Quality and sensory acceptability of fish fillet (Oreochromis niloticus) with alginate-based coating containing essential oils

Abstract

The quality and sensory acceptability of fish fillet (Oreochromis niloticus) with alginate-based coating containing ginger and oregano essential oils (EO) were evaluated. The antioxidant activity of essential oil, coating, and fish were also investigated. In relation to lipid oxidation, a decrease was observed in fish with the edible coatings compared to the control being the coating with oregano EO the most effective and also with the highest antioxidant activity. Loss in color and weight were significantly lower with coating. Fish with coating maintained firmness whereas fish without coating became softer. Fish with edible coating and oregano essential oils showed higher sensory acceptability regarding to odor evaluated by consumers. Thus, edible coatings with essential oils as natural antioxidant improved the product quality and sensory acceptability.

Introduction

Fish quality is associated with different attributes such as nutritional, physiochemical, microbiological and biochemical qualities. The freshness of fish reduces after death due to different reactions (such as changes in lipid and protein fractions, formation of hypoxanthine and amines) and also microbiological growth. This results in losses of sensory quality and nutritional value, making the preservation of the fish’s freshness of great importance (Mohan et al. 2012).

The most common form of preservation for fresh fish is ice; however, ice storage does not completely inhibit the reactions that lead to the deterioration of quality. Refrigeration is an important method for preservation of fish during storage. A low temperature can retard the growth of microorganisms and reduce undesirable chemical reactions. However, lipid oxidation still occurs in muscle, leading to rancid, discoloration and unpleasant odors (Sabeena Farvin et al. 2012; Sae-leaw and Benjakul 2014).

Aiming to improve the processed food products quality and extend its shelf life, synthetic antioxidants like the butylated hydroxytoluene (BHT) are usually used by the food industry. However, due to the concern about synthetic chemicals safety, studies are focused in the use of natural ingredients with bioactive compounds and antioxidants to improve the shelf life and food quality, associated to meet consumer wishes (Artés et al. 2007; Mohan et al. 2012).

The essential oils (EOs) are extracted from plants and are attracting interest as natural additives due to antimicrobial and antioxidant properties (Atarés and Chiralt 2016). Moreover, EOs are approved by the Food and Drug Administration and “generally recognized as safe” (FDA 2015).

Oregano (Origanum vulgare L.) and ginger (Zingiber officinale) essential oils have potential as natural antioxidants for food (Alexandre et al. 2016; Atarés and Chiralt 2016), and because they are extracted from well-known spices, their use in food could be widely accepted by consumers. Despite the benefits of EOs, they have a strong flavor which limits their direct use. Alternatives, such as the application in coating (suggested as an alternative food packaging), can be used to solve this problem (Acevedo-Fani et al. 2015; Ruiz-Navajas et al. 2013).

Edible coatings are generally formed using proteins and polysaccharides and the alginate is a natural polysaccharide (anionic), isolated from brown algae (Fabra et al. 2009; Lu et al. 2009). The alginate can be cross-linked by divalent ions and form strong gels, such as Ca²⁺, and these gels (coatings) can be used to prolong the shelf-life and preserve the food quality, decreasing the oxidation, reducing the contact with oxygen, increasing the water barrier and keeping the flavor of the product (Song et al. 2011).

Thus, the objective of this study was evaluated the effects of edible coatings (alginate-based) containing ginger and oregano essential oil on the quality characteristics of fish during storage and its sensory acceptability.

Material and methods

Material

The reagents 2,2′-azinobis-3-ethylbenzotiazoline-6-sulfonic acid (ABTS), potassium persulfate and 2,2-diphenyl-1-picrylhydrazyl (DPPH) were purchased from Sigma-Aldrich (USA). Calcium chloride—Anidrol (Brazil). Sodium alginate was from Dinamica (Brazil). Essential oils were from Ferquima^® (Brazil).

Fish fillet preparation

Tilapia (Oreochromis niloticus—Gift/Tilamax) from the genetic improvement program of the State University of Maringa grown in net pens were selected (average weight: 950 g) and slaughtered by section of the spinal cord, as recommended by the Ethics Committee on Animal Use (CEUA), eviscerated, and manually filleted. The fillets (skinless) were vacuum packed, transported in an insulated box with ice to the Laboratory (Animal Science and meat quality), and used for analysis.

Preparation of coating and treatments

The coating solution was prepared according to Groppo et al. (2009) modified by Vital et al. (2016). The sodium alginate (2% w/v) was dissolved in sterile distilled water 98 (70 °C). After the complete dissolution, the alginate solution was chilled (25 °C). For the coating with essential oil, the EOs were added to alginate solution and mixed under magnetic stirring. Tilapia fillets were divided into four treatments: uncoated fillet (CON); fillet with edible coating (EC); fillet with edible coating containing 0.1% of ginger EO (ECG) and fillet with edible coating containing 0.1% of oregano EO (ECO). The fillets were submerged in alginate solution, individually, during 1 min. After the fillets were allowed to drain (in order to remove the coating excess, suspended by a cloth of plastic material) during 1 min, and then, submerged in a crosslinking solution—calcium chloride solution (2% w/v) during 30 s. The samples were packaged individually in polystyrene tray, wrapped with a retractile film and stored at 2 ± 1 °C under light (12 h/day with fluorescent lamp—1200 lux), simulating the market conditions. Tilapia fillets (CON, EC, ECG and ECO) were randomly removed during storage at 1, 3, 7 and 14 days and analyzed.

Antioxidant activity

Firstly, it was determined the antioxidant activity of each essential oil using a methanol extraction (1:1000 v/v). Afterwards all the edible coatings used (with each essential oil or without) were analyzed after the solution preparation (1:4 v/v with methanol). Finally, fish samples of each treatment (CON, EC, ECG, ECO) were analyzed at day one of storage (1:1 w/v with methanol). The extracts used in the antioxidant analyses were obtained by homogenization (using an ultra turrax for fish extraction and tubes homogenizer and vortex for oil and coating), centrifugation (15 min, 4.000 rpm and 25 °C). For fish a filtration (filter paper) was also performed. The antioxidant activity was performed using the ABTS and DPPH radical scavenging assays.

ABTS radical scavenging activity

The ABTS activity was measured according to Re et al. (1999), with modifications. 7 mM ABTS (5 mL) was mixed with 88 µL of potassium persulfate (140 mM) to generate the ABTS⁺. The mixture remained in the dark during at least 16 h. Then, the radical was diluted with ethanol up to 0.70 ± 0.02 of absorbance. The activity (%) was read at 734 nm. The extract of samples (30 μL) were mixed with ABTS+ solution (3000 μL) and absorbance was read after 6 min. The activity was calculated as:

$$ABTSactivity\left(\%\right)=\left(1-\left(A_{samplet}/A_{samplet=0}\right)\right)\ast 100$$

where, A_{sample t=0}: absorbance of the sample at the beginning of analysis (time zero); A_{sample t}: absorbance of the sample after 6 min.

DPPH radical scavenging activity

The DPPH activity was measured according to Li et al. (2009), with modifications. Samples extract (150 µL) were mixed with a methanolic solution of DPPH (60 µM) (2850 µL). Absorbance was read at 515 nm after 30 min. The activity was calculated as:

$$DPPHactivity\left(\%\right)=\left(1-\left(A_{samplet}/A_{samplet=0}\right)\right)\ast 100$$

where, A_{sample t=0}: absorbance of the sample at the beginning of analysis (time zero); A_{sample t}: absorbance of the sample after 30 min.

Evaluation of lipid oxidation (TBARs)

The lipid oxidation was measured as equivalent of malonaldehyde (MDA) with the thiobarbituric acid (Souza et al. 2011; Vyncke 1970). The fish (5 g) and 10 mL of TCA solution was mixed (7.5% TCA, 0.1% gallic acid and 0.1% EDTA), homogenized (Ultra Turrax), and then centrifuged (15 min, 4000 rpm and 4 °C). The solution was filtered and mixed (1:1 v/v) with TBA solution (thiobarbituric acid—1%, TCA—15%, HCl—562.5 µM). The mixture was boiled (100 °C) during 15 min, cooled, and the absorbance measured at 532 nm. The MDA concentrations were calculated using a standard curve of 1,3,3-tetramethoxypropane (0–60 µM) and the results expressed as mg MDA/kg of fish.

Color parameter

The color parameters were evaluated over the coating in all fish samples. Lightness (L*), redness (a*) and yellowness (b*) were obtained by the CIELab system, using a Minolta CR-400 colorimeter (Konica Minolta Sensing, Osaka, Japan) with a D65 illuminant and 10º view angle.

Texture analysis

The firmness (N) of fish fillet was measured according to Sigurgisladottir et al. (1999) with modifications, using a CT3 Brookisfield (Texture Technologies 15 Corp., UK) texturometer, with a Warner–Bratzler blade. The speed was 2 mm/s, distance target was 30 mm and trigger was 10 g. Samples with 3.0 cm × 2.5 cm × 2.5 cm were prepared and obtained from the dorsal region of each fresh fillet.

Weight loss

The fish fillets had their individual weights recorded every day of analyses. The results, weight loss (WL), were calculated each day as a percentage relative to the initial weight according to the following equation:

$$WL\left(\%\right)=\frac{Wi-Wf}{\mathit{Wi}}\ast 100$$

where, Wi = initial weight (at the day of production) and Wf = final weight (at day 1, 3, 7 and 14 of storage).

pH analyses

The pH was measured using a digital pHmeter (Hanna—HI99163, Romania—Europe) equipped with a penetration electrode.

Microstructure

Fish samples (1 day of storage) were lyophilized according to Matumoto-Pintro et al. (2011). Samples were fixed on aluminum stubs and then coated with gold (sputter coater, Bal-Tec, SCD 050). The photographs were made at 15 kV with a scanning electron microscope (Superscan, Shimadzu SS-550).

Consumer test designs

The test was developed on the State University of Maringá (Brazil) in an adequately room to a sensory test. Consumers were distributed from 18 to 24 years, from 25 to 39 years, from 40 to 54 years, and older than 55 years, in accordance with the Brazilian national profile (IBGE, 2015), and divided by session, with 10 persons in each session (n of total consumers: 120). Each person evaluated odor, tenderness, flavor and overall acceptability of four codified samples per session, one of each treatment using a hedonic 9-point scale. Samples were served in a randomized design in order to avoid carry-over effects (Macfie et al. 2007).

For tilapia fillets, the dorsal part was segmented into 10 pieces of approximately 3 × 1 cm; the remaining part was used to monitor internal temperature to not damage samples. Both parts of the fillets were covered with aluminum foil, identified, and cooked in a pre-heated grill (Philco Grill Jumbo Inox, PHILCO S.A., Brazil) at 200 °C until they reached an internal temperature of 90 °C. Samples (10 pieces) were individually wrapped in aluminum foil and kept warm (50 °C).

The analysis was approved by the Committee on Ethics in Research of the Universidade Estadual de Maringá under the protocol CAAE: 58879716.7.0000.0104.

Statistical analyses

Fish data were evaluated by analysis of variance with the general linear model (GLM) with SPSS (v.23.0) (IBM SPSS Statistics). Means and standard deviation were presented. In a factorial design, the treatment and the storage time were considered fixed factors. The experiment was repeated two times with three replicates per treatment. When significantly differences were observed, a Tukey test was performed (p = 0.05). Acceptability of the sensory attributes was also assessed via analysis of variance using general lineal model (GLM). Treatment was considered as fixed effect and the consumer was included as a random effect on the consumer test. Mean and standard error were calculated for each acceptability variable. Differences between treatments were verified using Tukey test. Hierarchical cluster analysis with the Ward’s method was used to determine the different segments of consumers, according to the overall acceptability using XLSTAT (v.7.5.3). The number of clusters was selected by a dendrogram. Principal Components Analysis was used to verify the relationships between treatments and show it in a graphical way. Correlations between sensorial attributes were studied using Pearson´s correlation coefficient.

Results and discussion

Antioxidant activity

The main components present in the oils are: zingiberene (33%) for ginger essential oil and carvacrol (70%) for oregano essential oil (Ferquima^®). The ABTS and DPPH radical scavenging (1:1000 w/v) were 10.76 and 7.19% for ginger and 69.94 and 27.31% for oregano essential oil, respectively. These results demonstrated that the oregano EO showed a higher antioxidant activity (p < 0.001) than ginger EO. This behavior was also observed when the different edible coatings were analyzed and also when the assays were performed on coated fish fillet samples, where the samples containing the containing with oregano essential oil presented higher values of antioxidant activity. The results of ABTS radical scavenging for coatings without EO, with ginger EO and oregano EO were 12.06, 14.10 and 26.67% and 6.4, 8.21 and 10.38% for DPPH scavenging, respectively. Regarding the fish fillets treatments, samples with coatings and essential oil (ECG, ECO) presented higher antioxidant activity (p < 0.002) than CON and EC. Control fish samples (CON) presented values of 33.68 and 23.53% for ABTS and DPPH, while samples of fish with coating (EC) presented 33.33 and 22.05%. Addition of essential oils on the coating increase the antioxidant activity; with values on fish samples with ginger EO (ECG) of 36.82 and 22.05%, and oregano (ECO) 42.78 and 31.19%, for ABTS and DPPH scavenging, respectively. Oregano EO showed the best results for antioxidant activity in all cases (on the isolated essential oil, on the coating and in the samples of fish with ECO coating). CON and EC had (p > 0.05) similar antioxidant activity. Currently, natural antioxidants are being considered good substitutes for synthetic antioxidants, and the results also demonstrated that the EOs present antioxidant activity not only intrinsically, but also even after being added to the coating and applied on fish fillets.

Lipid oxidation

CON and EC showed higher lipid oxidation than fish with coatings and EO after 3 days of storage (Table 1). The lipid oxidation increased (p < 0.001) with the storage time, particularly in CON group, which showed the highest increase (Table 1). ECO was more effective than ECG after 14 days. This showed that natural antioxidant may be used to retard lipid oxidation in tilapia fillets during storage. Volpe et al. (2015) showed that carrageenan (extract from red and purple seaweeds) coating, with and without lemon essential oil reduced the lipid oxidation of fresh trout fillets during storage. Raeisi et al. (2015) found lower TBARS values in rainbow trout fillets coated with carboxymethylcellulose with Zataria multiflora Boiss EO and grape seed extract than uncoated samples during the storage time. Another study with quince seed mucilage edible films containing natural preservatives (oregano or thyme EO) prevented color and texture changes, and also the lipid oxidation in rainbow trout (Jouki et al. 2014). This protection against lipid oxidation may be associated to the synergism between the coating properties of an oxygen barrier and the antioxidant activity of the EO (Raeisi et al. 2015), fact that also occurs in our current results.

Table 1.

Effect of active edible coating on lipid oxidation (TBARS) expressed as mg malonaldehyde kg^-1 of fish during storage at 2 °C

Storage (days)	CON	EC	ECG	ECO	p value
1	0.19 ± 0.027aA	0.19 ± 0.014aA	0.17 ± 0.015abA	0.15 ± 0.008bA	0.030
3	0.25 ± 0.10aB	0.24 ± 0.018aB	0.18 ± 0.010bA	0.16 ± 0.003bA	< 0.001
7	0.33 ± 0.051aC	0.25 ± 0.026bB	0.19 ± 0.010cA	0.17 ± 0.002cA	< 0.001
14	0.41 ± 0.02aD	0.34 ± 0.018bC	0.28 ± 0.009cB	0.25 ± 0.020dB	< 0.001
p value	< 0.001	< 0.001	< 0.001	< 0.001

Means with different lowercase letters in the same line are significantly different (p < 0.05). Means with uppercase letters in the same column are significantly different (p < 0.05). CON—Fish without edible coating; EC—Fish with edible coating without essential oil; ECG—Fish with edible coating with ginger essential oil; ECO—Fish with edible coating with oregano essential oil

Color characteristics

Color values of tilapia fillets with and without coating during storage at 2 °C are shown in Table 2. The L* values ranged from 45.32 to 49.82, showing a significant difference (p > 0.001) only at day 14th of storage. The L* values increased with storage time in CON samples. Chéret et al. (2005) reported that the color of fish fillets is related to physical muscle structure, heme-based pigment, and the amount of unbound water, which influences light dispersion. The edible coating protected the fish fillet from discoloration, keeping it darker and less pale. Changes in color of fish over the storage period could be related to non-enzymatic (oxidation and microorganisms) and enzymatic reactions, which result in myofibrillar protein degradation and myofibrils disorganization, leading to changes in appearance, such as green and browning meat (Jung et al. 2003; Young and Whittle 1985). Jouki et al. (2014) evaluated the color of rainbow trout fillets wrapped with and without films of quince seed mucilage with natural preservatives during 18 days and storage at 4 °C and showed that the L* values generally increased with storage in all treatments. The authors also verified that the appearance of the fillets became less gray and whiter. Chéret et al. (2005), studying the muscle of sea bass (a white muscle), observed an increase in L* for a storage time of 7 days.

Table 2.

L*, a* and b* values of fish with and without an edible coating during cold storage

Storage (days)	CON	EC	ECG	ECO	p value
L*
1	47.28 ± 1.36B	46.39 ± 1.86	46.98 ± 1.65	46.12 ± 1.82	0.184
3	47.64 ± 2.30B	46.32 ± 1.66	46.60 ± 1.89	46.45 ± 1.49	0.132
7	47.14 ± 1.64B	45.63 ± 2.17	45.32 ± 2.86	46.43 ± 2.43	0.091
14	49.82 ± 1.91aA	46.60 ± 2.16b	47.03 ± 2.10b	46.46 ± 2.71b	< 0.001
p value	< 0.001	0.487	0.078	0.967
a*
1	− 2.27 ± 0.76A	− 2.63 ± 0.45	− 2.59 ± 0.25	− 2.60 ± 0.52	0.305
3	− 2.27 ± 0.47A	− 2.65 ± 0.43	− 2.56 ± 0.31	− 2.54 ± 0.66	0.278
7	− 2.28 ± 0.15A	− 2.40 ± 0.54	− 2.41 ± 0.26	− 2.26 ± 0.45	0.727
14	− 2.91 ± 0.67aB	− 2.25 ± 0.43b	2.31 ± 0.38b	− 2.32 ± 0.20b	0.004
p value	0.019	0.055	0.144	0.277
b*
1	2.01 ± 0.59B	2.23 ± 1.03	2.18 ± 1.40	2.18 ± 1.13	0.951
3	2.31 ± 1.18AB	2.15 ± 1.10	2.55 ± 0.70	2.05 ± 1.30	0.539
7	2.26 ± 1.09AB	2.16 ± 1.49	2.10 ± 1.33	2.05 ± 1.06	0.978
14	3.13 ± 0.57aA	2.72 ± 1.44ab	2.11 ± 1.14b	2.02 ± 0.95b	0.036
p value	0.033	0.518	0.626	0.984

Means with different lowercase letters in the same line are significantly different (p < 0.05). Means with uppercase letters in the same column are significantly different (p < 0.05). CON—Fish without edible coating; EC—Fish with edible coating without essential oil; ECG—Fish with edible coating with ginger essential oil; ECO—Fish with edible coating with oregano essential oil

In relation to a* values (redness), an increase in greenness was observed for CON samples, whereas the edible coating maintained the color during storage (Table 2). Some authors have reported that changes of color occur in parallel with TBARS development during storage (Wetterskog and Undeland 2004), and this study showed that coatings prevented the lipid oxidation compared to CON. The a* values of all groups were negative (− a*); this may be due to the processing, since the bleeding was done soon after slaughtering the fish, leaving the meat less red, as well as to the analysis, which was performed on the inside part of the fillet. Some authors have also found negative a* values and attributed them to the processing, which did not leave enough red muscle in the samples (Zhao et al. 2016).

Fish fillets with edible coating presented lower b* value than uncoated fish (CON) (Table 2). The b* values increased only for CON samples during storage period. This may be associated with the pigment oxidation (with high oxygen content), the lipid oxidation products, such as aldehydes, could be sources of carbonyl compounds for Maillard reaction (non-enzymatic browning reaction) (Jouki et al. 2014; Wu et al. 2011).

Generally, considering the evolution of color variables during storage it has been observed that the presence of an alginate-based coating preserve fish sample’s color, decreasing color losses which is one of the main variables considered by consumers on purchase decision.

Texture, weight losses, and pH measurement

Fish texture is considered an important quality attribute for palatability. Thus, texture measurement is important to verify the effect of the methods of preservation on fish quality (Ayala et al. 2011). The texture of fish muscle depends on intrinsic factors, such as collagen and fat content, as well as microbiological and autolytic processes caused by death, inducing myofibrillar degradation and softening of the muscle (Li et al. 2012b).

Some authors have reported that texture quality loss occurs in two phases during refrigerated storage. The first phase is mostly related to enzymatic autolysis, while the following is in relation to microbial action (Huss 1988; Vaz-Pires et al. 1995). A poor texture indicates that the shelf life of the fillets was limited by microbial activity and chemical reactions (Jouki et al. 2014). The treatments with coating were able to maintain the firmness of the fish during storage (p > 0.05), retarding softening. However, the firmness of CON decreased at the 14th day (p < 0.001), showing that the fillets from this treatment became softer (Table 3) and tending toward a poor texture, which is undesirable for consumers.

Table 3.

Shear force, weight loss analysis and pH measurement of fish during cold storage

	Days
	CON	EC	ECG	ECO	p value
Shear force (N)
1	71.83 ± 5.92A	70.22 ± 5.06	73.46 ± 4.38	70.92 ± 4.74	0.821
3	71.99 ± 3.43A	70.48 ± 2.28	71.17 ± 4.46	70.08 ± 1.98	0.843
7	69.67 ± 4.82AB	71.51 ± 6.49	69.66 ± 5.66	73.09 ± 4.20	0.730
14	63.68 ± 3.66bB	71.57 ± 2.74a	71.06 ± 1.07a	72.15 ± 2.11a	0.002
p value	0.048	0.961	0.660	0.638
Weight loss (%)
1	0.97 ± 0.31D	1.01 ± 0.19C	0.89 ± 0.24B	0.94 ± 0.03C	0.954
3	1.88 ± 0.09C	1.60 ± 0.22C	1.57 ± 0.18B	1.78 ± 0.16B	0.360
7	4.25 ± 0.80aB	2.87 ± 0.001bB	2.75 ± 0.07bA	2.37 ± 0.19bB	0.037
14	7.69 ± 0.44aA	3.74 ± 0.39bA	3.72 ± 0.41bA	4.29 ± 0.51bA	0.002
p value	0.001	0.001	0.001	0.001
pH
1	6.59 ± 0.04C	6.58 ± 0.05C	6.60 ± 0.02	6.61 ± 0.06	0.871
3	6.94 ± 0.05aB	6.67 ± 0.01bBC	6.67 ± 0.04b	6.65 ± 0.04b	< 0.001
7	7.03 ± 0.08aAB	6.73 ± 0.04bAB	6.69 ± 0.03b	6.67 ± 0.04b	< 0.001
14	7.23 ± 0.16aA	6.81 ± 0.04bA	6.70 ± 0.03b	6.72 ± 0.01b	< 0.001
p value	< 0.001	0.001	0.055	0.098

Means with different lowercase letters in the same line are significantly different (p < 0.05). Means with uppercase letters in the same column are significantly different (p < 0.05). CON—Fish without edible coating; EC—Fish with edible coating without essential oil; ECG—Fish with edible coating with ginger essential oil; ECO—Fish with edible coating with oregano essential oil

Weight loss was similar (p > 0.05) at days 1 and 3 for all groups studied (Table 3). The coating decreased weight loss in the fish after 7 days of storage (p = 0.037). The coating presents water-barrier properties (Song et al. 2011), keeping the water in the fish-coating system, and little or no exudates were visible from the coated fillets, which is an important attribute to consumers, as their presence is not attractive. Weight losses progressively increased (p < 0.001) for all samples during storage especially for CON, ranging from 0.97  to  7.69%, 1.01  to 3.74%, 0.89  to 3.72%, and 0.94  to 4.29% for CON, EC, ECG, and ECO, respectively.

The pH values are presented in Table 3. Usually, the pH of live fish is above 7.0, around 7.3, but this value decreases after death due to rigor mortis (glycogen is converted to lactic acid). In most fish species, the pH post mortem is between 6.0 and 6.8, and differences among these values may be due to the species, diet, level of stress during the catch, season, and type of muscle (Khalafalla et al. 2015).

At the first day of storage pH values of CON, EC, ECG, and ECO were between 6.58 and 6.61 (Table 3) and showed no statistical difference (p >0.05). After 3 days of storage and during the whole storage time, the pH of CON was higher than that EC, ECG, and ECO (p <0.001). This higher pH of CON may be due to an increase in volatile bases produced by endogenous or microbial enzymes (Li et al. 2012a, b). The coating (with or without EO) could effectively delay degradative processes, preserving the fish and maintaining lower pH values.

Microstructure

The microstructure (scanning electron micrographs) was used to verify the organization of droplet in the coating biopolymer matrix with and without EO and verify the structure of fish and fish with the coating (Fig. 1).

Fig. 1.

Scanning electron micrographs of the fish without coating (a)—40 × , alginate coating (b)—40 × , fish with alginate coating (c)—40 × , coating without oil (EC)—2400 × , coating with ginger essential oil (ECG)—2400 × , coating with oregano essential oil (ECO)—2400×

The edible coating without EO was homogeneous and smooth (Fig. 1—EC). Instead, the ECG and ECO showed heterogeneous structures, with droplets (oil) dispersed in the matrix. Similar behavior was observed in others studies with films and coatings containing EOs (Acevedo-Fani et al. 2015; Vital et al. 2016). Figure 1a shows the fish structure (CON), Fig. 1b shows the structure of the edible coating on its own, and Fig. 1c, at a comparable magnification (40×), shows the coating layer throughout the fish. These findings corroborate with the others results of this study. The presence of the essential oils in the coating structure reinforces the effect of their presence in relation to the greater antioxidant activity of the treatments with essential oils, as well as a greater action in relation to lipid oxidation. In addition, it is possible to observe that the oil is dispersed by the coating, despite the non-uniformity of the particles, and not all united in one place, facilitating its action as an antioxidant on the whole product.

Consumer acceptability

The effects of treatment on consumer sensory scores are presented in Table 4. Only the attribute odor showed significant differences (p < 0.05) between treatments, with oregano treatment (ECO) achieving the highest scores.

Table 4.

General acceptability of sensory attributes of tilapia with edible coating and acceptability scores among three consumer groups identified by cluster analysis (n = 120)

	CON	EC	ECG	ECO	SEM	p value
General acceptability
Odor	6.74b	6.76b	6.93ab	7.24a	0.075	0.014
Tenderness	7.81	7.63	7.60	7.80	0.057	0.100
Flavor	7.05	6.95	7.14	7.17	0.071	0.580
Overall accept.	7.23	7.07	7.27	7.31	0.064	0.393

	n	%	CON	EC	ECG	ECO	SEM	p value
Cluster analysis
Odor acceptability
Cluster 1	68	56.67	7.53b	7.53b	7.68b	8.12a	0.055	< 0.001
Cluster 2	11	9.17	5.91b	7.18a	7.45a	3.45c	0.293	< 0.001
Cluster 3	41	34.16	5.66b	5.37b	5.56b	6.80a	0.139	< 0.001
Flavor acceptability
Cluster 1	12	10.0	6.33a	6.75a	7.17a	3.25b	0.274	< 0.001
Cluster 2	36	30.0	5.89b	5.58b	5.56b	7.19a	0.139	< 0.001
Cluster 3	72	60.0	7.75	7.67	7.93	7.81	0.053	0.306
Overall acceptability
Cluster 1	73	60.83	7.82	7.75	7.93	7.89	0.049	0.498
Cluster 2	11	9.17	7.18a	7.00a	6.73a	3.55b	0.298	< 0.001
Cluster 3	36	30.00	6.06b	5.69b	6.08b	7.28a	0.118	< 0.001

Means with different lowercase letters in the same line are significantly different (p < 0.05). SEM: Standard Error of Mean. ^§Based on a 9-point scale (1: dislike extremely; 9: like extremely). CON–Fish without edible coating; EC–Coating and fish with edible coating without essential oil; ECG–Coating and fish with ginger essential oil; ECO–Coating and fish with oregano essential oil

According to Ojagh et al. (2010), coatings with essential oils should not present negative effects on the sensory characteristics. The results of the present study indicate that coatings were imperceptible or did not presented negative effect related to tenderness, flavor or overall acceptability, since there was no statistical difference respect to the CON group (Table 4) and coatings with essential oil did not produce undesirable sensory properties; on the contrary, coated products obtained higher scores in some attributes as odor acceptability.

According to Du et al. (2012), consumers are always in contact with a flavor blend, and some foods are more associated/related with a kind of spice. Oregano, for example, is present in different foods, favoring its acceptability.

All fish treatments had a good acceptance, with notes greater than 7 (on a 9-point scale) for most attributes analyzed, obtaining higher scores than those found in a previous study also conducted with tilapia (Kayan et al. 2015), indicating a good acceptance by the Brazilian population. In relation to Pearson´s correlation coefficients, all correlations were significant (p ≤ 0.001), and overall acceptability was highly correlated to flavor (r = 0.872), possibly due to the EO characteristics.

Cluster analysis

The acceptability scores from three clusters of consumers for tilapia fillets are described in terms of odor, flavor, and overall acceptability (Table 4).

Odor acceptability

There were three groups of consumers (Table 4), with all clusters showing differences (p ≤ 0.001) between treatments. Cluster 1 (56.7%) and cluster 3 (34.2%) presented similar behavior in odor acceptability. In both groups, the percentage of men and women was almost 50%, with 46–47% of the participants being younger than 25 years and 7.4% or 4.9% being older than 55 years, for cluster 1 and 3, respectively. In both groups, edible coat with oregano essential oil obtained the highest odor acceptability in relation to the other treatments. Differences between cluster 1 and 3 were in terms of scores, which were almost two points higher (between 8.1 and 7.5) for cluster 1.

However, some of the participants (cluster 2-representing 9.2%) rejected the odor from oregano edible coat, preferring treatments with ginger or only with edible coat. The control treatment presented intermediate scores (5.9 points). The 63.6% of this cluster were women under 40.

Flavor acceptability

For tilapia flavor acceptability (Table 4), 10% of the consumers were compiled in cluster 1, which rejected flavor from samples with oregano essential oil (3.25 points) compared to other treatments (p ≤ 0.001). On this group, the majority were women (58.3%), and the participants were mainly aged between 25 and 39 years (50%); no participant was older than 55 years.

A different profile was observed in cluster 2 (30% of the participants), with ECO obtaining the highest flavor acceptability (7.2 points). The percentage of men was 52.8%; 69.4% of the consumers were younger than 40 years and 8.3% older than 55 years.

The majority of the consumers (cluster 3–60%) did not report any differences ion flavor acceptability between treatments, accepting all flavors (scores between 7.6 and 7.9 on a 9-point scale).

Overall acceptability

For the main proportion of participants (cluster 1–60.8%), there were no differences on overall acceptability between treatments. Acceptability scores were higher than 7.7 for all samples. This group was characterized by a higher proportion of women in relation to men (58.3 vs. 41.7%), 16.7% between 40 and 54 years and without participants older than 55 years.

Cluster 2 (9.17% of consumers) reported significantly lower scores (p ≤ 0.001) for samples with oregano (3.55 points) compared to any of the other treatments. Consumers were 47.2% women, and 47.2% were aged between 25 and 54 years.

However, cluster 3 (30% of consumers) preferred samples with oregano essential oil (7.28 points) compared to the others treatments (p ≤ 0.001). In this cluster, 51.4% were women and 12.5% of the cluster were older than 40 years.

Principal components analysis

Information about coating and preferences by consumers is graphically shown in Fig. 2. The two principal components axes explained 91.96% of the total variance. The attributes flavor, odor, tenderness, and overall acceptability are also on the right-hand side of F1, close to ECO and ECG, while CON and EC are inversely related with these attributes. This data also demonstrating the positive correlation of the use of edible coating added with essential oils in fish with the sensory attributes.

Fig. 2.

Principal component analysis of the scores for odor, flavor, tenderness and overall acceptability of tilapia fillet with and without edible coatings. CON—Fish without edible coating; EC—Fish with edible coating without essential oil; ECG—Fish with edible coating with ginger essential oil; ECO—Fish with edible coating with oregano essential oil

Conclusion

The edible coatings (alginate-based) decreased weight and color losses, as well as lipid oxidation of fish fillet, and the coatings with EOs were more effective in preventing the latter two. At 14 days of storage coated fish (with EO) was redder, yellower, and less pale than control samples, and the pH did not change. The addition of EOs increased the antioxidant activity. ECO showed higher antioxidant activity and lower lipid oxidation in compare to ECG. In general, all treatments were well sensorial accepted, and samples with oregano in the coating received better notes regarding the odor. Thus, alginate-based coatings with EO could be used in fish products in order to maintain or improve their quality during the shelf life without adding undesirable sensorial characteristics to the product.