata breakages. Morover, in the case of these two species study identified gaping of the fillets, a phenomenon observed when myomers separate from one another leading to slits or holes in the fillet. Moderate degree of unfouvarable changes were observed for New Zealand sole and white bass and insignificant for thicklip grey mullet samples. Occurrence of gaping remarkably lowers quality of fishery products during cooling and freezing storage and highly depend on fish species, e.g. spotted wolffish (Anarhichas minor) fillets do not gape [31]. The different degree of fillet to gape depends also on connective tissue amount and stages of catching and post-catch processing (mostly chilling and freezing temperature and time). Bigeye sea perch and ridge scaled rattail fillets had the thinnest connective tissue surrounding single muscle fibres (endomysium) what eventually led to fillet gaping [26]. Fresh samples are devoid of structural changes, however gradually occur as storage time progresses [43, 44]. The traditional freezing process (used for tested materials freezing) is generally slow, resulting in large extracellular ice-crystal formations, which cause structure damage during storage and after thawing [3].

Fig. 2. Changes in meat structure of 5 fish species based on evaluation of cross-sections; a) bigeye sea perch, b) New Zealand sole, c) white bass, d) thicklip grey mullet, e) ridge scaled rattail.

Based on our study it can be assumed that not the size of muscle fibers and thickness of connective tissue, but percentage share of muscle fiber classes, stability of conective tissue and observed changes in the structure differentiate fillet texture between analysed species. Among analysed fish species the highest taste intensity (mainly odour intensity) had white bass fillets. Specific seafood flavour is strongly connected with lipid-derived aroma componets, which are produced by the enzymatic oxidation polyunsaturated fatty acids, especially arachidonic, eicosapentatenoic and docosahexaenoic acids [27]. White bass fillets were characterized by the highest level of fat and recommended fatty acids. Lower fishy taste and odour intensity of the bigeye sea perch fillets was related to the lowest level of fat content in meat. Meat of white bass was selected by panellists as the most desirable, in contrast to the meat of the New Zealand sole that was rated with the lowest scores. However, due to delicate texture of the sole fillets meat of this species might be intorduced in the diet of condumers who have problems with food comminuting (children, seniors). Described in the present study medium intensity of fishy taste and odour of fillets can increase popularity of the analysed species among potential consumers. Moreover, application of spices during meat processing will decrease undiserable fishy taste and create products which are attractive due to their sensory characteristics, convincing even the most challenging consumers – kids.

CONCLUSIONS

The aim of our study was to assist producers to appropriately utilize raw material and to offer products with desired nutritional and sensorial properties to customers. However, to make that possible a particular attention should be paid to catching and processing of fish. Results showed that meat of 5 fish species could serve as potential alternative for fish meat which is commonly utilised by fish processing industry. Meat of the analysed species had high level of protein (thicklip grey mullet) and low level of fat (while white bass and ridge scaled rattail). Additionaly, fat of white bass had preferable nutritional properties. i.e. high PUFA:SFA ratio (nearly two-fold higher that recommended by nutritionists), desirable atherogenic (AI) index, and also the highest content of EPA and DHA level in relation to other species. While meat of white bass was selected by panellists as the most desirable, owing to high level of hardness and chewing. Fillets of the New Zealand sole do not meet sensory panel preferences due to low meat quality i.e. high softness and low cohesiveness, springiness and juiciness. Fat of this species was also characterised by low nutritional, pro-health, quality due to the highest AI and TI values.

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Received: 26.01.2019
Reviewed: 11.05.2019
Accepted: 20.05.2019

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Remigiusz Panicz
Department of Meat Technology, Faculty of Food Technology and Fisheries,
West Pomeranian University of Technology, Szczecin, Poland
Kazimierza Królewicza 4
71-550 Szczecin
Poland

email: rpanicz@zut.edu.pl

Piotr Eljasik
Department of Meat Technology, Faculty of Food Technology and Fisheries,
West Pomeranian University of Technology, Szczecin, Poland
Kazimierza Królewicza 4
71-550 Szczecin
Poland
email: peljasik@gmail.com

Małgorzata Sobczak
Department of Meat Technology, Faculty of Food Technology and Fisheries,
West Pomeranian University of Technology, Szczecin, Poland
K. Królewicza 4, 71-550, Szczecin, Poland
phone: (+48 91) 423 10 61 ext. 332
email: msobczak@zut.edu.pl

Joanna Sadowska
2 Division of Human Nutrition Physiology, Faculty of Food Science and Fisheries, West Pomeranian University of Technology, Szczecin, Poland

email: jsadowska@zut.edu.pl

Sławomir Keszka
Division of Aquaculture, Faculty of Food Science and Fisheries, West Pomeranian University of Technology, Szczecin, Poland
Kazimierza Królewicza 4, 71-550 Szczecin, Poland, phone: +48 91 449 66 36
email: skeszka@zut.edu.pl

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