International Journal of Biological Macromolecules 150 (2020) 480–491 Contents lists available at ScienceDirect International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac Active biodegradable films based on the whole potato peel incorporated with bacterial cellulose and curcumin Yumei Xie a, Xuening Niu a, Jingwen Yang a, Runze Fan a, Jiahao Shi a, Niamat Ullah b, Xianchao Feng a, Lin Chen a,⁎ a b College of Food Science and Engineering, Northwest A&F University, No 22 Xinong Road, Yangling, Shaanxi 712100, China Department of Human Nutrition, The University of Agriculture Peshawar, Khyber Pakhtunkhwa 25000, Pakistan a r t i c l e i n f o Article history: Received 30 September 2019 Received in revised form 16 January 2020 Accepted 29 January 2020 Available online 30 January 2020 Keywords: Potato peel Bacterial cellulose Curcumin a b s t r a c t Development of biodegradable food packaging using biomass based materials derived from agricultural wastes has been a trend in recent years The biopolymer films were prepared using 3% and 5% (w/w) potato peel (PP) powder Bacterial cellulose (BC) (0, 5, 10 and 15% based on PP powder) was added as a reinforcement agent The scanning electron microscopy (SEM) revealed that 10% BC had a promising compatibility with the PP matrix X-ray diffraction (XRD) and thermogravimetric analysis (TGA) showed that the crystallinity and the thermal stability of films did not change with BC addition Fourier transform infrared spectroscopy (FTIR) indicated the hydrogen bonding interactions between the PP matrix and BC in the films BC addition significantly improved the tensile strength (TS), but reduced their water vapor permeability (WVP), oxygen permeability (OP) and moisture content (MC) of the PP films Addition of curcumin further increased the antioxidant properties of the PP films The PP films with 1–5% curcumin significantly reduced lipid oxidation in the fresh pork during storage with lower malondialdehyde (MDA) content © 2020 Elsevier B.V All rights reserved Introduction Biodegradable food packaging can reduce or even replace the synthetic plastic packaging Petroleum-derived polymeric packaging is hard to be degraded after use, which causes serious damage to our environment Every year, plentiful agricultural by-products can provide inexpensive and renewable biodegradable polymers for the producing of food packaging all over the world Potato is the fourth major crop after rice, wheat and maize over the world [1] Millions of tons of potato peel (potato peel, PP) waste are produced every year [2], leading to handling and storage problems [3] Conventional management strategies for elimination of the PP pollutants are cropland composting or producing feed for ruminant animals [4] Besides this, PP is also a good resource of biodegradable polymers, including carbohydrates (12 g/100 g) and low amounts of protein (2.56 g/100 g) [5] Development of the biodegradable films based on the whole PP therefore is a promising application for the surplus PP, which can increase the economic value and decrease the process steps of PP Nowadays, the peel wastes of certain fruits and vegetables have been already used to prepare the biodegradable films Previous studies have reported development of biopolymer films using the peel wastes, such as ripe banana peel flour [6], pomegranate peel powder [7], olive pomace residue [8], blueberry residue [9], and babassu mesocarp residue [10] Compared to pure biopolymers, such as starches, polysaccharides, cellulose, or proteins, extracted from agricultural sources, the https://doi.org/10.1016/j.ijbiomac.2020.01.291 0141-8130/© 2020 Elsevier B.V All rights reserved use of natural mixture of biopolymers directly obtained from agroindustrial byproducts has the advantage of using the whole agricultural sources, and decreasing the process steps, waste, and the costs of production Actually, using the whole agricultural sources is a convenient approach to avoid incompatibility between the different biopolymers extracted from agricultural sources However, application of PP or PPborn biopolymers to prepare the biodegradable or edible films has been scarcely documented As far as we know, only one published article has reported the way for the development of PP biopolymer films with glycerol as the plasticizer [3] The relatively poor mechanical properties are the common defect of biopolymer-based films Therefore, the mechanical properties of PP films are still needed to be improved Moreover, the antioxidative films based on the PP waste have never been studied Compared to plant cellulose, bacterial cellulose (BC) possesses higher tensile strength and water holding capacity, and better biocompatibility due to its distinct ultrafine fibrils of nanosized threedimensional network structure [11,12] Nowadays, BC has been widely applied to ameliorate moisture barrier, thermal stability and mechanical properties of the biocomposite films due to its promising characteristics [13–18] As aforementioned, BC is supposed to improve the properties of the PP films as a reinforcement agent in the present study Nowadays, active packaging films have been developed to prolong shelf life of food products and to maintain their quality Specifically, antioxidant active packaging films have been the focus to Ket-noi.com Ket-noi.com kho kho tai tai lieu lieu mien mien phi phi Y Xie et al / International Journal of Biological Macromolecules 150 (2020) 480–491 prevent or slow down the oxidation of lipids and proteins, leading to the deterioration of quality of food products, such as texture breakdown and off-flavor development Natural extracts obtained from food processing wastes have been widely used as antioxidant additives for active films, including mango kernel extract [19], grapefruit seed extract [20], protocatechuic acid [21], essential oils [22], green tea extract [23] and curcumin [24] In food industries, curcumin has been widely studied due to its well-known bioactives, such as anticancer, antioxidant and anti-inflammatory activities [25,26] Previous studies showed that biocomposite films containing curcumin have good antioxidant properties [24,27] As a result, the curcumin is going to be applied for development of the active PP films in the present study Materials and methods 2.1 Materials Fresh potato peel was provided by the student restaurant in Northwest A&F University (Shanxi, China) Curcumin (65% purity) was purchased from Sigma-Aldrich Crop (St Louis, MO, USA) Glycerol (99% purity) was purchased from Guanghua Sci-Tech Co., Ltd (Guangdong, China) The malondialdehyde (MDA) kit was purchased from Nanjing Institute of Bioengineering (Nanjing, China) All the other chemicals were at least analytical reagent 2.2 Production of bacterial cellulose (BC) BC was produced according to our previous study in our lab [16] The BC was first ground using a blender (Joyoung, Hangzhou, China), and then homogenized using a high-pressure homogenizer (SRH60-70, Shenlu Co., Ltd., Shanghai, China) The moisture content in the BC was measured after centrifugation (10,000 ×g, 15 min) 2.3 Preparation of potato peel powder 481 2.5 Characterization of the films 2.5.1 Thickness The thickness of the PP films was measured with an electronic digital caliper (MNT-150T, Germany) The thickness was the average of 10 points on the PP film [28] 2.5.2 Scanning electron microscopy (SEM) Microstructures of the PP films were observed by SEM according to a previous method [17] The films were first stuck to an aluminum tape on the SEM stubs Then, the surface and cross-section microstructures were observed with S-4800 SEM (Hitachi, Tokyo, Japan) 2.5.3 Fourier transform infrared (FTIR) spectroscopy Samples of the PP films were first sufficiently mixed with potassium bromide (KBr) FTIR spectra of samples were recorded in the range between 400 and 4000 cm−1 wavenumber with a Bruker Vertex-70 (Germany) using 16 scans at cm−1 resolution according to a previous method [17] 2.5.4 X-ray diffraction (XRD) Samples of the PP films were submitted to XRD using a diffractometer (D8-Advance Bruker, Germany) with Ni-filtered Kα Cu X-ray radiation (λ = 1.54 Å) XRD analysis was detected in the angular range from 2θ of to 60° with a step size of 0.02° The crystallinity index (CrI) of the PP films was then calculated by using the following empirical Eq (1) [29] CI ¼ I002 −Iam I 002 ð1Þ where I200 is the intensity value of crystalline cellulose, and Iam is the intensity value of amorphous region material (2θ = 18°) 2.5.5 Thermogravimetric (TGA) analysis Thermogravimetric analyses were carried out under a nitrogen atmosphere performed using a STA449F3 (Netzsch, Germany) with a heating rate of 10 °C·min−1 from 30 to 700 °C The weight loss and the different degradation phases of the samples were recorded The dirt on the potato peel (PP) was first cleaned with tap water After draining, the PP was dried in the oven at 50 °C for 24 h Then, the dried PP was ground to get PP powder particles (b75 μm) by a grinder The dried PP has about 9.22 ± 0.01% moisture content, 15.62 ± 0.07% protein, 6.31 ± 0.04% cellulose, 45.77 ± 0.07% Starch, and 6.82 ± 0.02% ash The rest is soluble polysaccharides and lignin 2.5.6 Oxygen permeability (OP) Oxygen permeability was analyzed by adsorption method of deoxidizer [30] Three grams of deoxidizing agent (sodium chloride: activated carbon: reduced iron powder = 1.5:1.0:0.5) were placed into a bottle (diameter × 20 mm, depth × 45 mm), and then sealed with the PP films All the bottles were put into a desiccator with BaCl2 solution to maintain the RH of 90% The OP of the films was then calculated using Eq (2) 2.4 Preparation of the potato peel films OP ¼ PP powder (6 or 10 g) was suspended into 194 or 190 g distilled water to get the 3% or 5% (w/w) PP suspension and stirred for 15 Glycerol (30% based on PP powder) was added into the PP suspensions as a plasticizer, and then the suspensions stirred for 30 at 85 °C A certain amount of BC (0, 5, 10 and 15% based on PP powder) was added into the PP suspensions The mixtures were homogenized at 10,000 rpm for with an Ultra-Turrax (T18, IKA-Works, Inc., Wilmington, NC, USA), and then mixtures were stirred for 60 at 90 °C to get uniform film-forming suspension (FFS; containing PP and BC) Then, the FFS were degassed by ultrasound The suspensions were poured into petri plates and dried at 40 °C for 72 h to prepare the PP films Finally, the PP films were conditioned at 25 °C for 48 h at 52% RH in a thermo-hygrostat The 3% PP films with 0%, 5%, 10% or 15% BC were named as 3-0, 3-5, 3-10 or 3-15, respectively The 5% films were named following the same rule where mt and m0 are the final and initial weights of the films, respectively t is time, and A is the permeation area mt −m0 tA ð2Þ 2.5.7 Water resistance To determine the moisture content (MC), the PP films were cut into square shapes of cm × cm, and subsequently dried at 105 °C for 24 h in an oven The MC of the PP films was then calculated using Eq (3) MC %ị ẳ initial sample weightdry sample weight 100% initial sample weight ð3Þ The water solubility (WS) was measured by the ratio of the weight of soluble matter to the initial weight of films [31] The PP film samples were first dried at 105 °C for 24 h to measure their initial weight, and then, were soaked in deionized water at 25 °C for 24 h After dried, the 482 Y Xie et al / International Journal of Biological Macromolecules 150 (2020) 480–491 weight of undissolved film samples was measured again The WS was then calculated using Eq (4) WS %ị ẳ initial weightfinal weight 100% initial weight ð4Þ The swelling degree (SG) of the PP films was determined following a previous method with modifications [32] The film specimens were immersed in deionized water The film specimens were weighed after removing the surface water The SG of the PP films was calculated as following: straight line divided by the area of the bottle mouth (m2) WVP ẳ WVTR Q S1 S2 ị X ð9Þ where X is the thickness of film (m), Q is the saturation vapor pressure (Pa) at 25 °C, S1 and S2 are the RH in the desiccator and in the bottle The driving force [Q(S1–S2)] is 3073.93 Pa under the conditions 2.6 Active PP films prepared by incorporation of curcumin SG %ị ẳ final weightinitial weight 100% initial weight ð5Þ 2.5.8 Color of the PP films The color of the PP films was measured using a Minolta Lab colorimeter (CM-5, Minolta Camera Co., Osaka, Japan) The CIELab color scale (L*, a* and b*) was applied The total color difference (ΔE) was calculated according to the Eq (6) qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 E ẳ L ị ỵ a ị2 þ ðΔb Þ ð6Þ where ΔL*, Δa*, Δb* are differences of color values between films and the standard white plate 2.5.9 Optical properties The light transparency was record in the range of 400 to 800 nm with a UV–visible spectrophotometer (UV-2550, Shimadzu, Japan) Rectangular PP film strips were placed into quartz cuvettes, and an empty quartz cuvette was used as the blank 2.5.10 Mechanical properties The PP films were first cut into 3.5 × cm strips The tensile strength (TS) and elongation at break (EAB) of the PP films were determined using a texture analyzer (XT PLUS/50, Stable Micro Systems Ltd.) [33] The initial grip separation was set to 40 mm The TS and EAB were calculated as following: TS ẳ F s EAB %ị ẳ 7ị LL0 100% L0 ð8Þ Combined with the results of the mechanical properties, barrier properties (OP and WVP), and hydrophobicity, the film-forming suspension (5-10) containing 5% PP powder with 10% BC was selected to prepare the active films In this study, curcumin solution (2%) was dissolved in film-forming suspension to prepare the active PP films (0, 1, and g curcumin/100 g PP powder) 2.6.1 Release of curcumin from the active PP films The curcumin release from the active PP films was measured using Folin-Ciocalteu method [35] Simulated alcoholic or fatty food systems were prepared using 10% ethanol or 95% ethanol to determine the release of curcumin from the active PP films Film samples (60 mg) were mixed with simulant (10 mL) in brown glass vials to obtain film extracts at room temperature Film extracts (0.1 mL) were taken at different time point (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60 min), and then mixed with 0.4 mL Folin-Ciocalteu reagent (0.1 mol/L) and 0.5 mL sodium carbonate solution (0.7 M) The absorbance of the mixture was measured at 765 nm after stayed in the darkness for h The concentration of released curcumin was expressed as gallic acid equivalents in mg of the film 2.6.2 Radical scavenging activity The radical scavenging activity of the PP films was measured using the free ABTS radical according to a previous study with slight modifications [19] ABTS solution was diluted with ethanol to an absorbance of 0.7 ± 0.02 at 734 nm Then, food simulant (990 μL) extracts was first mixed with ABTS working solution (10 μL), and then the mixture was mixed vigorously and left for 10 at 37 °C in the darkness The percentage of ABTS radical scavenging activity of the active film was calculated according to the Eq (10) ðAbs ABTS−Abs sampleÞ Abs ABTS where F is the maximum load, s is the initial cross-sectional area L0 is the initial length of the film, and L is the length of the film at the point of rupture ABTS scavenging activity %ị ẳ 2.5.11 Contact angle (CA) Contact angle of the film surface was analyzed using a contact angle analyzer (JY-PHa, China) to reflect the hydrophobicity of the PP film according to a previous method [10] The PP films were cut into strips and then fixed on the glass slide A drop of μL deionized water was deposited onto the surface of the film A CCD camera was used to record the degree of contact angle where AbsABTS and Abssample are the absorbance of the film samples mixed with and without ABTS solution, respectively 2.5.12 Water vapor permeability (WVP) WVP tests were determined according to a previous method with a minor modification [34] Bottles (diameter × 20 mm, depth × 45 mm) were sealed with the PP films containing g anhydrous CaSO4 for maintaining the RH of 0%, and then kept in a desiccator containing saturated K2SO4 solution at the bottom to maintain the RH of 97% Bottles were weighed with 30 intervals until constant weight The WVP was calculated by using the slope of weight loss versus time plot The vapor transmission rate (WVTR) was determined as the slope (g/h) of the ð10Þ 2.7 Evaluation of the antioxidant effect of the PP film incorporated with curcumin on fresh pork Pork was aseptically cut into small pieces with a similar shape Then pork pieces were wrapped by the active PP films containing curcumin (0%, 1%, 5%) All the wrapped samples were stored at ± °C Lipid oxidation in pork samples were tested on day and day The antioxidative effect of the active PP films on the lipid oxidation accelerated by UV was observed as well The wrapped pork samples were placed under UV illumination for 24 h The lipid oxidation of pork was analyzed according to the instruction of the malondialdehyde (MDA) kit (Nanjing Institute of Bioengineering, Nanjing, China) The lipid oxidation of pork was expressed as the concentrations of malondialdehyde (nmol per mg protein) Ket-noi.com Ket-noi.com kho kho tai tai lieu lieu mien mien phi phi Y Xie et al / International Journal of Biological Macromolecules 150 (2020) 480–491 2.8 Statistical analysis Experiments were run in at least three times The experimental data were subjected to analysis of variance, and significant differences between means were analyzed using Duncan's multiple range test (SPSS, Version 20.0, SPSS Inc., Chicago, IL, USA) A p value of b0.05 was used to determine the significance Results and discussion 3.1 Scanning electron microscopy (SEM) The microstructures of both surfaces and cross-sections of the PP films were observed by SEM There were obviously flaky particles on the surface of the PP films with low concentrations of BC (Fig 1A) The particles gradually vanished on the surface with the increase of BC concentrations A smooth and dense structure was observed in the PP film 483 incorporated with 10% BC (Fig 1A), indicating that 10% BC has good compatibility with the PP matrix However, agglomerates occurred again on the surface of the PP films when further addition of BC was up to 15% (Fig 1A) This result was in agreement with the previous report that some agglomerates presented in films for the addition of high concentration of nano-cellulose [36] Actually, there is an optimal dosage of cellulose incorporated into the biocomposite films [14,37,38] As showed in Fig 1A, the surface of the 5% PP films showed a similar change as that of the 3% PP films, but the 5% PP films were more rough than the 3% PP films The cracks of both the 3% and 5% PP films were gradually vanished with increasing of BC (0–10%) (Fig 1B) However, the cracks appeared again at 15% BC (Fig 1B) It seemed that low doses of BC (5% and 10%) properly dispersed in the PP matrix And the interactions between BC and biopolymers in the PP matrix might make the films denser Nevertheless, high doses of BC (15%) induced some agglomerations in the films This indicated that an additional amount of BC to the biopolymer Fig Scanning Electron Microscopy (SEM) images (3000 × magnification) of surface (A) and cross section (B) for the PP films incorporated with and without bacterial cellulose (BC) 484 Y Xie et al / International Journal of Biological Macromolecules 150 (2020) 480–491 matrix is not completely homogeneous These results were in agreement with previous studies [39,40] crystallinity of polymers, while a weaker and wider peak represents the amorphous region of the polymers [41] Fig A shows that pure BC exhibits typical reflection peaks at 2θ = 14.81°, 16.95° and 22.98°, 3.2 X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) which are related to the diffraction of planes 110, 110 and 002, respectively This was coincided with the previous report [18] In our present study, the PP films did not significantly display characteristic peaks of BC, which might be due to the low additions of BC in the PP films Compared to the control PP film (0% BC), the addition The diffractograms of the PP films with and without BC were showed in Fig 2A The sharper and more intense peak means a higher Fig X-ray diffraction (XRD) patterns (A) and Fourier transform infrared spectroscopy (FTIR) (B) of the PP films incorporated with and without bacterial cellulose (BC) Ket-noi.com Ket-noi.com kho kho tai tai lieu lieu mien mien phi phi Y Xie et al / International Journal of Biological Macromolecules 150 (2020) 480–491 Water vapor permeability ×10-4 (g / pa h m) Moisture content of films plays a basic role in the shelf life of packaged or coated materials Table shows the MC of both the 3% and 5% PP films decreased with increasing BC concentrations (0–15%) BC addition decreased the MC of 3% PP films from 0.195 ± 0.028% (0% BC, control) to 0.155 ± 0.007% (15% BC), and that of the 5% PP films from 0.238 ± 0.024% (0% BC, control) to 0.165 ± 0.004% (15% BC) (Table 1) Wang et al have reported that the MC values of films significantly decreased with the increase of BC [16] Abral et al have found that the WVP of the film drops with the addition of bacterial cellulose nanofibers [59] 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 a ab bc ab c c d 0% BC 5% BC 3% PP d 10% BC 15% BC 5% PP A kg / ms The water vapor barrier property of the PP films, indirectly expressed as WVP, is important for food preservation Water can transfer through the films, leading to the deterioration in quality of food products [47] The WVP values of the PP films with or without BC varied between 1.1 × 10−4 g m/Pa h m2 and 1.7 × 10−4 g m/Pa h m2, which was significantly lower than that of the wheat gluten film (2.05 × 1013 kg m/ Pa s m2) [48] and film prepared using powdered BC and poly(vinyl) alcohol (2.7 × 1011 g m/Pa s m2) [49] Hu, Wang, and Wang reported that the WVP values of quaternized chitosan/carboxymethyl cellulose blend films was from 2.95 to 12.08 × 10−6 g/Pa s cm [50] Samadani, Behzad, and Enayati reported that the WVP values of whey protein isolate films containing cellulose nanofibers was from to10 g m/kPa h m2 [51] The PP films in our study have better WVP properties compared the films prepared in the aforementioned studies As showed in Fig 3A, WVP of both the 3% and 5% PP films decreased with the increase of BC concentrations (0–10%) The addition of 10% BC decreased the WVP of the 3% PP films by 35%, and the 5% PP films by 30% compared to the control (Fig 3A) El et al reported that addition of cellulose nanocrystals (CNC) decreased the WVP values of bio-nanocomposit films [52] The interactions between BC and biopolymers in the PP matrix via hydrogen bonding, which lead to decrease of the number of hydrophilic free hydroxyl groups, and thus decrease of WVP of the PP films Moreover, the BC addition formed a more tortuous channel and denser matrix to block moisture transfer through the PP films [53,54] The moisture barrier properties are one of the major functions of films, the relatively high WVP would limit the use of the PP films to low or intermediate moisture foods Further addition of BC (15%) increased the WVP of the PP films Sirviö, Kolehmainen, Liimatainen, Niinimäki, & Hormi (2014) have found that nano-cellulose fibers addition reduces the WVP of the alginate-based films, but excessive nano-cellulose fibers accumulation enhanced the WVP transmission [55] Water vapor transmission through the film is a balance of its hydrophilic/hydrophobic ratio of the matrix, the tortuosity of the channel, and the presence of microstructural defects [56] The WVP of the 5% PP films was similar to that of the 3% PP films (Fig 3A) Food products are prone to be oxidized during processing, transport and preservation The oxygen barrier property of the PP films is also important for maintaining the quality of food products The OP value of the PP films in our study is from 6.2 to 7.0 × 10−3 g/m s Pa, which matches the results of previous studies Liu, Lin, Lopez-Sanchez, and Yang have reported that the OP value of the konjac glucomannan films reinforced 3.4 Moisture content (MC), swelling degree (SG), water solubility (WS), and contact angle (CA) 10-3 3.3 Water vapor permeability (WVP) and oxygen permeability (OP) by bacterial cellulose nanofibers is from 2.50 to 3.60 × 10−3 g/m s Pa [57] Wang et al have reported that the OP value of the chitosan/EGCG films reinforced with nano-bacterial cellulose is from to × 10−3 g/ m s Pa [17] For both the 3% and 5% PP films, addition of BC (0–10%) gradually decreased the OP (Fig 3B) due to a more indirect path for transmission of oxygen molecules The OP of the PP films reached to the lowest level at 10% BC, indicating the best oxygen barrier properties Fazeli, Keley, and Biazar have found that addition of cellulose nanofibers considerably reduce the OP values of the starch-based composite films [58] The formation of hydrogen bonds between the BC and the biopolymers in the PP matrix led to formation of a compact structure of the films Nevertheless, with further addition of BC (15%), the increase of OP values was observed, which was probably due to the formation of BC agglomerates [39] These results were supported by the SEM analysis as well (Fig 1) Oxygen permeability pa of BC (5%, 10%, 15%) did not significantly change the crystallinity of the 5% PP films (5-0, 20.05%; 5-5, 21.34%; 5-10, 22.83%; 5-15, 19.61%) This could be explained by that the PP is rich in cellulose and starch, which have the similar XRD profiles as BC [17,38] The intermolecular interactions between BC and the biopolymers in the PP matrix were characterized by the FTIR spectra A broad peak at 3200–3500 cm−1 was primarily assigned to O\\H stretching [42] Fig shows that the peak of the PP film incorporated with 10% BC (3392 cm−1) shifted to lower wavenumbers compared with the control PP film (0% BC) (3398 cm−1) The shifts could be attributed to the formation of hydrogen bonds between BC and the biopolymers in the PP matrix [17] Miri et al have reported that the broad peak of bionanocomposite films shifts to the lower frequencies due to hydrogen bonding interactions between biopolymers and reinforcing materials [43] The peak at 2870–2960 cm−1 is assigned to C\\H stretching vibration [42] The peak at 1632 cm−1 is primarily assigned to C_O stretching vibration [42] The absorption peak at 1250–1500 cm−1 is attributed to CH2 bending vibrations [44] The peak at 1038 cm−1 is attributed to C\\O stretching vibration in polysaccharides and glycerol [45] All the absorption peaks of the PP films with or without BC showed similar characteristic peaks, indicating that the interactions between BC and biopolymers in the PP matrix are physical but not chemical [46] 485 ab a a bc ab cd 5% BC 10% BC ab d 0% BC 3% PP 15% BC 5% PP B Fig Water vapor permeability (WVP) (A) and oxygen permeability (OP) (B) of the PP films incorporated with and without bacterial cellulose (BC) Results are the mean values ± standard deviation Different letters indicate significant differences (p b 0.05) 486 Y Xie et al / International Journal of Biological Macromolecules 150 (2020) 480–491 The interactions between hydroxyl groups of BC and hydrophilic sites of the biopolymers in the PP matrix, such as carbohydrates and proteins, resulting in a decreasing number of free O\\H groups This might prevent the interactions between hydrophilic group of biopolymers and water molecular This result was in accordance with previous studies [54,60] The SG presents the absorb moisture ability of the films The SG of the 3% PP films dropped from 1.11 ± 0.04% (0% BC, control) to 0.97 ± 0.03% (15% BC), and that of the 5% PP films from 1.12 ± 0.01% (0% BC, control) to 0.94 ± 0.08% (15% BC) (Table 1) These results could also be explained by the aforementioned reasons for changes of MC The amount of swelling of films was significantly reduced by adding nanocrystalline cellulose [61] The functional groups of the polymers in the PP matrix form hydrogen bonding with BC during the film formation process (as showed in FTIR test) and, consequently, their ability to store water molecules has been decreased Pavaloiu, Stoica-Guzun, Stroescu, Jinga, and Dobre have reported that BC acts as a reinforcement agent resulting in the decrease of hydration ability of the films [62] WS was measured to estimate the water resistance of films Table shows that the WS of the PP films gradually decreased with the increase of BC concentrations (Table 1) Wang et al have reported that the WS of films gradually decreased from 21.39% to 18.55% with increasing BC concentrations (0–10%) after 24 h immersion [16] Travalini, Lamsal, Magalhaes, and Demiate have reported that the solubility of the starch films decreased with the incorporation of the lignocellulose nanofibers [63] Salari, Khiabani, Mokarram, Ghanbarzadeh, & Kafil have found that the formation of hydrogen bonds between hydrophilic groups of chitosan and hydroxyl groups of cellulose nanocrystals increases the cohesiveness and decreases the water sensitivity of the films based on chitosan [14] The 5% PP films had higher WS than the 3% PP films, likely due to the hydrophilicity of the carbohydrates and proteins in the PP matrix [2] The relatively higher concentrations of glycerin in the 5% PP films could improve the WS as well Glycerin used as a plasticizer may reduce the interactions between biopolymer molecules, and hence, increases WS of films [16] Low WS of biocomposite films are required for their applications as food packaging The relatively high WS of the PP films containing BC indicates that low or intermediate moisture foods are more suitable for the application of the PP films than high moisture foods The CA was analyzed to investigate the wetability of the PP films High CA indicates high hydrophobicity, and vice versa Table shows all the PP films had relatively lower CA (b65°), indicating a hydrophilicity of the PP films This is likely due to hydrophilic groups of biopolymers in the PP matrix [37] The values of CA decreased with BC addition (Table 1) that might be attributed to the hydrophilic character of BC These results were explained by that the BC is more hydrophilic than biopolymers in the PP matrix Celebi and Kurt have reported that CA values of biocomposite films decreased with the addition of cellulose nanofiller [64] Research by Bahar et al (2012) has showed an increase in hydrophilicity of polypropylene nano-composite films reinforced with cellulose nano-whiskers [65] The 5% PP films had a lower CA than that of the 3% PP films, likely due to more hydrophilic polysaccharides in the 5% PP films 3.5 Thickness and mechanical properties Table shows the thickness and tensile properties of the PP films The average thickness of the PP films was 0.15 ± 0.01 mm The TS value of the PP films in our study was from to 11 MPa was higher than that of PP films (2–9 MPa) in a previous study [3] Samadani, Behzad, and Enayati reported that the TS values of whey protein isolate films containing cellulose nanofibers was from to10 MPa [51] Travalini, Lamsal, Magalhaes, and Demiate reported that the TS values of cassava starch films reinforced with lignocellulose nanofibers was from to MPa [63] The PP films in our study have better TS properties compared the films prepared in the aforementioned studies The tensile strength (TS) of the PP films was obviously increased due to BC addition (Table 2), indicating formation of more resistant and stretchable films The improvement of TS could be due to hydrogen bonding interactions between BC and biopolymers in the PP matrix Previous studies have reported that addition of cellulose nano-whiskers improves the mechanical properties of the biocomposite films [39,66] With increasing BC concentrations, the TS of the 3% PP films increased about 122.4% at 10% BC compared to 0% BC Nevertheless, a drastic decrease in TS was observed with further addition of BC (15%) George and Siddaramaiah have reported that the excess cellulose nanocrystals resulted in a decrease mechanical property of the gelatin-based films [54] Excess cellulose could form agglomeration, which disrupts interactions between biopolymers in the PP matrix, leading to a decrease of TS of the biocomposite films [42] This result was in agreement with the SEM results The EAB is an index of the flexibility or ductility of the films In most cases, the EAB of the film changes conversely to the TS [67] Table shows that the overall EAB of the PP films first decreased due to addition of BC (0–10% BC), and then increased slightly (15% BC) The BC incorporation in the films improved the mechanical properties of the PP films, however, reduced the flexibility or ductility of the films 3.6 Thermogravimetric analysis (TGA) Thermal stability of the PP films was tested using TGA The mass loss (%) and the temperature for maximum degradation (Tmax) of thermal decomposition of the PP films are showed in Fig There were three typical peaks of decomposition in DTG curves (Fig B) The first peak at 80–100 °C was caused by the water evaporation in the PP films [31] The second peak was occurred at 180–210 °C, where the mass loss was ascribed to degradation of starch and glycerol in the PP films [68] The main degradation of the PP films was occurred at around 300 °C, which was due to decomposition of the biopolymers in the PP films, such as hemicellulose and cellulose [68] All the PP films with or without BC showed a similar thermal behaviors in the temperature around 150–300 °C in TGA and DTG curves In other words, BC addition did not significantly improve thermal properties of the PP films, indicating the physical interactions between BC and biopolymers in the PP films This was supported by the results of FTIR spectra analysis (Fig 2B) Cellulose nano-whiskers addition does not change the thermal stability of the chitosan-xylan nanocomposite films [38], and the thermal degradation profile does not change even with higher cellulose nanocrystal contents [69] Wang et al have also reported that cellulose nanocrystals addition did not change the thermal degradation profile of the polylactic acid film according to DTG analysis [70] In addition, the 3% PP films had very similar thermal degradation profile as the 5% PP films 3.7 Film color and optical property Table shows L* value of the 3% PP films was lower than that of the 5% PP films, but the a* of the 3% PP films was higher than the 5% PP films The concentrations of the PP matrix may be the primary reason of differences between the 3% and 5% PP films Although there were some differences in L*, a*, and b* among the PP films with different concentrations of BC (Table 3), it was hard to tell the differences of colors between different films Table shows that ΔE between different films were b1 unit, which was beyond identification capacity of human eyes This could be due to the dark color of the PP matrix Wang et al have demonstrated that BC addition does not significantly change the color of the biocomposite films [17] Yu et al also have reported that cellulose nanofibrils addition (20–60%) did not significantly change the color of corn starch-chitosan films [71] Optical property of the PP films is important for the appearance of food products, and is also important to protect food products from direct illumination A higher opacity indicates a smaller transparency Addition of BC reduced the overall light transmitted through the films It Ket-noi.com Ket-noi.com kho kho tai tai lieu lieu mien mien phi phi Y Xie et al / International Journal of Biological Macromolecules 150 (2020) 480–491 487 Table Moisture content (MC), swelling degree (SG), water solubility (WS), and contact angle (CA) of the PP films with and without bacterial cellulose (BC) PP films MC (%) 3-0 3-5 3-10 3-15 5-0 5-5 5-10 5-15 0.195 0.182 0.171 0.155 0.238 0.207 0.184 0.165 ± ± ± ± ± ± ± ± SG (%) 0.028bc 0.018bcd 0.004cd 0.007d 0.024a 0.008b 0.007bcd 0.004d 1.11 1.05 1.00 0.97 1.12 1.09 1.06 0.94 ± ± ± ± ± ± ± ± WS (%) 0.04a 0.08abc 0.06bcd 0.03cd 0.01a 0.03ab 0.03abc 0.08d 0.335 0.326 0.306 0.302 0.371 0.334 0.322 0.311 ± ± ± ± ± ± ± ± CA (θ°) 0.017b 0.004bc 0.011cd 0.026d 0.008a 0.007b 0.006bcd 0.006bcd 33.43 30.13 28.47 26.77 26.80 24.57 23.20 22.37 ± ± ± ± ± ± ± ± 0.35a 0.06b 0.38c 1.05d 1.05d 0.23e 0.10f 0.15f Results are the mean values ± standard deviation Different letters in the same line indicate significant differences (p b 0.05) was obvious that the PP films became less transparent with increasing BC concentrations (Fig 5A), indicating more efficient in the light barrier A dispersed phase of BC increased the opacity, which could be ascribed to the differences in the refractive index between BC and PP matrix [17,32] The pictures of all PP films are presented in Fig 5B Oxidation of proteins and lipids in food products induced by light is a key factor for food spoilage Thus, great efforts have been made to develop lightbarrier packaging that can prevent photooxidation of foods [70] In this study, it was found that addition of BC enhanced the light barrier ability of the PP films This indicates that incorporating BC can improve light barrier property of biopolymer-based films 3.8 Antioxidant activity of the PP films incorporated with curcumin 3.8.1 Release of curcumin from the PP films into food simulant The polyphenol releasing from the active PP films was measured by alcoholic and fatty food simulants In general, polyphenols releasing from the films could be affected by many factors, such as hydration of matrix, swelling level, the interactions between polyphenols and biopolymers, and release time As showed in Fig 6, the polyphenols releasing from the control film increased up to 0.017 mg gallic acid/g film in alcoholic food simulant after 60 (Fig 6A), while there was almost no change in fat food simulant (Fig 6B) Polyphenols in the skin and cortex of potato are mainly chlorogenic acid, which accounts for 90% of total phenolic content of potato [72] Chlorogenic acid is water solubility but low liposolubility In Fig 6, the curcumin releasing from the PP films gradually increased with increase of curcumin concentrations (0–5%) The curcumin releasing from the films in fat food simulant was significantly higher than that of alcohol mimetics This could be explained by that curcumin is a lipophilic compound with low aqueous solubility 3.8.2 ABTS radical scavenging assay The ABTS radical scavenging activity was analyzed to observe antioxidant activity of the PP films incorporated with curcumin (Fig 6C) Curcumin has a hydrophobic nature, and hence, fatty food simulant was chosen to study the ABTS radical scavenging capacity The PP films without curcumin exhibited an increasing but low antioxidant capacity (Fig 6C) This could be ascribed to the phenolic acids in the PP Table Thickness, Tensile strength (TS), and elongation at break (%) of the PP films with and without bacterial cellulose (BC) PP films Thickness (mm) 3-0 3-5 3-10 3-15 5-0 5-5 5-10 5-15 0.16 0.16 0.16 0.17 0.15 0.14 0.14 0.15 ± ± ± ± ± ± ± ± 0.02bc 0.01bcd 0.01ab 0.02a 0.01de 0.01e 0.01e 0.01cde Fig TG (thermogravimetry, A) and DTG (derivation thermogravimetry, B) curves of the PP films incorporated with and without bacterial cellulose (BC) matrix Addition of curcumin significantly increased antioxidant activities of the PP films [27] The ABTS radical scavenging activity of the PP films was increased gradually from 4.72 ± 0.21% (control) to 20.32 ± 0.28%, 20.42 ± 0.23% and 32.03 ± 0.45% with increasing of curcumin Table Color parameters of the PP films with and without bacterial cellulose (BC) Tensile strength (MPa) Elongation at break (%) PP films L* 5.08 ± 0.57c 6.21 ± 1.12bc 11.30 ± 1.03a 7.07 ± 0.47b 7.52 ± 0.46b 10.01 ± 1.13b 13.87 ± 2.44a 10.04 ± 0.55b 0.16 ± 0.06a 0.15 ± 0.06ab 08 ± 0.03b 0.09 ± 0.01ab 0.14 ± 0.03ab 0.13 ± 0.01ab 0.11 ± 0.03b 0.12 ± 0.04ab 3-0 3-5 3-10 3-15 5-0 5-5 5-10 5-15 34.47 34.54 34.56 34.03 35.12 35.17 35.51 34.84 Results are the mean values ± standard deviation Different letters in the same line indicate significant differences (p b 0.05) a* ± ± ± ± ± ± ± ± 0.19d 0.21d 0.13d 0.14e 0.13b 0.03b 0.08a 0.14c 1.53 1.59 1.53 1.49 1.26 1.13 1.16 1.42 ΔE b* ± ± ± ± ± ± ± ± 0.08ab 0.09a 0.14ab 0.07ab 0.16c 0.06d 0.03cd 0.06b 11.22 11.44 11.60 11.22 11.47 11.40 11.86 11.61 ± ± ± ± ± ± ± ± 0.14c 0.20bc 0.07b 0.21c 0.12b 0.14bc 0.12a 0.21b 66.51 66.47 66.48 66.93 65.90 65.83 65.58 66.20 ± ± ± ± ± ± ± ± 0.19b 0.18b 0.13b 0.11a 0.12d 0.04d 0.07e 0.14c Results are the mean values ± standard deviation Different letters in the same line indicate significant differences (p b 0.05) 488 Y Xie et al / International Journal of Biological Macromolecules 150 (2020) 480–491 Fig Visible absorbance spectra (A) and visual appearance (B) of the PP films incorporated with and without bacterial cellulose (BC) Ket-noi.com Ket-noi.com kho kho tai tai lieu lieu mien mien phi phi Y Xie et al / International Journal of Biological Macromolecules 150 (2020) 480–491 489 concentrations (1, and 5%) The antioxidant capacity of curcumin arises mainly from the phenolic hydroxyl 3.8.3 Evaluation of antioxidant capacity of the PP films on preservation of fresh pork Oxidation has a very important influence on the deterioration of meat, which can result in sensorial degradations in aroma, flavor and the loss of nutritional value The extent of lipid oxidation is vital to evaluate the quality of meat The changes of malondialdehyde (MDA) as a lipid peroxidation marker in the fresh pork are showed in Fig The MDA contents of fresh pork packed with the PP films without curcumin increased from 0.44 ± 0.03 to 1.00 ± 0.01 nm/mg after storage for days at °C Incorporation of 1% and 5% curcumin significantly decreased the MDA contents of fresh pork on day (0.84 ± 0.07 and 0.72 ± 0.05 nm/mg, respectively) (Fig 7A) These results indicated the PP films with curcumin had good antioxidant effects on lipid in fresh pork Under UV illumination, the oxidation of lipids in pork was significantly prevented by the PP films incorporated with curcumin as well (Fig 7B) This indicated that the active PP films with curcumin could protect fresh pork from deterioration induced by illumination Moreover, Table S1 shows that the PP films containing curcumin (1% and 5%) did not greatly change the color, shearing force, taste and pH of fresh pork compared to the PP films without curcumin (0%) However, the color of fresh meat packaged by the PP films did become darker compared to the control samples (Table S1) This could be explained by that the yellow color of the PP can affect the color of the samples Conclusion The results of our present study demonstrated the possibility of using whole potato peel to develop the active PP films incorporated with BC and curcumin BC can interact with biopolymers in the PP matrix primarily through inter- and intra-molecular bonding interactions Consequently, incorporation of BC in the PP films increased mechanical properties of the PP films, and reduced oxygen permeability, water vapor permeability and the light transparency These positive changes indicate that the PP films incorporated with BC are a promising food packaging In particular, BC has the optimum dose in the PP films The PP films incorporated with 10% BC present a more homogeneous and dense structure observed by SEM Addition of curcumin endowed the PP films with good antioxidant capacity The PP films incorporated with curcumin effectively inhibited the lipid oxidation of fresh pork during storage In this way, the active PP films reinforced with BC produces a promising food packaging, preventing lipid oxidation and extending the shelf-life food products, especially the fatty foods Supplementary data to this article can be found online at https://doi org/10.1016/j.ijbiomac.2020.01.291 CRediT authorship contribution statement Yumei Xie: Investigation, Writing - original draft, Visualization, Data curation Xuening Niu: Investigation, Visualization, Data curation Jingwen Yang: Investigation, Visualization Runze Fan: Investigation, Visualization Jiahao Shi: Investigation, Visualization Niamat Ullah: Software, Visualization Xianchao Feng: Conceptualization, Methodology, Formal analysis Lin Chen: Conceptualization, Writing - review & editing, Supervision, Project administration Fig Release of polyphenols from the PP films into the alcoholic food simulant (A) and fatty food simulant (B) and ABTS radical scavenging activity of the active PP films (C) as functions of time 490 Y Xie et al / International Journal of Biological Macromolecules 150 (2020) 480–491 MDA (nmol / mg protein) 1.2 a 1.0 b c 0.8 0.6 d 0.4 0.2 0.0 control 0% curcumin day0 1% curcumin 5% curcumin day7 A MDA (nmol / mg protein) 1.2 a 1.0 0.8 0.6 b b 0.4 b 0.2 0.0 control 0% curcumin 0h 1% curcumin 5% curcumin 24h B Fig The malondialdehyde (MDA) contents of fresh pork after packaged with the active PP films at °C for days (A) or under UV illumination for 24 h (B) Results are 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