Assessment of Color and Sensory Evaluation of Frozen Fillets from Pangasius Catfish and Nile Tilapia Imported to European Countries

Abstract

The current study was designed to assess the color and sensory evaluation of frozen fillets of Pangasius catfish and Nile tilapia imported to Poland, Germany, and Ukraine in order to evaluate the possible impact of negative media image of these fish fillets on the sensory expectations of the consumers. The study used two trials of sensory evaluation: Blind and informed, on a group of untrained respondents and additional color analysis using the Commission Internationale de l’Éclairage (CIE) L*a*b* scale. The lowest grades during sensory evaluation in all measured parameters were given to Pangasius imported to Ukraine and Poland. The grades of Pangasius and tilapia imported to Germany were similar and significantly higher than of other Pangasius groups. Generally tilapia fillets were marked as better than Pangasius fillets. Moreover the present study found that there was considerable percentage of customers which are susceptible to various commercial frauds since they marked Pangasius higher when they thought they are consuming turbot. This study also showed that the negative image of Pangasius presented through various mass-media might have a negative impact on consumer’s perspective of Pangasius consumption.

Keywords:

• Pangasius
• Tilapia
• Fish consumption
• Color assessment
• Sensory evaluation

Introduction

Pangasius catfish (Pangasius hypophthalmus) is one of the most popular aquaculture products in the world. It reached the turnover of 1.4 million metric tons, from which 1.1 million are produced in Vietnam and was exported to over 80 different countries, including the United States, Canada, and the European Union countries. The Pangasius aquaculture significantly helped in the development of rural areas of Mekong delta, providing new workplaces and a source of income for hundreds of thousands people.^[12] Despite of being one of the most popular aquaculture products in the world, not many fish species received so much attention and negative marketing from the media as Pangasius catfish. Sensationalist documentaries aired on TV, political accusations on the forum of the European Parliament, and listing this fish on World Wildlife Fund’s (WWF’s) “red list.” All of this labeled the fish as controversial and unsafe in the eyes of the consumers, even though most of those allegations seem unfounded.^[3]

Tilapia is one of the most important aquaculture products in the world with the global yearly production of around 4.3 metric tons, from which 1 million tons is produced in China.^[4,5] Similar to Pangasius, tilapia has also been subjected to negative marketing by social and mass-media, especially in the United States, where it was often called “a poison.”^[6] Despite those controversies regarding both Nile tilapia and Pangasius catfish, the joint Food and Agriculture Organization of the United Nations (FAO) and The World Bank report predict that the aquaculture production of both those fish species will increase and in 2030 it will reach approximately 7.9 and 5 million tons per year.^[5]

The color of meat is an important parameter in assessing the quality of fish fillets.^[7] The color of meat and seafood is determined by the content of heme proteins and varies from red to brown.^[8] The Pangasius fillets color can differ from white to pink, red, or yellow. Those colors are associated with different quality of fish fillets and are the results of different methods used during farming and processing. The white fillets are regarded as the highest quality by the industry, while pink and red color means poor bleeding techniques. The yellow color is associated with the lowest quality and probably occurs due to poor water quality in which the fish were cultivated.^[9]

The current study was designed in order to evaluate if there could be an impact of negative media image of the frozen fillets of Pangasius catfish and Nile tilapia on the sensory expectations of consumers, by performing a sensory evaluation in two trials—blind and informed the current study was designed to assess the relationship between color and sensory evaluation of these fish imported to Poland, Germany, and Ukraine.

Materials and methods

Materials

Frozen fillets from Pangasius catfish and Nile tilapia imported to Poland (PP and TP, respectively) were obtained in February 2013, from retailers located in Cracow, Poland. Fillets were stored unpacked in freezers belonging to the retailer and immediately after purchase were transported to the freezers located in Food Technology Department of University of Agriculture in Cracow and stored at –18°C until further analysis.

Frozen fillets from Pangasius catfish and Nile tilapia imported to Germany (PG and TG, respectively) were obtained in January 2013, from retailers located in Berlin, Germany. Fillets were packed by the producer and stored in freezers belonging to the retailer. Frozen fillets from Pangasius catfish imported to Ukraine (PU) were obtained in December 2012, from retailers located in Lviv, Ukraine. Fillets were stored unpacked in open freezers belonging to the retailer. No analysis was performed on frozen fillets from Nile tilapia imported to Ukraine because no such fillets were available at the time of purchase.

Immediately after purchase, the frozen fillets from Germany and Ukraine were transported to the freezers located in Food Technology Department of University of Agriculture in Cracow and stored at –18°C until further analysis. During transport, fillets were stored in an icebox filled with ice bags. After transportation, the temperature of fillets was measured to ensure that it did not exceed –10°C and that the fillets were still fit for analyses. According to the ingredients list on the labels, the frozen fish fillets contained fillets and water (PG and TG); fillets, water and polyphosphates (E452; PU, PP) and fillets, water, and sodium triphosphate (E451; TP). Table 1 summarized the samples and ingredients list.

TABLE 1 Samples and the ingredients list

Name of samples	Code	Ingredients of the samples
Pangasius imported to Germany	PG	Fish fillets and water
Tilapia imported to Germany	TG	Fish fillets and water
Pangasius imported to Ukraine	PU	Fish fillets, water and polyphosphates (E452)
Pangasius imported to Poland	PP	Fish fillets, water and polyphosphates (E452)
Tilapia imported to Poland	TP	Fish fillets, water and sodium triphosphate (E451)

Color

The color analysis was performed using a CR 200 Minolta Chromameter (Osaka, Japan). The measurement of L* (lightness), a* (redness), and b* (yellowness)^[10] was performed in three different random locations on the fillets surface and calculated as an average. Before each measurement, the apparatus was calibrated on the standard white plate. The analysis was performed on 10 fillets from each group (10n × 2).

Sensory Evaluation

The organoleptic assessment was performed in sensory analysis laboratories located at Food Technology Faculty in University of Agriculture, Cracow. The experiment was designed according to rules of performing the sensory laboratory tests described by Resurreccion.^[11] The analysis was conducted on 30 untrained panelists aged 21–30 years old, who were neither vegetarians, nor allergic to fish and seafood. The respondents were informed that they should not consume any meals or drinks, smoke or use gums or mints for at least 30 min before the session. Each panelist was placed in individual cubical and instructed not to talk to other panelists during testing. Before the main analysis respondents were given a questionnaire, with preliminary questions like age, gender, weekly expenditures on food, and weekly fish consumption. At the end of questionnaire, the panelists were supposed to mark their knowledge about fish and fish products. Almost half of respondents were not responsible for the purchase of groceries in their household, while the rest spend 25–100 EUR weekly on food products. The majority of respondents (73%) consumed fish 0–1 times a week and marked their own knowledge about fish and fish products as average (84%).

Frozen fish fillets used in sensory evaluation were thawed, wrapped in aluminum foil and baked in the oven in 180°C until the temperature in the thermal center of the fish reached 80°C. Afterward, the fillets were cut into uniform transverse strips of 2–3 cm width and distributed to the panelists together with fresh mineral water.

The sensory evaluation consisted of two trials—blind and informed. In the first trial (blind) consumers were given five different fish samples labeled from A to E (Table 2) to hide their true name, and asked to evaluate five sensory parameters: color, odor, texture, taste, and overall acceptability on the 5-point unipolar hedonic scale, where 1 meant totally unacceptable and 5 meant perfect. Afterward, the respondents were asked to perform a preference test and mark which sample they thought was the best. The second trail (informed) measured if there might be a correlation between negative marketing of Pangasius catfish and Nile tilapia and the consumer’s perception of their sensory attributes. Fish samples were prepared in the same manner as in the previous trial with one difference. After cutting the cooked fish fillets onto transverse strips, those strips were cut in half and each half was labeled as one of the two different fish species: strips from Pangasius catfish were labeled as “Pangasius” and “turbot” and strips from Nile tilapia were labeled as “tilapia” and “sole” (Table 2). The samples were then evaluated by respondents for their color, odor, texture, taste, and overall acceptability using a 5-point unipolar hedonic scale, where 1 meant “not acceptable” and 5 “perfect.” Afterward the respondents were shown the estimated price of the labeled fish (Table 2) and asked to mark the price to quality ratio on the 5-point hedonic scale, where 1 meant “very bad” and 5 meant “very good.” The results from Trial 2 of sensory analysis were calculated in order to establish the differences in respondents grades depending on their knowledge of the name of fish they consumed (Table 2). The acquired results were shown as the result of subtraction of grades. Since the respondents, when grading both “Pangasius” and “turbot” or “tilapia” and “sole” were actually grading the same fish, from the same fillet, the expected difference in grades of each sensory parameter was 0. Because such grading is always subjective, the differences in grades by one point were still considered as minor differences. Due to this the differences in results between –1 and 1 of the sensory attribute of the consumed fish were considered as exactly the same or with small differences. The differences in results between (–2)–(–3) and 2–3 were considered as high differences in grades.

TABLE 2 Labeling of the fish samples for the sensory analysis

Trial 1
Code shown to respondents	Type of fish
A	Pangasius catfish imported to Germany
B	Pangasius catfish imported to Poland
C	Pangasius catfish imported to Ukraine
D	Nile tilapia imported to Germany
E	Nile tilapia imported to Poland
Trial 2
Name shown to the respondents	Price shown to the respondents [EUR/kg]	Real name of the evaluated fish
Pangasius	2–3	Pangasius imported to Poland
Tilapia	4–5	Tilapia imported to Poland
Sole	7–8	Tilapia imported to Poland
Turbot	10–12	Pangasius imported to Poland

Statistical Analysis

The statistical analysis was performed using Statistica software (StatSoft, Tulsa, OK, USA). The normality of results and homogeneity of variances were calculated using the Saphiro-Wilk and t-test. Variables with normal distribution and uniform variances were calculated by one-way analysis of variance (ANOVA) and the significance of differences was established using Tukey post hoc test. For variables without normal distribution or uniform variances, the significance of differences was calculated using non-parametric Kruskal-Wallis ANOVA, with multiple ranks comparison test. Significance of difference was established for α = 0.05 (the null hypothesis was discharged for α < 0.05). The results were labeled using a, b, c, d letters, where results with different letter differ significantly from each other (α < 0.05).

Results and discussion

Color

The color parameters of Nile tilapia and Pangasius catfish imported to different European countries are shown in Table 3. The color analysis showed significant differences between groups in all studied parameters. PU had the highest L*, a*, and b* of all studied groups, while TG had lowest redness and yellowness. No apparent trends and differences were visible between Pangasius and tilapia in general. The values found in this study were different then values found by other authors. Karl et al.^[12] measured the color of frozen fillets from Pangasius catfish imported to Germany from both conventional and organic farms. This study did not find any significant differences in color of fillets depending on the type of farming, but reported lower levels of L* and b* parameters then in this study. Analysis performed on frozen untreated tilapia fillets performed by Pivarnik et al.^[13] showed also lower values of L* and higher values of a* than in this study. According to their findings, yellowness of fish muscle increases during storage, while redness decreases. This increase in yellowness during storage can be attributed to heme oxidation which results in browning of the bloodline.

TABLE 3 The color parameters of frozen fillets from pangasius catfish and Nile tilapia

Fish	L*	a*	b*
PP	60.44^a ± 2.28	–1.42^a,b ± 0.41	4.88^a,b ± 1.51
PU	68.95^c ± 4.71	–0.21^c ± 0.10	9.55^d ± 0.61
PG	61.31^a,b ± 3.18	–1.32^b ± 0.17	6.63^b,c ± 0.37
TP	62.84^a,b ± 2.82	–0.94^b,c ± 0.42	8.21^c,d ± 1.79
TG	64.90^b,c ± 2.53	–2.61^a ± 0.20	4.49^a ± 1.28

^a,b,c,dSignificance of difference for α < 0.05.

Sensory Analysis

The differences in sensory evaluation between studied groups in the blind trial (Trial 1) are shown in Table 4. There is a clearly visible trend in all studied sensory attributes. The lowest grade was given to PU, then to PP and PG. Both tilapia groups received the highest score. The only exception was the color, where PG received the same score as TP. This trend is also reflected by the preference test, where panelists marked fish as “the best,” which clearly shows that tilapia was more preferable fish for the consumers. The differences in grading the Pangasius fillets is surprising because the fish used in sensory analysis were all cultivated and processed in Vietnam. On the other hand fillets imported to Ukraine and Poland also contained polyphosphates, which suggest that they were subjected to additional treatment in order to increase their water content and water holding capacities,^[14] which might change their sensory attributes. This, in turn, suggests that fillets exported to Germany, which has higher gross domestic product (GDP) per capita then Poland and Ukraine^[15] have better overall quality since the customers are able to pay more for the final product. No significant differences were found among the sensory evaluation of tilapia groups studied.

TABLE 4 Results from blind trial (Trial 1) of sensory evaluation

Name	PP	PG	PU	TP	TG
Color	3.6^a,b	3.8^b	3.1^a	3.8^a,b	4.0^b
Odor	2.9^b	3.6^a,b	2.8^b	4.0^a	3.9^a
Texture	3.3^a,b	3.9^b	2.8^a	4.1^b	4.0^b
Taste	3.2^a,b	3.5^a,b	2.8^b	3.9^a	3.7^a,b
Overall acceptability	3.3^a,b	3.6^a	2.7^b	3.9^a	4.1^a
The best [% of respondents]	9.5%	11.9%	4.8%	40.5%	33.3%

Results expressed as average of all the scores;

^a,bsignificance of differences (α < 0.05).

The results from the Informed Trial (Trial 2) of sensory evaluation (Table 5) showed that most of the respondents were consistent in giving their grades, and the differences were usually between –1 and 1. This would mean that most of the panelists were not affected by negative media image of Pangasius or by the price of the fish, in case where they compared “turbot” against “Pangasius.” On the other hand, in all the studied sensory attributes there was a group of consumers which graded the sample marked as “turbot” as much better than “Pangasius.” The highest difference was observed in the measurement of taste, where 33% of respondents graded “turbot” 2 or 3 points higher than “Pangasius” and 16.5% of respondents graded the taste of Pangasius higher, during the blind trial than during the informed trial. No such results were observed in the other direction. Only 3% of respondents graded “Pangasius” higher than “turbot” when marking its taste and texture and 6% of respondents marked “Pangasius” overall acceptability as higher than “turbot.” None of the respondents marked “Pangasius” odor and color higher than “turbot.” In the case of tilapia, only in single cases the differences in grades of each sensory attribute were higher when the fish was analyzed during the blind trial or labeled as “sole.”

TABLE 5 Differences in grades depending on the knowledge of fish name [% of respondents]

Color	Difference in grades
	–3	–2	–1	0	1	2	3
B-P	0	7	33	50	10	0	0
E-T	0	7	17	60	13	3	0
Tu-Pa	0	0	13	47	30	7	3
So-Ti	0	0	7	67	20	7	0
Odor	Difference in grades
	–3	–2	–1	0	1	2	3
B-P	0	10	20	40	13	17	3
E-T	0	7	13	57	13	7	3
Tu-Pa	0	0	13	30	27	13	17
So-Ti	0	3	17	43	27	7	3
Texture	Difference in grades
	–3	–2	–1	0	1	2	3
B-P	3	0	20	47	23	3	3
E-T	0	7	17	50	20	3	3
Tu-Pa	0	3	10	27	43	13	0
So-Ti	3	10	13	40	27	7	0
Taste	Difference in grades
	–3	–2	–1	0	1	2	3
B-P	3	0	13	33	33	13	3
E-T	0	10	20	50	20	0	0
Tu-Pa	0	3	10	33	20	27	7
So-Ti	3	0	33	43	20	0	0
Overall acceptability	Difference in grades
	–3	–2	–1	0	1	2	3
B-P	0	10	7	37	40	7	0
E-T	0	7	30	43	13	7	0
Tu-Pa	0	3	3	43	27	20	3
So-Ti	0	0	33	40	20	3	3
Price/quality ratio	Difference in grades
	–3	–2	–1	0	1	2	3
Tu-Pa	20	13	27	10	23	7	0
So-Ti	0	3	47	30	20	0	0

B-Pa: Score given to sample “B” from Trial 1 minus the score given to “Pangasius” in Trial 2; E-Ti: Score given to sample “E” from Trial 1 minus the score given to “Tilapia” in Trial 2; Tu-Pa: Score given to sample “Turbot” from Trial 2 minus the score given to “Pangasius” in Trial 2; So-Pa: Score given to sample “Sole” from Trial 2 minus the score given to “Tilapia” in Trial 2; Differences in grades between <–1,1> were treated as no or small differences; Differences in grades between <2,3> and <–3,–2> were treated as significant differences.

Even more surprising are the results which show the differences in marking the quality/price ratio. Since the price shown to the consumers of the sample labeled as “turbot” was 4–5 times higher than the sample labeled as “Pangasius,” the expected differences between their grades in quality/price ratio were from –3 to –2. However, only 33% of respondents marked this ratio as expected. Moreover, 30% of respondents graded the quality/price ratio of “turbot” as better than “Pangasius” and additional 10% as the same. This means that 40% of consumers were able to pay 4–5 times more for the same fish fillet, when its name was changed. When comparing the results of quality/price ratio between samples marked as “sole” and “tilapia” also 20% of respondents marked “sole” as better than “tilapia” and additional 30% as the same, even though the labeled price of the “sole” was 2 times higher than of “tilapia.”

ConclusionS

The study measured the consumer acceptance and preferences of Pangasius and tilapia fillets imported to various European countries by using two trials of sensory evaluation: blind and informed. Acquired results suggest that there are differences in sensory quality of Pangasius fillets, depending on their place of import. The respondents preferred more Pangasius fillets exported to Germany than fillets exported to Poland and Ukraine. No such differences were noted for tilapia. Moreover there is a considerable percentage of respondents which were susceptible to various commercial frauds, especially when it comes to frozen fillets from Pangasius. Although the study suggest that the negative image of Pangasius presented through various mass-media had a negative impact on consumer’s perspective of Pangasius consumption, additional study with larger respondents group should be performed in order to confirm this. The additional color analysis showed significant differences in all parameters between studied groups, with Pangasius imported to Ukraine having significantly different color parameters than other Pangasius groups and tilapia imported to Germany having significantly lower yellowness and redness then tilapia imported to Poland