Proximate and Nutritional Content of Rainbow Trout (Oncorhynchus mykiss) Flesh Cultured in a Tropical Highland Area

Abstract

The present study was performed to assess the proximate and nutrient content of rainbow trout flesh, cultured in the Doi Inthanon Fisheries Research Unit, Chiang Mai Inland Fisheries Research and Development Center, Thailand. 240 fish were randomly distributed across 12 cages with 20 fish cage-1. Sixteen individual fish from each cage were randomly collected at different ages of 10, 12 and 24 months. Body composition, pH, water-holding capacity, shear force, collagen content analysis, sensory, lipid oxidation, and fatty acids profile were evaluated. The results indicated that body composition and carcass length were significantly higher in fish aged 24 months, except for carcass and viscero-somatic index percentages (P ≤ 0.05). Fish at 24 months showed significantly higher pH, moisture, fat, shear force, thiobarbituric acid reactive substances, and total collagen content values than fish at 10 and 12 months (P ≤ 0.05). However, protein percentage, sensory measurement and water-holding capacity were significantly higher in younger fish at 10 and 12 months. The average lipid content and n-6/n-3 ratios were significantly greater in fish at 12 months and in ventral fillets (P ≤ 0.05). However, polyunsaturated fatty acids: saturated fatty acid ratio was higher in fish at 24 months and in dorsal fillets. In conclusion, rainbow trout cultured in sub-tropical, montane conditions can be valuable sources of protein, unsaturated fatty acids, eicosapentaenoic acids, and docosahexaenoic acids.

Keywords:
rainbow trout; flesh quality; age; muscle type; fatty acids

INTRODUCTION

Rainbow trout, Oncorhynchus mykiss is a fish native to the temperate climate of North America that can adapt very well to cool water conditions in mountainous areas of tropical or sub-tropical regions [¹]. This is one of the most commonly farmed fish species of the Salmonidae family, enjoying high demand from global markets [¹]. It was first introduced to Thailand in 1973 by His Majesty King Bhumibol Adulyadej to be cultured in the Northern highlands as an alternative form of livelihood opportunity and protein source for ethnic Karen people. Because of its tolerance to relatively high-water temperatures and its fast growth rate, excellent flesh, rainbow trout has become preferred popular salmonid species for aquaculture worldwide, including in several highland areas of tropical countries [¹]. However, the nutrient content in cultured fish flesh depends on several factors, such as species, seasonality, nutrition, area, and age [²]. It has been reported that salmonids are heterothermal animals and their body temperature can vary from 6 °C in winter to 20-22 °C in summer [³]. This usually implies a pronounced effect on both the general level of lipid metabolism and the lipid composition of poikilotherms. Cold temperatures are normally associated with an increased unsaturation degree in body fat, in particular with a conversion of saturated fatty acids of the biological membrane phospholipids typical of the warm season into the corresponding mono- and dienic fatty acids typical of the cold season [⁴]. In addition, fatty acids of fish flesh play an important role in human health, varies with season, age, and diet [⁵].

The variation in the fatty acid profile of fish may have effects on the nutritional value, texture and organoleptic properties [⁵]. Fish flesh is a well-known source of proteins with high biological value, polyunsaturated n-3 fatty acids (n-3 PUFA) such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), minerals and vitamins [⁶]. These fatty acids, particularly EPA and DHA, have been found to have anti-inflammatory and immunomodulatory properties. They also have been proved to be beneficial to cardiac, musculoskeletal, gastrointestinal, and immune systems in humans [⁷-⁹]. Evidence from epidemiological and preclinical studies indicates that n-3 fatty acids, especially EPA and DHA, have anti-cancer properties.

The identification of optimal rainbow trout fillet quality from cultured fish is complicated since quality infers a broad range of traits and variables [¹⁰]. It’s also complicated because the manufacture and sale of products involve many levels with varying perceptions of quality [¹⁰]. Therefore, a combination study between growth performance and fillet quality of rainbow trout in a tropical area could provide valuable information for fish farmers and food industry representatives in the determination of best harvest endpoints. Moreover, it would be particularly valuable for trout farmers using or planning to use innovative fish production technologies that recirculate water and optimize environmental variables. Therefore, the objective of this study was to investigate the proximate and nutritional contents in the flesh of rainbow trout, Oncorhynchus mykiss culture under Thai climate, and hydrological conditions.

MATERIAL AND METHODS

Experimental diets and design

Fingerlings rainbow trout were obtained from Doi Inthanon Fisheries Research Unit, Chiang Mai Inland Fisheries Research and Development Center, Thailand. The farming of rainbow trout (Oncorhynchus mykiss) is located at the height of about 1300 m above sea level on a small tributary stream of the River Klang near the base of Siriphum Waterfall on Doi Inthanon National Park. These raceways are supplied with water from the waterfall at the rate of 250 L/min in the summer season and 500 L/min in the rainy season. Two hundred and forty individual fish were allocated into 12 cages with 20 fish cage^-1 (2x2x1.5 m) in the same water body. The diet was hand-fed to the fish twice a day at 8:00 a.m. and 5:00 p.m. Water temperature was at 20 - 25 ^oC all year round. Fish were fed with a diet containing dry matter, crude protein, ether extract, crude fibre, ash percentage, and water content of 93.36, 42.63, 12.15, 0.62, 12.65%, 10%, respectively. The amount of feed was adjusted based on temperature and fish biomass according to the method described by Pornsopin [¹¹]. The feeding trial was last for 24 months.

Sampling method

Sixteen fish from each treatment were randomly selected at three different ages 10, 12 and 24 months. They were anesthetized, followed by slaughter, and packed on tissues in polystyrene iced boxes and transported to the laboratory within 4 hours. Upon arrival, the fish were weighed, measured, and the Viscera-somatic index (VSI) and Hepatosomatic index (HSI) were calculated. Fish were then filleted along the insertion line of the ribs to obtain a dorsal (DF) and ventral fillet (VF). These samples were stored at -25 ^oC and then analysed in duplicate for pH, water-holding capacity, moisture, crude protein, sensory, shear force, collagen content, total lipids, and fatty acids, as described in the next section.

Determination of fillet pH

The pH of dorsal and ventral fins fillets was determined at 5,45 min, and 24 hours (h) post-mortem, respectively, by using pH meter (pH meter model 191, Knick, Berlin, Germany). The electrode was inserted into the longissimus muscle at the anterior cut surface of the 10^th rib location.

Colour measurements

After the measurement of pH, the samples were kept in polyethylene bags, chilled at 4 °C for 48 h. They were then stored at 4 °C outside the bag for 1h (‘blooming’) before conducting colour measurements with the use of Chroma Meter (Minolta, CR-300, Osaka, Japan). The colour parameters included L* = Lightness; white=100, black=0, a* =redness; green=-80, red=100, b* =yellowness; blue=-50, yellow=70.

Water-holding capacity

Water-holding capacity was determined according to the method described by Honikel [¹²].

Shear force measurement

Shear force measurement was detected following the method of Roth and coauthors [¹³]. For boiled samples, shear force was measured using TA-XTplus Texture Analyzer from Stable Micro Systems equipped with a Warner-Bratzler test cell. For muscles, they were sliced at a constant speed of 2.0 mm/s, 45° angle inverted knife. The shear force was determined by the maximum force (N) and the total amount of work (J) after slicing through the sample.

Sensory analysis

For sensory measurement, 9 panelist testers were assigned to each group (total 3 groups), panelists being selected from students and faculty members who have taken sensory measurement training according to the methods of [¹⁴].

Chemical composition

Samples of the dorsal fillet (DF) and ventral fillet (VF) were minced and analysed in duplicate for moisture, fat and protein contents (Kjeldahl; 6.25 x N) according to Cunniff and Association of Official Analytical [¹⁵].

Lipid oxidation

Susceptibility of the lipids to oxidation was assessed by the 2-Thiobarbituric acid (TBARS, Thiobarbituric acid reactive substances) as the method described by Rossell [¹⁶].

Fatty acids profile

Fatty acids in the feed and fillet were extracted by a mixture of chloroform/methanol according to Folch and coauthors [¹⁷]. Fatty acid methyl esters were prepared, according to the method of Morrison and Smith [¹⁸].

Statistical analysis

All statistical analyses were performed using SAS version 6.12 19. Descriptive statistics of analysis results were calculated for each treatment. The results of meat quality were determined by two-way analysis of variance (ANOVA) considering slaughter ages and muscle types as fixed effects. When significant difference was found, the means were compared with Duncan’s New Multiple Range Test.

RESULTS

Body composition and biometric data of rainbow trout at different ages

Data on body composition and biometric data of rainbow trout at different ages are presented in Table 1. The results showed that body composition and carcass length of rainbow trout at 24 months were significantly higher than those at 10 and 12 months. However, carcass percentage was significantly higher in fish at 10 and 12 months compared to 24 months. In contrast, the dorsal and ventral fillets were significantly higher in fish at 24 months compared to fish at 10 and 12 months. Regarding the viscero-somatic index, the present study revealed that fish at 10 months had significantly higher VSI than those of 12 and 24 months. However, no significant difference in hepato-somatic was observed in fish at different ages.

Table 1
Body composition and biometric data of rainbow trout at different age

Criteria	Age (months)			SEM	P-Value
	10	12	24
Body composition
Whole body weight (g)	339.04c	500.74b	1133.64a	11.584	<0.001
Carcass weight1/ (g)	287.99c	432.44b	930.98a	10.307	<0.001
Visceral weight (g)	35.93b	34.43b	64.50a	1.438	<0.001
Liver weight (g)	4.52c	6.49b	15.15a	0.311	<0.001
Carcass (%)	84.94a	86.48a	81.90b	0.286	<0.001
Dorsal fillet weight (g)	77.63c	142.98b	261.40a	3.758	<0.001
Ventral fillet weight (g)	67.30c	129.53b	208.25a	3.557	<0.001
VSI2 (%)	11.23a	7.45b	7.02b	0.002	<0.001
HSI3 (%)	1.34	1.31	1.30	0.000	0.858
Carcass length
Total length (mm)	290.22c	334.68b	439.80a	2.006	<0.001
Standard length (mm)	263.15c	305.08b	387.45a	2.264	<0.001
Snout length (mm)	56.71c	66.55b	93.98a	0.628	<0.001
Depth (mm)	71.34c	80.83b	104.86a	0.596	<0.001
Thickness (mm)	36.53c	43.68b	60.20a	0.400	<0.001

• ^1/
  Carcass without visceral. ^2/ VSI (Viscero-somatic index) = 100[(weight of all visceral including gonads and heart)/body weight]. ^3/ HSI (Hepato-somatic index) = 100[(weight of liver)/body weight].

Flesh quality of different trout ages and muscle types

Meat quality of different trout ages and muscle types are shown in Table 2. pH at different slaughter ages and muscle types decreased 24 hours post-mortem, from 6.61 to 6.25 after 24 hours. Fish at 24 months of age had higher meat pH value than other groups, and ventral fillet showed significantly greater pH than ventral fillet. The chemical analysis showed that moisture and protein percentages were significantly higher in rainbow trout meat at 10 months compared with 12 and 24 months. However, rainbow trout at 10 and 12 months had a lower fat percentage than that of 24 months. For muscle types, dorsal fillets had higher moisture and protein than ventral fillets; however, fat content was lower in dorsal fillets compared to ventral fillets. For water-holding capacity, drip loss was higher in fish at 10 and 12 months, whereas thawing loss was higher in fish at an older age (24 months). Boiling loss was higher in fish at 10 months compared to fish at 12 and 24 months. No significant difference in water-holding capacity between dorsal and ventral fillets was observed. For fillet colour, it was found that the mean values of a^* and b^* was higher in fish fillets at 24 months compared with fish at 10 and 12 months of age. However, L^* was highest in fish at 12 months following by 10 and 24 months of age. For fillet colour, L^*, a^*, and b^* of the ventral fillet (VF) were significantly higher than dorsal fillet (DF) muscle. Results with regards to sensory measurement indicated that firmness and overall acceptability of rainbow trout at 10 months was significantly higher than other groups. Surprisingly, ventral fillets indicated more tenderness than dorsal fillets. Regarding the shear force, our results showed that maximum force and energy value increased as rainbow trout age increased. Additionally, boiled dorsal fillets presented a higher maximum shear force than ventral fillets. For lipid oxidation, a significant increase in TBARS was observed in rainbow trout flesh at all ages after 9 days of storage. Fish at older ages (24 and 12 months) showed higher TBARS compared to the younger age (10 months). In addition, TBARS in dorsal fillets was significantly higher than that of ventral fillets.

Table 2
Meat quality characteristics of rainbow trout at different age and muscle

Criteria	Age (months)			Muscle		SEM	P-Value
	10	12	24	DF1	VF2		Age	Muscle	Inter3
pH value
pHi (5 min pm4.)	6.53b	6.48b	6.61a	6.50y	6.55x	0.001	<0.001	0.267	<0.001
pHii (45 min pm.)	6.37b	6.38b	6.45a	6.38	6.40	0.001	0.018	0.596	0.060
pHu (24 h pm.)	6.34a	6.25b	6.37a	6.37	6.32	0.000	<0.001	0.828	0.779
Chemical composition, %
Moisture	72.49a	72.79a	71.30b	72.22x	71.83y	0.139	<0.001	<0.001	0.683
Protein	21.83a	20.03c	20.79b	20.38x	19.02y	0.074	<0.001	<0.001	<0.001
Fat	5.46c	6.48b	7.71a	7.07y	8.70x	0.148	<0.001	<0.001	0.064
Fillet color
L*5	48.51b	49.86a	46.90c	48.07y	48.86x	0.009	<0.001	0.054	0.002
a*	2.48b	2.10c	4.54a	2.31y	3.73x	0.004	<0.001	<0.001	0.365
b*	11.32c	13.07b	15.05a	12.27y	13.98x	0.009	<0.001	<0.001	0.828
Sensory evaluation6
Firmness	5.94a	5.24b	5.51b	5.48	5.70	0.003	<0.001	0.089	0.244
Odour	5.29	5.33	5.37	5.41	5.24	0.004	0.887	0.287	0.248
Juiciness	5.68	5.73	6.03	5.85	5.76	0.004	0.097	0.515	0.798
Tenderness	5.89	5.96	6.01	5.80y	6.10x	0.003	0.742	0.021	0.894
Acceptability	5.94a	5.54b	5.55b	5.63	5.76	0.003	0.006	0.232	0.541
Shear force
Max. Force (N)
Raw fillet	5.04b	5.89b	11.52a	7.35	7.04	0.068	<0.001	0.718	0.947
Boiled muscle	9.73c	14.20b	19.01a	16.30x	12.82y	0.139	<0.001	0.040	0.840
Work (J)
Raw fillet	0.11b	0.13b	0.25a	0.16	0.16	0.002	<0.001	0.951	0.957
Cook fillet	0.35b	0.52b	0.72a	0.58	0.45	0.006	<0.001	0.183	0.860
Water holding capacity, %
Drip loss	9.75a	11.26a	6.38b	8.65	9.60	0.057	0.002	0.385	0.277
Thawing loss	8.13b	6.08c	9.53a	7.02	8.86	0.056	0.024	0.076	0.544
Grilling loss	14.75	13.06	12.18	13.94	13.13	0.150	0.681	0.659	0.100
Boiling loss	8.07a	4.65b	3.26b	3.94	6.72	0.053	<0.001	<0.001	0.077
TBARS, mg malondaldehyde/ kg fillet
Day 0	1.71b	5.00a	5.49a	3.5y	4.64x	0.124	<0.001	<0.001	0.178
Day 3	7.35c	8.44b	9.91a	7.95y	9.19x	0.103	<0.001	<0.001	<0.001
Day 6	10.12b	8.68c	10.72a	9.64y	10.05x	0.098	<0.001	0.039	<0.001
Day 9	9.47b	9.49b	12.53a	9.26y	11.7x	0.101	<0.001	<0.001	<0.001

• ^a,b,c
  Mean within the same row with different superscripts differ significantly (P<0.001) by age effect.
• ^x,y
  Mean within the same row with different superscripts differ significantly (P<0.001) by muscle effect.
• ¹
  Dorsal fillet
• ²
  Ventral fillet
• ³
  Interaction between age and muscle
• ⁴
  post mortem,
• ⁵
  L* = Lightness; white=100, black=0, a* =redness; green=-80, red=100, b* =yellowness; blue=-50, yellow=70 and
• ⁶
  1= low intensity 10= high intensity

Lipid and fatty acid composition

Results of lipid and fatty acid composition are presented in Table 3. The results indicated that the highest value was SFAs (67.079, 66.546, and 61.952%), followed by MUFAs (18.333, 18.881, and 20.103%), and PUFAs (14.592, 14.577, and 17.951%) at 10, 12, and 24 months, respectively. Furthermore, SFAs were higher in the ventral compared to the dorsal fillets, whereas PUFAs were significantly greater in the dorsal compared to ventral fillets. However, no significant difference in MUFAs was observed between dorsal and ventral fillets. The concentration of individual fatty acids showed that the highest amount was the C18:0, followed by C16:0 and C22:6 (n-3) when fish weights increased. In the case of muscle types, C18:0 was higher in VF than DF muscles, whereas C22:6 (n-3) was higher in DF than VF muscles. However, no significant difference in C16:0 was observed between DF and VF muscles. Regarding the PUFA: SFA ratio, the present study revealed that differences in the PUFA: SFA ratio occurred among different slaughter ages and two muscle parts of the rainbow trout were observed. Fish at age 24 months had significantly higher PUFA/SFA ratio than those at 10 and 12 months, and the DF was significantly higher than that for the VF. For n-6/n-3 PUFA, the highest value was observed in rainbow trout at 12 months (0.144). This value was significantly higher than in fish at 10 and 24 months. However, no significant difference in n-6/n-3 ratio was found between 10 and 24 months. This value in VF was higher than that in the DF muscle.

Table 3
Fatty acids profile and total fatty acids (TFA, mg/100g fillet) of rainbow trout at different age and muscle

Criteria	Age (months)			Muscle		SEM	P-Value
	10	12	24	DF	VF		Age	Muscle	Inter3
C 14:0	2.634a	2.609a	2.119b	2.405y	2.503x	0.001	<0.001	<0.001	0.248
C 14:1	0.491a	0.488a	0.431b	0.466y	0.474x	0.000	<0.001	0.024	0.661
C 15:0	9.314a	7.785b	6.612c	7.682	8.125	0.015	<0.001	0.265	0.995
C 15:1	0.343b	0.220c	0.403a	0.326	0.319	0.001	<0.001	0.657	0.614
C 16:0	23.270a	23.820a	20.152b	22.437	22.391	0.015	<0.001	0.911	0.451
C 16:1	0.580b	0.614a	0.603a	0.593	0.604	0.000	<0.001	0.127	0.326
C 17:0	0.563a	0.520b	0.484c	0.513y	0.532x	0.000	<0.001	0.009	0.017
C 17:1	7.156a	6.474b	6.163b	6.700	6.496	0.009	0.002	0.372	0.378
C 18:0	29.609c	30.063b	30.628a	29.480y	30.720x	0.007	<0.001	<0.001	<0.001
C 18:1	9.475c	10.925b	12.304a	10.800	11.003	0.006	<0.001	0.187	0.085
C 18:2n6c	0.209b	0.226a	0.220ab	0.216	0.220	0.000	0.011	0.326	0.425
C 18:2n6t	0.343b	0.914a	0.277b	0.475y	0.547x	0.001	<0.001	0.023	<0.001
C 18:3n6	0.772b	0.281c	0.922a	0.677	0.639	0.001	<0.001	0.217	<0.001
C 18:3n31	0.723a	0.443b	0.496b	0.494y	0.613x	0.001	<0.001	<0.001	<0.001
C 20:0	0.164c	0.191b	0.206a	0.180y	0.194x	0.000	<0.001	<0.001	<0.001
C 20:1	0.232a	0.215b	0.227a	0.220y	0.230x	0.000	<0.001	0.005	<0.001
C 20:3n3	1.270b	1.293b	1.535a	1.431x	1.301y	0.001	<0.001	<0.001	0.777
C 20:3n6	0.158c	0.204b	0.327a	0.216y	0.243x	0.000	<0.001	0.027	0.002
C 20:4n6	0.126ab	0.118b	0.128a	0.123	0.125	0.000	0.037	0.631	0.677
C 20:5n32	0.748b	0.633c	0.870a	0.788x	0.713y	0.001	<0.001	<0.001	0.026
C 21:0	1.694c	1.892b	2.020a	1.776y	1.961x	0.001	<0.001	<0.001	0.041
C 22:1	0.114a	0.085b	0.103a	0.106x	0.096y	0.000	<0.001	0.034	<0.001
C 22:2	0.268c	0.658a	0.386b	0.470x	0.404y	0.001	<0.001	<0.001	<0.001
C 22:6n33	9.991b	9.825b	12.809a	11.838x	9.911y	0.008	<0.001	<0.001	0.015
C 24:0	0.279a	0.182c	0.251b	0.239	0.236	0.000	<0.001	0.571	<0.001
C 24:1	0.061a	0.048b	0.035c	0.045y	0.050x	0.000	<0.001	<0.001	<0.001
SFA4	67.079a	66.546a	61.952b	64.211y	66.174x	0.012	<0.001	<0.001	0.214
MUFA	18.333b	18.881b	20.103a	19.081	19.130	0.009	<0.001	0.844	0.076
PUFA	14.592b	14.577b	17.951a	16.713x	14.700y	0.009	<0.001	<0.001	0.182
PUFA:SFA	0.218b	0.219b	0.292a	0.263x	0.224y	0.000	<0.001	<0.001	0.088
n-6 PUFA	1.599c	1.731b	1.861a	1.697y	1.764x	0.001	<0.001	0.013	0.442
n-3 PUFA	12.725b	12.189b	15.705a	14.547x	12.532y	0.009	<0.001	<0.001	0.233
n-6:n-3	0.128b	0.144a	0.122b	0.119y	0.144x	0.000	<0.001	<0.001	0.422
TFA	5242.2b	6516.5a	4054.6c	4050.4y	6491.7x	6.852	<0.001	<0.001	<0.001

• ^{a, b, c}
  Mean within the same row with different superscripts differ significantly (P<0.001) by age effect. ^x,y Mean within the same row with different superscripts differ significantly (P<0.001) by muscle effect. ¹ALA=alphalinolenic acid, ² DHA= docosahexaenoic acid, ³ EPA= eicosapentaenoic acid. ⁴SFA=saturated fatty acids, MUFA=monounsaturated fatty acids; PUFA = polyunsaturated fatty acids.

DISCUSSION

Carcass quality of different trout age and muscle type

The present study indicated that both body composition and the carcass length of rainbow trout at 24 months were significantly higher than those at 10 and 12 months. It is natural that older fish accumulate more protein and lipid compared to young ones. However, carcass percentage was significantly higher in fish at 10 and 12 months compared to 24 months. The result agreed with previous results in rainbow trout, O. mykiss [¹]. The reason for this may be attributable to the evolution of gonadal development since gonad size increases with the growth of the animal during its reproductive stage [²⁰]. Moreover, carcass yield also depends on the animal’s sex [¹]. For carcass yield, our result indicated that fish at 10 months had significantly higher VSI than those of 12 and 24 months. Similarly, rainbow trout at 14 months had higher VSI than those of 24 and 26 months, according to Davidson and coauthors [¹⁰]. By contrast, rainbow trout at weights ranging from 371-440 g showed significantly higher VSI than fish at weights between 300-370 g^-1. The differences in these findings may be due to the different culture conditions and diets.

Flesh quality of different trout age and muscle type

The present study revealed that pH at different slaughter ages and muscle types rapidly decreased after 24 hours post-mortem. Similar results were observed in cod, Gadus morhua [²¹]. In contrast, no significant decrease of pH after post mortem was observed in the same species, O. mykiss [²²]. It has been well-documented that the rapid decrease in pH more or less depends on the fish stage, slaughter, and storage conditions [²³]. This could be a reason for significant differences in pH at different slaughter ages found in the present study.

For chemical analysis, present results revealed that younger trout had significantly higher moisture and protein percentages than older fish, whereas lipid content was higher in older fish compared to younger fish. The results were in line with previous results in rainbow trout, O. mykiss [¹]. However, no significant differences in moisture and fat percentages have been reported in rainbow trout at different weights [⁶]; and in yellow croaker, Pseudosciaena crocea at ages of 1 and 2 years [²⁴]. The decrease in protein content of older fish may be attributable to the conversion of protein into fat or protein used for energy [²⁵]. Nargis [²⁶] reported that protein content in moderate-sized Koi carp, Cyprinus carpio was higher in older fish. This decrease in muscle protein may be due to the use of energy for growth [²⁷]. Significant differences in moisture, protein, and fat percentages between different types of fillet were also observed in the present study. Similarity, significant differences in chemical composition of different muscle types and portions were observed in Pacific bluefin tuna, Thunnus orientalis [²⁸] and Asian catfish, Pangasius bocourti [⁵]. The fluctuations in fish chemical composition are linked to intake-rations since protein rates in muscle tissue slightly increased in feeding time and consequently increased fat rates [²⁹].

In terms of water holding capacity (WHC), our study indicated that the mean value WHC of 10 months rainbow trout was higher than those at 12 and 24 months. Several factors have been reported to affect the WHC of fish flesh. Suárez and coauthors [³⁰] have demonstrated that the WHC directly related to muscle structure, which was strongly influenced by the structural changes in the proteins comprising the muscle, fibre contraction and by water distribution both intra- and extra-cellular. The free water is maintained in the interior of the tissue by capillary action and surface tension, while loss results from changes in myofibrils volume. Ang and Haard [³¹] indicated that the rate of pH decline in the muscle post-mortem was important, as a rapid pH decline may cause soft texture and poor water holding capacity of the meat. Nonetheless, a negative correlation between flesh pH and WHC has been revealed by Toldrá [³²]. Recently, Roth and coauthors [³³] have proven that stressed fish with softer flesh texture and drip loss almost 3-fold higher than rested fish, presumably as a result of physical stress of muscle fibrils or connective tissue combined with protease-mediated muscle tissue degradation. These could be the reason for a significant difference in WHC amongst different aged fish in the present study.

Our study showed that the mean values of a^* and b^* were higher in fish fillets at 24 months compared with fish at 10 and 12 months of age. However, L^* was highest in fish at 12 months, followed by 10 and 24 months of age. Our results were in agreement with previous results reported by Werner and coauthors [³⁴]. For muscle colour, L^*, a^*, and b^* of VF were significantly higher than the DF muscle. Significant differences in colour parameters of different body parts of fish have been demonstrated in previous studies. Suárez and coauthors [³⁵] indicated that higher values of L^* in dorsal and ventral muscles of rainbow trout compared with other lots were observed. In addition, b^* values were higher in the skin and flesh of fish. Many factors have been demonstrated to be responsible for colour changes in fish flesh. It has been well-documented that higher muscle fat contents resulted in higher L^* and b^* values [³⁶]. Stress also affected the flesh colour [³⁶] by an isolubilization of muscle proteins concerning intense muscle activity before death [³⁷]. Different culture conditions also affected lightness and skin colour distribution of gilthead seabream [³⁸].

Regarding the sensory measurement, in this trial, firmness and overall acceptability of 10 months trout were significantly higher than the other groups. Surprisingly, ventral fillets were tenderer than dorsal fillets. The reason for this could be attributable to the fat content in fish flesh. Valente and coauthors [³⁸] indicated that significant differences between lipid content and both fatty flavour and the perception of fatty texture was observed in gilthead seabream from different production systems. It has been reported that ventral fillets of turbot showed a more pronounced odour than dorsal ones. The reason was probably attributable to the relatively higher fat content of the ventral fillet [³⁹]. Fillet lipid content shows a correlation with flesh texture and affects texture attributes [⁴⁰]. Moreover, fillet fatty acid composition may be connected to fattiness and a so-called “juiciness experience” [⁴¹]. Izquierdo and coauthors [⁴²] revealed that slightly lower hardness was found in the fillets of gilthead seabream fed with vegetable oils due to slightly higher lipid content and a significantly lower percentage of saturated fatty acids found in their flesh. Nonetheless, no significant differences between the firmness degree during chewing and resistance to force applied in the mouth, despite the differences in lipid content and fatty acid profile were observed [⁴³]. For blackspot seabream, Pagellus bogavaveo “firmness” was similar in wild and farmed fish [⁴⁴]. Fat-rich tissues normally tasted smooth and juicy, whereas dryness was found in the tissue with low fat. Lipid content, water content, and fibre characteristics are thought to contribute to the juiciness of the fish in organoleptic tests [45].

For shear force, present results were similar to those observed in triploid brown trout, Salmo trutta [⁴⁶]. The shear force was affected by several factors, such as firmness, collagen content, and others. It has been reported that flesh firmness was positively related to the collagen content of muscle in Atlantic salmon [⁴⁷]. Roth and coauthors [¹³] studied shear-force of salmon flesh according to different pre-slaughter techniques and found that carbon oxides technique showed minimum force (N) and lowest energy (J). An increase in shear force due to handling stress has been indicated in the raw muscle of farmed cod, although no significant difference was observed [⁴⁸]. Similarly, cod chased to exhaustion in reduced water level tended to have a softer texture than dip-netted cod [⁴⁹].

In the present study, a significant increase in TBARS was observed in rainbow trout flesh at all ages after 9 days of storage. In addition, TBARS in dorsal fillets was significantly higher than that of ventral fillets. These findings were in agreement with previous results reported by Daniel and coauthors [⁵⁰] and Secci and coauthors [⁵¹]. The significant increase of TBARS after storage may be due to the stress pre-slaughter. It has been reported that stressful killing methods influence oxidative stress during frozen storage, both reducing the length of the induction phase and increasing the rate of lipid oxidation [⁵¹]. There is well-documented literature concerning the interaction between peroxides and lipid oxidation. Therefore, it could be assumed that the higher level of hydroperoxide might have negatively affected the flesh oxidative stability.

Lipid and fatty acid composition

Fish, especially sea fish, have considerable amounts of n-3 PUFAs, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) [²⁸]. The present study indicated that the level of SFAs (average value, 67.079%) was highest, followed by MUFAs (18.333%), and PUFAs (14.592%). Similar trends were also observed in previous studies in blackspot seabream, Pagellus bogaraveo [⁴⁴] and yellow croaker, Pseudosciaena crocea [²⁴]. However, the results disagreed with findings on O. mykiss [⁶,⁵¹], and Pacific Bluefin tuna, Thunnus orientalis [²⁸], and Chinook salmon, O. tshawytscha [⁵²]. Interestingly, our result indicated that fish at 10 and 12 months of age had significantly higher SFAs compared to 24 months and VF muscle was greater than DF muscle. These were in line with previous findings in O. tshawytscha [²⁸]. In contrast, yellow croaker at 2-year old had significantly higher SFAs than the one-year-old fish [²⁴]. In terms of MUFAs, the older fish (24 months) contained more than younger fish (10 and 12 months). The result agreed with findings in chinook salmon [⁵²] but disagreed with results in yellow croaker [²⁴]. For PUFAs, our result agreed with findings in yellow croaker [²⁴] but disagreed with results in chinook salmon [⁵²]. We also found that DF muscle had significantly higher PUFAs than VF muscle. However, no significant difference in PUFAs between DF and VF was observed in Pacific bluefin tuna [²⁸]. The difference in these findings may be attributable to the differences in culture condition, diet, species, and sex.

Regarding the concentrations of individual fatty acids in the lipid fraction, the C18:0 content was dominant compared to the other ones. A similar order in the level of these fatty acids were observed in O. mykiss [⁵¹] and Asian catfish, Pangasius bocourti [⁵]. In contrast, a different predominance order in fatty acids were observed in yellow croaker, P. crocea [²⁴] and blackspot seabream, P. bogaraveo [⁴⁴], where the predominant fatty acid orders were C16:0, C18 and C22:6 n-3 or C22:6 n-3, C16:0 and C18, respectively. This could be due to the use of different lipid sources in the diet because the fatty acid composition of the muscular tissue in fish reflects that of the diet [⁵³]. Significant differences in individual predominance fatty acid were observed in rainbow trout at different ages. The results were similar to those reported by Kiessling and coauthors [⁵²]. Finding a different order, Tang and coauthors [²⁴] indicated that yellow croaker at 2-year old had significantly greater C16:0 than that of fish at 1 year old. In terms of C18, fish at 24 months of age showed significantly higher levels than those at 10 and 12 months. This was similar to the result reported by Kiessling and coauthors [⁵²], but disagreed with the result of Tang and coauthors [²⁴]. Another predominant fatty acid was C22: 6 (n-3), with our result indicating that older fish accumulated more fatty acid than younger fish. This agreed with the previous result in yellow croaker [²⁴], but it did not agree with results obtained in Chinook salmon [⁵²]. Different parts of the fish body also had an effect on the fatty acids profile. Rebolé and coauthors [⁶] have reported that lipid content and the saturated fatty acids/polyunsaturated fatty acids and n-6/n-3 ratios were higher in the skin than in the muscle; whereas, the proportion of docosahexaenoic acid (C22:6 n-3) was higher in the muscle. The present results also indicated that C18:0 was higher in VF than DF muscles, whereas C22:6 n-3 was higher in DF than VF muscles. However, no significant differences between these fatty acids were observed between dorsal ordinary muscles and ventral ordinary muscles in Pacific Bluefin tuna [²⁸] or between different body parts of Asian catfish [⁵].

It indicated that differences in the PUFA/SFA ratio occurred among the three different slaughter ages and two muscle parts of the rainbow trout fillet were observed. The results agreed with a previous study in rainbow trout [⁶], where significant differences in PUFA/SFA and n-6/n-3 of fish at different ages and muscle types were detected. However, the PUFA/SFA and n-6/n-3 ratios of present study were lower than those reported by Rebolé and coauthors [⁶] and Secci and coauthors [⁵¹]. It was well-documented that fatty acid compositions of fish vary due to several factors such as the geographical location, season, food availability, water temperature, age, and size of the fish and the maturation status [⁵⁴]. In addition, diets containing only n-3 PUFA-poor vegetable oils such as soybean and palm oil as lipid sources could lead to a decrease of EPA and DHA in farmed fish with an increase of SFA and n-6 PUFA [⁵⁵]. All in all, the n-6/n-3 PUFA ratios of different age and muscle types are in the recommended range for a healthy diet.

CONCLUSION

The three age stages and two flesh body parts tested for rainbow trout showed different nutrient compositions. The younger fish contained significantly higher carcass percentage, protein, moisture content and acceptability; however, lower long-chain PUFA and MUFA were lower than older fish. Based on body composition, meat quality, and the n-6/n-3 PUFA ratio, the nutritional quality of younger fish is better than the older ones tested. It may be concluded that rainbow trout cultured under highland water source conditions in a tropical or sub-tropical region may be considered a valuable food source for human consumption.

Acknowledgments

We would like to thank the staff of the Department of Animal Science and Aquaculture and the central laboratory of the Faculty of Agriculture, Chiang Mai University. Thanks are also due to Dr. David J.H. Blake for providing English proofreading of the draft.

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HIGHLIGHTS

• • Body composition and carcass length were significantly higher in fish aged 24 months
• • Higher pH, moisture, shear force, TBARS, and collagen were found in 24-month fish
• • Lipid content and n-6/n-3 ratios were higher in 12-month fish and in ventral fillets
• • PUFA: SFA ratio was higher in 24-month fish and in dorsal fillets

• Funding:

  This research was supported by the Royal Project Foundation, Chiang Mai, Thailand and Functional Food Research Center for Well-being, Chiang Mai University.

Publication Dates

• Publication in this collection
  18 Sept 2020
• Date of issue
  2020

History

• Received
  11 May 2018
• Accepted
  27 Feb 2020