Improvement of starch films for food packaging through a three-principle approach: Antioxidants, cross-linking and reinforcement

This study uses sunflower hulls, a by-product from the sunflower snack industry, to recover both, valuable phenolic compounds and cellulose fibers, for the production of antioxidant reinforced starch films as potential food packaging material. The phenolic extract provided antioxidant properties to the films with EC50 values of 89 mg film/mg DPPH.

Carbohydrate Polymers 250 (2020) 116828 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Improvement of starch films for food packaging through a three-principle approach: Antioxidants, cross-linking and reinforcement T Carolin Menzel* Division of Glycoscience, Department of Chemistry, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, AlbaNova University Centre, Stockholm, Sweden ARTICLE INFO ABSTRACT Keywords: Potato starch Citric acid Molecular weight DPPH radical scavenging assay Compression molding Molar mass distribution Crystallinity This study uses sunflower hulls, a by-product from the sunflower snack industry, to recover both, valuable phenolic compounds and cellulose fibers, for the production of antioxidant reinforced starch films as potential food packaging material The phenolic extract provided antioxidant properties to the films with EC50 values of 89 mg film/mg DPPH The cellulose fibers reinforced the starch films with a threefold increase in Young´s modulus Furthermore, citric acid was added to induce cross-linking of the starch polymers and improve film integrity The addition of citric acid induced both, starch polymer hydrolysis and cross-linking, seen in a shift in chain-length distribution after debranching with iso-amylase This is the first study that focuses on a threeprinciple approach to improve edible starch films, and follows UN goals on sustainability to reduce waste and increase value in by-products as a step forward to functionalize packaging material Introduction According to the Plastics Europe Market Research Group, the global plastic production reached about 350 million tonnes in 2017, of which about million tonnes were calculated to reach the ocean with an predicted increase by an order of magnitude by 2025 (Jambeck et al., 2015) Plastics are mainly used for packaging (about 40 %), however, only 50 % of all plastics produced are considered to be disposable and only % are recycled (UNenvironmentprogramme, 2019) During the last two decades, many efforts have been made to find renewable, biodegradable and non-toxic alternatives to the conventional plastics like polyethylene (PE), polypropylene (PP), polystyrene (PS) or polyethylene terephthalate (PET) Natural polymers like proteins, polysaccharides or lipids offer a great potential to replace synthetic plastics Starch is a biopolymer well known for its biocompatibility, degradability, availability and can easily be converted into a thermoplastic material Starch occurs naturally in form of semi-crystalline granules and consists of the two main polymers amylose and amylopectin, which contribute to around 25 % and 75 %, respectively Both macromolecules are built up by chains of α- (1–4) linked D-glucose monomers with molar masses up to around 106 Da for amylose and 109 Da for amylopectin Amylose is considered to be mainly built up by linear glucose chains with only few side branches, whereas amylopectin exhibits (1–6) α-linked branching points that built up a repetitive cluster ⁎ structure (Pérez & Bertoft, 2010) In order to use starch for packaging applications, starch has to be plasticized and/or physico-chemical modified to overcome its hydrophilic character Chemical modification can include etherification, esterification or cross-linking reactions, preferably with food-grade additives in case of food packaging applications Citric acid has been shown to work as excellent plasticizer and cross-linker to improve starch films in terms of their water sensitivity, thermal stability and tensile strength (Menzel et al., 2013; Olsson, Menzel et al., 2013; Seligra, Medina Jaramillo, Famá, & Goyanes, 2016) Hence, citric acid cross-linked starch films are considered suitable food packaging materials Another way to improve starch-based films is the addition of natural fibers as reinforcing component for thermoplastic materials Starch-based films with cellulose fibers showed increased tensile strength and lower water vapor permeability (Müller, Laurindo, & Yamashita, 2009; Wilpiszewska & Czech, 2014) Another approach to improve food packaging include the concept of active packaging, in which an active compound added to the matrix enhances properties such as antioxidant or antimicrobial activity and hence, improve the shelf-life of a product (Valdés, Mellinas, Ramos, Garrigós, & Jiménez, 2014) Especially natural compounds with antibacterial and antioxidant properties gained attention to replace synthetic additives such as butylated hydroxytoluene (BHT) Several studies showed the successful incorporation of antioxidants into starch films (Luchese, Uranga, Spada, Tessaro, & de la Caba, 2018; Menzel, Corresponding author at: KTH Royal Institute of Technology, Roslagstullsbacken 21, SE-10044, Stockholm, Sweden E-mail address: cmenzel@kth.se https://doi.org/10.1016/j.carbpol.2020.116828 Received 21 March 2020; Received in revised form 24 June 2020; Accepted 23 July 2020 Available online 01 August 2020 0144-8617/ © 2020 The Author Published by Elsevier Ltd This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Carbohydrate Polymers 250 (2020) 116828 C Menzel González-Martínez, Chiralt, & Vilaplana, 2019; Menzel, GonzálezMartínez, Vilaplana, Diretto, & Chiralt, 2019) The recovery of active compounds and biopolymers from food waste and agro-industrial by-products is a current target of sustainable functional materials and of much interest in recent research (BenitoGonzález, López-Rubio, & Martínez-Sanz, 2019; Chiralt, Menzel, Hernandez-García, Collazo, & Gonzalez-Martinez, 2020; Galanakis, 2012; Valdés et al., 2014), and in line with the European Commission Horizon 2020 work program on circular economy and the United Nations Sustainable development goals set on food waste reduction (Economic & Affairs, 2018) In this study, three approaches were combined to produce innovative starch films for food packaging with improved properties Firstly, citric acid was used as cross-linking and plasticizing agent in starch films to decrease water sensitivity and improve film integrity Secondly, cellulose fibers were added to reinforce the starch network and increase film strength and decrease water vapor permeability And thirdly, an antioxidant extract was added to enhance the antioxidant properties of the starch films Furthermore, the importance of this study lies in the choice of active compounds and fiber origin Both, the cellulose fibers and antioxidant compounds were extracted from an agroindustrial by-product, sunflower hulls, which is currently regarded as waste and is underutilized Processing conditions that simulate industrial parameters were chosen to produce the films, hence, meltblending and compression-molding were studied Table Film sample abbreviation including film composition [g] of starch films with cellulose fibers (Ce), citric acid (Ca) and antioxidant extract (A) Sample Starch Glycerol Cellulose fibers Ce Ca CeCa CaA CeCaA Ref S Ref A 40 40 40 40 40 40 40 10 10 10 10 10 10 1 Citric acid Antioxidant Extract 3 3 2 treatment and bleaching were repeated two more times until the material was completely white The yield of extracted cellulose fibers was about 27 % by weight The purity was determined after sulfuric acid hydrolysis and glucose determination using HPAEC-PAD analysis of neutral sugars 2.3 Starch film forming process: melt blending and compression molding This study is a continuation of previous work with the aim to further improve the properties of starch films Antioxidant starch films by Menzel, González-Martínez, Chiralt et al (2019) showed low extensibility and remaining high water vapor permeability Hence, citric acid cross-linking and cellulose fiber addition targets to improve these properties Two reference samples from the previous study by the same authors were included in to better interpret and compare the results (Ref A as starch film with antioxidant extract and Ref S with is only starch-glycerol film) For each film, 40 g of potato starch was blended with 10 g glycerol (Table 1) In case of citric acid addition, g crystalline citric acid was blended with 10 g glycerol and then mixed with the starch powder Same procedure was applied to the antioxidant extract, where g of the extract were added directly to the glycerol and then mixed with the starch The cellulose fibers (1 g) were added directly to the starch powder and then blended with glycerol The pre-mixed blends were mixed using a using a Haake PolyLab QC internal mixer (Thermo Fisher Scientific, Germany) and heated to 160 °C for at 50 rpm and torque-time curves were recorded (Supplementary Fig S1) The blend was milled using a mill (Moulinex A320R1, 700 W, France) and then the fine powder was conditioned at 53 % RH at room temperature for days before film molding The compression-molding was carried out according to Menzel, González-Martínez, Chiralt et al (2019) The starch films were stored at 33 % RH at room temperature before further analysis 2.1 Material and chemicals 2.4 Physico-chemical characterization of starch films Sunflower hulls were kindly provided by Grefusa (Alzira, Spain) The hulls were washed, dried and milled A phenolic extract was extracted with aqueous methanol and freeze-dried as described elsewhere (Menzel, González-Martínez, Chiralt et al., 2019) In brief, milled sunflower hulls were extracted with aqueous methanol at room temperature and the extract was freeze-dried All chemicals and reagents were of analytical grade if not further described and purchased from SigmaAldrich (USA) For starch films production, potato starch was used, which was purchased from Roquette (France) and had an amylose content of 27 % Glycerol, sodium carbonate, methanol and ethanol were from purchased from PanReac Quimica (Spain) Iso-amylase was purchased from Megazyme (E.C 3.2.1.68 from Pseudomonas sp., 180 U/ mg) Molar mass and chain length distribution of debranched starch films The molar mass distribution of the starch films dissolved in DMSO/LiBr 0.5 % (w/w) was measured according to Vilaplana and Gilbert (2010) using same size-exclusion parameters The chain length distribution of starch was studied after debranching with iso-amylase About mg starch film was dispersed in 4.5 mL distilled water (1 h in boiling water bath) To the cold dispersion 10 μL 10 ppm sodium azide and 0.5 mL acetate buffer (0.1 M, pH 3.5) was added Iso-amylase (25 μL, 20 U/ ml, h at 37 °C) was added and left at 37 °C for h Afterwards, the solution was precipitated with 25 mL of absolute ethanol and centrifuged to discard the supernatant The precipitant was dissolved in DMSO/LiBr 0.5 % (w/w) for h at around 60 °C and injected to a SEC-MALLS/DRI system and molar mass was measured using a standard calibration with pullulan standards with molecular weight of 342–708,000 Da The degree of polymerization was calculated using the molar mass of anhydroglucose (162 Da) Starch film appearance and microstructure Digital pictures of films as well as field emission scanning electron microscope (FESEM, ZEISS ULTRA 55 model, Germany) images of cross-sections of each film (1.5 kV acceleration voltage) and cellulose fibers Therefore, films were previously dehydrated, cryo-fractured and then gold-coated Thickness of films was measured at six points using a digital electronic micrometer with an accuracy of 0.001 mm (Palmer model COMECTA, Barcelona) Transparency and color Color measurement were carried out using a MINOLTA spectrocolorimeter (Model CM-3600d, Tokyo, Japan) as Material and methods 2.2 Extraction of phenolic extract and cellulose fibers from sunflower hulls The extraction of the phenolic compounds was in accordance to Menzel, González-Martínez, Chiralt et al (2019) Afterwards, the dried and methanol-extracted sunflower hulls were treated with alkaline % H2O2 solution at a sample:solvent ratio of 1:10 (w/v) and heated under stirring to 60 °C for h to extract the hemicellulose and lignin part of the material, which was used in another study The peroxide mixture was filtered under vacuum and washed with distilled water until pH was reached Afterwards, the material was alkali treated with % (w/ v) NaOH solution at a ratio 1:20 (w/v) under reflux for h, and finally bleached using 1.7 % (w/v) NaClO2 under reflux for h The alkali Carbohydrate Polymers 250 (2020) 116828 C Menzel described previously (Menzel, González-Martínez, Chiralt et al., 2019) Three measurements on three films were taken The CIELab color coordinates (illuminant D65, observer 10°) were obtained from the reflectance of an infinitely thick layer of the material (Hutchings, 1999) The transparencies of the films at wavelengths ranging from 400 to 800 nm were investigated Water vapor permeability All films were pre-conditioned at 53 % RH at 23 °C before measurement Water vapor permeability was determined at 53 %–100 % RH gradient according to the ASTM E96 Standard method (cup method), including the corrections for the air gap by Gennadios, Weller, and Gooding (1994) The measurement was carried out in duplicate Moisture content and swelling in water The moisture content of films conditioned at 33 % RH were determined gravimetrically by drying at 60 °C for 48 h using vacuum until constant weight The swelling was measure by immersing a 20 × 20 mm piece of dried film in 15 mL of water for 24 h at room temperature Afterwards, the films were wiped of and weight was measured for the swelling determination Mechanical properties Six to nine replicates of each film formulation were used to determine tensile properties following the ASTM standard method (D882.ASTM D882, 2001) Therefore, the conditioned films were cut into 25 mm x 80 mm pieces, mounted into a Universal testing machine (Stable Micro System TA, XT plus, Haslemere, England) and a stretching of 50 mm/ was applied The stress-strain curves were recorded and tensile strength, elongation at break and Young´s modulus was calculated Thermogravimetric analysis A thermogravimetric analyzer from Mettler Toleda (TGA/SDTA 851e, Switzerland) was used to heat film samples from 25 °C to 600 °C at a heating interval of 10 K/ and to analyze the onset and peak temperature as well as the loss of weight at this point The measurements were done in triplicates X-ray diffraction (XRD) analysis Crystalline structure was characterized by X-ray diffraction (XRD) measurements in air at room temperature The patterns were recorded in the range of and 60 2θ angles with a PANalytical X′Pert PRO diffraction system operated at 40 mA, using CuKα radiation (λ =1.5418 Å) Based on the recorded diffractogram, a simple crystallinity index (Xc) was obtained based on the method described by Hulleman, Kalisvaart, Janssen, Feil, and Vliegenthart (1999) using the following Eq (1): Xc = Hc , Hc + Ha 3.2 Compression molding of films All films were easy to process using melt blending and subsequent compression molding The torque-time curves during the melt-blending of the starch mixtures are shown in supplementary Fig S1 As expected, there was in increase in torque, which is attributed to increasing internal pressure and viscosity Starch granules were fractured and swelled Then the torque decreases which indicates the agglutination of starch as studied in detail by Castaño et al (2017) until a constant molten steady state was reached The addition of citric acid resulted in lower torque, which could be connected to its plasticizing effect but also partial hydrolytic action during the heat and shear treatment as discussed below in detail In contrast, the addition of cellulose fibers increased the maximum torque The changes during the melt-blending have not been studied in detail for the here described starch blends and, hence, could help in future to optimize the thermo-plasticization process in future applications and industrial up-scaling 3.3 Appearance of films and optical properties All starch formulations formed homogenous, transparent and consistent films as shown in the digital images in Fig The thermocompression conditions were suitable to produce alone-standing films with a thickness of 0.15−0.19 mm (Table 2), which decreased when citric acid was added The addition of antioxidant extract resulted in slightly yellowbrownish colored films as seen in higher a* and b* color coordinates (supplementary Table S1), and slightly lower internal transmittance (lower Ti values, Table 2) The formation of color is due to the light absorbance of phenolic compounds in the antioxidant extract The addition of citric acid did not negatively affect the transmittance of the films, but rather seemed to improve it (Ca film with Ti of 85.8 %) compared to starch reference films with only glycerol (Ref S of 83.7 %) Higher values of Ti indicate greater homogeneity The incorporation of cellulose fibers had no negative effect on the optical properties 3.4 Molar mass and chain length distribution using SEC Results of the molar mass distribution using SEC-MALLS/DRI are given in Table as weight-average molecular weight Mw, numberaverage molecular weight Mn and polydispersity D In general, the Mw decreased drastically in starch films when citric acid was added, which has been reported previously (Menzel et al., 2013; Shi et al., 2007) The low Mw in all films indicates extensive hydrolysis of glycosidic bonds in starch Furthermore, starch films were debranched using iso-amylase to determine changes in the chain-length distribution of the starch films during processing Fig showed the typical bimodal distribution of the reference starch film (Ref S) with amylopectin branches at DP 26 and 48, as well as the amylose branches at DP > 100 Starch-glycerol films, cellulose fiber and antioxidant extract incorporation slightly changed the profile towards shorter chains of amylose, but did not affect the amylopectin bimodal distribution (Menzel, González-Martínez, Chiralt et al., 2019) However, citric acid incorporation into the starch films significantly affected the starch structure and showed a monomodal distribution of chains (Ca, CeCa, CaA, CeCaA in Fig 3), i.e the bimodal branch chain length distribution of amylopectin was not detectable but shifted towards higher chain length with DP 69–78 and a shoulder at DP 180 In addition, the amylose peak seemed to vanish or shift completely Citric acid is known to hydrolyze glycosidic bonds in starch films when prepared by melt blending (Shi et al., 2007), but at the same time form ester bonds with starch If starch had only been hydrolysed, part of the branch chain-length population would have moved towards lower values, however, longer chains with a maximum DP at around 75 indicate that intermolecular cross-linkages of starch by citric acid were formed Iso-amylase cleaves specifically at the α- (1-6) glycosidic linkages in starch Hence, chains with a DP 70 might be a result of two (1) where Hc and Ha are the intensities for the crystalline and amorphous profiles with typical baselines at a value of 2Ɵ between 17° and 18° (Supplementary Fig S2) In-vitro antioxidant activity of starch films A DPPH radical assay was used to determine the antioxidant activity of the films according to Menzel, González-Martínez, Chiralt et al (2019) In brief, films were dispersed in water for at least 12 h and a filtered aliquot of the solution was used for the DPPH* assay Absorbance was read at 515 nm using a spectrophotometer (Evolution 201 VisibleUV, ThermoScientific, Germany) The measurements were carried out in triplicates The results were expressed as efficient concentration of antioxidant in the raw material to decrease the initial DPPH* concentration by 50 % (EC50, mg film/ mg DPPH*) Results and discussion 3.1 Extraction of cellulose fibers and characterization One target of the study was to recover cellulose as biopolymer from the agro-industrial by-product sunflower hulls The extraction of cellulose fibers using alkaline extraction and bleaching was successful and resulted in a yield of 27 % and a purity of 89 % Cellulose fibers were white and characterized by FESEM and particle size distribution as shown in Fig Carbohydrate Polymers 250 (2020) 116828 C Menzel Fig FESEM image of extracted cellulose fibers after alkaline extraction and bleaching Digital image of cellulose material in upper right corner and particle size distribution of cellulose fibers in lower right corner adjacent branches being esterified by the tricarboxylic acid The shoulder peak with DP 180 was expected to originate from long chain amylose hydrolyzed by the competing acid action of citric acid during the thermal processing of melt blending and compression molding as has been reported before (Menzel et al., 2013; Shi et al., 2007) In addition, starch films with only added citric acid showed lowest branch chain lengths compared to starch-citric acid films with cellulose fibers and antioxidant extract due to the dilution factor and probably steric hindrance showing very low crystallinity due to the complete disruption and gelatinization of starch granules during film processing and the storage conditions at 33 % RH (van Soest, Hulleman, de Wit, & Vliegenthart, 1996) Only slight signals of typical B-type and V-type crystalline structure were observed The small amounts of detected Vh-type structures (Ref S and Ref A) are linked to amylose crystallization into single helical structure Interestingly, starch-glycerol films with antioxidant showed predominant Vh-type crystallinity, which suggested that the phenolic compounds of the antioxidant extract might favored crystallization of amylose into single helical structure The formation of B-type crystallinity originates from the recrystallisation of amylose and the amyloseinduced recrystallisation of amylopectin by co-crystallizing into the same B-type lattice (Hulleman et al., 1999) The B-type crystallinity indexes Xc for values of 2Ɵ between 17° and 18° are between 0.46 and 0.03 (Table 3) The B-type crystallinity is known to be strongly dependent on the processing (temperature, water content), as well as storage conditions (relative humidity, time, temperature), which finally influences and correlates with the stress-strain behavior of thermoplastic starch films Typical stress-strain curves of the starch films are shown in Fig and tensile strength, elongation at break and Young´s modulus parameters are given in Table As expected, the reference starch-glycerol film (Ref S) showed plastic behavior until fracture (Fig 4) The mechanical properties were affected by the different film formulations and crystallinity In general, a high crystallinity index showed an increase in Young´s modulus and tensile strength, which can be explained by physical cross-links by amylose and amylopectin co-crystallization that reduce intermolecular interactions and cause cracking The incorporation of cellulose fibers resulted in reinforced starch films with resistance to high stress (1086 MPa) However, these films were quite brittle, 3.5 Antioxidant activity of films by DPPH radical scavenging assay The antioxidant extract from sunflower hulls has been described in detail in a previous study and chlorogenic acid was identified as the main active compound (Menzel, González-Martínez, Chiralt et al., 2019) The calculated EC50 values using DPPH radical scavenging assay of these films are given in Table for the films with antioxidant extract and citric acid (CaA) and cellulose fibers (CeCaA) As expected, the phenolic compounds in the extract showed radical scavenging activity, which was higher compared to the reference (Ref A) Citric acid significantly enhanced the antioxidant properties of the films seen as lowering of EC50 values by a third The exact action and migration of citric acid in combination with the phenolic compound to protect food against oxidation will have to be studied in more detail However, these films showed superior activity and, hence, are considered a valuable source for future food contact applications 3.6 Crystallinity and mechanical properties The crystallinity in native granular potato starch is of the B-type (data not shown) The diffractograms of all films are given in the Fig 3, Fig FESEM cross-sections (upper row) of starch films and digital images below (a – Ce films, b – Ca film, c – CeCa film, d – CaA film, e – CeCaA film) Carbohydrate Polymers 250 (2020) 116828 C Menzel Table Film thickness, internal transmittance Ti at 500 nm, moisture content, swelling in water, water vapor permeability WVP and EC50 values of starch films Sample Thickness [mm] Ti (500 nm) Moisture [g/100 g] Swelling [%] WVP [gmm/kPahm2] EC50 values [mg/mg DPPH] Ce Ca CeCa CaA CeCaA Ref S Ref A 0.149 0.153 0.199 0.136 0.155 0.188 0.181 0.826 0.858 0.837 0.730 0.752 0.837 0.473 5.91 ± 0.31 4.66 ± 0.05 5.07 ± 0.07 4.99 ± 0.10 5.28 ± 0.14 6.03 ± 0.01 6.04 ± 0.08 327 ± 50 95 ± 14 87 ± 14 114 ± 1.4 121 ± 0.2 145 ± 11 375 ± 28 11.8 ± 1.26 10.1 ± 0.62 11.3 ± 1.03 11.0 ± 0.66 9.2 ± 0.43 9.95 ± 1.90 11.5 ± 0.54 – – – 89.6 88.6 – 122 Table Molar mass distribution (weight-average molecular weight Mw, number-average molecular weight Mn and polydispersity D), crystallinity index Xc, Young´s modulus, tensile strength and elongation at break of all starch films Sample Ce Ca CeCa CaA CeCaA Ref S Ref A Molecular weight [x 104 g/mol] Crystallinity index Xc Mw Mn D 187 ± 7.10 0.85 ± 0.10 1.41 ± 0.26 2.56 ± 0.40 0.44 ± 0.23 357 188 102 ± 11.6 0.12 ± 0.04 0.16 ± 0.01 0.32 ± 0.06 0.29 ± 0.01 236 118 1.8 6.9 8.5 8.1 1.5 1.4 1.6 0.40 0.24 0.28 0.46 0.23 0.17 0.03 Young´s modulus Tensile strength Elongation at break [MPa] [MPa] [%] 1086 ± 157 147 ± 21.3 145 ± 38.1 424 ± 93.7 170 ± 42.9 55 ± 20 441 ± 134 7.50 ± 2.19 3.09 ± 1.10 2.18 ± 1.45 4.83 ± 1.03 3.43 ± 0.86 4.73 ± 1.11 8.24 ± 1.54 0.82 ± 0.17 22.1 ± 8.98 16.6 ± 5.10 1.72 ± 0.93 11.2 ± 9.01 26.0 ± 5.95 4.23 ± 3.82 which can be linked to starch-cellulose interactions decreasing starch chain mobility (Avérous, Fringant, & Moro, 2001) In addition, starch films with incorporated antioxidant extract (CaA, CeCaA, Ref A) showed high Young´s modulus and high tensile strength but lower extensibility The internal microstructure might block dislocation motion due to interactions of the phenolic compounds with the starch chains and hence, the results in more brittle material The addition of citric acid (Ca, CeCa, CeCaA) had different effects on the mechanical properties of the starch films As expected, the addition of citric acid decreased the extensibility but resulted in stronger films compared to starch-glycerol films (Ref S) in terms of resistance to stress seen as higher Young´s modulus (Fig 4) Chemical cross-links between starch chains and citric acid could have resulted in a reinforced network but leads to cracking of the material However, at the same time films with cellulose fibers, citric acid and antioxidant extract show favorable properties in terms of good extensibility and high tensile strength Citric acid has been shown to work as plasticizer and cross-linker in starch films as well as acid hydrolyzing glycosidic bonds causing lower molecular weight, which seemed not to significantly affect the mechanical properties (Menzel et al., 2013; Olsson, Menzel et al., 2013) Fig SEC weight distribution of debranched starches as function of their degree of polymerization (DP) Fig Diffractograms of starch films Carbohydrate Polymers 250 (2020) 116828 C Menzel Table Thermogravimetric analysis results with onset and peak temperature and weight loss Sample Ce Ca CeCa CaA CeCaA Ref S Ref A Onset Peak Weight loss [%] [°C] [°C] < 150 °C 150 °C- onset Until 350 °C At 550 °C 276 ± 1.00 273 ± 0.41 275 ± 0.78 271 ± 1.47 273 ± 0.10 279 ± 0.32 264 ± 1.25 309 ± 1.8 308 ± 0.2 311 ± 1.2 315 ± 0.8 317 ± 0.2 305 ± 0.88 309 ± 0.51 5.2 3.2 5.0 4.6 3.0 3.5 3.5 18.3 20.8 25.6 24.3 26.1 17.6 16.8 77.9 76.4 74.5 70.3 74.5 77.9 71.2 86.3 84.7 84.2 80.0 83.8 83.6 79.3 3.7 Thermal behavior of starch films an increase in branch-chain length of citric acid cross-linked starch has been reported These films are perfect candidates as coating materials to prevent lipid oxidation of food like nuts or cereals In addition, the study showed the successful utilization of sunflower hulls as agro-industrial waste to gain added-value products in terms of active compounds and fibers to produce green and edible food packaging Thermal stability of films was determined using thermal gravimetric analysis The weight loss as function of temperature is shown in the supplementary (Fig S2) with four distinct zones of weight loss First, temperature zone < 150 °C, which corresponds to the evaporation of water and volatile compounds of the antioxidant extract As expected, starch films with antioxidant extract showed higher weight loss in that area (Table 4) The second zone until the onset of starch degradation (Table 4, weight loss 150 °C until onset at ∼270 °C) represents the weight loss due to the degradation of glycerol and citric acid, which decompose above 148 °C (Barbooti & Al-Sammerrai, 1986) All starch films with citric acid showed higher weight loss up to ∼270 °C The third degradation zone from onset temperature to 350 °C is typical for the decomposition of starch The fourth zone above 350 °C, represents the residual weight of starch films after decomposition, which differed due to the higher ash content from the antioxidant extract (Menzel, González-Martínez, Chiralt et al., 2019) The thermal properties of starch films were not negatively affected by the addition of citric acid Author contribution The author, Carolin Menzel, wrote the project, which was financially covered by the Swedish research council by a personal grant to the same person All analysis and interpretation including figures and tables were carried out by the author The manuscript was entirely written and revised by the author Acknowledgement This work was supported by the Swedish Research Council Formas [2015-00550] 3.8 Barrier properties and swelling Appendix A Supplementary data Changes in water vapor permeability of all films were not significantly affected and were between 9.2 and 11.8 gmm/kPahm2, which is expected for starch-glycerol films (Menzel, González-Martínez, Chiralt et al., 2019) Previous reports showed that water vapor permeability was improved in solution cast starch films with added citric acid (Olsson, Hedenqvist, Johansson, & Järnström, 2013) However, the film processing by melt-blending and compression molding did not improve film properties in the same way, which has to be studied in further detail The swelling of starch film in water is given in Table and calculated as weight increase The glycerol-starch films were able to swell slightly (145 %) The incorporation of citric acid in all film formulations showed a decrease in swelling, which indicates a higher integrity of the films through ester bond and cross-links between starch The increase in swelling of films with cellulose fibers was expected due to the swelling capability of the fibers However, the reference film containing only antioxidant extract and glycerol showed largest degree of swelling (375 %), which was unexpected and has to the best of our knowledge not been reported yet Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2020.116828 References Avérous, L., Fringant, C., & Moro, L (2001) Plasticized starch–cellulose interactions in polysaccharide composites Polymer, 42(15), 6565–6572 Barbooti, M M., & Al-Sammerrai, D A (1986) Thermal decomposition of citric acid Thermochimica Acta, 98, 119–126 Benito-González, I., López-Rubio, A., & Martínez-Sanz, M (2019) High-performance starch biocomposites with celullose from waste biomass: Film properties and retrogradation behaviour 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González-Martínez, Chiralt et al., 2019) The calculated EC50 values using DPPH radical scavenging assay of these films are given in Table for the films with antioxidant extract and citric acid (CaA) and. .. typical baselines at a value of 2Ɵ between 17° and 18° (Supplementary Fig S2) In-vitro antioxidant activity of starch films A DPPH radical assay was used to determine the antioxidant activity of