Sustainable Fish Gelatin Films: from Food Processing Waste to

Research Article pubs.acs.org/journal/ascecg

Sustainable Fish Gelatin Films: from Food Processing Waste to Compost Alaitz Etxabide,† Itsaso Leceta,‡ Sara Cabezudo,§ Pedro Guerrero,† and Koro de la Caba*,† †

BIOMAT Research Group, Department of Chemical and Environmental Engineering, ‡BIOMAT Research Group, Department of Applied Mathematics, and §BIOMAT Research Group, Department of Business Management, Engineering College of Gipuzkoa, Plaza de Europa 1, 20018 Donostia-San Sebastián, Spain ABSTRACT: In recent years there has been an increasing concern about the huge amount of plastic waste generated in daily life. In order to reduce the impact of petroleum-derived plastics, transparent and colorless fish gelatin films were prepared by solution casting. The effect of solution pH on film structure and consequently on mechanical, barrier, and optical properties was analyzed. Results showed that films prepared at basic conditions were hydrophobic and presented high light barrier and high tensile strength. Furthermore, environmental assessment demonstrated that composting as end of life scenario provided an environmental benefit in all the impact categories, highlighting the potential of these films as alternative raw materials to manufacture sustainable products.

KEYWORDS: Gelatin films, pH, Characterization, Composting, Life cycle assessment

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INTRODUCTION Concerns over the fossil fuels depletion and the environmental pollution caused by human activity lead to the development of novel products based on renewable materials with lower environmental charge in its whole life cycle.1 The European plastic demand in 2014 reached 47.8 million tones, of which 39.5% were used in packaging applications, most of them for short-term-applications. Furthermore, 25.8 million tones of consumed plastics ended up in the waste stream, of which 30.8% went to landfills because waste burial is still the first option in many EU countries.2 Different kinds of plastics are used in an extensive range of needs, being that the synthetic polymers, such as polypropylene (PP), low density polyethylene (LDPE), high density polyethylene (HDPE), or polyvinyl chloride (PVC), are the most common materials used in a wide variety of markets. In order to reduce the impact of petroleum-derived plastics, considerable attention is focused on biobased films,3 among them, those prepared with proteins from food processing wastes, such as gelatin.4,5 The production of gelatin from fish wastes is a topic that has recently gained much attention. Since fish skins and bones contribute almost 30% of the total fish weight, the use of these fishery industry wastes can be considered a promising way for gelatin production.6 Furthermore, their valorization could reduce the problems associated with waste management,7 avoiding serious ecological problems and environmental pollution due to inappropriate waste handling. Gelatins are water-soluble proteins produced from the hydrolysis of collagen. These proteins are heteropolymers built from amino acids joined together by peptide bonds, amide linkages formed by the condensation reaction of the amino © XXXX American Chemical Society

acids. The sequence of those amino acids constitutes the primary structure of proteins. The properties of proteins depend not only on the sequence of amino acids but also on the way in which protein chains are folded in space, such as αhelix, β-sheets, or unordered random coil structures, which are referred to as secondary structures.8 Most proteins do not adopt completely uniform conformations, and full descriptions of their preferred three-dimensional arrangements are defined as tertiary structures. Tertiary structures depend on noncovalent interactions, such as hydrogen bonding, hydrophobic interactions, and charge attractions and repulsions.9 In addition to the tertiary structure, the way in which polypeptide structures may aggregate constitutes the quaternary structure.10 These native structures of proteins may be altered by treatments that do not disrupt the primary structure but alter the secondary, tertiary, and quaternary structures. The structural unfolding of proteins, commonly known as denaturation, can be done by changing solution pH during the film forming process. Gelatins show some functional properties, such as film forming ability, biocompatibility, and commercial availability at a relatively low cost. These properties make gelatin suitable materials for different applications, such as food packaging, drug delivery, or wound dressing, among others.11,12 However, gelatin films show high brittleness, which limits their commercial application. Since the addition of plasticizers reduces protein chain-to-chain interactions and, thus, induces Received: April 12, 2016 Revised: July 20, 2016

A

DOI: 10.1021/acssuschemeng.6b00750 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering film flexibility,13 polyols can be incorporated into film forming solutions. With this regard, glycerol is widely used in food, cosmetic, and pharmaceutical formulations due to its nontoxicity and abundance as a byproduct of biodiesel production.14 Besides plasticizer addition, processing conditions must be optimized in order to denature protein and facilitate the interactions with the plasticizer. Denaturation can occur by means of controlling solution pH. When it occurs, the protein unfolds, affecting the secondary, tertiary, and quaternary structures of the protein, while the primary structure remains intact.15 All these structures determine the final shape of the protein and, so, its functionality. As the final properties of gelatin films are structure sensitive, changes undergone by the modification of solution pH were studied in this work. Some functional properties, such as mechanical, optical, and barrier properties, are the most relevant properties required for food packaging materials.16 Mechanical properties, such as strength and flexibility, are required to maintain film integrity over time and protect the packaged product from deterioration. Optical properties, such as color, transparency and gloss, are closely related to the appearance of the product and, thus, to consumer acceptability. Finally, barrier properties, such as UV light resistance, water contact angle, and water vapor permeability, are important in order to preserve the packaged product’s quality from environmental agents and extend its shelf life. In addition to the improvement of the above-mentioned properties, the treatment of films after disposal has attracted increasing interest, mainly due to the municipal policies carried out to reduce the huge amount of plastics sent to landfills.17 With this regard, natural polymers can be biodegraded through natural processes, such as hydrolysis in the presence of enzymes or under composting conditions. There are a few studies on polymer degradation in the environment; most of them are carried out with poly(lactic acid) (PLA).18,19 These soil burial studies followed the changes that occurred in the films over time, highlighting the potential use of composting as a waste treatment in order to reduce pollution and disposal costs. Regarding the environmental impact of these renewable and biodegradable films, the life cycle assessment (LCA) is a useful tool to determine the environmental charge of the products and processes from the extraction of natural resources through their manufacture until final disposal. At every stage of the life cycle, consumption of energy and resources, such as water and materials, and the emissions of greenhouse gases are considered.20 To the best of our knowledge, there is no information related to the environmental assessment of fish gelatin films in addition to film preparation and characterization. In this context, the goal of this work was to analyze the effect of preparation conditions on the functional properties of fish gelatin films, such as mechanical, optical, and barrier properties. The changes observed in properties were related to the changes which occurred in a gelatin structure, analyzed by spectroscopic and diffraction methods. Furthermore, environmental impacts were measured in order to identify both the benefits and those aspects that need further improvement.

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(1999/724/CE). Glycerol, used as a plasticizer, and NaOH and HCl, used to fix solution pH, were provided by Panreac (Barcelona, Spain). Film Preparation. Fish gelatin films were prepared by solution casting. First, 5 g of gelatin was dissolved in 100 mL of distilled water for 30 min at 80 °C under continuous stirring to obtain a good blend. After that, 10 wt % glycerol (on gelatin dry basis) was added to the solution, and the pH was adjusted to 2.0 (acid pH) with HCl (1 N) or up to 10.0 (basic pH) with NaOH (1 N). Three solution pHs were analyzed: acid pH, basic pH, and control pH (when solution pH was not modified). After fixing pH, the solution was maintained at 80 °C for other 30 min under stirring. Then, 20 mL of film forming solution was poured onto each Petri dish (ø = 17 cm) and left drying 48 h at room temperature to obtain films with a thickness of 50 μm. Finally, all films were conditioned in a controlled biochamber (ACS Sunrise 700 V, Madrid, Spain) at 25 °C and 50% relative humidity for 48 h before testing. Film Characterization. Ultraviolet−Visible (UV−vis) Spectroscopy. A V-630 UV−vis spectrophotometer (Jasco, Madrid, Spain) was used to determine the light barrier properties of films. Light absorption was measured at wavelengths from 200 to 800 nm, and three specimens were tested for each sample. The transparency of the films was measured at 600 nm and normalized considering the film thickness. Gloss and Color Measurements. Gloss was measured at 60° incidence angle according to ASTM D-523,21 using a flat surface Multi Gloss 268 plus gloss meter (Konika Minolta, Valencia, Spain). Color was determined with the CR-400 minolta Croma Meter colorimeter (Konika Minolta, Valencia, Spain). Films specimens were placed on the surface of a white standard plate (calibration plate values L* = 97.39, a* = 0.03, and b* = 1.77), and color parameters L*, a*, and b* were measured using the CIELAB color scale: L* = 0 (black) to L* = 100 (white), −a* (greenness) to +a* (redness), and −b* (blueness) to +b* (yellowness). Measurements were taken ten times for each sample at 25 °C. Water Contact Angle (WCA). Water contact angle measurements were performed using an OCA 20 contact angle system (DataPhysics Instruments, Eibar, Spain). A 3 μL droplet of distilled water was placed on the film surface to estimate the hydrophobic character. The image of the drop was carried out using SCA20 software. Five measurements were made for each sample at 25 °C. Water Vapor Permeability (WVP). WVP measurements were carried out in a controlled humidity environment chamber PERME W3/0120 (Labthink Instruments Co. Ltd., Shandong, China). Each film was cut in samples of 7.4 cm diameter and a test area of 33 cm2. Films were maintained at a temperature of 38 °C and a relative humidity of 90%, according to ASTM E96-00.22 WVP was determined gravimetrically until constant weight. First, water vapor transmission rate (WVTR) was calculated by eq 1

⎛ g ⎞ G ⎟ = WVTR⎜ ⎝ s·cm 2 ⎠ t × A

(1)

where G is the change in weight (g), t is time (h), and A is the test area (m2). Water vapor permeability (WVP) was determined by eq 2 ⎛ g ⎞ WVTR × L ⎟ = WVP⎜ ⎝ cm·s·Pa ⎠ ΔP

(2)

where L is the thickness of the samples, and ΔP is the partial pressure difference of water vapor across the film. Measurements were carried out in triplicate. Tensile Testing. Samples were cut into strips of 4.75 mm × 22.25 mm. Tensile strength (TS) and elongation at break (EB) were measured for each film at least five times on an electromechanical Insight 10 testing system (MTS Systems, Barcelona, Spain) at 25 °C, according to ASTM D1708-93.23 X-ray Diffraction (XRD). XRD analysis was performed with a diffraction unit (PANalytic Xpert PRO, Madrid, Spain) operating at 40 kV and 40 mA. The radiation was generated from a Cu−Kα (λ = 1.5418 Å) source. The diffraction data were collected from 2θ values

EXPERIMENTAL SECTION

Materials. A commercial cod fish gelatin type A was employed in this study. It has bloom 200, 11.06% moisture, and 0.147% ash. Fish gelatin was kindly supplied by Weishardt International (Liptovsky Mikulas, Slovakia) and meets the quality standard for edible gelatin B


Research Article

ACS Sustainable Chemistry & Engineering from 2.5° to 50°, where θ is the incidence angle of the X-ray beam on the sample. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectra of the films were carried out on a Nicolet Nexus FTIR spectrometer using ATR Golden Gate (Thermo Scientific, Madrid, Spain). A total of 32 scans were performed at 4 cm−1 resolution. Measurements were recorded between 4000 and 800 cm−1. All spectra were smoothed using the Savitzky-Golay function. Second-derivative spectra of the amide region were used at peak position guides for the curve fitting procedure, using OriginPro 9.1 software. Film Disintegration. Indoor soil burial was carried out as a simulation test at laboratory scale in order to evaluate film disintegration.24 Briefly, a series of Petri dishes were used as soil containers, and natural microflora present in soil was used as the degrading medium. The initial microbial composition of a soil is known to be pH dependent. The initial soil pH was 7.3, determined by a mass/volume ratio of 1:5 (1 g soil/5 mL distilled water). The fish gelatin films were cut into a rectangular shape (2 cm × 3 cm), and then the specimens were buried at the depth of 4 cm from the soil surface. The samples were incubated at room temperature, and the soil humidity was 100% during the whole study (15 days). The disintegration of fish gelatin films was followed by FTIR spectroscopy. Environmental Assessment. Environmental assessment was performed according to ISO 14040 guidelines and recommendations.25 Therefore, it was divided into four phases: goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA), and results interpretation. The aim of this analysis was to calculate the environmental load of fish gelatin films, identifying their environmental impacts in order to propose correcting measures when designing packaging films and, thus, increase their commercial potential. As also shown in recent analyses for packaging films,26 the functional unit selected was 1 m2 (50 μm film thickness). In this work, a ’cradle to grave’ perspective was considered, including three stages: raw material extraction, film manufacture, and disposal. The software used to carry out this analysis was SimaPro 7.3.3. LCI data for the extraction stage, which was not carried out in our laboratories, were obtained from the literature, in the case of fish gelatin,27 and from Ecoinvent 2.2 database, available in SimaPro software, in the case of glycerol and the additives used in the extraction processes. As the manufacture of the films was performed in our laboratories, LCI data were collected based on this work. Finally, the data for the end of life stage was provided by Lapatx composting facility located in the Basque Country. Some assumptions were considered when performing the environmental assessment. The glycerol employed as a plasticizer was considered a byproduct of the soy oil production destined for biodiesel. In the end of life stage, out of every 1 kg of film waste, 200 g was considered as compost, whereas the remaining organic matter was considered to be digested by microorganisms that take part in the biodegradation process. In addition, generated compost for soil improvement purposes was considered an avoided product. Based on the inventory data, the final results were presented in the impact categories used in the Eco-indicator 99 methodology. This methodology groups the environmental impacts in three damage categories or end points: human health, ecosystem quality, and resources.28 In this work midpoint measurements were carried out since midpoints are considered to be links in the cause-effect chain of an impact category in which characterization factors reflect the importance of the emissions.29 Human health category accounts for 6 midpoint impact categories: carcinogens, respiratory organics, respiratory inorganics, climate change, and ozone layer. These impact categories use DALY scale (Disability Adjusted Life Years). Ecosystem quality accounts for ecotoxicity, acidification/eutrophication, and land use. Ecotoxicity and land use are expressed in terms of Potentially Affected Fraction (PAF m−2 year−1), and acidification/eutrophication is expressed as Potentially Disappeared Fraction (PDF m−2 year−1). Additionally, resources category accounts for minerals and fossil fuels; both of them are expressed as MJ surplus energy. In order to consider the relative importance of the studied impact categories, normalized

impact values were represented. Normalization was performed by determining the ratio of the category indicator result and that of a reference based on the European level with updates of the most relevant emissions. Statistical Analysis. Data were subjected to one-way analysis of variant (ANOVA) by means of a SPSS computer program (SPSS Statistic 20.0). Post hoc multiple comparisons were determined by the Tukey’s test with the level of significance set at P < 0.05.

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RESULTS AND DISCUSSION Packaging technology must balance product protection with other issues such as appearance, consumer acceptability, and social and environmental consciousness. Taking these aspects into consideration, characterization of optical, barrier, and mechanical properties of fish gelatin films was carried out and related to the material structure to analyze the effect of solution pH on film properties. Film Characterization. A good protection from UV light could prevent the packaged product deterioration as a consequence of oxidation reactions caused by UV light.30 As shown in Figure 1, fish gelatin films exhibited high UV light

Figure 1. UV−vis spectra of fish gelatin films as a function of solution pH.

absorption due to the presence of peptide bonds (200−250 nm) and chromophores such as tyrosine and phenylalanine (250−300 nm), common aromatic amino acids found in fish gelatins.31,32 In addition to UV light resistance, other optical properties, such as transparency and color, affect consumers’ willingness to purchase, and, thus, those are properties that must be taken into consideration. As can be seen in Figure 1, films were transparent since there is not absorbance at 600 nm, irrespective of the solution pH used to prepare the films. In the same manner, no significant (P > 0.05) change was observed for color parameters, as shown in Table 1, where L*, a*, and b* values are listed. Also in Table 1, gloss values are shown. Gloss is directly related to the surface roughness of the film, so gloss values are lower when surface is rougher. Gloss values higher than 70 measured at an incidence angle of 60° indicate glossy and smooth surfaces.33 When solution pH was modified, films showed higher gloss and, thus, a smoother surface than control films, suggesting that pH modifies the film structure. C


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Optical Properties of Fish Gelatin Films as a Function of Solution pHa pH

L*

a*

b*

gloss (deg)

control acid basic

96.13 ± 0.53a 96.14 ± 0.44a 96.19 ± 0.25a

−0.18 ± 0.09a −0.18 ± 0.04a −0.10 ± 0.03a

2.86 ± 0.41a 2.65 ± 0.10a 3.01 ± 0.10a

35.89 ± 2.85a 152.00 ± 1.58c 119.75 ± 6.90b

a Two means followed by the same letter in the same column are not significantly (P > 0.05) different through the Turkey’s multiple range test. N = 4 was the minimum number of replications.

In order to relate the functional properties of fish gelatin films with the structure changes promoted by solution pH, Xray diffraction (XRD) was carried out since it is a common technique to determine the tertiary structure of proteins. The diffraction bands of fish gelatin films as a function of solution pH are shown in Figure 2. All patterns displayed two diffraction

In addition to barrier properties against UV light, barrier properties against water were measured. The water contact angle (WCA) is a measure of the wettability of the surface and defines the hydrophilic or hydrophobic character of the material. Low WCA values, lower than 90°, correspond to high wettability or hydrophilicity, while high values, higher than 90°, correspond to low wettability or hydrophobicity.34 The WCA values of fish gelatin films are shown in Table 2. As can Table 2. Water Contact Angle (WCA) and Water Vapor Permeability (WVP) Values of Fish Gelatin Films as a Function of Solution pHa WVP 1012 (g cm‑1 s‑1 Pa‑1)

pH

WCA (deg)

control acid basic

76.85 ± 1.36 91.35 ± 3.26b 88.61 ± 3.36b a

1.48 ± 0.16a 1.52 ± 0.10a 1.81 ± 0.16b

a

Two means followed by the same letter in the same column are not significantly (P > 0.05) different through the Turkey’s multiple range test. N = 3 was the minimum number of replications.

be seen, higher WCA values were obtained when solution pH was modified. This fact could be explained by protein conformational changes that expose nonpolar groups toward the surface. In order to control moisture transfer to and from the surrounding environment, films must show water vapor permeability suitable for their use. WVP values of fish gelatin films are shown in Table 2. As can be seen, WVP values increased (P < 0.05) at basic pH, probably due to a higher degree of protein denaturation and, thus, higher unfolding that facilitates permeation of water vapor molecules. Finally, mechanical properties were measured since appropriate tensile strength and elongation at break are of great importance when handling and applying materials in order to maintain their integrity. As can be seen in Table 3, the best

Figure 2. XRD patterns of fish gelatin films as a function of solution pH.

peaks, the peak at 21°, related to the crystallinity of gelatin, and the peak at 7.4°, corresponding to the residual triple-helix from native collagen.35,36 When solution pH was modified, lower crystallinity was observed, indicating that a less ordered structure was obtained as a consequence of a higher denaturation degree, which resulted in changes in the tertiary structure of gelatin.37 As the tertiary structure of proteins is governed by noncovalent interactions, such as hydrogen bonding, hydrophobic interactions, and charge attractions and repulsions, the change in solution pH could disrupt these noncovalent interactions, leading to the decrease in crystallinity observed in Figure 2. These results are in accordance with WVP values, which showed that a higher degree of denaturation facilitated water vapor permeation. Otherwise, the content of residual triple-helix was lower at control and acid pHs than at basic conditions. Although gelatin is obtained by a partial hydrolysis of collagen, which breaks its triple-helix structure and forms random coils, gelatin is able to recover partially the native structure of collagen, a phenomenon known as coil-to-helix transition.38 Results showed that the refolding of the typical collagen triple-helix could be favored at basic conditions during drying of fish gelatin films. Infrared spectroscopy is also a valuable tool for the investigation of protein structure due to the sensitivity of the amide I vibration to the secondary structure of proteins.39 Therefore, FTIR spectroscopy was used to study changes in the secondary structure of gelatin in terms of α-helix/unordered

Table 3. Tensile Strength (TS) and Elongation at Break (EB) of Fish Gelatin Films as a Function of Solution pHa pH

TS (MPa)

EB (%)

control acid basic

36.52 ± 2.98a 43.02 ± 0.52b 52.39 ± 3.16c

1.79 ± 0.54a 2.31 ± 0.33a 2.88 ± 0.68a

a

Two means followed by the same letter in the same column are not significantly (P > 0.05) different through the Turkey’s multiple range test. N = 3 was the minimum number of replications.

mechanical properties were obtained when a basic pH was used to prepare films. TS values significantly (P < 0.05) increased when pH was modified, and a TS value higher than 50 MPa was obtained at basic pH. However, the elongation at break was not significantly (P > 0.05) affected by the solution pH. D


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ACS Sustainable Chemistry & Engineering and β-sheet conformations by means of analyzing the amide I band (1600−1700 cm−1), associated with the vibration of the carbonyl group (CO) of the peptide bond in proteins. As above-mentioned, the secondary structure of gelatin films consists mainly of α-helix/unordered conformations, associated with the band at 1650 cm−1, and antiparalel β-sheet conformations, related to the bands at 1630−1615 cm−1 and 1700−1680 cm−1.40,41 As shown in Figure 3, the predominant structure was α-helix, irrespective of solution pH. However, films prepared at basic conditions showed higher area (Table 1)

and, thus, higher quantity of α-helix structure than the films prepared at acidic conditions. Otherwise, an area with absorbance reduction in β-sheet structure (1700−1680 cm−1) was seen when films were prepared at acidic conditions. Therefore, results showed that solution pH altered the secondary structure of fish gelatin films. In aqueous solutions, protein residues are charged, and hence protein mobility depends on pH. When the pH is far from the isoelectric point, the protein net charge changes toward positive or negative charges depending on solution pH. As the same charges repel one another, this prevents protein from aggregating. Therefore, the internal interactions are altered, forcing protein structure to open up (denaturalization or unfolding).42 Due to these phenomena, the film structure changes and so do some functional properties of the films, as above-mentioned. In addition to functional properties of gelatin films, their biodegradability is a relevant benefit in comparison with commercial nonbiodegradable polymers, which cause problems and costs associated with their waste management after disposal. Since gelatin and glycerol are biodegradable compounds and no chemical reaction occurs between them, a biodegradation test is not necessary to certify the film biodegradability.43 However, film disintegration tests were carried out by burying films into soil under controlled conditions to analyze the disintegration of gelatin films by FTIR spectra. There was no variation in the spectrum of the films prepared at different pHs (data not shown). Therefore, as a higher unfolding and, so, more available sites for interactions appeared at basic conditions, the disintegration tests were carried out with the films prepared at basic conditions, and results are shown in Figure 4. The main absorption peaks were

Figure 4. FTIR spectra of fish gelatin films before and after soil burial disintegration.

located in the spectral range from 1630 to 800 cm−1. Gelatin bands were related to CO stretching at 1630 cm−1 (amide I), N−H bending at 1530 cm−1 (amide II), and C−N stretching at 1230 cm−1 (amide III).36 The main absorption bands of glycerol were related to the five peaks corresponding to the vibrations of C−C bonds at 850, 940, and 1000 cm−1 and C−O bonds at 1050 and 1100 cm−1.44 No shift of the characteristic bands was observed, but the decrease in intensities of amides I, II, and III was shown during film disintegration. This fact indicates the cleavage of bonds, which is the principle of

Figure 3. Curve fitting spectra of amide I (black line) for fish gelatin films as a function of solution pH. Curve fitting spectra are related to β-sheet structure (gray and green lines) and α-helix structure (red line). E


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ACS Sustainable Chemistry & Engineering degradation. Besides, the decrease of absorption band intensity was more notable for amide II and III bands than for amide I band. Since the biodegradation rate depends on the accessibility of the microorganisms to polymer chains, this process depends on the flexibility of chains and on the amorphous regions of the protein, among others.45 Results indicated that microorganisms mainly attacked the C−N stretching bond, following with N−H bending bond, and hardly acting on the CO stretching bond. Therefore, the hydrolyzable functional groups of amide III (C−N) and amide II (N−H) presented better accessibility than the amide I (C O). These results are in accordance with the energy required to break the above-mentioned bonds related to the amide group. Specifically, 305, 390, and 799 kJ/mol are the bond energies for C−N, N−H, and CO, respectively. Furthermore, it is worth noting that the band corresponding to glycerol barely changed due to the fact that microorganisms present in the soil did not attack the plasticizer. In fact, a number of microorganisms are able to grow on glycerol,46 so proliferation of microorganisms on the plasticizer could promote the disintegration of protein. As a consequence, there was a slight increase of the soil pH up to 7.9. This rise in pH may be caused by the ammonification of proteins from the film. Environmental Assessment. Concerning the environmental impacts of fish gelatin films, three main stages were assessed: material extraction, film manufacture, and end of life. A schematic diagram of the system boundaries is shown in Figure 5. The preparation of fish gelatin films is a way of adding value to the waste from the fish processing industry. About 30% of such waste consists of skins and bones with a high collagen content.6 The extraction method consists of a mild swelling in sodium hydroxide and afterward in acetic acid. Subsequently, gelatin extraction is carried out in distilled water at 70 °C during 90 min, and then the sample is dried in an oven for 18 h at 50 °C. Based on the average extraction yield of fish gelatins, the yield considered in this work was 15%.27 This process was not carried out in our laboratories, and data were taken from literature.27 Furthermore, the production of the edible part of the fish was out of the system boundaries. Concerning the manufacture of fish gelatin films, data were obtained from the process employed in our laboratories, described above in the Experimental Section. In the film manufacture the environmental load of the process from turning raw materials into films was considered, so the additives employed and the energy consumed during the process were taken into account. Since glycerol was considered as a byproduct of soybean oil production to obtain biodiesel, the esterification process of soybean oil to methyl ester and glycerol in this oil production process was considered. Finally, the disposal scenario was composting, a process based on the aerobic degradation of the waste. Out of every 1 kg of film waste, 200 g was considered as compost, and residual organic matter was considered to be digested by microorganisms within the biodegradation process. It was considered that the composting facility is energetically self-sufficient, and no electric consumption was taken into account. This process was not carried out in our laboratories, and the abovementioned data were provided by the composting facility in the Basque Country. Furthermore, transportation was not considered in any stage of the assessment. Eco-indicator 99 was the method used to assess the environmental impact of fish gelatin films. This methodology

Figure 5. System boundaries for the environmental assessment of fish gelatin films.

has been broadly employed in Europe due to the fact that it links results via midpoints or impact categories to end point or damage categories, providing the negative environmental effects due to the substances emitted and resources used.47 According to the goal of this environmental assessment, the impact categories in relation to each of the three product stages are shown in Figure 6. As can be seen, the raw material extraction was the most pollutant phase in the majority of the impact categories analyzed. Specifically, fossil fuels and respiratory inorganics were the impact categories in which fish gelatin films caused higher environmental damage. The energy consumption during gelatin extraction was mainly responsible for the environmental load in these categories. In relation to the manufacture stage, the energy consumption together with the soya cultivation process to obtain glycerol were the main reasons for the environmental burden in fossil fuels, land use, and respiratory inorganics. With regard to land use, the environmental burden was due to the glycerol used as plasticizer since it is considered as a coproduct from the F


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ACS Sustainable Chemistry & Engineering

carried out with polysaccharides like the above-mentioned3,26,49 or biopolyesters like polylactic acid (PLA),50 and very few studies have determined the environmental impacts for proteinbased films51,52 or for different stages from cradle to grave, as the present work did. These results highlight the importance of this study to support decision making related to the improvements that must be incorporated into the different phases of the film development in order to achieve materials with good functional properties that are really environmentally more sustainable. However, it must be emphasized that some improvements could be expected when scaling up production from laboratory to large-scale facilities.

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CONCLUSIONS Homogeneous, colorless, and transparent fish gelatin films were developed in this work, irrespective of solution pH. Furthermore, films showed high resistance against UV light, which prevents the product packaged from oxidation reactions caused by light, maintaining its quality for longer periods of time. However, it was observed that solution pH affected gelatin structure. In particular, the films prepared at a basic pH show high tensile strength with values higher than 50 MPa. These good functional properties of fish gelatin films and the positive environmental effect of composting as the end of life scenario highlight the potential use of these films as alternative materials for packaging applications in order to reduce the huge consumption of nonrenewable and nonbiodegradable materials.

Figure 6. Environmental impact values normalized for fish gelatin films.

production of soybean oil to obtain biodiesel, and, thus, cultivation of soybean was considered. The environmental impact of the assembly, in which raw material extraction stage and manufacture stage were gathered, and the impact values related to the end of life phase are clearly differentiated in Table 4. By this way, the environmental benefit provided by composting scenario in all the environmental categories under analysis can be properly appreciated. Since composting transfers biodegradable waste into useful soil products, the carbon dioxide produced does not contribute to an increase in greenhouse gases because it is already part of the biological carbon cycle.48 Moreover, avoiding the production of generated compost for soil conditioner and the associated emissions provides a positive effect on the environmental impact. The results explained above agree with recent environmental assessments in which energy consumption for extraction and manufacturing processes were mainly responsible for the environmental impact of the biocomposite films.26 Although biopolymers are considered ecofriendly materials due to their renewable and biodegradable character, extraction and production practices can cause environmental burdens, as shown for some polysaccharide-based films such as starch,26 chitosan,49 or agar.3 It is worth noting that the majority of the environmental assessments related to biopolymers has been

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AUTHOR INFORMATION

Corresponding Author

*Phone: +34 943 017188. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the University of the Basque Country UPV/ EHU (research group GIU15/03) as well as the Provincial Council of Gipuzkoa (OF221/2015 (ES)) for their financial support. The authors also thank Advanced Research Facilities (SGIker) from the UPV/EHU. Alaitz Etxabide thanks UPV/ EHU (fellowship PIF13/008).

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REFERENCES

(1) Yates, M. R.; Barlow, C. Y. Life cycle assessments of biodegradable, commercial biopolymers- A critical review. Resources, Conversation and Recycling 2013, 78, 54−66.

Table 4. Normalized Results for the End of Life Stage of Fish Gelatin Films impact category fossil fuels minerals land use acidif/eutrop. ecotoxicity ozone layer radiation climate change resp. inorganics resp. organics carcinogens

assembly 4.17583 4.54575 9.90902 1.60126 7.04678 2.11478 2.54337 4.46968 2.00957 1.13861 1.85121

× × × × × × × × × × ×

end of life −05

−1.44962 −3.11713 −4.49753 −8.53697 −2.80816 −6.95414 −3.69787 −2.76574 −2.18191 −2.85968 −2.54244

10 10−07 10−06 10−06 10−07 10−09 10−07 10−06 10−05 10−08 10−06 G

× × × × × × × × × × ×

total −07

10 10−09 10−09 10−08 10−09 10−12 10−10 10−07 10−07 10−10 10−09

4.16133 4.51458 9.90452 1.51589 7.01870 2.10782 2.53968 4.19311 1.98775 1.11001 1.84866

× × × × × × × × × × ×

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Research Article

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Sustainable Fish Gelatin Films: from Food Processing Waste to

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