Food aroma mass transport properties in renewable hydrophilic polymers

Document downloaded from: This paper must be cited as: The final publication is available at Copyright Additional Information https://dx.doi.org/10.1016/j.foodchem.2011.07.052 http://hdl.handle.net/10251/75317 Elsevier Balaguer, MP.; Gavara Clemente, R.; Hernández Muñoz, P. (2012). Food aroma mass transport properties in renewable hydrophilic polymers. Food Chemistry. 130(4):814-820. doi:10.1016/j.foodchem.2011.07.052. Accepted Manuscript Food aroma mass transport properties in renewable hydrophilic polymers M. Pau Balaguer, Rafael Gavara, Pilar Hernández-Muñoz PII: S0308-8146(11)01013-2 DOI: 10.1016/j.foodchem.2011.07.052 Reference: FOCH 11294 To appear in: Food Chemistry Received Date: 28 January 2011 Revised Date: 11 April 2011 Accepted Date: 14 July 2011 Please cite this article as: Pau Balaguer, M., Gavara, R., Hernández-Muñoz, P., Food aroma mass transport properties in renewable hydrophilic polymers, Food Chemistry (2011), doi: 10.1016/j.foodchem.2011.07.052 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. 1 1 Food aroma mass transport properties in 2 renewable hydrophilic polymers 3 4 M. Pau Balaguer, Rafael Gavara, Pilar Hernández-Muñoz* 5 6 Institute of Agrochemistry and Food Technology (IATA). CSIC. 7 Avda. Agustín Escardino 7, 46980 Paterna, Valencia, Spain. 8 9 Corresponding author 10 Pilar Hernández-Muñoz: Tel: +34 96 3900022 – Fax: +34 96 3636301 11 [email protected].es 12 2 ABSTRACT 13 The sorption and transport properties of gliadin and chitosan films with respect to four 14 representative food aroma components (ethyl caproate, 1-hexanol, 2-nonanone and α-15 pinene) have been studied under dry and wet environmental conditions. The partition 16 coefficients (K) of the selected volatiles were also obtained using isooctane and 17 soybean oil as fatty food simulants. The results showed that gliadin and chitosan films 18 have very low capacities for the sorption of volatile compounds, and these capacities 19 are influenced by the nature of the sorbate, the environmental relative humidity and the 20 presence of glycerol as a plasticizer in the polymeric matrix. The volatile compounds 21 also present a low partitioning in the biopolymer film/food stimulant system. Given the 22 low levels of interaction observed with the volatiles, gliadin and chitosan films are of 23 potential interest for the packaging of foods in which aroma is one of the most 24 important quality attributes. 25 26 Keywords 27 Biopolymer films; gliadins; chitosan; aroma compounds; diffusion coefficient; solubility 28 coefficient; partition coefficient 29 3 1. Introduction 30 Plastics obtained from polysaccharides and proteins are attracting considerable 31 interest in food packaging applications. These biopolymers fulfil the criteria of 32 sustainability since they are extracted directly from renewable resources and their 33 biodegradability is in keeping with environmental protection. Whilst the film forming 34 capacity of these biomacromolecules has been employed in the development of edible 35 coatings to preserve the quality of minimally processed foods, these polymers also can 36 be processed into self-standing plastics for food packaging applications. However, 37 major disadvantages of these polymeric films include their solubility in water and the 38 lack of mechanical strength, especially under wet environments, which limits their 39 application as packaging materials (Woerdeman et al., 2004). Water and mechanical 40 resistance however can be improved by blending with higher performance polymers, 41 incorporating fillers or developing polymer nanocomposites, and efforts are currently 42 being made in this direction (Rhim & Ng, 2007). Finally, many natural biopolymers 43 cannot be melt-processed and although some, such as starch and proteins, are 44 thermoplastics their thermal processing presents certain difficulties (Hernandez-45 Izquierdo & Krochta, 2008). 46 Whilst synthetic polymers dominate the food packaging market, natural polymers can 47 occupy a niche in this area, replacing the former when packaging is required for just 48 short periods (film wrappings, laminated papers, containers for fast food, bags, etc). In 49 these applications, hydrophilic biopolymers present attractive properties such as good 50 oxygen barrier properties at low and intermediate humidities, grease resistance, and 51 aroma barrier (Bordenave, Grelier, Pichavant & Coma, 2007). It is well known that 52 flavour is a key factor in determining food quality and exerts a direct effect on 53 consumer acceptance. The sorption of aroma compounds into a packaging material 54 that is in contact with a food can produce an imbalance in the food’s flavour profile 55 4 thereby deteriorating the sensorial quality of the packaged product, a phenomenon 56 known as flavour scalping. Flavour scalping by plastics in contact with foods, 57 particularly polyethylene and polypropylene, is well-documented in the literature 58 (Sajilata, Savitha, Singhal & Kanetkar, 2007). These polymers are usually employed as 59 interior linings in contact with foods but their olefinic structure endows them with a 60 considerable affinity for apolar compounds. In contrast, hydrophilic polymers can be 61 expected to present low affinities for apolar compounds and hence a reduced tendency 62 to cause flavour scalping. There is, however, little information in the literature regarding 63 this issue. 64 Films made from gliadins, a fraction of wheat gluten soluble in 70% (v/v) ethanol, are 65 glossy, transparent and possess good oxygen barrier properties in low and 66 intermediate relative humidity environments (Hernandez-Munoz, Kanavouras, Ng & 67 Gavara, 2003). Chitosan (poly -(1,4)N-acetyl-D-glucosamine) is a biodegradable 68 natural polymer produced industrially by the chemical deacetylation of chitin, a major 69 component of crab and shrimp shells and the second most abundant biopolymer 70 present in nature after cellulose. Chitosan is soluble in aqueous acidic solutions, 71 becoming a cationic polyelectrolyte with antimicrobial properties. It also possesses 72 excellent film-forming characteristics, with the resulting films demonstrating good 73 mechanical properties and low permeability to oxygen, a property which (as is the case 74 in protein films) is largely dependent on the relative humidity (Clasen, Wilhelms & 75 Kulicke, 2006). 76 Given the favourable film-forming and high oxygen barrier properties of gliadins and 77 chitosan they are biopolymers of potential interest for use as food-contact packaging 78 materials. Although the gas and water vapour barrier properties of these polymers have 79 been extensively analysed, no work has been reported in the literature on their 80 interaction with food aroma components. 81 5 The aim of the current work has been to study the sorption behaviour of different aroma 82 compounds into gliadin and chitosan films. For this purpose four volatile molecules, 83 ethyl caproate, 1-hexanol, 2-nonanone and -pinene were chosen to represent the 84 main chemical families of volatile compounds found in foodstuffs. Due to the 85 hydrophilic nature of these biopolymers, the kinetics of aroma sorption were assayed at 86 room temperature at different relative humidities. The partition coefficients for each 87 volatile between a fatty food model and the film were also studied. 88 89 2. Materials and methods 90 2.1. Materials 91 Crude gluten from wheat (80% protein, 7% fat and 8.1% moisture content on a dry 92 weight basis), high molecular weight chitosan, glycerol, ethanol and glacial acetic acid 93 were all of laboratory grade and obtained from Sigma-Aldrich (USA). The aroma 94 compounds ethyl caproate, 1-hexanol, 2-nonanone and -pinene (each with a 95 minimum purity of 98%) and the fatty food simulants isooctane and soya oil, were also 96 supplied by Sigma-Aldrich (USA). 97 2.2. Film preparation 98 Gliadins were extracted from wheat gluten in 70% ethanol solution as described 99 elsewhere (Hernandez-Munoz et al., 2003) and glycerol was added as a plasticizer to 100 the film-forming solution at 25% (g/100 g dry protein). The use of glycerol was 101 necessary to facilitate film handling at 23 ºC and 50% relative humidity (standard 102 conditions). Chitosan was dissolved in 0.5% (w/w) aqueous acetic acid at a 103 concentration of 1.5% (w/w). The solution was filtered with cheesecloth under vacuum 104 to remove residues of insoluble particles. Polymer solution was poured onto a 105 6 horizontal flat polystyrene tray and dried at 37 ºC. Chitosan films were neutralized with 106 0.1 M NaOH. 107 2.3. Equilibrium distribution of volatile compounds in the film/food simulant system 108 For equilibrium distribution experiments, 20 cm2 of film, previously conditioned in 109 standard conditions, was cut into 4 cm2 squares which were threaded onto a stainless 110 steel wire with alternating glass tube spacers to prevent the films sticking together. The 111 specimens were located in glass vials filled with a solution of isooctane or soybean oil, 112 and the corresponding volatile compound was added at each simulant in a 1% (w/w) 113 concentration. The vials were completely filled with liquid to avoid headspace and 114 hermetically sealed. In order to reach equilibrium samples were stored in the dark at 23 115 ºC for three months. A blank consisting of the aroma solution without film was prepared 116 to control aroma loss caused by degradation or volatility. 117 2.4. Vapour phase sorption of volatile organic compounds in films 118 Dry films were placed in hermetically sealable 250 ml glass jars and conditioned to the 119 desired relative humidity with phosphorus pentoxide (dry environment) or saturated salt 120 solutions of magnesium nitrate (52.9 ± 0.2 RH) and sodium chloride (75.3 ± 0.1 RH); 121 films were allowed to equilibrate for one week at 23 ºC. Thereafter, a 2 ml vial with the 122 corresponding volatile compound was placed in the jar. Equilibrium moisture content of 123 the films was determined by drying moisture-equilibrated samples in a vacuum oven at 124 70 ºC for 24 h. Uptake of the volatile compound into the polymer film was measured at 125 different times until equilibrium was reached. 126 2.5. Analysis of volatiles sorbed in a film 127 The amount of volatile compound sorbed in a film was quantified by thermal desorption 128 using a Dynatherm Thermal Desorber (Supelco Teknokroma, Barcelona, Spain) 129 7 coupled to a Hewlett Packard model P5890 gas chromatograph equipped with a flame 130 ionization detector. A strip of the polymer sample was wiped dry with a tissue and 131 placed in the thermal desorption tube, which was inserted in the desorption oven. 132 Tubes were desorbed for 7 minutes at 140 ºC and helium was used as the carrier gas 133 at a flow rate of 1 ml·min-1. Desorbed compounds were transferred from the desorber 134 oven to an Ultra2 column (25m x 0.2 mm x 0.33 m) through a nickel transfer line 135 maintained at 200 ºC. After desorption, the film sample was recovered and weighed on 136 an analytical balance. The thermal desorption-gas chromatography system was 137 calibrated with polyethylene containing known amounts of the volatile compounds 138 under study (measured independently by gravimetry). 139 2.6. Analysis of volatiles in food simulants 140 The amount of volatile compound in a food simulant was measured by gas 141 chromatography with the same chromatograph described above using a HP-1 column 142 (25 m x 0.53 mm x 2.65 m). 143 2.7. Solubility parameters 144 The solubility parameter of a substance () is defined as the square root of the 145 cohesive energy density (CED) (Hildebrand & Scott, 1949): 146  = (CED)1/2 = (Ecoh / V)1/2 147 where Ecoh is the cohesive energy, and V the molar volume. 148 Hildebrand et al. (1949) correlated the heat of mixing (Hm) in a binary system with the 149 cohesive energy of the components through the equation: 150 Hm = V (1 – 2)2 12 151 14 where is the initial concentration of sorbate in the polymer ( ) and is the 307 concentration of the sorbate in both surfaces of the plane sheet which is assumed to be 308 constant throughout the experiment. 309 The solution under these conditions is (Crank, 1975): 310 311 where is the concentration of the sorbate in the polymer (kg/m3) at time (s), and 312 is the concentration of the sorbate in the polymer at equilibrium (kg/m3), is the 313 thickness of the film (m) and the diffusion coefficient (m2/s). 314 3.2.1. Solubility coefficient 315 The estimated solubility coefficient of volatiles in both glycerol-plasticized and non-316 plasticized gliadin films conditioned at different relative humidities is presented in Table 317 2. Glycerol-plasticized gliadin films showed poor affinities for volatile organic 318 compounds as revealed by the low values obtained for their solubility coefficients. 319 Under each relative humidity condition studied, the solubility coefficients followed the 320 order: 1-hexanol > 2-nonanone > ethyl caproate > -pinene, and this pattern was 321 similar to that found for the liquid fatty food simulant/polymer systems tested. The 322 binding strength was greater for the alcohol 1-hexanol while the hydrocarbon -pinene 323 interacted only weakly with gliadins. Zhou et al. (2006) also reported a decrease in the 324 sorption of volatile compounds in dry soy protein in the following order: 1-hexanol > 325 hexanal > 2-hexanone > ethyl butyrate > hexane. 326 As can be seen in Table 2 when glycerol was not present in the film, the solubility of 327 the volatiles decreased considerably as compared to glycerol-plasticized gliadin films. 328 This behaviour was similar to that previously described in the liquid fatty food 329 simulant/polymer system. Moreover the addition of glycerol promoted a greater 330 15 increase in the solubility of the more polar molecule 1-hexanol compared to ethyl 331 caproate. Thus, glycerol not only plasticizes the film favouring the sorption of organic 332 volatiles, it also increases the affinity of the protein for relatively polar compounds. 333 The effect of relative humidity on the sorption of volatile compounds was only 334 evaluated in gliadins plasticized with glycerol. The composition (wet basis) of gliadin 335 films plasticized with 25% glycerol and when conditioned at 53% RH was 70.2% 336 protein, 17.5% glycerol and 12.3% water, whereas for films kept at 75% RH this was 337 63% protein, 16% glycerol and 21% water. The solubility coefficients of the volatiles 338 were affected by the relative humidity in different manners depending on their polarity. 339 In this regard, humidity had a negative effect on the binding of 1-hexanol to gliadin films 340 plasticized with glycerol. Compared to dry environments, when films are exposed to 341 53% and 75% RH, the solubility coefficient values were reduced by 50% and 85%, 342 respectively. Gliadins present a great affinity to water, which is even increased by the 343 high hygroscopicity of glycerol incorporated to the film. Water molecules interact 344 strongly with the polar groups from proteins and glycerol, and thus reduce the number 345 of free sites for 1-hexanol-film interactions. The decrease in the sorption of 1-hexanol in 346 the hydrated matrix indicates that its retention involves hydrogen bonding and dipole-347 dipole interactions through the hydroxyl group of the alcohol. At 53% RH the hydration 348 of films slightly increased the solubility coefficients of the volatiles ethyl caproate and 2-349 nonanone, whereas the solubility coefficient of the weakly retained volatile -pinene 350 was practically unaffected. Protein hydration confers flexibility to the polypeptide chains 351 and promotes protein conformational changes (Lefèvre et al., 2005) which can modify 352 the binding of volatile compounds depending on their molecular structure and chemical 353 nature. A further increase in the RH to 75% gave rise to lower retention of the four 354 volatiles studied. At high RH, gliadin films containing 25% glycerol are highly 355 plasticized by water imbibed by the protein matrix, which could hinder the accessibility 356 of protein binding sites for interaction with weakly retained compounds. 357 16 Several authors have reported the effect of the degree of protein hydration on the 358 binding of volatile aroma compounds. In this sense Seuvre et al. (2000) examined the 359 importance of beta-lactoglobulin hydration for the binding of 2-nonanone and linalool, 360 and the authors found that the retention of these two flavour molecules did not increase 361 significantly at humidities across the range of 11% to 43%. These volatiles were 362 however, highly retained when the protein was present at 3% in an aqueous solution. 363 Zhou et al. (2006) reported that the weak interactions of the apolar compounds hexane, 364 1-hexene and limonene with soy protein isolate were not affected by the environmental 365 relative humidity tested across the range 0-50%, whereas the sorption of the relatively 366 polar compounds hexanol, trans-2-hexen-1-ol and cis-3-hexen-1-ol tested at high 367 partial vapour pressure increased gradually when the RH was increased from 30% to 368 50%. However at low partial vapour pressures the sorption of the three alcohols 369 decreased in the humidity range 0-50%, suggesting competition for protein binding 370 sites between flavour compounds and water. 371 3.2.2. Diffusion coefficient 372 The diffusion coefficients of volatile compounds in gliadin films are given in Table 2. In 373 general, the diffusion coefficients of volatiles through protein films were found to be 374 much lower than those for conventional films used in contact with foodstuffs 375 (Hernandez-Munoz, Gavara & Hernandez, 1999). 376 The chemical structure and the free volume of a polymer play a major role in the 377 diffusion behaviour of small organic molecules. Increasing the free volume of a polymer 378 by plasticization or swelling is expected to increase diffusion. As can be observed in 379 Table 2, for films kept in a dry environment plasticization of gliadins with 25% glycerol 380 provoked a slight increase in the diffusivity of the volatiles evaluated under these 381 conditions (1-hexanol and ethyl caproate) compared to films without glycerol. The 382 diffusion coefficient experienced a further increase with the incorporation of water in the 383 17 films. At intermediate and high relative humidities, moisture acts as a strong plasticizer 384 in protein films, disrupting hydrogen bonds between polypeptide chain segments giving 385 rise to an increase in chain mobility and free volume for mass transport. 386 The diffusion coefficient is also affected by the shape, size and chemical nature of the 387 diffusing molecule, as shown in Table 2. For the same film and environmental 388 conditions, variations in the diffusivity should be related to the penetrant including 389 molecular geometry, molar volume and penetrant-penetrant and polymer-penetrant 390 specific interactions. The effect of penetrant size and shape on diffusion in a polymer 391 has been the object of numerous studies. Although most of these have been carried 392 out using alkanes, in general, it has been found that for a homologous series of linear 393 molecules diffusivity decreases with increasing molecular weight and molar volume 394 (Kwan, Subramaniam & Ward, 2003), whereas rigid molecules with cyclic and 395 branched geometries are expected to diffuse more slowly compared to flexible and 396 linear ones (Sakellariou & Kapadia, 1996). In the present study, it is difficult to compare 397 the diffusivity of molecules belonging to different series of compounds, and although 398 differences in the diffusion coefficients were not great, the lower diffusion coefficient 399 was obtained for -pinene because of the rigid bicyclic backbone and despite 400 possessing a lower molar volume than ethyl caproate and 2-nonanone. This behaviour 401 was maintained in all the relative humidity environments evaluated. Regarding 402 elongated molecules, the diffusion coefficient could be expected to increase as the 403 molar volume decreases if the functional group of the molecule is not taken into 404 account; however, 2-nonanone presented a lower diffusion coefficient than 1-hexanol 405 whereas ethyl caproate diffusivity was similar to the alcohol. Clustering of 1-hexanol 406 and specific interactions with the film could have decreased its diffusivity in the film. 407 The sorption kinetics of the compounds in the chitosan film could not be accomplished 408 since the experimental method used was not sensitive enough to discern differences in 409 the initial sorption values. Thus, only sorption values at equilibrium were obtained and 410 18 these are shown in Table 2. The results reveal the low affinity of chitosan films for the 411 organic vapours and a preference for alcohols and ketones. The retention of the 412 volatiles studied was greater under humid conditions. The moisture content of the 413 chitosan films at 50% and 75% RH was 16.5% and 22.4% (g water/g dry film) 414 respectively. Since chitosan films were not plasticized with glycerol, the effect of the 415 humidity on the retention of volatiles varies with respect to the effect observed in gliadin 416 films incorporating glycerol. As can be seen in Table 2, the solubility coefficients of the 417 volatiles increased considerably when dry films were conditioned at 75% RH, whilst 418 sorption was not affected at a moderate humidity level of 50%. At low to moderate 419 humidities water molecules are strongly adsorbed as monolayers through specific 420 interactions with the hydroxyl and amine groups of chitosan. At 75% HR the water 421 content in the film exerts a plasticizing effect favouring the sorption of vapours, 422 especially those carrying highly polar functional groups. 423 4. Conclusions 424 The results from this study show that hydrophilic biobased films made from gliadins or 425 chitosan have low sorption capacities for aroma compounds. The extent of sorption 426 depends on several factors including the chemical structure of the volatile organic 427 compound, the film composition, and the degree of film hydration. It has also been 428 shown that aroma compounds have a low partitioning in gliadin and chitosan 429 bioplastics employing soybean oil and isooctane as fatty food simulants. Given the low 430 levels of interaction observed with the organic volatiles studied, gliadins and chitosan 431 show great potential for use in the packaging of foods in which aroma is one of the 432 most important quality attributes. 433 Acknowledgments 434 19 This research has been supported from the Spanish Ministry of Science and Innovation 435 through the projects AGL2006-02176, AGL2009-08776 and FUN-C-FOOD Consolider 436 Ingenio. The authors would like to thank A. P. 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Journal of Agricultural and Food Chemistry, 54(5), 528 1838-1843. 529 530 531 27 558 RESEARCH HIGHLIGHTS 559 560 • Sorption kinetics and equilibrium partitioning of food aroma compounds in 561 bioplastics. 562 • Gliadin and chitosan films show low sorption and partitioning capacities of food 563 aroma compounds. 564 • Sorption and diffusion depend on volatile chemical structure, film composition 565 and moisture. 566 • Great potential in packaging of foods in which aroma is an important quality 567 attribute. 568 569 570 571