Effect of dragon fruit peel powder on quality and acceptability of fish nuggets stored in a solar cooler (5 ± 1 °C)

Abstract

Fish nuggets were prepared with dragon fruit peel powder (1.0, 1.5 and 2.0% w/w) to evaluate its quality and improvement in shelf-life during 15 days storage in a prefabricated solar cooler (5 ± 1 °C). Antioxidative and antimicrobial effects of dragon fruit peel powder in fish model system were also evaluated during storage. Results showed that dragon fruit peel is a good source of dietary fibre (59.8%) and phenolic compounds [65.7 mg Gallic Acid Equivalent (GAE)/100 g of sample] and contained 6.03% protein, 6.14% fat and 4.34% ash. Use of dragon fruit peel powder significantly (p < 0.05) improved the emulsion stability and cooking yield and nuggets with peel powder had lower pH value than control. Fish nuggets with peel powder showed gradual decrease (p < 0.05) in hardness, springiness, cohesiveness, gumminess and chewiness with advancement of storage period. Nuggets with 1.5% dragon fruit peel showed better sensory attributes compared to the others. Dragon fruit peel powder significantly inhibited (p < 0.05) the lipid oxidation and microbial load in fish nuggets during the storage period. So, it can be concluded that dragon fruit peel powder may be used as antioxidant dietary fibre for improved quality and acceptability of fish nuggets in prefabricated solar cooler. 1.5% level of incorporation showed better results in terms of antioxidant activity and better shelf-life of the fish nuggets.

Introduction

Having a huge water resource, world has got plenty of fish varieties which are highly nutritive and easily digestible. Fish is considered as a valuable source of protein in the human diet and is an excellent alternative choice for meat, mainly because of its naturally high content of essential n-3 polyunsaturated fatty acids and relatively low content of cholesterol (Uçak et al. 2011). Due to high content of polyunsaturated fatty acid, fish is highly prone to oxidative rancidity resulting loss of quality and spoilage (Seema et al. 2018). Different spoilage mechanisms reported to be involved in this quality loss which includes microbial load, endogenous enzyme activity, lipid oxidation, non-enzymatic browning and enzymatic browning (Medina et al. 2009). So in the fish and fish products, use of antioxidants and antimicrobial agents is often required to increase their shelf-life in addition to obtaining other beneficial effects. But that has also a lot of adverse effects on consumers’ health particularly when such agents are of chemical in origin. Hence, the demand towards natural or phyto-materials as anti-microbial and antioxidants is increasing day by day.

Plants are the natural sources of valuable bio-active substances and are known to possess antimicrobial, antifungal and antioxidant properties (Vaquero et al. 2010) and have a vast source of dietary fibre (Das et al. 2020). So, adding suitable plant extracts in such value added food items (nuggets, sausage, patties etc.) results into making them functionally low in calories and help to reduce the risk of coronary heart disease, obesity, diabetes, hypertension and gastrointestinal disorders besides giving rise a better shelf-life of such products.

Dragon fruit (Hylocereus polyrhizus) is mainly found in South East Asia and considered as a functional food due to its health benefits. Dragon fruit is gaining much attention recently because of its micronutrient enrichment regulated by the phenolics that possess antioxidant and anti-proliferative activities (Wu et al. 2006). Recently, Madane et al. (2019) reported that dragon fruit peel could be used as antioxidant dietary fibre and its incorporation significantly improved the quality, acceptability and shelf life of chicken nuggets. Siregar et al. (2020) also reported that red dragon fruit skin can be utilized as natural preservative for extending shelf life of tilapia by way of inhibiting microbial growth and developing oxidative rancidity. However, available literature on use of dragon fruit peel or extracts in the fish product is not available.

In tropical areas, the main challenge of marketing fish products from Point of Sales (POS) to doorstep level is to maintain the cold chain. Sometimes, the retailer cannot even afford the refrigeration cost from their marginal profit amount. This leads to a discouraging business model for small scale entrepreneurs. As a result, the marketing model of fish products adopted by the industry in underdeveloped and developing countries so far has never grown up to a satisfactory level.

Considering the above, an approach of developing a model solar cooler has been conceptualized in the present study as per method described by Biswas et al. (2020) for use in household as well as in commercial practices from the point of storage up to the end point customer. It will benefit the small scale entrepreneurs as well as explore the marketing opportunities of such perishable commodities in an improved manner. The initial capital investment for this cooler is also affordable and recurring expenses will be lowered down to almost zero, compared to the huge electricity consumption and capital requirement of conventional refrigerators. A novel approach of developing such type of portable model solar cooler can bring out a marketing boost in fish products industry particularly for small scale vendors engaged in selling street foods or sea shore fish-products in the ‘off-grid’ areas besides fetching the multidimensional benefits using such ‘green energy’ based portable solar cooler in harvesting of marine fishes.

Hence, an attempt has been made in the present study to incorporate dragon fruit peel powder (DFPP) at different levels (1%, 1.5% and 2% w/w) in Pangasius fish nuggets and to evaluate their storage stability in a fabricated portable solar cooler (5 ± 1 °C).

Materials and methods

Preparation of dragon fruit peel

The fresh and whole dragon fruits were procured from the local market and brought to the laboratory of Agricultural Engineering department, Visva-Bharati, Santinikatan. Fruits were washed thoroughly under running water and wiped until dry. Now the peels were separated from the fruits, cut into small pieces and dried in a hot air oven at 50 °C for approximately 24 h following the procedure of Madane et al. (2020). Dried peels were ground into powder and these samples were then aerobically packed in sterilized airtight plastic containers for further use and analysis.

Preparation of fish nuggets

Fresh, whole and mature Pangasius fishes (Pangasius bocourti) were brought from the local market and carried to the department in an insulated box with ice flakes to maintain the cold chain. The whole fish was de-scaled, cut, gutted and de-headed manually and washed thoroughly under running water. Fish fillets were prepared and washed again with cold water. Fish mince was prepared as per the procedure outlined by Tokur et al. (2006). Fish mince was used for nugget formulation. One kg formulation of emulsion was made for each treatment. Control formulation consisted of 70% fish mince, 10% refined mustard oil, 10% ice flakes, 1.6% salt, 0.3% Sodium tripolyphosphate (STPP), 0.3% sugar, 1.8% dry spices mixture, 3% condiments (ratio of onion and garlic in 3:1) and 3% refined wheat flour. The reasons for taking vegetable oil at the rate of 10% were manifold. Firstly Pangasius spp of fish is considered as lean fish as it contains very low fat content (Kulawik et al. 2016; Sokamte et al. 2020), as low as approx 1 ± 0.02% of its wet basis. Secondly, during preparation of the fillets, skin and belly fats were trimmed off, so it was important to incorporate some vegetable oil in the formulation to obtain the desired emulsion consistency. Moreover, the products so developed, should not taste bland and have to ensure to satiate the consumers' palate in the sensory panel. Hence, considering the existing literatures on preparation of chicken nuggets (Ayman et al. 2020), 10% mustard oil was used in the formulation in the present study. The ingredients used for preparing dry spice mixture were turmeric powder-20%, coriander powder-20%, cumin powder-20%, black pepper powder-20% and ‘Agmark’ grade meat masala-20%. Sodium nitrite at 150 ppm was also added to the above formulations. To the minced fish dragon fruit peel powder was added on w/w basis at the rate of 0% for control (CR), 1.0% (S1), 1.5% (S2) and 2.0% (S3) replacing the fish mince portion.

Minced fish, salt, sodium tripolyphosphate and sodium nitrite were added and chopped for about 1–2 min. After addition of ice flakes, it was chopped again for 2 min. Dragon fruit peel powder was added for uniform dispersion in the minced fish and chopping was continued for another 1–2 min. Refined mustard oil was slowly added while chopping for proper dispersion. Condiment paste, dry spice powder and refined wheat flour were added and chopping was continued until a uniform dispersion of all ingredients and the desired emulsion consistency was obtained. Final temperature of the fish emulsion was 10–12 °C. Fish emulsion was placed into stainless steel molds (18 × 12 × 4 cm), packed compactly and covered. The emulsion filled molds from all the treatments were clipped and cooked in a steam oven at atmospheric pressure for 35 min. The fish blocks were cooled to room temperature and cut into slices. The slices were manually cut into nuggets. About 200 g nuggets were packed in polyethylene pouches. The nuggets were kept in a pre-fabricated solar refrigerator (5 ± 1 °C) for 15 days to study the quality changes during storage. The samples were analyzed on 0th, 3rd, 6th, 9th and 15th days for different parameters.

Fabrication of solar cooler

A typical solar cooler was designed and fabricated as per method described by Biswas et al. (2020). The cooling chamber was made from food grade plastic material with volume of 25,047 cc and with proper provision of insulating foam with 1 inch or 2.54 cm thickness. Two heat sinks, made up of aluminum finned radiators were fixed inside (size 4 × 4 × 2.6 cm) and outside (size 10 × 10 × 3 cm) the cooling chamber. The heat sinks helped to absorb and dissipate heat from the cooling chamber into the atmosphere for the purpose of making the chamber cool. Two inbuilt small fans (12 V, 0.28 A) were fitted with heat sink to remove heat from the cooling chamber. Heat transfer takes place from the chamber through a module called thermoelectric module (TM) which is a solid-state heat pump that functions on the principle of heat conductivity called ‘Peltier’s effect’. In the fabricated cooling chamber here the TM has a capacity of 12 V, 50 W, 50 A. A battery (12 V, 10A, 20 h) was used. In this solar cooler, one polycrystalline type solar panels (12 V, 80 W) were used. The temperature recorded after 6 h of charging the battery under open sunlight was 5 ± 1 °C.

Analysis of dragon fruit peel powder

For proximate composition, the percentage of moisture, crude protein, fat and ash content of dragon fruit peel were determined as per the procedure of AOAC (1995).

Dietary fibres (DF)

Dietary fibre was estimated by using enzymatic–gravimetric method (Prosky et al. 1988). Briefly, samples were diffused in phosphate buffer and then digested by heat stable enzymes such as alpha-amylase, protease and amyloglucosidase. After the completion of the digestion, the insoluble dietary fibre (IDF) was filtered and washed with warm distilled water. The solution found from enzymatic digestion, were precipitated with 95% ethanol of four volumes. From the precipitation residue, the soluble dietary fibre (SDF) was determined. By drying the collected residue, the total dietary fibre was calculated by adding up of IDF and SDF. The protein and ash contents were determined for corresponding corrections.

Determination of total phenolics content

The concentration of total phenolics in dragon fruit peel powder was determined by the Folin–Ciocalteu (F–C) assay (Escarpa and Gonzalez 2001). It is based on a chemical reduction of the reagent, a mixture of tungsten molybdenum oxides. The product of the metal oxide reduction has a blue colour that exhibits a broad light absorption with a maximum at 765 nm. The intensity of light absorption at that wavelength is proportional to the concentration of phenols. Suitable aliquots of extracts were taken in a test tube and the volume was made to 0.5 ml with distilled water followed by the addition of 0.25 ml F–C (1 N) reagent and 1.25 ml sodium carbonate solution (20%). The tubes were vortexed and the absorbance recorded at 725 nm (Thermo Electron Corporation, England, Model No. AQA 133204) after 40 min. The total phenolics content was calculated using gallic acid as standard and results were expressed as mg gallic acid equivalent per 100 g of sample.

Total carotenoid content (TCC)

The method used for determining the TCC was performed on a UV–Vis spectrophotometer (Prache et al. 2003). Tee and Lim (1991) reported that the total carotenoids in dragon peel powder were measured at 450 nm. The absorbance for beta carotene (BC) was scanned from 400 to 550 nm, and the λ max was confirmed as 450 nm. Absorbance for both extracts and BC were read at 450 nm using Secomam UV–Vis spectrophotometer (RS232, Cedex, France). The TCC in the samples was determined by BC standard calibration curve: A = mB + c; where, A = absorbance value, m = slope of the BC standard calibration curve, B = concentration of BC (μg/ml), c = y intercept of the curve. There is a limitation of the method as no prior separation process is carried out. The absorbance taken at selected wavelengths is not specific for the compounds of interest. Therefore, the results obtained would be referred as TCC.

Determination of vitamin C

Sample preparation, chromatography conditions, identification and quantification of vitamin C in HPLC method was performed using the method described by Najwa and Azlan (2017). The homogenous solid sample was measured around 10–30 g and mixed with 60–80 ml of 3% metaphosphoric acid (HPO3) for one minute. The obtained extract was filtered through filtration paper and washed for few times by using vacuum pump filtration. Next, the filtrate was quantitatively transferred into a 100 ml volumetric flask and 3% of metaphosphoric acid (HPO3) was added up to 100 ml volumetric mark. All the sample solutions were filtered again through 0.45 μm syringe filter. After that, the samples were run in the HPLC system.

Identification was performed by the comparison of retention time of analyte in the analyzed sample with the retention time of the calibration standard. Quantification was carried out with the external standard method (Vitamin C standards at various concentrations) using the following equation formula to calculate the Vitamin C concentration on samples.

$$\text{CA}\left(\text{mg}/100\text{g}\right)=\frac{\text{Aa}\times \text{D}\times \text{RF}}{m}\times 100$$

where A_a = Peak area of the analyze; D = Sample factor dilution; RF = Response factor (way to adjust the proportionality of the detector) response to the concentration of vitamin C and is calculated the following formula:

$$\text{RF =}\frac{{\text{C}}_{\text{st}}}{{\text{A}}_{\text{st}}}$$

where C_st = standard working solution concentration 50 μg/ml; A_st = corresponding peak area; m = weight of samples.

Analysis of fish nuggets

For proximate composition, the percentage of moisture, crude protein, fat and ash content of nuggets were determined as per the procedure of AOAC (1995). Texture profile analysis was conducted using TA-HDi texture analyzer (stable micro system, UK), following the procedure described by Bourne (1978).

For pH estimation, 10 g of sample was homogenized with 50 ml of distilled water and pH was determined separately using a digital pH meter (Model 330i, WTW®, Germany) with a standardized electrode.

Emulsion stability was estimated as per the method followed by Das et al. (2006). About 25 g of emulsion was heated for 20 min at 80 °C in a polypropylene bag and the exudate was drained out. After draining out the cooked mass was cooled down and weighed.

Cooking yield was estimated by weighing of each fish block and recorded before and after cooking. The cooking yield was calculated and expressed as percentage by weight of cooked fish block/weight of raw fish block × 100.

Thiobarbituric acid and reactive substances (TBARS) values of the fish nuggets were estimated to study the antioxidative effect of dragon fruit peel during 0th, 3rd, 6th, 9th and 15th days of refrigerated storage period (Tarladgis et al. 1960).

For antimicrobial activity of dragon fruit peel, total plate count (TPC), total psychrophilic count (TPSC), total coliform count (TCFC) and yeast and mould count (YMC) for fish nuggets were performed (APHA 1984). All the microbial counts were done using readymade media (Hi-Media, Mumbai) and microbial counts were expressed in log cfu/g.

The sensory evaluation was carried out using the 8 point hedonic scale where ‘8’ denotes extreme desirability and ‘1’ denotes as unacceptable. A team of 7 well trained panelists evaluated different sensory attributes of fish nuggets. The products were deep fried in the rice bran oil at 150–160 °C for getting the desired golden brown colour and until the core temperature of the product reaches 65 °C. Water was provided to rinse the mouth between sample evaluations.

Statistical analysis

For statistical analysis, six sets of experiments were conducted (n = 6). Each experiment was conducted in duplicate and data obtained were analyzed by two-way analysis of variance (ANOVA) and the means were compared by the Tukey’s Post hoc test at the 5% level of significance (p < 0.05) through a SPSS-20^® version of software package, using a general linear model, considering the treatments and time (processing and storage) as a fixed effect and the replicates as a random effect. All the data obtained were represented as mean along with standard deviation (Mean ± SD).

Results and discussion

Proximate composition of dragon fruit peel powder

In the present study, the dragon fruit peel was used to determine the nutritious value and its incorporation as functional ingredient in fish nuggets to improve the shelf life and healthiness of fish product. Dragon fruit peels are often discarded in the beverage processing industries. The chemical composition of dragon fruit peel power is presented in Table 1. Results indicate that dragon fruit peel powder contained 9.99% moisture, 6.03% protein, 6.14% fat and 4.34% ash contents. As the peel is not considered as the edible part of the fruit, its proximate composition varies with different nutritional reports on dragon fruits. Recently, Madane et al. (2019) reported protein, ash and lipid content of dragon fruit peel powder were 10.36%, 2.34% and 4.48%, respectively.

Table 1.

Mean ± S.D. of dietary fibre, total phenolics, total carotenoids, vitamin C and proximate composition of dragon fruit peel powder

Parameters	Mean ± S.D. (n = 6)
Dietary fibre (DF) (g/100 g) (insoluble dietary fibre + soluble dietary fibre)	59.83 ± 0.19
Insoluble dietary fibre (g/100 g)	9.42 ± 0.22
Soluble dietary fibre (g/100 g)	50.41 ± 0.21
Total phenolic (mg GAE*/100 g)	65.71 ± 0.12
Total carotenoid (mg/kg)	1.87 ± 0.04
Vitamin C (mg/kg)	2.62 ± 0.06
Moisture (%)	9.99 ± 0.13
Protein (%)	6.03 ± 0.06
Fat (%)	6.14 ± 0.04
Ash (%)	4.34 ± 0.16

*GAE, Gallic acid equivalent

As shown in Table 1, the DFPP used in this study contained total 59.83% dietary fibre (9.42% soluble dietary fibre and 50.41% insoluble dietary fibre). This amount of DF in DFPP was higher than the byproducts of asparagus (49% DF: 10.4% SDF and 38.6% IDF), orange (37.8% DF: 13.6% SDF and 24.2% IDF) and peach (31.7% DF: 9.7% SDF and 22% IDF) (Grigelmo-Miguel and Martín-Belloso 1999). Due to the presence of good amount of DF, it can be said that DFPP may have a very good physicochemical effect even better than wheat bran (44% DF) and oat bran (23.8% DF) as reported by Grigelmo-Miguel and Martín-Belloso (1999).

Phenolic compounds and vitamin C in dragon fruit peel powder

The amount of total phenolic compounds in DFPP is also presented in the Table 1. Dragon fruit peel contained 65.71 mg GAE/100 g total phenolic compounds. This result can be supported by the results of Manihuruk et al. (2017), who recorded similar Phenolics content in red DFPP. The amount of total phenolic compounds and flavonoids may influence the antioxidant activity (Das et al. 2016). According to Siregar et al. (2020), red dragon fruit peel is full of polyphenols and contains active compounds like alkaloids, terpenoids, flavonoids, tianine, niacin, pyridoxine, cobalamin, phenolic, carotene and phytoalbumin, giving rise to its superior antioxidant and antibacterial ability.

In the DFPP, the amount of total carotenoid and vitamin C were 1.87 mg/kg and 2.62 mg/kg respectively. Presence of these two organic micronutrients causes various health benefits like boosted immunity, reduced risk of coronary heart disease and stroke, as well as certain types of cancer (Schieber et al. 2001).

Effect of dragon fruit peel on quality of fish nuggets

Effect on proximate composition of fish nuggets

The results of proximate composition study like moisture, protein, fat and ash percentage of control and treated fish nuggets are shown in Table 2. For moisture content, a significant difference (p < 0.05) noticed between the control and treated nuggets. For the control sample, the moisture content was 66.80% whereas a moisture content of 68.01% in S₁, 68.47% in S₂ and 68.90% in S₃ sample were recorded. This could be due to the addition of DFPP as it absorbed more water during emulsion preparation (Madane et al. 2019). The similar results were found by Das et al. (2015) where they used Bael pulp residue in Goat meat nuggets. DFPP incorporation does not affect (p > 0.05) in the amount of protein content in both control and treated samples. These results can be supported by Madane et al. (2019) where they used the DFPP in chicken nuggets and found similar results. The ash content was found to be higher (p > 0.05) in treated samples due to addition of dietary fibers in the form of DFPP. In control sample the ash content was 2.11% and 3.26% in S1, 3.54% in S2 and 3.66% in S3. The observation of Madane et al. (2019) on the ash content of fish nuggets differs from the present study where though they noticed an increasing trend in the ash% with the increased level of DFPP incorporation but this change was non-significant (p > 0.05). Such significant difference in ash content between control and treatment groups may be due to type, content and maturity of the dragon fruit peel used for the study. The fat content was found to be significantly lower (p < 0.05) in treated nuggets. The reduction in fat content might be attributed to the higher amount of fibre rich components present in dragon fruit peel replacing the fish mince portion in the treated nuggets. Also, increased amount of moisture absorption from the emulsion may lead to relative reduction in the fat% as moisture and fat content are inversely proportional in any muscle food emulsion. This observation is supported by Yasarlar et al. (2007), who reported that fat content in meatballs gradually decreased with an increase in level of wheat and oat bran.

Table 2.

Mean ± S.D. of proximate composition of fish nuggets incorporated with different levels of dragon fruit peel powder (DFPP) (n = 6)

Samples	Moisture%	Protein%	Fat%	Ash%
Control	66.80 ± 0.183c	13.74 ± 0.620	17.35 ± 0.493a	2.11 ± 0.260b
S1 (1.0% DFPP)	68.01 ± 0.289b	13.65 ± 0.107	15.37 ± 0.329b	3.26 ± 0.167a
S2 (1.5% DFPP)	68.47 ± 0.219a	13.71 ± 0.616	13.92 ± 0.218c	3.54 ± 0.407a
S3 (2.0% DFPP)	68.90 ± 0.384a	13.41 ± 0.254	14.03 ± 0.417c	3.66 ± 0.478a

Means bearing different superscripts within the column differ significantly (p < 0.05)

Means bearing no superscripts within a column do not differ significantly (p > 0.05)

Control = Emulsion without phytochemicals

Effect on lipid oxidation

The effect of DFPP in the fish nuggets on lipid oxidation, in the form of thiobarbituric acid reactive substances (TBARS) is tabulated in Table 3. An increasing trend in TBARS value (p < 0.05) of the control and treated samples was seen with advancement of time. Lipid oxidation in presence of oxygen and production of volatile metabolites increased the TBARS value (Uçak et al. 2011). After 15 days of storage, the TBARS value of the control increased from 0.33 to 1.51 mg malonaldehyde/kg and among the treated samples, S₂ sample (1.5% DFPP added) showed the least increment (p < 0.05) in TBARS value, which increased from 0.25 to 1.31 mg malonaldehyde/kg. Chicken meat nuggets treated with 3.0% dragon fruit peel powder retarded the lipid oxidation process more efficiently by maintaining TBARS values during 20 days storage (Madane et al. 2019). The lower lipid oxidation rate of the DFPP treated samples may be attributed to the natural antioxidants present in DFPP. It is already reported that, the DFPP contains phytochemical compounds, antibacterial agents and natural antioxidant properties which in turn help in controlling lipid oxidation and increase in TBARS value (Manihuruk et al. 2017). Literatures on fish nuggets incorporated with such plant additives are scanty. However, Biswas et al (2017) also noticed TBARS values in fish mince from 0.402 mg malonaldehyde/kg in 0 day to 1.273 mg malonaldehyde/kg in 14th day of refrigerated storage.

Table 3.

Mean ± S.D. of thiobarbituric acid reactive substances (TBARS) (mg MDA/kg) and microbiological parameters (log10 cfu/g) of fish nuggets incorporated with different levels of dragon fruit peel powder (DFPP) stored at fabricated solar cooler (5 ± 1 °C): (n = 6)

Samples	0 day	3rd day	6th day	9th day	15th day
TBARS (mg/MDA/kg)
Control	0.33 ± .032aA	0.52 ± .061aB	0.65 ± .041aB	1.08 ± .128aC	1.51 ± .182aD
S1 (1.0% DFPP)	0.26 ± .036abA	0.41 ± .065bAB	0.52 ± .048bB	0.61 ± .062bB	1.34 ± .297bC
S2 (1.5% DFPP)	0.25 ± .032bA	0.40 ± .055bAB	0.51 ± .070bB	0.60 ± .106bB	1.31 ± .256bC
S3 (2.0% DFPP)	0.27 ± .067abA	0.42 ± .068bAB	0.53 ± .038bB	0.63 ± .083bB	1.34 ± .339bC
Total plate count (log10 cfu/g)
Control	1.94 ± 0.212aA	2.31 ± 0.246aAB	2.75 ± 0.219aB	3.48 ± 0.334aC	6.15 ± 0.455bD
S1 (1.0% DFPP)	1.87 ± 0.163aA	2.07 ± 0.139bA	2.65 ± 0.448aB	3.17 ± 0.180bC	5.10 ± 0.506bD
S2 (1.5% DFPP)	1.85 ± 0.184aA	2.09 ± 0.400bA	2.63 ± 0.433aB	3.14 ± 0.215bC	5.00 ± 0.460bD
S3 (2.0% DFPP)	1.86 ± 0.155aA	2.10 ± 0.331bA	2.64 ± 0.326aB	3.15 ± 0.494bC	5.13 ± 0.360bD
Coliform count (log10 cfu/g)
Control	ND	ND	ND	ND	2.65 ± 0.06 a
S1 (1.0% DFP)	ND	ND	ND	ND	1.88 ± 0.04 b
S2 (1.5% DFP)	ND	ND	ND	ND	1.87 ± 0.01 b
S3 (2.0% DFP)	ND	ND	ND	ND	1.89 ± 0.01b
Psychrophilic count (log10 cfu/g)
Control	ND	ND	ND	2.05 ± 0.01	4.12 ± 0.01a
S1 (1.0% DFP)	ND	ND	ND	1.79 ± 0.03	2.86 ± 0.01b
S2 (1.5% DFP)	ND	ND	ND	1.76 ± 0.02	2.84 ± 0.01b
S3 (2.0% DFP)	ND	ND	ND	1.78 ± 0.02	2.86 ± 0.01b
Yeast and mould count (log10 cfu/g)
Control	ND	ND	ND	ND	2.66 ± 0.02a
S1 (1.0% DFP)	ND	ND	ND	ND	1.93 ± 0.02b
S2 (1.5% DFP)	ND	ND	ND	ND	1.89 ± 0.05b
S3 (2.0% DFP)	ND	ND	ND	ND	1.91 ± 0.05b

Means bearing different superscripts (lower case) within the same column differ significantly (p < 0.05)

Means bearing different superscripts (upper case) within the same row differ significantly (p < 0.05)

Control = emulsion without phytochemicals; #ND not detected

While identifying the best level of incorporation of DFPP in the fish nuggets, it is observed that 1.5% of level showed the best result in controlling the oxidative reaction (p < 0.05) within the product even to the extent of 15th day of storage under solar cooler. On the 0th, 3rd, 6th, 9th and 15th day the arithmetic values of TBARS of S₂ group were lowest among the four samples which were 0.25 mg malonaldehyde/kg, 0.40 mg malonaldehyde/kg, 0.51 mg malonaldehyde/kg, 0.60 mg malonaldehyde/kg, and 1.31 mg malonaldehyde/kg respectively. This happened due to the presence of phenolic compounds which can inhibit the lipid oxidation (Manihuruk et al. 2017) and the findings can be justified by the findings of Uçak et al. (2011) who reported similar results by using 0.4% and 0.8% rosemary extract in Atlantic mackerel fish burgers.

In the present study, the effect of dragon fruit peel powder has been examined under polyethylene pouches where oxygen transmission rate is not controlled, just to focus on specific objective of the present study and to understand the antioxidant and antimicrobial effects of DFPP in the fish nuggets stored at 5 ± 1 °C. A better result is expected in commercial or industrial set up, where laminates, pre-formed trays or other combinations of smart packaging technologies can be used.

Effect on microbial quality of fish nuggets

The results of microbial quality of fish nuggets during storage days are shown in Table 3. The total plate counts (TPC) increased in a significant way (p < 0.05) with advancement of storage period in both control and treated nuggets. In the control sample, the TPC value increased from 1.94 to 6.15 log cfu/g from 0 to 15th day of storage under solar cooler. Among the DFPP treated samples, the S₂ sample showed the least TPC value and the count increased from 1.85 to 5.00 log cfu/g during storage period. Apart from overall count, the S₂ sample showed the best results with least counts i.e. 1.85 log cfu/g, 2.09 log cfu/g, 2.63 log cfu/g,3.14 log cfu/g and 5.00 log cfu/g on 0, 3rd, 6th, 9th and 15th days respectively under solar cooler storage. Fish nuggets with dragon fruits peel powder (1.5% incorporation) showed relatively better results among the four samples with the count of 1.87 log cfu/g in Coliform count, 2.84 log cfu/g in psychrophilic count and 1.89 log cfu/g in yeast and mould count at 15th day of storage under solar cooler. The initial low microbial counts of the treated samples might be attributed to the fact that the nuggets were steam-cooked at atmospheric pressure for 35 min. Also, sterile environment and good handling practice in the laboratory set up might have exerted added effect into this. Various reports authenticated the results of this study (Bhat et al. 2011; Chidanandaiah et al. 2009; Kumar and Tanwar 2011). Similar results were found in pork nuggets by Thomas et al. (2016). It was due to the low pH of peel powder which had a combined effect on the nuggets and the presence of flavones and glycosides as anti-microbial components. These factors might increase the shelf life of the nuggets by causing cell injury which ultimately leads to a prolonged lag phase and a retarded log phase of microbes.

Effect on texture of fish nuggets

In textural properties such as hardness, springiness, cohesiveness, gumminess and chewiness of control and treatment groups of fish nuggets were presented in Table 4. All the parameters showed a significant decreasing trend (p < 0.05) on the subsequent storage days especially beyond 3rd days of storage, but not in a very drastic way. For the control product, after 15 days of storage the hardness decreased from 45.29 to 33.01 N/cm², springiness decreased from 0.65 to 0.48 cm, cohesiveness decreased from 0.27 to 0.13, gumminess decreased from 12.20 to 4.29 N/cm² and chewiness decreased from 7.94 to 2.08 N/cm. Whereas in nuggets with dragon fruit peel powder showed almost similar textural properties. Among the three treated samples, the 1.5% showed relatively best results for 15 days of overall storage. In nuggets with 1.5% peel powder, after 15 days the hardness decreased from 45.97 to 34.41 N/cm², springiness decreased from 0.68 to 0.54 cm, cohesiveness decreased from 0.25 to 0.18, gumminess decreased from 11.49 to 6.19 N/cm² and chewiness decreased from 7.81 to 3.34 N/cm. In respect to each of the storage days observation, the S₂ sample showed the best results in case of every parameters clearly showed in Table 4. For all the TPA parameters, the overall interaction between control and treatment groups was found to be non-significant (p > 0.05) whereas interaction between storage days was found to be significant (p < 0.05).

Table 4.

Mean ± S.D. of different textural parameters of fish nuggets incorporated with different levels of dragon fruit peel powder (DFPP) stored at fabricated solar cooler (5 ± 1 °C): (n = 6)

Samples	0 day	3rd day	6th day	9th day	15th day
Hardness (N/cm2)
Control	45.29 ± 1.114aA	44.59 ± 0.427aA	42.84 ± 0.826bB	38.10 ± 1.024aC	33.01 ± 0.679aD
S1 (1.0% DFPP)	45.43 ± 0.469aA	45.31 ± 0.416aA	43.86 ± 0.832abB	38.16 ± 0.910aC	33.48 ± 1.093aD
S2 (1.5% DFPP)	45.97 ± 0.192aA	45.85 ± 0.212aA	44.96 ± 0.555aB	39.47 ± 1.612aC	34.41 ± 1.625aD
S3 (2.0% DFPP)	44.85 ± 1.558aA	44.64 ± 0.368aA	43.84 ± 1.516abB	38.77 ± 0.774aC	33.64 ± 1.056aD
Springiness (cm)
Control	0.65 ± .012abA	0.63 ± .041abAB	0.59 ± .042aB	0.54 ± .008abC	0.48 ± .026bD
S1 (1.0% DFPP)	0.64 ± .014bA	0.61 ± .008bAB	0.60 ± .008aAB	0.56 ± .018abB	0.52 ± .022abC
S2 (1.5% DFPP)	0.68 ± .014aA	0.66 ± .016aAB	0.62 ± .029aB	0.59 ± .008aB	0.54 ± .036aC
S3 (2.0% DFPP)	0.63 ± .022bA	0.61 ± .026bAB	0.59 ± .022aAB	0.53 ± .022bB	0.50 ± .018abC
Cohesiveness
Control	0.27 ± .018aA	0.25 ± .008aA	0.21 ± .024abB	0.18 ± .018abC	0.13 ± .018cD
S1 (1.0% DFPP)	0.23 ± .018bcB	0.21 ± .014bBC	0.20 ± .008abC	0.17 ± .012bD	0.15 ± .018bcD
S2 (1.5% DFPP)	0.25 ± .008bbA	0.25 ± .008aA	0.23 ± .022aA	0.20 ± .018aB	0.18 ± .014aC
S3 (2.0% DFPP)	0.21 ± .027cA	0.22 ± .014bA	0.19 ± .016bB	0.18 ± .008abB	0.16 ± .018abC
Gumminess (N/cm2)
Control	12.20 ± 0.899aA	11.15 ± 0.395aA	8.99 ± 0.948bB	6.87 ± 0.867abC	4.29 ± 0.536cD
S1 (1.0% DFPP)	10.45 ± 1.152bA	9.52 ± 0.529bAB	8.77 ± 1.216bB	6.49 ± 0.372bC	5.02 ± 0.348bD
S2 (1.5% DFPP)	11.49 ± 0.458abA	11.45 ± 1.131aA	10.34 ± 0.488aB	7.89 ± 0.456aC	6.19 ± 0.184aD
S3 (2.0% DFPP)	9.42 ± 0.228cA	9.82 ± 0.120bA	8.33 ± 0.222bB	6.93 ± 0.812abC	5.38 ± 0.404bD
Chewiness (N/cm)
Control	7.94 ± 0.685aA	7.02 ± 0.437aB	5.33 ± 0.935bC	3.71 ± 0.493bD	2.08 ± 0.184bE
S1 (1.0% DFPP)	6.69 ± 0.397bB	5.81 ± 0.259bC	5.26 ± 0.567bC	3.63 ± 0.353bD	2.61 ± 0.624abE
S2 (1.5% DFPP)	7.81 ± 0.347aA	7.56 ± 0.565aB	6.41 ± 0.437aC	4.65 ± 0.318aD	3.34 ± 0.540aE
S3 (2.0% DFPP)	5.93 ± 0.324cB	5.99 ± 0.382bC	4.91 ± 0.480bC	3.67 ± 0.401bD	2.69 ± 0.528abE

Means bearing different superscripts (lower case) within the same column differ significantly (p < 0.05)

Means bearing different superscripts (upper case) within the same row differ significantly (p < 0.05)

Control = emulsion without phytochemicals

These results may be due to the lowering of pH with advancement of storage period. Chicken nuggets with dragon fruit peel had no significant effect (p > 0.05) on various textural parameters such as hardness, cohesiveness, gumminess and chewiness values of the control and treated groups, though all the values decreased with increasing level of DFPP (Madane et al. 2019). In another way, low pH of peel powder might cause high denaturation of proteins and thus all parameters showed a decreasing trend by reducing the strength of protein-gel matrix. Due to the addition of peel powder, the pH decreasing trend became a little better, so the parameters also showed a better trend as compared to the control product. The results were similar to the findings of Vivar-Vera et al. (2018).

Effect on physicochemical properties of fish nuggets

In the study of physicochemical properties of fish nuggets emulsion (Table 5), the emulsion pH, emulsion stability and the cooking yield of the nuggets were examined on the 0th day. On day 0, the emulsion pH of the control was 6.24 and for the treated samples, these were 6.11, 6.21 and 6.35 for S₁, S₂ and S₃ respectively. The effect of addition of DFPP in fish emulsion on the pH values was found to be non-significant (p > 0.05). In the case of emulsion stability and cooking yield, addition of dragon fruit peel powder significantly (p < 0.05) improved both emulsion stability and cooking yield of fish nuggets and this could be due to the presence of high dietary fibre having water retention properties. Similar results were also reported, where due to the high dietary fibre in bael pulp residue and dragon fruit peel powder, the emulsion stability and cooking yield of meat nuggets were improved (Madane et al. 2019; Rajkumar et al. 2014). For the addition of DFPP, there was a good amount of fibre, which helps to increase the water holding capacity and thereby to reduce the cooking loss (Thebaudin et al. 1997). The studies conducted by various workers also stated that the water present in the insoluble parts of the plant fibres, like polysaccharides binding to water by hydrogen, ionic and/or hydrophobic interactions and by surface tension in the pores of the matrix, effect in the positive cooking yield (Hughes et al. 1997).

Table 5.

Mean ± S.D. of different PHYSICO-chemical parameters of emulsion of fish nuggets (at 0th day)

Samples	Emulsion pH	Emulsion stability	Cooking yield
Control	6.24 ± 0.181	91.84 ± 0.351c	92.14 ± 0.782b
S1 (1.0% DFPP)	6.11 ± 0.328	93.90 ± 0.387b	94.15 ± 0.241a
S2 (1.5% DFPP)	6.21 ± 0.368	94.09 ± 0.450b	94.92 ± 0.566a
S3 (2.0% DFPP)	6.35 ± 0.410	94.75 ± 0.327a	94.76 ± 0.446a

Means bearing different superscripts within the column differ significantly (p < 0.05)

Means bearing no superscripts within a column do not differ significantly (p > 0.05)

Control = emulsion without phytochemicals

Effect on sensory attributes of fish nuggets

In sensory evaluation, parameters like appearance, flavor, tenderness, juiciness and overall acceptability of fish nuggets were examined and values are presented in Fig. 1a–e. For all the parameters, the differences among the control and the fruit peel powder treated samples were prominent. All the parameters showed a decreasing trend (p < 0.05) from 0 to day 15. Recently Madane et al. (2019) reported that sensory score of all the treated chicken nuggets with dragon fruit peel decreased with time, whereas control nuggets received much lower score and was acceptable only up to 10th day of storage. In this study, the appearance of control fish nuggets decreased from 6.11 to 1.88, flavor deceased from 6.09 to 1.10, tenderness decreased from 6.15 to 1.36, juiciness decreased from 5.52 to 1.05 and overall acceptability value changed from 6.13 to 1.01.

Fig. 1.

a Sensory (appearance) scores of different fish nuggets. b Sensory (flavour) scores of different fish nuggets. c Sensory (tenderness) scores of different fish nuggets. d Sensory (juiciness) scores of different fish nuggets. e Sensory (overall acceptability) scores of different fish nuggets

When comparison was made within the treatment groups, the S₂ sample shows better results (p < 0.05) in overall acceptability. The nuggets with 1.5% peel powder showed significantly (p < 0.05) better tenderness than even control. For all the sensory parameters, the overall interaction between control and treatment groups was found to be non-significant (p > 0.05) whereas interaction between storage days was found to be significant (p < 0.05).

But according to the judges of sensory panel, all the products were acceptable up to 6th day of storage. The treated nuggets showed deterioration in sensory attributes as the storage day progressed. A gradual decrease in values of sensory parameters was seen due to loss of volatile flavor from the condiments and spices during the storage (Das et al. 2008). A significant water loss from the fish muscle matrix model system during storage system may be the probable cause of juiciness and texture loss of the nuggets. The treated products are more stable than the control one. This may be due to the fact that DFPP acts as an inhibitor of lipid oxidation and acted as a stabilizing agent (Madane et al. 2019).

Conclusion

The results of the present study indicate that the dragon fruit peel is rich in dietary fibers and natural antioxidants. Incorporation of dragon fruit peel powder improved the emulsion stability and cooking yield and maintained the quality and stability of fish nuggets during storage in prefabricated solar cooler. Dragon fruit peel powder incorporation at the rate of 1.5% in fish nuggets showed better quality attributes and consumer acceptability up to 6 days of storage at 5 ± 1 °C. From this study, it could be observed that, DFPP is rich in natural antioxidants and dietary fibers and had positive influence on the physicochemical, microbial and sensory qualities of fish nuggets and could also be used as functional additive in fish food industry without affecting the quality and acceptability of the final products. Future research and improvisation on the designing and setting up of the present model of solar cooler may exert better results in further lowering the temperature below 4 °C. Also, the performance of model solar cooler employed in this study could further be explored in rural belts of the country particularly in off-grid areas to utilize this technology for a better marketing approach of perishable livestock commodities.

Author contributions

OB conducted the research trials, collected the raw data and performed data analysis, PK guided the whole work and designed the trial, SKD helped in statistical analysis and in writing the manuscript as per journal’s guideline.

Funding

Being it the part of Ph.D. thesis work of first and corresponding author, no separate funding was required.

Data availability

Data produced in the manuscript are original and derived from the present study only.

Declarations

Conflict of interest

No conflict of interest lies between the authors and their affiliated institutions regarding submission of this manuscript in JFST for possible publication.