Original papers
Effects of various organic salts on the properties of edible films prepared from North Pacific krill (Euphausia pacifica) protein
2022 Volume 28 Issue 2 Pages 133-140
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2022 Volume 28 Issue 2 Pages 133-140
The present study aimed to clarify the potential of edible films prepared from North Pacific krill (Euphausia pacifica) protein and the effects of various salt types (sodium chloride, sodium acetate, sodium benzoate, sodium citrate, and sodium tartrate) as solubilizers on the film properties. A film containing sodium chloride was used as a control. The physical properties and protein composition of films were determined. Results showed that all types of organic salts could be used to produce films with North Pacific krill protein. All film types were observed to have excellent ultraviolet barrier properties. The film containing sodium citrate had the highest mechanical properties and protein solubility, and the lowest absorbance of ultraviolet-visible light, indicating that sodium citrate is the most useful salt type assessed in this study. Thus, North Pacific krill represents a potential resource in the production of edible films for active food packaging.
North Pacific krill (Euphausia pacifica) is an important zooplankton with a stock size of 700 to 1 400 million metric tons, and it provides a sustainable harvest of approximately 70 to 200 million metric tons per year (Landymore et al., 2019). Krill is known to contain high quality proteins that include the nine essential amino acids, chitin, proteolytic enzymes, carotenoids and essential fatty acids; thus, it can be defined as a perfect nutrient supplement (Gigliotti et al., 2011; Sun et al., 2014; Landymore et al., 2019). Despite the various beneficial nutritional qualities of krill, it has several disadvantages, such as a small body size, fluoride in the exoskeleton and rapid deterioration of quality, making it inconvenient for use in food processing (Sun et al., 2014). Several researchers have demonstrated that the recovered protein from krill exhibits a gel-forming capacity following the removal of the inedible parts by appropriate methods (Burri and Johnsen, 2015; Amano et al., 2018). Krill protein was recovered by pH manipulation and saltwater treatment, and it was clarified that the saltwater treatment could more effectively recover krill protein (Amano et al., 2017). However, there has been no research on the preparation of edible films using recovered krill protein.
Bio-based polymeric films have become an increasingly popular alternative to synthetic plastic materials in the development of eco-friendly and non-toxic materials for food packaging (Muppalla et al., 2014). It is possible that the use of commercial films can be minimized or even replaced in the coming years. Consequently, the environmental impact of single-use packaging materials would be reduced (Kaewprachu et al., 2018). Hydrocolloids (proteins, polysaccharides), lipids (waxes, fatty acids) and their composites are film-forming materials that are commonly used for biodegradable films. Proteins are widely used as film-forming materials because of their high nutritional value, relative abundance and film-forming ability (Le et al., 2015). Fish myofibrillar proteins are often used to produce biodegradable films, and because they consist of high molecular weight proteins, films with strong film-forming characteristics are produced (Nuanmano et al., 2015). It is well-known that fish myofibrillar proteins can be solubilized not only by adjusting the pH of the film-forming solution with acidic or alkaline solutions, but also by salts (Leerahawong et al., 2011).
Based on the above background, this study aimed to clarify the suitability of recovered krill protein for the production of edible films and the effects of various organic salts on the properties of edible films prepared from recovered krill protein.
Materials The samples used in this study were kept frozen (−40° C) at Tokyo University of Marine Science and Technology after being delivered on April 2018.
Sucrose, sodium chloride (Na-chloride), sodium acetate (Na-acetate), sodium benzoate (Na-benzoate), sodium citrate (Na-citrate), and sodium tartrate (Na-tartrate) were purchased from FUJIFILM Wako Chemical Corp. (Osaka, Japan). Tween 20 was purchased from Amersham Biosciences Corp. (Amersham, United Kingdom). Glycerol was purchased from KOKUSAN CHEMICAL Co., Ltd. (Yokohama, Japan). Hydrochloric acid (HCl), trichloroacetic acid (TCA), sodium hydroxide (NaOH), Tris-HCl (pH 8.8), and sodium dodecyl sulfate (SDS) were purchased from FUJIFILM Wako Chemical Corp. Urea was purchased from KISHIDA CHEMICAL Co., Ltd. (Osaka, Japan). 2-Mercaptoethanol was purchased from Sigma-Aldrich (St. Louis, MO, USA). Methanol and acetic acid were purchased from KOKUSAN CHEMICAL Co., Ltd. Coomassie brilliant blue R-250 was purchased from FUJIFILM Wako Chemical Corp. All reagents used in this study were of analytical grade.
Preparation of krill sample When the krill arrived at our laboratory, they were ground with 7.5% (w/w) sucrose by using a meat chopper (M-22A, Nantsune, Osaka, Japan). The minced krill meat was packed into a polyethylene bag and stored at −30 °C until use. The sample temperature was kept below 10 °C during all steps of the experiment.
Extraction of protein from krill sample The extraction of protein from krill was conducted as described by Amano et al. (2018) with a slight modification. The krill sample was mixed with a pre-cooled 8% Na-chloride solution in a ratio of 1:1 (w/v) to obtain 4% Na-chloride (final concentration) and homogenized at 10 000 rpm for 1 min on ice, followed by shaking with a seesaw shaker at 40 rpm and 4 °C for 10 min. Then, the homogenate was centrifuged at 2 °C and 15 000 g for 10 min to collect the supernatant. The supernatant was filtered through a double layer of gauze, then mixed with ion exchanged water (IEW) in a ratio of 1:9 (v/v). Then, the diluted supernatant was stirred with a magnetic stirrer at 400 rpm and 4 °C for 30 min. This was followed by centrifugation at 2 °C and 15 000 g for 10 min. Finally, the precipitate was collected after centrifugation and the protein content of the recovered protein was measured by using Lowry's method (Lowry et al., 1951).
Preparation of film Briefly, a film-forming solution (FFS) was prepared with recovered krill protein, salts, Tween 20, glycerol, and IEW. The recovered protein was mixed with Na-chloride, Na-acetate, Na-benzoate, Na-citrate, or Na-tartrate and Tween 20 in IEW to obtain a FFS containing 2% (w/w) protein, 0.5% (w/w) salt, and 1% (w/w) Tween 20. Glycerol was added as a plasticizer at 30% (w/w of protein). Subsequently, the pH of the FFS was adjusted to 7 using 1 N HCl, and the air bubbles were removed. Then, 4 g of each FFS was cast onto individual rimmed silicone plates (5 × 5 cm), followed by drying at 25 °C under 50% relative humidity in a ventilated oven (KCL-2000A; Tokyo Rikakikai Co., Ltd., Japan) for 24 h. The dried films were carefully peeled off manually and conditioned for another 24 h at 25 °C under 50% relative humidity before the film properties were examined.
Protein solubility A method by Suzuki et al. (2013) and Yu et al. (2017) with a slight modification was used to evaluate the effects of Na-chloride and organic salts (Na-acetate, Na-benzoate, Na-citrate, and Na-tartrate) on the protein solubility of the FFS. In brief, 100 mL of FFS was prepared and TCA was added to the FFS to obtain a final TCA concentration of 5% (w/v) in FFS. The FFS containing 5% TCA was vortexed and left at 4 °C for 30 min, followed by centrifugation at 4 °C and 10 000 g for 20 min. Then, the supernatant was removed and the collected precipitate was washed with 5% TCA solution three times. The collected and washed precipitate was then mixed with 10 mL of 1 N NaOH solution and shaken with a seesaw shaker at 40 rpm and 4 °C for 18 h. The protein concentrations of the solution and supernatant were measured by Lowry's method (Lowry et al., 1951). Protein solubility (%) was calculated by the following equation:
 |
where PA and PB are the protein concentrations of the mixture after and before centrifugation, respectively.
Thickness The thickness was measured by using a dial-type gauge (Series 7360, Mitsutoyo Co., Kanagawa, Japan) at eight random positions on each film.
Moisture content The moisture content was determined according to the standard AOAC (AOAC, 1990) methods. The samples were dried in an oven (DRA430, Advantec, Tokyo, Japan) at 105 °C until the weight was constant.
Mechanical properties The tensile strength (TS) and elongation at break (EAB) point of films were determined according to the ASTM method (ASTM, 1999). A tensipresser (TTP-50BX II, Taketomo Electric Inc., Tokyo, Japan) was used to determine the TS and EAB of films. Rectangular strips (20 × 45 mm) were prepared from the films. A 30-mm initial grip length and 1-mm/s cross-head speed were set, and the films were strained until they became disrupted.
Protein pattern SDS-polyacrylamide gel electrophoresis of the raw krill and recovered protein was performed with the method of Laemmli (1970) by using 5% to 20% gradient polyacrylamide gels (e-PAGEL, ATTO Corporation, Tokyo, Japan). A 0.5-g sample was dissolved in 20 mL of 20 mM Tris-HCl (pH 8.8), 2% SDS, 8 M urea and 2% 2-mercaptoethanol. In each sample, 10 µg of the protein was loaded onto a precast gel and subjected to electrophoresis at a constant current of 20 mA per gel. The PageRuler Unstained Protein Ladder (Thermo Fisher Scientific Inc., MA, USA) was used as the standard for protein molecular weight determination. After electrophoresis, the gel was stained with 0.1% (w/v) Coomassie brilliant blue R-250 in a 30:10:60 volume ratio of methanol:acetic acid:water, then de-stained in a 30:10:60 volume ratio of methanol:acetic acid:water.
Water vapor permeability (WVP) The WVP of films was determined according to the ASTM method (1983). Each type of the film was used with grease to seal a glass cup containing silica gel. Then, the cups with film were put in a desiccator containing distilled water and placed in a ventilated chamber at 30 °C. The weight of the cups was recorded at 1-h intervals over a 9-h period. The WVP value was calculated by the following equation:
 |
where w is the weight gain of the cup (kg), L is the film thickness (m), A is the area of the exposed films (m2), t is the time of the gain (s) and P2 – P1 is the vapor pressure differential (Pa) across the film.
Water solubility The water solubility of films was determined according to the method of Kaewprachu et al. (2018). The film sample was weighed and placed in tubes with 20 mL of distilled water. Then, it was shaken at 200 rpm and 25 °C for 24 h. The solution was filtered through 5A filter paper, which was then desiccated at 105 °C for 24 h to recover the remaining undissolved film and dried at 105 °C for 24 h. The water solubility value was calculated by the following equation:
 |
where W0 is the initial weight of the film expressed as dry matter (g) and Wf is the weight of the undissolved dried film residue (g).
Optical properties The color and light transmission of films were determined according to the method of Nguyen et al. (2019). An ultraviolet (UV)-Visible Recording Spectrophotometer (UV-1800, Shimadzu Co., Japan) was used to measure the light transmission of film samples at a wavelength between 200 and 800 nm. A CIE colorimeter (CR-13, Konica Minolta Inc., Japan) and the calibration plate of the colorimeter, which is a white board used as the background, were used to measure the color values [L* (lightness), a* (redness), and b* (yellowness)] of the films. The whiteness value was calculated by the following equation according to Nguyen et al. (2019):
 |
Statistical analysis Data obtained from the study were analyzed by Minitab Software (19.0). One-way analysis of variance (ANOVA) and Tukey's multiple comparison tests were performed. Differences between groups were considered to be significant when p was < 0.05.
Effect of various salt types on protein solubility The protein solubility ranged from 47.73% to 76.31% (Fig. 1). Among the salt types, the solution containing Na-citrate showed the highest solubility (76.31%), and the solution containing Na-acetate showed the lowest solubility (47.73%). It is known that the dissolution of myosin filaments is related to increases in ionic strength (Leerahawong et al., 2011). The ionic strength of salts at the same molarity was in the order of Na-citrate > Na-chloride > Na-tartrate > Na-acetate > Na-benzoate. Among the types assessed in this study, at the same concentration, Na-citrate showed the highest ionic strength. This might be the reason why the solution containing Na-citrate showed the highest protein solubility. The results revealed that all salt types could effectively solubilize fish myofibrillar protein, but Na-citrate was the most effective.
Effect of various salts on protein solubility of film forming solutions. Error bars represent standard deviation (n = 4). Vertical bars with different letters are significantly different (p ≤ 0.05). “Na-Chloride”, “Na-Acetate”, “Na-Benzoate”, “Na-Citrate”, and “Na-Tartrate” mean film forming solutions containing Na-chloride, Na-acetate, Na-benzoate, Na-citrate, and Na-tartrate, respectively.
Film thickness It is known that the film thickness affects the properties of edible films, i.e., the mechanical properties, light transmission and WVP (Kaewprachu and Rawdkuen, 2014).
The thickness of the films ranged from 37.22 to 42.33 µm (Table 1) and was dependent on the salt type. One possible reason for this observation might be related to the moisture content of the films, since the drying condition and relative humidity were kept constant in this study. The film containing Na-tartrate significantly thicker than the other films and had the lowest moisture content.
| Thickness (µm) | Moisture Content (%) | Tensile Strength (MPa) | Elongation at Break (%) | |
|---|---|---|---|---|
| Na-chloride | 37.22b ± 0.41 | 24.93ab ± 0.41 | 1.99c ± 0.29 | 14.69bc ± 2.46 |
| Na-acetate | 38.14b ± 0.47 | 25.11ab ± 0.59 | 1.47d ± 0.14 | 43.29a ± 7.69 |
| Na-benzoate | 37.25b ± 0.37 | 24.46b ± 0.75 | 2.52b ± 0.21 | 21.92b ± 7.23 |
| Na-citrate | 37.11b ± 0.28 | 26.62a ± 0.52 | 3.63a ± 0.29 | 18.16bc ± 6.39 |
| Na-tartrate | 42.33a ± 0.25 | 19.18c ± 0.86 | 2.61b ± 0.50 | 9.94c ± 2.44 |
Means ± standard deviation (n = 6 for thickness, n = 3 for moisture content, n = 10 for tensile strength and elongation at break).
Means with different letters within the row are significantly different (p ≤ 0.05).
“Na-chloride”, “Na-acetate”, “Na-benzoate”, “Na-citrate”, and “Na-tartrate” mean films containing Na-chloride, Na-acetate, Na-benzoate, Na-citrate, and Na-tartrate, respectively.
Moisture content The moisture content of the films ranged from 19.18% to 26.62% (Table 1). The film containing Na-citrate had the highest moisture content (26.62%) and it was similar to those of the films containing Na-chloride, Na-acetate and Na-benzoate, whereas the film containing Na-tartrate had the lowest moisture content (19.18%). This may be because Na-citrate and Na-acetate are more hydrophilic than the other salt types assessed in this study and the hygroscopic nature of Na-chloride (Suzuki et al., 2013). Thus, it is possible that Na-tartrate and Na-citrate could bind to water less and more effectively, respectively, resulting in the lowest moisture content in the film containing Na-tartrate and the highest moisture content in the film containing Na-citrate.
Mechanical properties of films The TS and EAB of the films ranged from 1.47 to 3.63 MPa and 9.94% to 43.29%, respectively (Table 1). The film containing Na-citrate had the highest TS (3.63 MPa) while the film containing Na-acetate had the lowest TS (1.47 MPa) and the highest EAB (43.29%). The film containing Na-tartrate showed the lowest EAB (9.94%). These results are lower than those for low density polyethylene film (11.40 MPa, 622.67%) (Kaewprachu et al., 2017) and polyvinyl chloride film (46.92 MPa, 268.31%) (Kaewprachu et al., 2018).
The mechanical properties of films are generally related to their three-dimensional structure (Shiku et al., 2004). The main reason for a low TS is the presence of a low amount of high molecular weight proteins, such as myosin heavy chain (MHC). In other words, the presence of a high amount of low molecular weight proteins deteriorates the chain-to-chain integration, resulting in a weaker film network (Hoque et al., 2011). The presence of a high amount of low molecular weight proteins was observed in all film samples (Fig. 2). The higher TS of the film containing Na-citrate when compared to the other films is attributed to its higher protein solubility (76.31%), even though there was no significant difference in protein composition. It is thought that there was more entanglement of dissolved proteins, and more hydrophobic bonds were formed between protein molecules (Suzuki et al., 2013). In addition, a lower molecular weight additive is thought to enter more easily between proteins, resulting in a lower TS and higher EAB (Kaewprachu et al., 2018). The molecular weights of the salt types assessed in this study were 58.44, 82.03, 144.11, 294.10, and 230.08 g/mol for Na-chloride, Na-acetate, Na-benzoate, Na-citrate, and Na-tartrate, respectively. Thus, the low molecular weight of Na-acetate might be the reason for the lowest TS and highest EAB values in this study. Even though Na-citrate is known to be more hydrophilic than the other salt types, and the film containing Na-citrate had the highest moisture content, the reason of the film containing Na-citrate had the lowest EAB result might be explained by the high molecular weight of Na-citrate. In other words, a low molecular weight additive may exhibit a greater plasticizing effect than high molecular weight additives when used at the same weight concentration, since the molar concentration of the low molecular weight additive would be higher (Leerahawong et al., 2011).
Protein patterns of krill meat, krill protein, and films containing various salts. M: high molecular weight marker, R: krill meat, P: krill protein, films containing 1: Na-chloride, 2: Na-acetate, 3: Na-benzoate, 4: Na-citrate, and 5: Na-tartrate.
Protein pattern The SDS-polyacrylamide gel electrophoresis results for films containing different salt types are shown in Fig. 2. It was clearly demonstrated that the salt type did not affect the protein composition of films. A higher amount of low molecular weight proteins (from 10 to 15 kDa) was observed in all films when compared to krill protein. This suggests that protein degradation occurred during film formation. Consequently, the films contained fewer crosslinks and had weaker bonding, which caused fewer interactions between molecules, distorting the protein structure and resulting in a lower TS and higher WVP. The main reason for protein degradation was previously investigated, and it was clarified that serine-type proteinases were the dominant proteinases of North Pacific krill (E. pacifica) (Sun et al., 2014). In addition, there is a high correlation between the amount of MHC and the physical properties of films (Suzuki et al., 2013); this might be a reason for the low mechanical properties of the developed films in this study.
WVP of films The WVP of films ranged from 2.81 × 10−13 to 3.68 × 10−13 kgm−1s−1Pa (Table 2). The film containing Na-tartrate had the highest WVP (3.68 × 10−13 kgm−1s−1Pa), whereas the film containing Na-benzoate had the lowest WVP (2.81 × 10−13 kgm−1s−1Pa). The low vapor permeability of the film containing Na-benzoate may be due to its low hydrophilicity when compared to the other salts, but the reason for the high WVP of the film containing Na-tartrate remains unclear, and it cannot be explained by its hydrophilicity or hydrophobicity. The greater thickness of the film containing Na-tartrate may be a reason for its high WVP, as it could absorb more water (Kaewprachu et al., 2015). These results were higher than those reported for low density polyethylene film (0.04 × 10−12 kgm−1s−1Pa) (Kaewprachu et al., 2017) and polyvinyl chloride film (0.23 × 10−13 kgm−1s−1Pa) (Kaewprachu et al., 2018).
| Water Vapor Permeability (10−13kgm−1s−1Pa) | Film Solubility (%) | |
|---|---|---|
| Na-chloride | 2.94bc ± 0.12 | 64.32b ± 1.48 |
| Na-acetate | 3.10b ± 0.04 | 60.50c ± 1.94 |
| Na-benzoate | 2.81c ± 0.14 | 65.78b ± 2.70 |
| Na-citrate | 2.87bc ± 0.11 | 74.37a ± 1.48 |
| Na-tartrate | 3.68a ± 0.24 | 58.51c ± 1.07 |
Means ± standard deviation (n = 5).
Means with different letters within the row are significantly different (p ≤ 0.05).
“Na-chloride”, “Na-acetate”, “Na-benzoate”, “Na-citrate”, and “Na-tartrate” mean films containing Na-chloride, Na-acetate, Na-benzoate, Na-citrate, and Na-tartrate, respectively.
The characteristics of proteins affect the water barrier properties of protein-based films, which are inferior to commercial films. The high hydrophilicity of protein-based films is thought to be the main reason for their poor barrier properties. High hydrophilicity is generally attributed to a high amount of polar amino acids and the presence of plasticizers, such as glycerol and sorbitol. This can lead to structural changes in the polymer network, i.e., the expansion of space in the polymeric matrix and movement of polymer chains, resulting in a natural decrease in the density and increase in the water permeability of films (Limpan et al., 2010).
Water solubility of films The solubility of the films in water ranged from 58.51% to 74.37% (Table 2). The film containing Na-citrate had the highest water solubility (74.37%) while the film containing Na-tartrate had the lowest water solubility (58.51%). It is known that the water solubility of films is affected by the hydrophilicity and molecular weight of additives, such as plasticizers and protein solubilizers (Kaewprachu et al., 2018). The hydrophilic nature and high molecular weight of Na-citrate might explain why the film containing Na-citrate showed the highest water solubility. A high molecular weight additive may disrupt protein-protein interactions, resulting in a film with high water solubility (Yang and Paulson, 2000). Although the molecular weight of Na-tartrate is higher than that of the other salts, except for Na-citrate, its lower hydrophilicity might explain why the film containing Na-tartrate had the lowest water solubility. The solubility of films is an important property, and it is known that water solubility is an indicator of the hydrophilicity of films. A low or high water resistance can be selected depending on the application or requirement. On the other hand, protein-based films are known to be highly sensitive to water, i.e., these films show low water resistance, which represents an environmental benefit due to their more rapid degradation.
Optical properties of films Light transmission in the UV-visible range and color values are shown in Tables 3 and 4, respectively. All film samples blocked UV light transmission in the 200 to 280-nm range regardless of salt type; this range is known to be related to the induction of lipid oxidation in food systems (Coupland and McClements, 1996). Several researchers have reported that UV light absorption can be explained by the aromatic amino acid content of protein-based films (Nguyen et al., 2019). Our results showed 13.49% to 70.25% light transmission (Table 3) in the visible range (200 to 800 nm). The film containing Na-citrate had the highest light transmission (22.40% to 69.84%) in the UV-visible range, whereas the film containing Na-chloride had the lowest (13.49% to 56.39%). Light transmission has been shown to increase with increasing protein dissolution and uniformity (Suzuki et al., 2013). This might be the reason why the film containing Na-citrate and Na-chloride showed the highest and the lowest light transmission, respectively, in the UV-visible range. The transparency values of the films ranged from 5.52 to 9.01 (Table 3). The film containing Na-chloride the lowest transparency (9.01; higher value of transparency), whereas the film containing Na-tartrate had the highest transparency (5.52). The transparency of films is one of the most important properties for food applications as it can accentuate the appearance of packaged food products.
| Wavelengths (nm) | Transparency | ||||||
|---|---|---|---|---|---|---|---|
| 200 | 280 | 350 | 400 | 600 | 800 | ||
| Na-chloride | 0.00 | 0.00 | 13.49c ± 1.39 | 20.73d ± 1.81 | 46.52d ± 3.26 | 56.39c ± 3.59 | 9.01a ± 0.83 |
| Na-acetate | 0.00 | 0.00 | 18.55b ± 2.37 | 27.55bc ± 3.06 | 60.52ab ± 4.33 | 70.72a ± 4.20 | 5.76c ± 0.82 |
| Na-benzoate | 0.00 | 0.00 | 21.00a ± 2.94 | 30.16ab ± 3.61 | 56.94c ± 3.19 | 66.61b ± 2.60 | 6.63b ± 0.68 |
| Na-citrate | 0.00 | 0.00 | 22.40a ± 4.32 | 31.77a ± 5.21 | 62.59a ± 3.95 | 69.84a ± 2.98 | 5.52c ± 0.75 |
| Na-tartrate | 0.00 | 0.00 | 17.77b ± 1.88 | 27.02c ± 2.28 | 58.59bc ± 2.78 | 68.20ab ± 2.75 | 5.54c ± 0.50 |
Means ± standard deviation (n = 6).
Means with different letters within the row are significantly different (p ≤ 0.05).
“Na-chloride”, “Na-acetate”, “Na-benzoate”, “Na-citrate”, and “Na-tartrate” mean films containing Na-chloride, Na-acetate, Na-benzoate, Na-citrate, and Na-tartrate, respectively
| L* | a* | b* | Whiteness | |
|---|---|---|---|---|
| Na-chloride | 59.95c ± 0.97 | 32.56b ± 0.72 | 36.72ab ± 0.64 | 36.65c ± 1.29 |
| Na-acetate | 60.88bc ± 2.37 | 30.38c ± 0.96 | 35.80b ± 1.18 | 38.85b ± 1.98 |
| Na-benzoate | 64.28a ± 2.01 | 28.15d ± 1.86 | 32.91d ± 1.98 | 43.84a ± 3.06 |
| Na-citrate | 61.73b ± 1.59 | 34.67a ± 1.67 | 34.47c ± 1.51 | 37.90bc ± 2.71 |
| Na-tartrate | 59.62c ± 1.66 | 35.21a ± 1.34 | 37.41a ± 2.30 | 34.64d ± 2.73 |
Means ± standard deviation (n = 6).
Means with different letters within the row are significantly different (p ≤ 0.05).
“Na-chloride”, “Na-acetate”, “Na-benzoate”, “Na-citrate”, and “Na-tartrate” mean films containing Na-chloride, Na-acetate, Na-benzoate, Na-citrate, and Na-tartrate, respectively.
The color values of films ranged from 59.62 to 64.28, 28.15 to 35.21, 32.91 to 37.41, and 34.64 to 43.86 for L* (lightness), a* (redness), b* (yellowness), and whiteness, respectively (Table 4). The high redness and yellowness of films might be due to natural pigments of the raw material, and the low lightness value could help prevent food deterioration (Loranty et al., 2010). Based on the observed results, all salt types affected the appearance, transparency and color of the films.
The results of this study showed that all salt types affected the properties of the developed films. Na-citrate was the most suitable salt type to use as a protein solubilizer for preparing films from North Pacific krill (E. pacifica) protein, because of its high mechanical properties and low absorbance in the UV-visible light range. We demonstrated that North Pacific krill protein can be applied to the production of edible films. While the high proteinase activity of these films resulted in lower mechanical properties when compared to commercial films, it might be possible to develop edible films from North Pacific krill by suppressing MHC autolysis.
Acknowledgements This study was supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and a project of the Policy Research Institute, Ministry of Agriculture, Forestry and Fisheries (PRIMAFF), a study on constructing an efficient supply system of domestic seafood based on domestic and overseas institutions-research with collaboration scheme: contracting research on policy for agriculture, forestry and fisheries.
Conflict of interest There are no conflicts of interest to declare.
