Abstract
The objective of this research was to recover protein and water from tuna defrosting wastewater. Tuna defrosting wastewater (TDW) was concentrated, and salt protein residue (PR) was separated from concentrate TDW (cTDW). Protein in the cTDW was precipitated (PP). Salt was removed from PR and PP by using hot water (60 °C). PR and PP were dried at 50 °C before analysis for total protein, amino acid profile and salt content. Salty protein solution (PS) following salt removal from the precipitate was collected and concentrated. Then salt cPS was desalted by Sephadex G-25, and the elution was collected and concentrated. The resulting cPS was analyzed for pH, total protein, salt content, amino acid profile and antioxidant properties. Water from the protein recovery procedure was collected and analyzed for biological qualities (heterotrophic plate count, coliform bacteria, E. coli, Staphylococcus aureus, Salmonella spp. and Clostridium perfringens), physical qualities (apparent color, turbidity, pH) and chemical qualities (total dissolved solids, total hardness and sulfate).The results showed that cTDW contained 11.57 ± 0.03 % protein and 3.36 ± 0.03% NaCl. After salt was removed, the dried PR and PP contained 33.10 ± 0.16% and 6.92 ± 0.13% protein, respectively, and 0.23 ± 0.00% and 0.05 ± 0.00% NaCl, respectively. Dried PR contained 9 essential amino acids at higher concentrations than in PP. Concentrated PS contained 3.15 ± 0.12% protein and no NaCl. Histidine (254.15 mg/100 g) was the dominant amino acid in cPS. Antioxidant properties are shown by values for DPPH, ABTS and FRAP. The physical, chemical and bacterial parameters of recovered water met the guidelines for drinking water quality. These results indicate that recovery of protein and water is possible in fish processing, which could reduce costs for processors and benefit the environment.
Keywords: Protein recovery, Fish protein, Wastewater, Waste utilization, Water recycling
1. Introduction
The main processes used during industrial fish processing are defrosting, washing, cooking, canning and cleaning. These processes require high quantities of water leading to the release of significant volumes of wastewater [1]. The characteristics of wastewater are determined by the operations and production lines of the factory. The polluting effect of these wastes is due to their high content of soluble organic matter and suspended solids, which vary depending on the raw material and the characteristics of the industrial process [2]. Defrosting wastewater contains protein, giving it a high organic nitrogen concentration, which is converted anaerobically into ammonium and can cause water pollution. This wastewater can also contain high concentrations of salts (Na+, K+, Ca2+, Mg2+) due to onboard freezing of fish and can lead to high-salinity waste. A tuna processing plant may consume 400 m3/day of water for defrosting. Outgoing wastewater contains high salinity and organic matter (protein, carotenoids, minerals and flavor) that presents a serious environmental problem [1]. These plants therefore need a suitable wastewater treatment system for pollution control. Presently, wastewater treatment systems consume a large portion of the operating budget for processors, while no byproducts are recovered from the waste, and the treated water is released to natural waterways. Thus, fish processing plants must continuously take in large volumes of clean freshwater to support their activities.
Recently, two-thirds of the world's population was found to be under stressful conditions due to water shortages. Fish processing industries both contribute to and are impacted by declining supplies of clean and fresh water. Some researchers attempted to conserve water during canned tuna processing by educating staff about water conservation and modifying three processing steps, namely spray cooling, can washing and floor washing. Following these measures, net water consumption was reduced from 13 m3/ton of raw material to 8.8m3/ton, representing a 32% reduction in overall water consumption [3]. Many researchers have tried to recycle water from fish processing. Raw sardines were washed to stimulate wastewater from fish industries, and then the wastewater was passed through a 500 Da MWCO polyamide membrane. Protein recoveries of 19–58% were observed [4]. Skipjack tuna wash water was precipitated with different pH and temperature combinations and found that the best processing conditions for protein precipitation were 4 °C and pH 4.5, 5.5 or 6.5, with protein recoveries of appoximately 94% [5]. In contrast, 63% the protein in surimi wash water was precipitated at pH 5 and heat treatment at 60 °C precipitated almost all of the remaining protein [6].
Microbiological safety issues are associated with water recycling during the production of shrimps in brine, but these hazards may be effectively controlled using a HACCP approach. Following these procedures, processed water recovered from peeling during shrimp processing and treated by means of reverse osmosis could be recycled within the same food unit operation [7]. A continuous ohmic heating system has been developed to coagulate protein from surimi wastewater to reduce the biological oxygen demand of the wastewater. After heating the samples were centrifuged and the remaining protein in the supernatants was measured and compared with the results from previous batch experiments. Heating under a higher electric field strength and lower flow rate resulted in higher temperatures of the samples. The lab-scale ohmic heating system possessed good performance in coagulating protein (60%) from surimi wastewater [8]. Water use (water balance) was measured at six fish processing steps, and physicochemical and bacteriological parameters of the wastewater were analyzed. Water recycling and reuse can potentially be applied after a simple primary treatment of the wastewater and disinfection from the freezing tunnel and cooling chamber defrosting. This practice may reduce the total average water consumption of the processing unit by 11%, and, if the wastewater from the cooling tower purges were also reused for other purposes, this reduction could reach 21.9%, enhancing the competitiveness of this industry and conserving fresh drinking water [9].
The objective of this research was to recover protein and water from tuna defrosting wastewater. The results can be applied in tuna processing plants to create value-added products and to save costs by reducing water consumption. These modifications would benefit the tuna processing industry and other fishery industries.
2. Materials and methods
2.1. Tuna defrosting wastewater characterization
Tuna defrosting wastewater (TDW) from a tuna processing plant in Samutsakorn Province, Thailand, was used in this study. The TDW was first filtered through cotton fabric to remove any fish particle. The pH was measured using a Metrohm 744 pH meter. The dissolved solids in the liquid fractions were measured using an RHB-32ATC refractometer. Total protein was determined according to Ref. [10], and salt (sodium chloride) content was determined according to Ref. [11].
2.2. Crude protein harvesting
TDW was concentrated by using Buchi Rotavapor R-220 Pro/V-600 Pro until the solid content of an aqueous solution was 30 °Bx. Salty protein residue (PR) was separated from concentrated TDW (cTDW) by using Tomy Refrigerated Centrifuge Suprema 25 at 10,000×g and 4 °C for 30 min and then dried in a hot air oven at 50 °C for 3 h. The pH, total protein [10] and salt content [11] in cTDW and dried salty PR were determined.
2.3. Protein precipitation
Concentrated TDW was precipitated by ethanol and heat precipitation. The salty protein precipitate (PP) was collected by using a Hettich Universal 32 centrifuge at 6000×g and 4 °C for 30 min. Salty PP was dried in hot air oven at 50 °C for 3 h. The total protein [10] and salt content [11] in the dried salty PP were determined.
Ethanol precipitation: Absolute ethanol was chilled at −20 °C for 3 h before use. Cold ethanol was added to cTDW at a ratio of 1:3 and kept at −20 °C for 3 h.
Heat precipitation: cTDW was heated in a water bath at 60 °C for 30 min.
2.4. Salt removal from precipitate
Salt was removed from salt PR and PP by using hot water. Water (60 °C) was added to salt PR and PP at a ratio 9:1 and heating continued at 60 °C in a water bath for 30 min. The mixture was then centrifuged at 6000×g and 4 °C for 30 min, and washed 2–3 times until no sodium chloride was detected. PR and PP were dried at 50 °C in a hot air oven for 3 h. Dried PR and PP were analyzed for total protein [10], amino acid profile [12] and salt content [11].
2.5. Desalting of protein solution
Salty protein solution (PS) following salt removal from precipitate was collected. Salty PS was concentrated by using a BuchiR-124 evaporator until the solid content of an aqueous solution was 30 °Bx. Concentrated salty PS was desalted as follows.
Ten milliliters of salty cPS was loaded into a desalting column (1.2 × 58.5 cm) packed with Sephadex G-25 at a flow rate of 1.3 ml/min and equilibrated with distilled water. The fractions were eluted from the column using distilled water and 1.5 ml fractions were collected until no additional fractions contained salt. The absorbance at 280 nm and salt content [11] of each fraction were determined. The fractions with high absorption at 280 nm were pooled and then concentrated by using a Buchi R-124 evaporator.
Concentrate protein solution (cPS) was analyzed for pH, total protein [10], soluble protein [13,14], salt content [11], and amino acid profile [12]. The antioxidant properties DPPH [15], ABTS [16], and FRAP [17] were also determined using Trolox as a positive control. The metal chelating assay was modified from Ref. [18], using EDTA as a positive control.
2.6. Water recovery from tuna defrosting wastewater
Water from the protein recovery procedure was collected and analyzed according to Ref. [19] for biological qualities, namely standard plate count, coliform bacteria, E. coli, Staphylococcus aureus, Salmonella spp. and Clostridium perfringens; physical qualities, namely apparent color using an Agilent 8454 UV–Vis spectrophotometer, turbidity using a Hach 2100Q turbidity meter, pH and chemical qualities, namely, total dissolved solids, total hardness as CaCO3, and sulfate, soluble protein [13,14] and salt content [11].
2.7. Statistical analysis
A completely randomized design (CRD) was used, with three replications. Data are presented as the mean ± standard deviation (SD). Means among treatments were compared by Duncan's multiple range test and analysis of variance (ANOVA) at a statistical significance of 95%.
3. Results and discussion
3.1. Tuna defrosting wastewater characterization
The characteristics of the tuna defrosting wastewater (TDW) are summarized in Table 1. The pH of TDW (7.45 ± 0.1) was in the range of 6.45–8.65 reported from other fish processing plants for tuna wastewater [1], surimi wastewater [4] and shrimp deicing water [7]. Ammonia emission and proteinaceous matter decomposition from different fish processing activities are mostly pH dependent [20]. The amount of dissolved solids in TDW was 0.20 ± 0.00 °Bx, and was related to the protein and sodium chloride content in TDW (0.17 ± 0.01 % protein and 0.14 ± 0.02 % NaCl). The small amount of dissolved solids in TDW indicated that water recovered from TDW is the best potential source for water recycling. Fish processing wastewater contains small particles of fish, soluble protein, blood and other elements that produce a C: N: P ratio approximately 100:4:1, which is favorable for promoting microbial growth under optimum conditions and causing water pollution [1].
Table 1.
Total protein and salt content in material, crude protein, protein precipitation, salt removal and desalting.
| Sample | % protein | % Salt |
|---|---|---|
| Characterization of material | ||
| TDW | 0.17 ± 0.01a | 0.14 ± 0.02a |
| Crude Protein harvesting | ||
| cTDW | 11.57 ± 0.03e | 3.36 ± 0.03b |
| Dried salty PR | 22.11 ± 1.50g | 78.36 ± 7.78e |
| Protein precipitation | ||
| Dried ethanol precipitate | 16.28 ± 0.79f | 79.32 ± 0.99e |
| Dried heat precipitate | 25.73 ± 0.28h | 43.40 ± 0.02d |
| Salt removal from precipitate | ||
| Dried PR | 33.10 ± 0.16i | 0.23 ± 0.00a |
| Dried PP | 6.92 ± 0.13c | 0.05 ± 0.00a |
| Desalting of protein solution | ||
| Salty cPS | 8.05 ± 0.08d | 24.49 ± 0.44c |
| cPS | 3.15 ± 0.00b | ND |
ND: Not detected.
TDW: tuna defrosting wastewater; cTDW: concentrated tuna defrosting wastewater; Dried salty PR: Dried salty protein residue; Dried PR: Dried protein residue; Dried PP: Dried protein precipitation; Salty cPS: Salty concentrated protein solution; cPS: concentrated protein solution.
The data are presented as the mean ± standard deviation.
abc different superscript letters within a column for each category represent significant differences between samples (P ≤ 0.05).
3.2. Crude protein harvesting
The protein and salt contents in cTDW were 11.57 ± 0.03% and 3.36 ± 0.03%, respectively as shown in Table 1. Concentrate TDW may cause serious environmental problems due to its high concentration of protein and high salinity [1]. Salty PR contained high concentrations of protein (22.11 ± 1.50%) and salt (78.36 ± 7.78%). General sewage sludge requires treatment at plants due to its high nutrient levels [21]. The characteristic of cTDW and salt PR were similar to those of sewage sludge from tuna processing waste treatment. This sludge may impair the waste water treatment process due to the effects of high salinity on plant growth and other physiological processes [22]. Salt removal is an important process for protein utilization.
3.3. Protein precipitation
Concentrate TDW was precipitated by ethanol and heat methods. The percentage of dried yield from ethanol precipitation was 4.95 ± 0.32%. It contained 16.28 ± 0.79% protein and 79.32 ± 0.99% salt (Table 1). Ethanol, an organic solvent, affects protein solutions by lowering protein solubility [23]. Heat precipitation gave a higher dried yield (7.64 ± 0.22%) than ethanol. It contained 25.73 ± 0.28 % protein and 43.4 ± 0.02 % salt. Tuna protein was denatured at 60 °C, similar to the temperature for denaturing protein in Atlantic cod (58–68 °C) [24], then protein was coagulated and separated from aqueous solution. Heat protein precipitation showed higher potential than alcohol both in yield and protein content due to the higher sodium chloride solubility in water than in ethanol [25]. Both precipitates contained high salt content, and would pose a risk for human and animal health.
3.4. Salt removal from precipitate
After salt was removed, the dried PR and PP contained 33.10 ± 0.16% and 6.92 ± 0.13% protein, respectively and 0.23 ± 0.00% and 0.05 ± 0.00% salt, respectively (Table 1). The salt content in both PR and PP was less than 1%, a level that is considered safe for human and animal intake [26].
Water, salt and blood are found in defrosting wastewater [27]. Tuna is a vertebrate, and the hemoglobin in red cells gives defrosting wastewater its red color [28]. The color of the dried PR was dark brown, the protein pigment was harvested and the resulting dried PP was light brown, as shown in Fig. 1. The amino acid profiles of PR and PP are shown in Table 2. Dried PR contained 20 amino acids, and the concentrations of 9 essential amino acids were much higher than those in PP. Protein and amino acids were water soluble because the concentration of protein and amino acids in PP was lower than that in PR after salt removal. The concentrations of protein and amino acids in PR were similar to those of tuna flesh [29], and make them potentially useful in human food or animal feed.
Fig. 1.
Process of protein and water recovery from tuna defrosting wastewater.
Table 2.
Amino acid profile of dried protein residue and protein precipitate.
| Amino acid profile (mg/100g sample) | Dried sample |
|
|---|---|---|
| Protein residue (PR) | Protein precipitate (PP) | |
| Histidine | 2245.98 | 1521.48 |
| Isoleucine | 1240.75 | <100 |
| Leucine | 2677.95 | 126.92 |
| Lysine | 2035.69 | 207.99 |
| Methionine | 745.89 | ND |
| Phenylalanine | 1753.13 | <250 |
| Threonine | 1315.18 | <200 |
| Tryptophan | 451.94 | ND |
| Valine | 1838.20 | <100 |
| Sum of EAA | 14304.7 | 1856.39 |
| Aspartic acid | 3000.13 | 364.15 |
| Alanine | 1988.6 | 171.09 |
| Arginine | 1435.57 | <250 |
| Cystine | 367.77 | ND |
| Glutamic acid | 3218.34 | 1153.05 |
| Glycine | 1270.61 | 251.9 |
| Hydroxylysine | ND | ND |
| Hydroxyproline | ND | ND |
| Proline | 1011.11 | <200 |
| Serine | 1238.28 | <200 |
| Tyrosine | 1163.07 | ND |
| Total AA | 28998.19 | 3796.58 |
ND: Not detected.
3.5. Desalting of protein solution
Histidine (661.18 mg/100 g), was the dominant amino acid in salty cPS, it is important for human health and may be necessary as a supplement in some populations [30]. Moreover, histidine is a pharmaceutical agent because of its antioxidant and anti-inflammatory properties [31]. A high concentration of salt (24.49 ± 0.44%) may pose a human health risk. After desalting, the color of cPS was dark brown, as shown in Fig. 1.
The concentrated PS remained protein (3.15 ± 0.12%) and histidine (254.15 mg/100 g). Its antioxidant properties shown in terms of DPPH, ABTS and FRAP were 48.34 ± 1.78, 602.73 ± 8.14 and 672.36 ± 4.71 μmol TE/mg protein, respectively, as shown in Table 3. The odd electron of the nitrogen atom in DPPH is reduced by receiving a hydrogen atom from histidine to the corresponding hydrazine [32], and histidine donates a hydrogen atom to the ABTS radical [33]. Histidine donates electrons to Fe3+ for ferric reducing antioxidant power (FRAP) [34], whereas no metal chelating activity is observed for histidine. No research has examined amino acids in TDW, but the amount of histidine in this research was higher than that in tuna protein hydrolysate (136.99 mg/100 g sample) [35], which showed antioxidant activities in FRAB, ABTS and DPPH.
Table 3.
Antioxidant properties of salty concentrated protein solution (Salty cPS) and concentrated protein solution (cPS).
| Sample | Antioxidant properties (μmol TE/mg protein) |
|||
|---|---|---|---|---|
| DPPH | ABTS | FRAP | Metal chelating | |
| Salty cPS | 58.57 ± 1.36b | 97.14 ± 10.13b | 1442.99 ± 20.24b | ND |
| cPS | 48.34 ± 1.78a | 602.73 ± 8.14a | 672.36 ± 4.71a | ND |
ND: Not detected.
The data are presented as the mean ± standard deviation.
abc different superscript letters within a column for each category represent significant differences between samples (P ≤ 0.05).
3.6. Water recovered from tuna defrosting wastewater
The physical, chemical and microbiological qualities of the tap water and water remaining following the protein recovery procedure are shown in Table 4. The pH of water recovered from TDW was 7.53, it contained 0.04 ± 0.00% protein and 0.03 ± 0.00% salt. These small amounts of protein and salt would not have an effect on food processing due to GHP chlorinated water sanitation [36]. Physical parameters of appearance color and turbidity were measured as < 0.1 Pt–Co Unit and 0.43 NTU, respectively. The chemical parameters, namely, total dissolved solid, total hardness and sulfate, of the recovery water were <0.5 mg/l, <2.5 mg/l and <0.68 mg/l, respectively. Bacteria were not detected. The physical, chemical and bacterial parameters of recovered water were less than those of tap water, and met the guidelines for drinking water quality [37]. Recovered water was suitable for reuse in fish processing. The yield of water recovered from this research was approximately 82% of the total TDW. The quality of water is better than that of water from other wastewater treatment, which would benefit recycling in processing plants.
Table 4.
Physical, chemical and microbiological qualities of tap water and water remaining following the protein recovery procedure.
| Parameters | Unit | Drinking water quality (FAO, 2011) | Tap water | Recovery water |
|---|---|---|---|---|
| 1. Physical | ||||
| Appearance color | Pt–Co Unit | <15 | 8.33 | <0.1 |
| Turbidity | NTU | <4 | 0.58 | 0.43 |
| pH | – | 6.5–8.5 | 7.0 | 7.53 |
| 2. Chemical | ||||
| Total dissolved solid | mg/L | <600 | 156 | <0.5 |
| Hardness as CaCO3 | mg/L | <300 | 87.34 | <2.5 |
| Sulfate | mg/L | <250 | 30.74 | <0.68 |
| 3. Bacterial | ||||
| Standard plate count | CFU/mL | 500 | <1 | 2.3 |
| Total coliform bacteria | MPN/100 mL | ND | <1 | <1 |
| E. coli | MPN/100 mL | ND | <1 | <1 |
| Staphylococcus aureus | CFU/100 mL | ND | <1 | <1 |
| Salmonella spp. | CFU/100 mL | ND | <1 | <1 |
| Clostridium perfringens | CFU/100 mL | ND | <1 | <1 |
ND: Not detected.
Total coliform bacteria, E. coli, Staphylococcus aureus, Salmonella spp. and Clostridium perfringens < 1 means not detected.
4. Conclusion
Our experiments have shown that it is possible to recover protein from tuna defrosting wastewater, and that the protein is suitable as a food ingredient in seasoning or protein powder. Water was a by-product generated from this process, and was clean enough for reuse in food processing. This finding could be applied by the food industry to achieve zero waste release and to recycle water and reduce production costs.
Data availability statement
Data included in article/supp. material/referenced in article.
CRediT authorship contribution statement
Jirapa Hinsui: Writing – review & editing, Writing – original draft, Supervision, Resources, Project administration, Methodology, Funding acquisition, Data curation, Conceptualization. Kornkanok Krasae: Methodology, Investigation, Formal analysis. Nuttapong Jantaratch: Methodology, Investigation, Formal analysis. Nopparat Mahae: Writing – review & editing.
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Jirapa Hinsui reports financial support was provided by Kasetsart University Research and Development Institute. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This research was funded by Kasetsart University Research and Development Institute (KURDI), FF (KU) 7.65.
Contributor Information
Jirapa Hinsui, Email: jirapa_h@hotmail.com.
Kornkanok Krasae, Email: sbbam77@gmail.com.
Nuttapong Jantaratch, Email: nuttapong.ju@ku.th.
Nopparat Mahae, Email: nopparat.rmutsv@gmail.com.
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