- Email: [email protected]
Flavor scalping by polyethylene sealants
Food Packaging and Shelf Life 21 (2019) 100371
Contents lists available at ScienceDirect
Food Packaging and Shelf Life journal homepage: www.elsevier.com/locate/fpsl
Flavor scalping by polyethylene sealants a,⁎
a
a
T b
a
An Adams , Stefan van Bloois , Bart Otte , Ester Caro , Dibyaranjan Mekap , Peter Sandkuehler a b c
c
Dow Benelux B.V., Herbert H. Dowweg 5, 4542 NM Terneuzen, the Netherlands Dow Chemical Iberica S.L., Autovia Tarragona - Salou s/n, Spain Dow Europe GmbH, Bachtobelstrasse 3, 8810 Horgen, Switzerland
A R T I C LE I N FO
A B S T R A C T
Keywords: Flavor scalping Polyethylene sealants Food packaging interaction
Flavor scalping, i.e. the sorption of food aroma compounds by packaging materials, can lead to a loss of product quality and potential damage to the package. In this study, the flavor scalping performance of 14 different Linear Low Density Polyethylene sealant resins was comparatively evaluated in stand-up pouch applications. In sunflower oil as the matrix, no significant flavor scalping was observed. In aqueous model systems clear indications of flavor scalping were observed for limonene, decanal and 2(E)-nonenal. The degree of scalping of a specific flavorant was found to be inversely related with the water solubility. Comparing the flavor scalping data obtained for the different sealant resins in aqueous model systems showed significantly higher scalping for the sealants with the lowest density. Differences between the tested sealant resins are limited, and are not expected to generate sensorial differentiation between the sealant resins, in the density range (0.902 – 0.919 g/ml) evaluated.
1. Introduction The quality and shelf life of packaged food products and beverages depend strongly on the type of packaging and on the interactions between food components and packaging materials. The interaction of materials used for food packaging with food components is an important quality criterion for food packaging. Different phenomena describe the various possible ways of this interaction (Fig. 1). Flavor scalping, defined as the sorption of food aroma compounds by packaging materials, can lead to a loss of product quality by decreasing the global aroma intensity or by generating an unbalanced flavour profile. In addition, flavor scalping can result in damage to the package, for instance as a result of the plasticizer activity of specific sorbed flavor compounds. Many literature studies report the sorption of flavor compounds from solutions by plastic materials [Sajilata, Savitha, Singhai, & Kanetkar, 2007;Van Willige, Linssen, Meinders, Van der Stege, & Voragen, 2002; Van Willige, Schoolmeester, van Ooij, Linssen, & Voragen, 2002]. As polyethylene (PE) is the most common plastic on the inner surface of food-contact polymer containers and has a high lipophilicity, the degree of flavor scalping is a relevant quality criterion for PE food packaging materials [Sajilata et al., 2007; Nielsen & Jägerstad, 1994]. In model systems designed to study flavor scalping, limonene is typically included and is an appropriate model flavor compound to
⁎
discuss some of the most important literature findings [Sajilata et al., 2007;Nielsen & Jägerstad, 1994]. Limonene is an unsaturated hydrocarbon terpene, ubiquitous in citrus flavor and a very common flavorant. Due to its nonpolar properties, limonene has a high affinity for many polymeric packaging materials, such as PE. Literature observations from diverse flavor scalping studies allow the following conclusions:
• The sorption of limonene from model solutions or orange juice in•
• •
Corresponding author. E-mail address: [email protected] (A. Adams).
https://doi.org/10.1016/j.fpsl.2019.100371 Received 3 May 2019; Received in revised form 11 July 2019; Accepted 16 July 2019 Available online 23 July 2019 2214-2894/ © 2019 Elsevier Ltd. All rights reserved.
creases with temperature. In the case of scalping from the vapor phase this can be different [Van Willige, Linssen, Meinders et al., 2002; Van Willige, Schoolmeester et al., 2002]. Scalping of limonene by linear low density polyethylene (LLDPE) depends on the matrix, and is more pronounced from model solutions than from orange juice. Juice pulp particles hold flavor compounds in equilibrium with the watery phase resulting in a decrease in sorption of these compounds by plastics. Flavor compounds can be dissolved, absorbed, bound, entrapped, encapsulated or diffusion limited by the presence of a variety of food constituents [Van Willige, Linssen, & Voragen, 2000]. The sorption of limonene increases with increasing film thickness (increasing amount of polymer available) [Cava, Catala, Gavara, & Lagaron, 2004]. Limonene scalping depends on the density of the polymer, e.g.
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 1. Possible interactions between foodstuff, polymer film, and the environment, and their adverse consequences [adapted from Sajilata et al., 2007].
•
scalping by LLDPE has been observed to be more pronounced than by HDPE [Charara, Williams, Schmidt, & Marshall, 1992]. The extent of limonene sorption can be reduced by increasing the crystallinity of the polymer (biaxially drawn films). Crystalline regions can reduce sorption or act as impermeable barriers for diffusion through the polymer [Ikegami, Nagashima, Shimoda, Tanaka, & Osajima, 1991].
Table 1 Overview and labeling of different PE grade materials.
The impact of limonene sorption has been shown to be of minor importance for the sensory properties of the food product in different studies [Van Willige, Linssen, Legger-Huysman, & Voragen, 2003]. As limonene can act as a precursor for off-flavor compounds in orange juice, such as α-terpineol, scalping of limonene has sometimes been indicated as beneficial for the overall sensory properties of the product. However, diverse possible adverse effects on the packaging material have been reported, such as swelling of LLDPE films leading to a higher oxygen permeability, a decrease in modulus of elasticity and tensile strength, and delamination of multilayer packages [Van Willige, Linssen, Meinders et al., 2002; Van Willige, Schoolmeester et al., 2002]. Various studies have compared the flavor scalping performance of different polymers used for plastic packaging, but differences within LLDPE sealants with varying properties have not yet been investigated. For this study, a comparative evaluation was performed of the flavor scalping performance of a series of selected PE sealants from different suppliers and with different properties. The variety of sealants were applied as part of a multilayer structure pouch representing a real application, i.e. a Polyethylene Terephthalate (PET)/adhesive/PE/PE/PE sealant. The intent is to show whether any of the varying parameters of the PE sealant layer in this application influence the flavor scalping performance relevant for food packaging purposes.
Sample Code
Performance Segment
Density (g/ml)
GP 1 GP 2 GP 3 GP 4 GP 5 GP 6 GP 7 MHP MHP MHP MHP MHP MHP MHP
General Purpose General Purpose General Purpose General Purpose General Purpose General Purpose General Purpose Medium & High Performance Medium & High Performance Medium & High Performance Medium & High Performance Medium & High Performance Medium & High Performance Medium & High Performance
0.919 0.918 0.918 0.918 0.917 0.917 0.916 0.913 0.912 0.912 0.912 0.909 0.902 0.902
1 2 3 4 5 6 7
2.2. Method optimization A storage temperature of 30 °C was selected to perform the flavor scalping experiments as a “worst case scenario” for food storage conditions. This temperature is somewhat higher than general room temperature, but can occur in specific regional conditions. Flavor scalping processes are expected to be delayed at lower temperatures [Van Willige, Linssen, Meinders et al., 2002; Van Willige, Schoolmeester et al., 2002]. For optimization of the HS-SPME procedure, an extraction temperature of 30 °C was chosen for the water solutions to be consistent with the storage temperature. For the oil samples, the extraction temperature was increased to 60 °C to enhance the partitioning of the flavor compounds to the fiber. For extraction of the different flavor compounds under study, a 50/30 μm divinylbenzene/carboxen on polydimethylsiloxane fiber (DVB/Car/PDMS) was selected, as it is suitable for a broad range of analytes as shown in previous experience and literature reports [Keršiene, Adams, Dubra, De Kimpe, & Leskauskaite, 2008]. Standard solutions of the target flavor compounds varying in concentration between 2.5 and 150 ppm were analyzed to determine the ranges of sensitivity and solubility, taking into account realistic values for in-use applications. It was decided not to use an emulsifier or co-solvent to enhance the solubilization of poorly soluble compounds, as this may interfere with the flavor scalping processes under study. For each data point, three different pouches were used, and from each pouch two different samples were taken for SPME analysis. Thus, each data point in the graphs below represents the average of six measurements. The relative standard deviation (RSD) on the SPME measurements for the different flavor compounds and sample points ranged between 0.1 and maximum 13%, with an average of 3.4%. Refined sunflower oil was used as the oil matrix of choice, as this is a relatively odor-free oil. The headspace composition of the oil was evaluated before analysis and compared with olive oil. In refined sunflower oil, some volatiles were detected at very low trace levels
2. Material and methods 2.1. Samples An overview of the LLDPE sealants is shown in Table 1. Samples were delivered to the laboratory as empty sealed Stand-Up Pouches (SUP). The sealants were applied as part of a multilayer structure pouch representing a true application (PET/adhesive/PE/PE/PE sealant). Polyethylene samples were selected from different suppliers with differences in:
• production process (solution/gas phase); • catalyst (metallocene/Ziegler-Natta); • melt index; • additive package; • density.
2
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Table 2 Overview of flavor compounds used in the different experiments. Compound
CAS#
Supplier
Min purity (%)
Concentration range (vol ppm)
Ethyl butyrate 2,5-Dimethylpyrazine (R)-(+)-Limonene Linalool Eugenol 2(E)-Nonenal 6-Methyl-5-hepten-2-one (MHO) Octanal Decanal
105-54-4 123-32-0 5989-27-5 78-70-6 97-53-0 18829-56-6 110-93-0 124-13-0 112-31-2
Sigma-Aldrich Sigma-Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich
99 98 97 97 99 97 99 99 98
25 - 100 25 - 100 2.5 - 10 25 - 100 25 - 100 25 - 100 25 - 100 25 - 100 2.5 - 10
the inner surface was dried carefully by gently tapping with a lint-free paper towel. After this drying step, 50 ml of dichloromethane, containing 5 ppm (vol) of internal standard n-decane, was poured into the bag. The top opening was again folded 3 times and further closed with a paperclip. The bags were put into a bag holder to ensure close contact of the solvent with the inner SUP layer and placed onto a shaking device, where they were shaken for 1 h, at approx. 200 rpm and room temperature. The samples were then analyzed by means of liquid injection-GCFID. The GC column had a DB-1701 stationary phase (30 m x 0.32 mm x 1 μm) and was heated in the oven at 80 °C, followed by heating to 180 °C at 25 °C/min and to 260 °C at 20 °C/min with 7 min hold. Analysis was performed by FID detection. A 1 μl aliquot was automatically injected at a 1/20 split ratio. Carrier gas was He at constant pressure of 52.8 kPa.
(nonanal, undecanal), but this was significantly lower than olive oil, and not interfering with the experiments performed. Levels of oxidation products were monitored carefully during the course of the experiments to exclude interaction with the volatiles under study. For the solvent extraction procedure, including an evaporation step was evaluated. However, as losses of the most volatile flavor compounds cannot be excluded, and the sensitivity of the optimized GC method was sufficient, the evaporation step was omitted in the final experimental procedure. It must be noted that the deviation on the solvent extraction data was quite high, with a relative standard deviation (RSD) varying between 1 and 47% (n = 3).
2.3. Procedure for aqueous flavor solutions To prepare the stock solution of the relevant compounds (Table 2), a mixture of the different pure flavor compounds was prepared in the appropriate ratios and mixed well. An aliquot of this flavor mixture was then transferred to a large volume of milliQ water in a 5 L Erlenmeyer flask and stirred intensively on a magnetic stirrer overnight. A small corner was cut from the sealed bags with a clean pair of scissors and 250 ml of the stock solution was then transferred into the sample bag with a funnel and sealed by folding the opening 3 times and placing a metal paperclip on the opening. The bags were then placed in a thermostated oven with a constant temperature of 30 °C. After a preset time, 3 replicate bags were taken from the oven for each sample point. The solution was poured out of the sample bag and transferred to a 250-ml glass beaker. The solution was stirred for 2 min at 250 rpm, and 5 ml of the solution was transferred into a Gerstel SPME vial and capped (Septum: Butyl red/PTFE grey, 55° shore A for Seal size 20 mm). The samples were then analyzed by means of Headspace Solid-Phase Microextraction - Gas Chromatography - Flame Ionization Detection (HS-SPME-GC-FID) (Agilent GC 7890). A Mass Selective detector (Agilent 5975C MSD) was used to identify unknowns. As SPME fiber a DVB/Car/PDMS (Divinylbenzene/Carboxen/ Polydimethylsiloxane) fiber (Supelco U-57329; 24 gauge; 50/30 μm; 1 cm length) was used. Samples were incubated for 5 min at 30 °C for water and 60 °C for oil with agitation (Agitator on 10 s 250 rpm; off 2 s), extracted for 20 min at the same temperature and desorbed in the inlet at 250 °C for 1 min. SPME sampling and extraction was all automated using an MPS autosampler (Gerstel). The GC column had a DB-1701 stationary phase (30 m x 0.32 mm x 1 μm) and was heated in the oven at 50 °C, followed by heating to 120 °C at 5 °C/min and to 260 °C at 20 °C/min with 5 min hold. Analysis was performed by FID detection. Carrier gas was He at constant pressure of 52.8 kPa. For each data point, 3 different pouches were used, and from each pouch 2 different samples were taken for SPME analysis. Thus, each data point in the graphs represents the average of 6 measurements. As a control, glass 50-ml vials were used. These were filled with 50 ml of solution, closed with a septum covered on the inside with aluminum foil, and treated the same way as the SUP pouches. After emptying the bag, the bag was cut open on the top side, and
2.4. Procedure for flavor in oil solutions Refined sunflower oil (from the local supermarket Jumbo) was used as an oil matrix. For these experiments, the flavor in oil mixture was prepared directly in the pouches: 200 ml of oil was poured into a measuring cylinder and 140 μl of the flavor scalping mix was added. The flavor scalping mix was prepared with equal volumetric parts (e.g. 2 ml) of the compounds of interest. Samples were treated as the aqueous model systems discussed above.
2.5. Overview of the different experiments discussed Table 3 shows the sealants, flavor compounds and sampling scheduling applied for the different experiments that are discussed in this report.
2.6. Statistical data evaluation In a first step, the Brown-Forsythe test was used to test for equal variances of the data obtained for the different flavorants. In most cases, the p values were greater than 0.05 so we concluded that there were no significant differences between the group variances. In such cases, analysis of variance (ANOVA) method was used to test for differences between the means. In case differences in variance were concluded, Welch’s t-test was applied. When ANOVA results show that there is at least one significant difference (p < 0.05) among the different resins for the interaction with all flavorants, Tukey’s HSD test was used as a follow-up test to compare means. JMP Pro 12 software was used to perform these calculations.
3
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Table 3 Overview of the different experiments discussed. Experiment number
Sealants
Flavor compounds
Time of experiment (days)
Number of sampling points
1
MHP 3 MHP 4 MHP 5
28
7
2
GP 1 GP 3 GP 4 MHP 1 MHP 6
14
6
3
GP 2 GP 6 MHP 2 MHP 7
14
6
4
GP 1 GP 2 GP 3 GP 4 GP 6 MHP MHP MHP MHP GP 5 GP 7 GP 8
100 ppm ethyl butyrate 100 ppm 2,5-dimethylpyrazine 100 ppm linalool 100 ppm eugenol 10 ppm limonene 25 ppm ethyl butyrate 25 ppm 2,5-dimethylpyrazine 25 ppm 6-methyl-5-hepten-2-one 25 ppm linalool 25 ppm 2(E)-nonenal 25 ppm eugenol 2.5 ppm limonene 2.5 ppm decanal 25 ppm ethyl butyrate 25 ppm 2,5-dimethylpyrazine 25 ppm 6-methyl-5-hepten-2-one 25 ppm linalool 25 ppm 2(E)-nonenal 25 ppm eugenol 2.5 ppm limonene 2.5 ppm decanal 25 ppm ethyl butyrate 25 ppm 2,5-dimethylpyrazine 25 ppm 6-methyl-5-hepten-2-one 25 ppm linalool 25 ppm 2(E)-nonenal 25 ppm eugenol 2.5 ppm limonene 2.5 ppm decanal
14
1
28
6
5
1 2 6 7
100 ppm 100 ppm 100 ppm 100 ppm 100 ppm 100 ppm 100 ppm
ethyl butyrate 2,5-dimethylpyrazine linalool eugenol limonene octanal 2(E)-nonenal
3. Results and discussion 3.1. Selection of flavor compounds and model systems For the purpose of this work, a general model system was selected to evaluate flavor scalping in Stand-Up Pouches (SUPs). Regarding the matrix, two different food-simulating liquids were selected: (1) water and (2) a food-grade oil (100% refined sunflower oil). A variety of foodrelevant flavor compounds were selected, with different chemical functionalities to enable the assessment of the influence of chemical structure on potential flavor scalping effects. The flavor compounds selected are shown in Scheme 1:
• a hydrocarbon monoterpene (R)-(+)-limonene (1), • a terpene alcohol linalool (2), • an aromatic alcohol eugenol (3), • a nitrogen-containing heterocyclic compound 2,5-dimethylpyrazine (4), • an aliphatic ester ethyl butyrate (5), • two ketones 6-methyl-5-hepten-2-one (6) and 2-nonanone (8), • two aliphatic aldehydes octanal (7) and decanal (10), • an α,β-unsaturated aldehyde 2(E)-nonenal (9).
Scheme 1. Structures of the different flavor compounds used in this study.
illustrate the conclusions and trends that were discovered. The degree of flavor scalping by the SUP pouches from different LLDPE resins was evaluated by two analytical methods:
Table 4 lists the odor characteristics, and some relevant physical constants of these model flavor compounds. Multiple experiments were performed, comparing from three up to nine different sealants in one experiment, in an aqueous or sunflower oil model system, for a period of time ranging from two weeks up to eight weeks. Only a selection of experiments is discussed here to
(1) At regular time intervals, the model solutions in the pouches were analyzed by means of Headspace Solid-Phase Microextraction Gas Chromatography Flame Ionization Detection (HS-SPME-GC-FID) to monitor the possible decrease in concentration of scalped 4
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Table 4 Overview and properties of flavor compounds used in the flavor scalping experiments1. Flavorant
Odor
Average max ppm in food (safety in use FEMA GRAS2)
Vapor pressure (mm Hg at 25 °C)
Log P (o/w)
Water solubility (mg/l @ 25 °C)
(R)-(+)-Limonene 1 Linalool 2
Citrus orange fresh sweet Citrus, floral, sweet, bois de rose, woody, green, blueberry Sweet, spicy, clove, woody Nutty, peanut, musty, earthy, powdery and slightly roasted with a cocoa powder nuance Sweet, fruity, tutti-frutti Citrus, green, musty, lemongrass, apple Aldehydic, waxy, citrus, orange with a green peely nuance Fresh, sweet, green weedy, earthy, herbal Fatty, green, waxy, cucumber, melon Sweet, aldehydic, waxy, orange peel, citrus, floral
31 - 2,300 2.0 - 90
1.5 0.1
4.57 2.97
13.8 1,590
0.6 - 2,000 10
0.0 4.0
2.27 0.63
2,460 31,970
28 - 1,400 1.1 - 1.3 0.1 - 6.1
13.9 1.3 2.1
1.80 1.95 2.95
4,900 3,351 560
0.1 - 4.0 0.2 0.6 – 6.6
0.6 0.3 0.2
3.14 3.32 3.97
371 205 43.5
Eugenol 3 2,5-Dimethylpyrazine (DMP) 4 Ethyl butyrate 5 6-Methyl-5-hepten-2-one (MHO) 6 Octanal 7 2-Nonanone 8 2-(E)-Nonenal 9 Decanal 10
1 Data extracted from (The Good Scents Company Information System 2019) (http://www.thegoodscentscompany.com/) and Chemspider (2019) (http://www. chemspider.com/). 2 All flavorants have been determined to be ‘generally recognized as safe’ (GRAS) under conditions of intended use as flavor ingredients by the FEMA (Flavor and Extracts Manufacturers Association of the United States) Expert Panel.
flavorants. (2) The inside of the emptied pouches was extracted with dichloromethane to detect the sorption of flavorants by the internal LLDPE layer.
control vials. However, SPME peak areas are the result of a complex equilibrium and partitioning of flavor compounds between the sample liquid phase, sample headspace, and SPME fiber surface. Therefore, SPME competition effects can play a role as well, when changing concentrations of scalped flavorants can influence sorption of others. Percentages higher than 100% as sometimes observed are the result of the complex interplay between the different processes occurring simultaneously. For the same experiment, the results of the solvent extraction of the emptied pouches at different time intervals are shown in Fig. 3. From the glass control vials, no detectable amounts of flavor compounds were extracted. In all cases, amounts of flavor compounds extracted from the SUPs were significantly higher than from the control, but no significant differences were observed between the different resins. There is a general increase of the amounts extracted during the course of the experiments, reaching a plateau near the end of the experiment. The amounts extracted after the full length of the experiment (28 days) can be considered as a measure for the degree of flavor scalping. Table 5 shows a clear correlation with the water solubility of the flavor compound: the lower the water solubility, the more a flavorant is retained by the lipophilic LLDPE layer and consequently, the higher is the relative amount extracted. Flavor scalping decreased in the following order: limonene > eugenol > linalool > ethyl butyrate > 2,5-dimethylpyrazine. As discussed before, significant amounts of limonene were lost due to evaporation. Varying losses of flavor compounds during the experiment are due to differences in volatility. Table 5 also indicates a higher deviation on the data with increased volatility, which can be explained by losses from evaporation.
Results of both analytical methods are discussed below, for the aqueous model solutions and oil media, respectively. 3.2. Results flavor scalping aqueous solutions: experiment 1 For experiment 1, Stand-Up Pouches prepared with three different PE sealants of the medium/high performance segment (i.e. MHP 3, MHP 4 and MHP 5) were filled with an aqueous model system containing 5 different flavor compounds (ethyl butyrate, 2,5-dimethylpyrazine, linalool, eugenol at 100 ppm, and limonene at 10 ppm), and kept for 28 days at 30 °C. Changes in the headspace flavor concentrations with time were measured by means of HS-SPME-GC-FID, and are shown in Fig. 2, displayed as percentage of the initial concentration at initial time t0. For most flavorants, i.e. 2,5-dimethylpyrazine, linalool, and eugenol, the compound concentration in the pouches did not change significantly over time, and was not significantly different from the control. Therefore, no flavor scalping can be concluded and no significant differentiation between the different sealant resins can be shown. On the contrary, flavor scalping was clearly observed for limonene for all three resins. From the first sampling points onwards (t > 0), only very low levels of limonene were observed in the aqueous solutions. The almost instantaneous sorption of limonene by LDPE is known in literature [Sajilata et al., 2007], and is the result of the apolar nature of limonene leading to its fast partitioning from the polar water phase to the apolar internal LLDPE layer of the pouches. However, in the sealed control vials the concentration of limonene decreased rapidly as well. As solvent extraction of the emptied vials afterwards did not recover significant amounts of limonene (cf. discussion below), losses from the control vials are mostly due to the evaporation to the headspace, resulting from the poor water solubility and high volatility of limonene (Table 4). Comparing the data obtained with the glass control vials with the results of the SUPs demonstrates that the losses of limonene are due to a combination of evaporation and scalping. A quantitative differentiation between both effects cannot be made based on the present data. In case of ethyl butyrate, the flavor concentration in the SUPs was higher than in the glass control vials. One hypothesis to explain this effect would be the selective evaporation of ethyl butyrate from the
3.3. Results flavor scalping aqueous solutions: SPME experiment 2 As scalping was observed only for limonene, additional compounds with a wider range of water solubility and log P values (Table 4) were included in the aqueous model system for the following experiment to investigate flavor scalping behavior further. Five different PE sealant layers were included in experiment 2. Eight flavor compounds were added to an aqueous model system at lower concentration to ensure complete solubilization, i.e. 25 ppm of ethyl butyrate, 2,5-dimethylpyrazine, 6-methyl-5-hepten-2-one, linalool, 2(E)-nonenal, eugenol, and 2.5 ppm of limonene and decanal. Six sampling points were collected over a period of 14 days. For most flavor compounds, the concentration did not decrease significantly during the course of the experiment, and was in some cases higher than in the glass control vials (data not shown). Therefore, no scalping could be concluded for these 5
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 2. Changes in concentration, expressed as % of initial headspace-SPME-GC-FID peak areas, for the different flavor compounds and PE sealants in an aqueous model system, experiment 1.
significantly from aqueous solutions by all five sealant resins tested in this experiment. Profiles of these scalped flavorants are shown in Fig. 4. The final SPME-GC-FID peak areas for the eight flavor compounds and five sealants are compared statistically by means of ANOVA in Table 6.
compounds. Only for limonene, 2(E)-nonenal and decanal the concentration in the SUP was significantly lower than in the glass control vials during the complete course of the experiment. These data demonstrate that limonene, 2(E)-nonenal and decanal were scalped 6
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 3. Amounts of flavorants extracted from the emptied pouches (mg) at different time intervals for the different flavor compounds and PE sealants in an aqueous model system, experiment 1.
The flavorant concentration reached an equilibrium after several days and this final concentration was lower for limonene as compared to decanal and 2(E)-nonenal, respectively. Comparison of the final
concentrations for the different resins showed no significant differences for limonene, which was scalped almost completely. For decanal and especially 2(E)-nonenal, differences are observed for the five sealant
Table 5 Overview of amounts extracted from SUP after 28 days for experiment 1: average and RSD (n = 3) in comparison with physical constants of flavor compounds. Flavorant
Average amount extracted (% of initial amount)
RSD (%)
Pvap (mm Hg @ 25 °C)
Log P (o/w)
Water solubility (mg/l @ 25 °C)
2,5-dimethylpyrazine Ethyl butyrate Eugenol Linalool Limonene
0.1 2.1 6.6 4.4 14.0
19.8 23.1 9.0 13.8 17.8
4.0 13.9 0 0.1 1.5
0.63 1.80 2.27 2.97 4.57
31,970 4,900 2,460 1,590 13.8
7
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 4. Changes in headspace-SPME-GC-FID peak areas (% of initial solution) for the different flavor compounds and PE sealants in an aqueous model system, experiment 2.
between the different sealant resins were noted. For 2(E)-nonenal scalping was intermediate and allows some differentiation. As it is difficult and ambiguous to compare data obtained in two different experiments, an additional experiment, experiment 4, was set up to evaluate nine PE sealant resins in one experiment. Due to the large amounts of SUPs required to do this (three replicates for each sample point) only one measurement was performed, after 14 days of storage. The HS-SPME-GC-FID data from this experiment are gathered in Fig. 5. The results confirm what was found in experiments 2 and 3: flavor scalping was observed for limonene, 2(E)-nonenal and decanal, and not for the other flavor compounds. Statistical comparison of the scalping performance of all the different sealants showed differentiation only for 2(E)-nonenal scalping which was most pronounced for the sealants with lowest density: MHP 6 (d = 0.902) and MHP 7 (d = 0.902). This corresponds with literature findings: the amount of flavor compounds sorbed in polymers has been shown to decrease with an increase in polymer density [Sajilata et al., 2007]. To complement the SPME data, the amounts of flavorant extracted from the inner surface of the emptied pouches, expressed as percentage of the initial amount, are shown in Table 7 for experiments 2, 3 and 4. The value shown is an average of all sealant SUPs. No significant differences between the nine sealants were noted. These data show that
Table 6 Statistical evaluation of day 14 SPME-GC-FID data of the scalped flavor compounds and LLDPE sealants in experiment 2 by means of ANOVA (Tukey’s HSD test) – Connecting letters report. Resin
Density (g/ml)
limonene
2(E)-nonenal
decanal
GP 4 GP 1 GP 3 MHP 1 MHP 6 Glass control
0.918 0.919 0.918 0.913 0.902
B B B B B A
B BC BC C D A
B C C C C A
Different letters indicate statistically significant differences.
resins evaluated in this experiment. Similar results were obtained from experiment 3, evaluating scalping of the same flavorants by a different set of 4 PE resins (data not shown). The scalping of limonene, 2(E)-nonenal and decanal from the aqueous solution took place for all sealant resins evaluated in experiment 3. Differentiation of the different sealants was evaluated by statistical comparison of all HS-SPME-GC-FID peak areas at the end of the experiment (day 14) by means of ANOVA analysis (Tukey’s honest significance difference, or HSD test). For limonene and decanal, scalping was clear and fast for all resins and no significant differences 8
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 5. HS-SPME-GC-FID measurements of experiment 4 showing the % of flavorant scalped from aqueous model solutions after 14 days of storage for nine sealant resins (* indicates significant difference α 0.05).
100%). In most cases, the data show a relatively stable concentration of the flavor compounds in the oil during the experiments, and no flavor scalping was observed for all flavor compounds evaluated, even after eight weeks. A selection of graphs is shown in Fig. 7. The absence of flavor scalping in the oil model system can be due to the lower polarity difference between the matrix and the sealant, associated with a lower driving force for repartitioning [Van Willige, Linssen, Meinders et al., 2002; Van Willige, Schoolmeester et al., 2002]. The oil itself can be absorbed by the polyethylene sealant layer, thus making it very difficult to separate flavor absorption from oil absorption, with flavor compounds dissolved in it [Nielsen, 1992]. For all flavorants, the amount extracted from the emptied pouches by solvent extraction is about 0.03 – 0.08 mg. At a concentration of 100 ppm, this corresponds to about 0.3 – 0.8 ml of oil, which may very well correspond to the volume of oil remaining in the pouches. Considering this, it was concluded that solvent extraction of emptied pouches is not an appropriate technique to evaluate flavor scalping from oil media by LLDPE resins. In all additional experiments performed with the oil model system, no significant scalping was observed for any of the flavor compounds in the model, and for any of the LLDPE sealant resins evaluated, even after a prolonged experiment of eight weeks. Therefore, no further testing was performed with this model system.
Table 7 Overview of average amounts of flavor compounds recovered from the inside of the SUPs by extraction after 14 days for three experiments with aqueous model systems. Flavorant1
Amount (% of initial amount added) extracted from SUP after 14 days
Experiment
2
3
4
2,5-dimethylpyrazine Ethyl butyrate MHO Linalool 2-Nonenal Decanal Limonene
nd 2.6 3.4 4.7 28.1 49.5 28.1
nd 1.5 2.2 3.6 20.1 38.2 16.0
nd 2.1 3.3 4.9 26.7 38.0 14.0
Pvap (mm Hg @ 25 °C)
Log P (o/ w)
Water solubility (mg/l @ 25 °C)
4.0 13.9 1.3 0.1 0.3 0.2 1.5
0.63 1.80 1.95 2.97 3.32 3.97 4.57
31,970 4,900 3,351 1,590 205 43.5 13.8
the degree of flavor scalping was inversely proportional to the water solubility of the flavorants, illustrated in Fig. 6. A lower water solubility/high log P is associated with a higher affinity for the lipophilic LLDPE layer and a higher degree of scalping. These data correspond with literature findings indicating that compounds with lower polarity are absorbed more by nonpolar polyolefins [Sajilata et al., 2007]. Losses of flavorants during the experiment are due to evaporation. This explains, for instance, that amounts of limonene recovered from the SUPs were lower than expected, although the scalping was almost complete.
4. Conclusions From the data discussed above, the following conclusions can be drawn:
• Flavor scalping performance was comparatively evaluated for 14
3.4. Results flavor scalping oil model system: experiment 5 Besides the properties of the flavorant and the polymer, the composition of the food product also plays an important role in influencing the sorption of flavor compounds. Therefore, also sunflower oil was evaluated as a model matrix to examine flavor scalping. Because of better solubility of the flavorants in the oil, no evaporation was observed from the control vials, and results are expressed as percentage of flavorant in the pouches as compared to the control vial (referenced at
• •
polyethylene sealant resins in Stand-Up pouches for an aqueous and a sunflower oil model system. In the aqueous model solutions, clear flavor scalping was observed for limonene, decanal and 2(E)-nonenal. The degree of scalping of a specific flavorant was inversely related with the water solubility: a lower water solubility/higher log P was associated with a higher affinity for the lipophilic polyethylene layer
Fig. 6. Inverse relationship between amounts extracted and water solubility for the different flavor compounds (average of 4 experiments, each multiple PE sealants), aqueous model systems. 9
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 7. Changes in headspace-SPME-GC-FID peak areas (% of initial solution) for the different flavor compounds and PE sealants in an oil model system, experiment 5.
and a higher degree of scalping.
Funding sources
lower degree of scalping for general purpose as compared to medium and high performance sealants, which can be explained by differences in density/crystallinity. In case sunflower oil was used as the matrix, the polarity difference with the lipophilic sealant layer was much smaller, and no significant flavor scalping was observed, even after a period of 8 weeks. Flavor scalping by stand-up pouches made of laminates with polyethylene sealant resins does occur, and depends on the lipophilicity of the flavorant. Differences between various LLDPE sealant resins were noted but are limited, and are not expected to generate sensorial differentiation between the sealant resins, in the density range (0.902 – 0.919 g/ml) evaluated.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
• The data obtained for flavorants tested in aqueous solutions show a • • •
Declaration of Competing Interest The authors are all affiliated to the Dow Chemical Company, with no declaration of interest.
Acknowledgments The authors would like to thank Shayne Green and Dave Albers for valuable discussions, and Edwin Mes for his support.
10
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Sajilata, M. G., Savitha, K., Singhai, R. S., & Kanetkar, V. R. (2007). Scalping of flavors in packaged foods. Comprehensive Reviews in Food Science and Food Safety, 6, 17–35. The Good Scents Company Information System, http://www.thegoodscentscompany. com/ Accessed July 8th, 2019. Van Willige, R. W. G., Linssen, J. P. H., & Voragen, A. G. J. (2000). Influence of food matrix on absorption of flavor compounds by linear low-density polyethylene: Proteins and carbohydrates. Journal of the Science of Food and Agriculture, 80, 1779–1789. Van Willige, R. W. G., Linssen, J. P. H., Legger-Huysman, A., & Voragen, A. G. J. (2003). Influence of flavor absorption by food-packaging materials (low-density polyethylene, polycarbonate and polyethylene terephthalate) on taste perception of a model solution and orange juice. Food Additives and Contaminants, 20, 84–91. Van Willige, R. W. G., Linssen, J. P. H., Meinders, M. B. J., Van der Stege, H. J., & Voragen, A. G. J. (2002). Influence of flavor absorption on oxygen permeation through LDPE, PP, PC and PET plastics food packaging. Food Additives and Contaminants, 19, 303–313. Van Willige, R. W. G., Schoolmeester, D., van Ooij, A., Linssen, J. P. H., & Voragen, A. G. J. (2002). Influence of storage time and temperature on absorption of flavor compounds from solutions by plastic packaging materials. Journal of Food Science, 67(6), 2023–2031.
References Cava, D., Catala, R., Gavara, R., & Lagaron, J. M. (2004). Testing limonene diffusion through food contact polyethylene by FT-IR spectroscopy: Film thickness, permeant concentration and outer medium effects. Polymer Testing, 24, 483–489. Charara, Z. N., Williams, J. W., Schmidt, R. H., & Marshall, M. R. (1992). Orange flavor absorption into various polymeric packaging materials. Journal of Food Science, 57(963-966), 972. Chemspider, Royal Society of Chemistry, http://www.chemspider.com/ Accessed July 8th, 2019. Ikegami, T., Nagashima, K., Shimoda, M., Tanaka, Y., & Osajima, Y. (1991). Sorption of volatile compounds in aqueous solution by ethylene-vinyl alcohol copolymer films. Journal of Food Science, 56, 500. Keršiene, M., Adams, A., Dubra, A., De Kimpe, N., & Leskauskaite, D. (2008). Interactions between flavour release and rheological properties in model custard desserts: Effect of starch concentration and milk fat. Food Chemistry, 108, 1183–1191. Nielsen, T. (1992). Study of factors affecting the absorption of aroma compounds into low-density polyethylene. Journal of the Science of Food and Agriculture, 60, 377–381. Nielsen, T., & Jägerstad, M. (1994). Flavour scalping by food packaging. Trends in Food Science & Technology, 5, 353–356.
11
Contents lists available at ScienceDirect
Food Packaging and Shelf Life journal homepage: www.elsevier.com/locate/fpsl
Flavor scalping by polyethylene sealants a,⁎
a
a
T b
a
An Adams , Stefan van Bloois , Bart Otte , Ester Caro , Dibyaranjan Mekap , Peter Sandkuehler a b c
c
Dow Benelux B.V., Herbert H. Dowweg 5, 4542 NM Terneuzen, the Netherlands Dow Chemical Iberica S.L., Autovia Tarragona - Salou s/n, Spain Dow Europe GmbH, Bachtobelstrasse 3, 8810 Horgen, Switzerland
A R T I C LE I N FO
A B S T R A C T
Keywords: Flavor scalping Polyethylene sealants Food packaging interaction
Flavor scalping, i.e. the sorption of food aroma compounds by packaging materials, can lead to a loss of product quality and potential damage to the package. In this study, the flavor scalping performance of 14 different Linear Low Density Polyethylene sealant resins was comparatively evaluated in stand-up pouch applications. In sunflower oil as the matrix, no significant flavor scalping was observed. In aqueous model systems clear indications of flavor scalping were observed for limonene, decanal and 2(E)-nonenal. The degree of scalping of a specific flavorant was found to be inversely related with the water solubility. Comparing the flavor scalping data obtained for the different sealant resins in aqueous model systems showed significantly higher scalping for the sealants with the lowest density. Differences between the tested sealant resins are limited, and are not expected to generate sensorial differentiation between the sealant resins, in the density range (0.902 – 0.919 g/ml) evaluated.
1. Introduction The quality and shelf life of packaged food products and beverages depend strongly on the type of packaging and on the interactions between food components and packaging materials. The interaction of materials used for food packaging with food components is an important quality criterion for food packaging. Different phenomena describe the various possible ways of this interaction (Fig. 1). Flavor scalping, defined as the sorption of food aroma compounds by packaging materials, can lead to a loss of product quality by decreasing the global aroma intensity or by generating an unbalanced flavour profile. In addition, flavor scalping can result in damage to the package, for instance as a result of the plasticizer activity of specific sorbed flavor compounds. Many literature studies report the sorption of flavor compounds from solutions by plastic materials [Sajilata, Savitha, Singhai, & Kanetkar, 2007;Van Willige, Linssen, Meinders, Van der Stege, & Voragen, 2002; Van Willige, Schoolmeester, van Ooij, Linssen, & Voragen, 2002]. As polyethylene (PE) is the most common plastic on the inner surface of food-contact polymer containers and has a high lipophilicity, the degree of flavor scalping is a relevant quality criterion for PE food packaging materials [Sajilata et al., 2007; Nielsen & Jägerstad, 1994]. In model systems designed to study flavor scalping, limonene is typically included and is an appropriate model flavor compound to
⁎
discuss some of the most important literature findings [Sajilata et al., 2007;Nielsen & Jägerstad, 1994]. Limonene is an unsaturated hydrocarbon terpene, ubiquitous in citrus flavor and a very common flavorant. Due to its nonpolar properties, limonene has a high affinity for many polymeric packaging materials, such as PE. Literature observations from diverse flavor scalping studies allow the following conclusions:
• The sorption of limonene from model solutions or orange juice in•
• •
Corresponding author. E-mail address: [email protected] (A. Adams).
https://doi.org/10.1016/j.fpsl.2019.100371 Received 3 May 2019; Received in revised form 11 July 2019; Accepted 16 July 2019 Available online 23 July 2019 2214-2894/ © 2019 Elsevier Ltd. All rights reserved.
creases with temperature. In the case of scalping from the vapor phase this can be different [Van Willige, Linssen, Meinders et al., 2002; Van Willige, Schoolmeester et al., 2002]. Scalping of limonene by linear low density polyethylene (LLDPE) depends on the matrix, and is more pronounced from model solutions than from orange juice. Juice pulp particles hold flavor compounds in equilibrium with the watery phase resulting in a decrease in sorption of these compounds by plastics. Flavor compounds can be dissolved, absorbed, bound, entrapped, encapsulated or diffusion limited by the presence of a variety of food constituents [Van Willige, Linssen, & Voragen, 2000]. The sorption of limonene increases with increasing film thickness (increasing amount of polymer available) [Cava, Catala, Gavara, & Lagaron, 2004]. Limonene scalping depends on the density of the polymer, e.g.
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 1. Possible interactions between foodstuff, polymer film, and the environment, and their adverse consequences [adapted from Sajilata et al., 2007].
•
scalping by LLDPE has been observed to be more pronounced than by HDPE [Charara, Williams, Schmidt, & Marshall, 1992]. The extent of limonene sorption can be reduced by increasing the crystallinity of the polymer (biaxially drawn films). Crystalline regions can reduce sorption or act as impermeable barriers for diffusion through the polymer [Ikegami, Nagashima, Shimoda, Tanaka, & Osajima, 1991].
Table 1 Overview and labeling of different PE grade materials.
The impact of limonene sorption has been shown to be of minor importance for the sensory properties of the food product in different studies [Van Willige, Linssen, Legger-Huysman, & Voragen, 2003]. As limonene can act as a precursor for off-flavor compounds in orange juice, such as α-terpineol, scalping of limonene has sometimes been indicated as beneficial for the overall sensory properties of the product. However, diverse possible adverse effects on the packaging material have been reported, such as swelling of LLDPE films leading to a higher oxygen permeability, a decrease in modulus of elasticity and tensile strength, and delamination of multilayer packages [Van Willige, Linssen, Meinders et al., 2002; Van Willige, Schoolmeester et al., 2002]. Various studies have compared the flavor scalping performance of different polymers used for plastic packaging, but differences within LLDPE sealants with varying properties have not yet been investigated. For this study, a comparative evaluation was performed of the flavor scalping performance of a series of selected PE sealants from different suppliers and with different properties. The variety of sealants were applied as part of a multilayer structure pouch representing a real application, i.e. a Polyethylene Terephthalate (PET)/adhesive/PE/PE/PE sealant. The intent is to show whether any of the varying parameters of the PE sealant layer in this application influence the flavor scalping performance relevant for food packaging purposes.
Sample Code
Performance Segment
Density (g/ml)
GP 1 GP 2 GP 3 GP 4 GP 5 GP 6 GP 7 MHP MHP MHP MHP MHP MHP MHP
General Purpose General Purpose General Purpose General Purpose General Purpose General Purpose General Purpose Medium & High Performance Medium & High Performance Medium & High Performance Medium & High Performance Medium & High Performance Medium & High Performance Medium & High Performance
0.919 0.918 0.918 0.918 0.917 0.917 0.916 0.913 0.912 0.912 0.912 0.909 0.902 0.902
1 2 3 4 5 6 7
2.2. Method optimization A storage temperature of 30 °C was selected to perform the flavor scalping experiments as a “worst case scenario” for food storage conditions. This temperature is somewhat higher than general room temperature, but can occur in specific regional conditions. Flavor scalping processes are expected to be delayed at lower temperatures [Van Willige, Linssen, Meinders et al., 2002; Van Willige, Schoolmeester et al., 2002]. For optimization of the HS-SPME procedure, an extraction temperature of 30 °C was chosen for the water solutions to be consistent with the storage temperature. For the oil samples, the extraction temperature was increased to 60 °C to enhance the partitioning of the flavor compounds to the fiber. For extraction of the different flavor compounds under study, a 50/30 μm divinylbenzene/carboxen on polydimethylsiloxane fiber (DVB/Car/PDMS) was selected, as it is suitable for a broad range of analytes as shown in previous experience and literature reports [Keršiene, Adams, Dubra, De Kimpe, & Leskauskaite, 2008]. Standard solutions of the target flavor compounds varying in concentration between 2.5 and 150 ppm were analyzed to determine the ranges of sensitivity and solubility, taking into account realistic values for in-use applications. It was decided not to use an emulsifier or co-solvent to enhance the solubilization of poorly soluble compounds, as this may interfere with the flavor scalping processes under study. For each data point, three different pouches were used, and from each pouch two different samples were taken for SPME analysis. Thus, each data point in the graphs below represents the average of six measurements. The relative standard deviation (RSD) on the SPME measurements for the different flavor compounds and sample points ranged between 0.1 and maximum 13%, with an average of 3.4%. Refined sunflower oil was used as the oil matrix of choice, as this is a relatively odor-free oil. The headspace composition of the oil was evaluated before analysis and compared with olive oil. In refined sunflower oil, some volatiles were detected at very low trace levels
2. Material and methods 2.1. Samples An overview of the LLDPE sealants is shown in Table 1. Samples were delivered to the laboratory as empty sealed Stand-Up Pouches (SUP). The sealants were applied as part of a multilayer structure pouch representing a true application (PET/adhesive/PE/PE/PE sealant). Polyethylene samples were selected from different suppliers with differences in:
• production process (solution/gas phase); • catalyst (metallocene/Ziegler-Natta); • melt index; • additive package; • density.
2
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Table 2 Overview of flavor compounds used in the different experiments. Compound
CAS#
Supplier
Min purity (%)
Concentration range (vol ppm)
Ethyl butyrate 2,5-Dimethylpyrazine (R)-(+)-Limonene Linalool Eugenol 2(E)-Nonenal 6-Methyl-5-hepten-2-one (MHO) Octanal Decanal
105-54-4 123-32-0 5989-27-5 78-70-6 97-53-0 18829-56-6 110-93-0 124-13-0 112-31-2
Sigma-Aldrich Sigma-Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich
99 98 97 97 99 97 99 99 98
25 - 100 25 - 100 2.5 - 10 25 - 100 25 - 100 25 - 100 25 - 100 25 - 100 2.5 - 10
the inner surface was dried carefully by gently tapping with a lint-free paper towel. After this drying step, 50 ml of dichloromethane, containing 5 ppm (vol) of internal standard n-decane, was poured into the bag. The top opening was again folded 3 times and further closed with a paperclip. The bags were put into a bag holder to ensure close contact of the solvent with the inner SUP layer and placed onto a shaking device, where they were shaken for 1 h, at approx. 200 rpm and room temperature. The samples were then analyzed by means of liquid injection-GCFID. The GC column had a DB-1701 stationary phase (30 m x 0.32 mm x 1 μm) and was heated in the oven at 80 °C, followed by heating to 180 °C at 25 °C/min and to 260 °C at 20 °C/min with 7 min hold. Analysis was performed by FID detection. A 1 μl aliquot was automatically injected at a 1/20 split ratio. Carrier gas was He at constant pressure of 52.8 kPa.
(nonanal, undecanal), but this was significantly lower than olive oil, and not interfering with the experiments performed. Levels of oxidation products were monitored carefully during the course of the experiments to exclude interaction with the volatiles under study. For the solvent extraction procedure, including an evaporation step was evaluated. However, as losses of the most volatile flavor compounds cannot be excluded, and the sensitivity of the optimized GC method was sufficient, the evaporation step was omitted in the final experimental procedure. It must be noted that the deviation on the solvent extraction data was quite high, with a relative standard deviation (RSD) varying between 1 and 47% (n = 3).
2.3. Procedure for aqueous flavor solutions To prepare the stock solution of the relevant compounds (Table 2), a mixture of the different pure flavor compounds was prepared in the appropriate ratios and mixed well. An aliquot of this flavor mixture was then transferred to a large volume of milliQ water in a 5 L Erlenmeyer flask and stirred intensively on a magnetic stirrer overnight. A small corner was cut from the sealed bags with a clean pair of scissors and 250 ml of the stock solution was then transferred into the sample bag with a funnel and sealed by folding the opening 3 times and placing a metal paperclip on the opening. The bags were then placed in a thermostated oven with a constant temperature of 30 °C. After a preset time, 3 replicate bags were taken from the oven for each sample point. The solution was poured out of the sample bag and transferred to a 250-ml glass beaker. The solution was stirred for 2 min at 250 rpm, and 5 ml of the solution was transferred into a Gerstel SPME vial and capped (Septum: Butyl red/PTFE grey, 55° shore A for Seal size 20 mm). The samples were then analyzed by means of Headspace Solid-Phase Microextraction - Gas Chromatography - Flame Ionization Detection (HS-SPME-GC-FID) (Agilent GC 7890). A Mass Selective detector (Agilent 5975C MSD) was used to identify unknowns. As SPME fiber a DVB/Car/PDMS (Divinylbenzene/Carboxen/ Polydimethylsiloxane) fiber (Supelco U-57329; 24 gauge; 50/30 μm; 1 cm length) was used. Samples were incubated for 5 min at 30 °C for water and 60 °C for oil with agitation (Agitator on 10 s 250 rpm; off 2 s), extracted for 20 min at the same temperature and desorbed in the inlet at 250 °C for 1 min. SPME sampling and extraction was all automated using an MPS autosampler (Gerstel). The GC column had a DB-1701 stationary phase (30 m x 0.32 mm x 1 μm) and was heated in the oven at 50 °C, followed by heating to 120 °C at 5 °C/min and to 260 °C at 20 °C/min with 5 min hold. Analysis was performed by FID detection. Carrier gas was He at constant pressure of 52.8 kPa. For each data point, 3 different pouches were used, and from each pouch 2 different samples were taken for SPME analysis. Thus, each data point in the graphs represents the average of 6 measurements. As a control, glass 50-ml vials were used. These were filled with 50 ml of solution, closed with a septum covered on the inside with aluminum foil, and treated the same way as the SUP pouches. After emptying the bag, the bag was cut open on the top side, and
2.4. Procedure for flavor in oil solutions Refined sunflower oil (from the local supermarket Jumbo) was used as an oil matrix. For these experiments, the flavor in oil mixture was prepared directly in the pouches: 200 ml of oil was poured into a measuring cylinder and 140 μl of the flavor scalping mix was added. The flavor scalping mix was prepared with equal volumetric parts (e.g. 2 ml) of the compounds of interest. Samples were treated as the aqueous model systems discussed above.
2.5. Overview of the different experiments discussed Table 3 shows the sealants, flavor compounds and sampling scheduling applied for the different experiments that are discussed in this report.
2.6. Statistical data evaluation In a first step, the Brown-Forsythe test was used to test for equal variances of the data obtained for the different flavorants. In most cases, the p values were greater than 0.05 so we concluded that there were no significant differences between the group variances. In such cases, analysis of variance (ANOVA) method was used to test for differences between the means. In case differences in variance were concluded, Welch’s t-test was applied. When ANOVA results show that there is at least one significant difference (p < 0.05) among the different resins for the interaction with all flavorants, Tukey’s HSD test was used as a follow-up test to compare means. JMP Pro 12 software was used to perform these calculations.
3
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Table 3 Overview of the different experiments discussed. Experiment number
Sealants
Flavor compounds
Time of experiment (days)
Number of sampling points
1
MHP 3 MHP 4 MHP 5
28
7
2
GP 1 GP 3 GP 4 MHP 1 MHP 6
14
6
3
GP 2 GP 6 MHP 2 MHP 7
14
6
4
GP 1 GP 2 GP 3 GP 4 GP 6 MHP MHP MHP MHP GP 5 GP 7 GP 8
100 ppm ethyl butyrate 100 ppm 2,5-dimethylpyrazine 100 ppm linalool 100 ppm eugenol 10 ppm limonene 25 ppm ethyl butyrate 25 ppm 2,5-dimethylpyrazine 25 ppm 6-methyl-5-hepten-2-one 25 ppm linalool 25 ppm 2(E)-nonenal 25 ppm eugenol 2.5 ppm limonene 2.5 ppm decanal 25 ppm ethyl butyrate 25 ppm 2,5-dimethylpyrazine 25 ppm 6-methyl-5-hepten-2-one 25 ppm linalool 25 ppm 2(E)-nonenal 25 ppm eugenol 2.5 ppm limonene 2.5 ppm decanal 25 ppm ethyl butyrate 25 ppm 2,5-dimethylpyrazine 25 ppm 6-methyl-5-hepten-2-one 25 ppm linalool 25 ppm 2(E)-nonenal 25 ppm eugenol 2.5 ppm limonene 2.5 ppm decanal
14
1
28
6
5
1 2 6 7
100 ppm 100 ppm 100 ppm 100 ppm 100 ppm 100 ppm 100 ppm
ethyl butyrate 2,5-dimethylpyrazine linalool eugenol limonene octanal 2(E)-nonenal
3. Results and discussion 3.1. Selection of flavor compounds and model systems For the purpose of this work, a general model system was selected to evaluate flavor scalping in Stand-Up Pouches (SUPs). Regarding the matrix, two different food-simulating liquids were selected: (1) water and (2) a food-grade oil (100% refined sunflower oil). A variety of foodrelevant flavor compounds were selected, with different chemical functionalities to enable the assessment of the influence of chemical structure on potential flavor scalping effects. The flavor compounds selected are shown in Scheme 1:
• a hydrocarbon monoterpene (R)-(+)-limonene (1), • a terpene alcohol linalool (2), • an aromatic alcohol eugenol (3), • a nitrogen-containing heterocyclic compound 2,5-dimethylpyrazine (4), • an aliphatic ester ethyl butyrate (5), • two ketones 6-methyl-5-hepten-2-one (6) and 2-nonanone (8), • two aliphatic aldehydes octanal (7) and decanal (10), • an α,β-unsaturated aldehyde 2(E)-nonenal (9).
Scheme 1. Structures of the different flavor compounds used in this study.
illustrate the conclusions and trends that were discovered. The degree of flavor scalping by the SUP pouches from different LLDPE resins was evaluated by two analytical methods:
Table 4 lists the odor characteristics, and some relevant physical constants of these model flavor compounds. Multiple experiments were performed, comparing from three up to nine different sealants in one experiment, in an aqueous or sunflower oil model system, for a period of time ranging from two weeks up to eight weeks. Only a selection of experiments is discussed here to
(1) At regular time intervals, the model solutions in the pouches were analyzed by means of Headspace Solid-Phase Microextraction Gas Chromatography Flame Ionization Detection (HS-SPME-GC-FID) to monitor the possible decrease in concentration of scalped 4
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Table 4 Overview and properties of flavor compounds used in the flavor scalping experiments1. Flavorant
Odor
Average max ppm in food (safety in use FEMA GRAS2)
Vapor pressure (mm Hg at 25 °C)
Log P (o/w)
Water solubility (mg/l @ 25 °C)
(R)-(+)-Limonene 1 Linalool 2
Citrus orange fresh sweet Citrus, floral, sweet, bois de rose, woody, green, blueberry Sweet, spicy, clove, woody Nutty, peanut, musty, earthy, powdery and slightly roasted with a cocoa powder nuance Sweet, fruity, tutti-frutti Citrus, green, musty, lemongrass, apple Aldehydic, waxy, citrus, orange with a green peely nuance Fresh, sweet, green weedy, earthy, herbal Fatty, green, waxy, cucumber, melon Sweet, aldehydic, waxy, orange peel, citrus, floral
31 - 2,300 2.0 - 90
1.5 0.1
4.57 2.97
13.8 1,590
0.6 - 2,000 10
0.0 4.0
2.27 0.63
2,460 31,970
28 - 1,400 1.1 - 1.3 0.1 - 6.1
13.9 1.3 2.1
1.80 1.95 2.95
4,900 3,351 560
0.1 - 4.0 0.2 0.6 – 6.6
0.6 0.3 0.2
3.14 3.32 3.97
371 205 43.5
Eugenol 3 2,5-Dimethylpyrazine (DMP) 4 Ethyl butyrate 5 6-Methyl-5-hepten-2-one (MHO) 6 Octanal 7 2-Nonanone 8 2-(E)-Nonenal 9 Decanal 10
1 Data extracted from (The Good Scents Company Information System 2019) (http://www.thegoodscentscompany.com/) and Chemspider (2019) (http://www. chemspider.com/). 2 All flavorants have been determined to be ‘generally recognized as safe’ (GRAS) under conditions of intended use as flavor ingredients by the FEMA (Flavor and Extracts Manufacturers Association of the United States) Expert Panel.
flavorants. (2) The inside of the emptied pouches was extracted with dichloromethane to detect the sorption of flavorants by the internal LLDPE layer.
control vials. However, SPME peak areas are the result of a complex equilibrium and partitioning of flavor compounds between the sample liquid phase, sample headspace, and SPME fiber surface. Therefore, SPME competition effects can play a role as well, when changing concentrations of scalped flavorants can influence sorption of others. Percentages higher than 100% as sometimes observed are the result of the complex interplay between the different processes occurring simultaneously. For the same experiment, the results of the solvent extraction of the emptied pouches at different time intervals are shown in Fig. 3. From the glass control vials, no detectable amounts of flavor compounds were extracted. In all cases, amounts of flavor compounds extracted from the SUPs were significantly higher than from the control, but no significant differences were observed between the different resins. There is a general increase of the amounts extracted during the course of the experiments, reaching a plateau near the end of the experiment. The amounts extracted after the full length of the experiment (28 days) can be considered as a measure for the degree of flavor scalping. Table 5 shows a clear correlation with the water solubility of the flavor compound: the lower the water solubility, the more a flavorant is retained by the lipophilic LLDPE layer and consequently, the higher is the relative amount extracted. Flavor scalping decreased in the following order: limonene > eugenol > linalool > ethyl butyrate > 2,5-dimethylpyrazine. As discussed before, significant amounts of limonene were lost due to evaporation. Varying losses of flavor compounds during the experiment are due to differences in volatility. Table 5 also indicates a higher deviation on the data with increased volatility, which can be explained by losses from evaporation.
Results of both analytical methods are discussed below, for the aqueous model solutions and oil media, respectively. 3.2. Results flavor scalping aqueous solutions: experiment 1 For experiment 1, Stand-Up Pouches prepared with three different PE sealants of the medium/high performance segment (i.e. MHP 3, MHP 4 and MHP 5) were filled with an aqueous model system containing 5 different flavor compounds (ethyl butyrate, 2,5-dimethylpyrazine, linalool, eugenol at 100 ppm, and limonene at 10 ppm), and kept for 28 days at 30 °C. Changes in the headspace flavor concentrations with time were measured by means of HS-SPME-GC-FID, and are shown in Fig. 2, displayed as percentage of the initial concentration at initial time t0. For most flavorants, i.e. 2,5-dimethylpyrazine, linalool, and eugenol, the compound concentration in the pouches did not change significantly over time, and was not significantly different from the control. Therefore, no flavor scalping can be concluded and no significant differentiation between the different sealant resins can be shown. On the contrary, flavor scalping was clearly observed for limonene for all three resins. From the first sampling points onwards (t > 0), only very low levels of limonene were observed in the aqueous solutions. The almost instantaneous sorption of limonene by LDPE is known in literature [Sajilata et al., 2007], and is the result of the apolar nature of limonene leading to its fast partitioning from the polar water phase to the apolar internal LLDPE layer of the pouches. However, in the sealed control vials the concentration of limonene decreased rapidly as well. As solvent extraction of the emptied vials afterwards did not recover significant amounts of limonene (cf. discussion below), losses from the control vials are mostly due to the evaporation to the headspace, resulting from the poor water solubility and high volatility of limonene (Table 4). Comparing the data obtained with the glass control vials with the results of the SUPs demonstrates that the losses of limonene are due to a combination of evaporation and scalping. A quantitative differentiation between both effects cannot be made based on the present data. In case of ethyl butyrate, the flavor concentration in the SUPs was higher than in the glass control vials. One hypothesis to explain this effect would be the selective evaporation of ethyl butyrate from the
3.3. Results flavor scalping aqueous solutions: SPME experiment 2 As scalping was observed only for limonene, additional compounds with a wider range of water solubility and log P values (Table 4) were included in the aqueous model system for the following experiment to investigate flavor scalping behavior further. Five different PE sealant layers were included in experiment 2. Eight flavor compounds were added to an aqueous model system at lower concentration to ensure complete solubilization, i.e. 25 ppm of ethyl butyrate, 2,5-dimethylpyrazine, 6-methyl-5-hepten-2-one, linalool, 2(E)-nonenal, eugenol, and 2.5 ppm of limonene and decanal. Six sampling points were collected over a period of 14 days. For most flavor compounds, the concentration did not decrease significantly during the course of the experiment, and was in some cases higher than in the glass control vials (data not shown). Therefore, no scalping could be concluded for these 5
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 2. Changes in concentration, expressed as % of initial headspace-SPME-GC-FID peak areas, for the different flavor compounds and PE sealants in an aqueous model system, experiment 1.
significantly from aqueous solutions by all five sealant resins tested in this experiment. Profiles of these scalped flavorants are shown in Fig. 4. The final SPME-GC-FID peak areas for the eight flavor compounds and five sealants are compared statistically by means of ANOVA in Table 6.
compounds. Only for limonene, 2(E)-nonenal and decanal the concentration in the SUP was significantly lower than in the glass control vials during the complete course of the experiment. These data demonstrate that limonene, 2(E)-nonenal and decanal were scalped 6
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 3. Amounts of flavorants extracted from the emptied pouches (mg) at different time intervals for the different flavor compounds and PE sealants in an aqueous model system, experiment 1.
The flavorant concentration reached an equilibrium after several days and this final concentration was lower for limonene as compared to decanal and 2(E)-nonenal, respectively. Comparison of the final
concentrations for the different resins showed no significant differences for limonene, which was scalped almost completely. For decanal and especially 2(E)-nonenal, differences are observed for the five sealant
Table 5 Overview of amounts extracted from SUP after 28 days for experiment 1: average and RSD (n = 3) in comparison with physical constants of flavor compounds. Flavorant
Average amount extracted (% of initial amount)
RSD (%)
Pvap (mm Hg @ 25 °C)
Log P (o/w)
Water solubility (mg/l @ 25 °C)
2,5-dimethylpyrazine Ethyl butyrate Eugenol Linalool Limonene
0.1 2.1 6.6 4.4 14.0
19.8 23.1 9.0 13.8 17.8
4.0 13.9 0 0.1 1.5
0.63 1.80 2.27 2.97 4.57
31,970 4,900 2,460 1,590 13.8
7
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 4. Changes in headspace-SPME-GC-FID peak areas (% of initial solution) for the different flavor compounds and PE sealants in an aqueous model system, experiment 2.
between the different sealant resins were noted. For 2(E)-nonenal scalping was intermediate and allows some differentiation. As it is difficult and ambiguous to compare data obtained in two different experiments, an additional experiment, experiment 4, was set up to evaluate nine PE sealant resins in one experiment. Due to the large amounts of SUPs required to do this (three replicates for each sample point) only one measurement was performed, after 14 days of storage. The HS-SPME-GC-FID data from this experiment are gathered in Fig. 5. The results confirm what was found in experiments 2 and 3: flavor scalping was observed for limonene, 2(E)-nonenal and decanal, and not for the other flavor compounds. Statistical comparison of the scalping performance of all the different sealants showed differentiation only for 2(E)-nonenal scalping which was most pronounced for the sealants with lowest density: MHP 6 (d = 0.902) and MHP 7 (d = 0.902). This corresponds with literature findings: the amount of flavor compounds sorbed in polymers has been shown to decrease with an increase in polymer density [Sajilata et al., 2007]. To complement the SPME data, the amounts of flavorant extracted from the inner surface of the emptied pouches, expressed as percentage of the initial amount, are shown in Table 7 for experiments 2, 3 and 4. The value shown is an average of all sealant SUPs. No significant differences between the nine sealants were noted. These data show that
Table 6 Statistical evaluation of day 14 SPME-GC-FID data of the scalped flavor compounds and LLDPE sealants in experiment 2 by means of ANOVA (Tukey’s HSD test) – Connecting letters report. Resin
Density (g/ml)
limonene
2(E)-nonenal
decanal
GP 4 GP 1 GP 3 MHP 1 MHP 6 Glass control
0.918 0.919 0.918 0.913 0.902
B B B B B A
B BC BC C D A
B C C C C A
Different letters indicate statistically significant differences.
resins evaluated in this experiment. Similar results were obtained from experiment 3, evaluating scalping of the same flavorants by a different set of 4 PE resins (data not shown). The scalping of limonene, 2(E)-nonenal and decanal from the aqueous solution took place for all sealant resins evaluated in experiment 3. Differentiation of the different sealants was evaluated by statistical comparison of all HS-SPME-GC-FID peak areas at the end of the experiment (day 14) by means of ANOVA analysis (Tukey’s honest significance difference, or HSD test). For limonene and decanal, scalping was clear and fast for all resins and no significant differences 8
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 5. HS-SPME-GC-FID measurements of experiment 4 showing the % of flavorant scalped from aqueous model solutions after 14 days of storage for nine sealant resins (* indicates significant difference α 0.05).
100%). In most cases, the data show a relatively stable concentration of the flavor compounds in the oil during the experiments, and no flavor scalping was observed for all flavor compounds evaluated, even after eight weeks. A selection of graphs is shown in Fig. 7. The absence of flavor scalping in the oil model system can be due to the lower polarity difference between the matrix and the sealant, associated with a lower driving force for repartitioning [Van Willige, Linssen, Meinders et al., 2002; Van Willige, Schoolmeester et al., 2002]. The oil itself can be absorbed by the polyethylene sealant layer, thus making it very difficult to separate flavor absorption from oil absorption, with flavor compounds dissolved in it [Nielsen, 1992]. For all flavorants, the amount extracted from the emptied pouches by solvent extraction is about 0.03 – 0.08 mg. At a concentration of 100 ppm, this corresponds to about 0.3 – 0.8 ml of oil, which may very well correspond to the volume of oil remaining in the pouches. Considering this, it was concluded that solvent extraction of emptied pouches is not an appropriate technique to evaluate flavor scalping from oil media by LLDPE resins. In all additional experiments performed with the oil model system, no significant scalping was observed for any of the flavor compounds in the model, and for any of the LLDPE sealant resins evaluated, even after a prolonged experiment of eight weeks. Therefore, no further testing was performed with this model system.
Table 7 Overview of average amounts of flavor compounds recovered from the inside of the SUPs by extraction after 14 days for three experiments with aqueous model systems. Flavorant1
Amount (% of initial amount added) extracted from SUP after 14 days
Experiment
2
3
4
2,5-dimethylpyrazine Ethyl butyrate MHO Linalool 2-Nonenal Decanal Limonene
nd 2.6 3.4 4.7 28.1 49.5 28.1
nd 1.5 2.2 3.6 20.1 38.2 16.0
nd 2.1 3.3 4.9 26.7 38.0 14.0
Pvap (mm Hg @ 25 °C)
Log P (o/ w)
Water solubility (mg/l @ 25 °C)
4.0 13.9 1.3 0.1 0.3 0.2 1.5
0.63 1.80 1.95 2.97 3.32 3.97 4.57
31,970 4,900 3,351 1,590 205 43.5 13.8
the degree of flavor scalping was inversely proportional to the water solubility of the flavorants, illustrated in Fig. 6. A lower water solubility/high log P is associated with a higher affinity for the lipophilic LLDPE layer and a higher degree of scalping. These data correspond with literature findings indicating that compounds with lower polarity are absorbed more by nonpolar polyolefins [Sajilata et al., 2007]. Losses of flavorants during the experiment are due to evaporation. This explains, for instance, that amounts of limonene recovered from the SUPs were lower than expected, although the scalping was almost complete.
4. Conclusions From the data discussed above, the following conclusions can be drawn:
• Flavor scalping performance was comparatively evaluated for 14
3.4. Results flavor scalping oil model system: experiment 5 Besides the properties of the flavorant and the polymer, the composition of the food product also plays an important role in influencing the sorption of flavor compounds. Therefore, also sunflower oil was evaluated as a model matrix to examine flavor scalping. Because of better solubility of the flavorants in the oil, no evaporation was observed from the control vials, and results are expressed as percentage of flavorant in the pouches as compared to the control vial (referenced at
• •
polyethylene sealant resins in Stand-Up pouches for an aqueous and a sunflower oil model system. In the aqueous model solutions, clear flavor scalping was observed for limonene, decanal and 2(E)-nonenal. The degree of scalping of a specific flavorant was inversely related with the water solubility: a lower water solubility/higher log P was associated with a higher affinity for the lipophilic polyethylene layer
Fig. 6. Inverse relationship between amounts extracted and water solubility for the different flavor compounds (average of 4 experiments, each multiple PE sealants), aqueous model systems. 9
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Fig. 7. Changes in headspace-SPME-GC-FID peak areas (% of initial solution) for the different flavor compounds and PE sealants in an oil model system, experiment 5.
and a higher degree of scalping.
Funding sources
lower degree of scalping for general purpose as compared to medium and high performance sealants, which can be explained by differences in density/crystallinity. In case sunflower oil was used as the matrix, the polarity difference with the lipophilic sealant layer was much smaller, and no significant flavor scalping was observed, even after a period of 8 weeks. Flavor scalping by stand-up pouches made of laminates with polyethylene sealant resins does occur, and depends on the lipophilicity of the flavorant. Differences between various LLDPE sealant resins were noted but are limited, and are not expected to generate sensorial differentiation between the sealant resins, in the density range (0.902 – 0.919 g/ml) evaluated.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
• The data obtained for flavorants tested in aqueous solutions show a • • •
Declaration of Competing Interest The authors are all affiliated to the Dow Chemical Company, with no declaration of interest.
Acknowledgments The authors would like to thank Shayne Green and Dave Albers for valuable discussions, and Edwin Mes for his support.
10
Food Packaging and Shelf Life 21 (2019) 100371
A. Adams, et al.
Sajilata, M. G., Savitha, K., Singhai, R. S., & Kanetkar, V. R. (2007). Scalping of flavors in packaged foods. Comprehensive Reviews in Food Science and Food Safety, 6, 17–35. The Good Scents Company Information System, http://www.thegoodscentscompany. com/ Accessed July 8th, 2019. Van Willige, R. W. G., Linssen, J. P. H., & Voragen, A. G. J. (2000). Influence of food matrix on absorption of flavor compounds by linear low-density polyethylene: Proteins and carbohydrates. Journal of the Science of Food and Agriculture, 80, 1779–1789. Van Willige, R. W. G., Linssen, J. P. H., Legger-Huysman, A., & Voragen, A. G. J. (2003). Influence of flavor absorption by food-packaging materials (low-density polyethylene, polycarbonate and polyethylene terephthalate) on taste perception of a model solution and orange juice. Food Additives and Contaminants, 20, 84–91. Van Willige, R. W. G., Linssen, J. P. H., Meinders, M. B. J., Van der Stege, H. J., & Voragen, A. G. J. (2002). Influence of flavor absorption on oxygen permeation through LDPE, PP, PC and PET plastics food packaging. Food Additives and Contaminants, 19, 303–313. Van Willige, R. W. G., Schoolmeester, D., van Ooij, A., Linssen, J. P. H., & Voragen, A. G. J. (2002). Influence of storage time and temperature on absorption of flavor compounds from solutions by plastic packaging materials. Journal of Food Science, 67(6), 2023–2031.
References Cava, D., Catala, R., Gavara, R., & Lagaron, J. M. (2004). Testing limonene diffusion through food contact polyethylene by FT-IR spectroscopy: Film thickness, permeant concentration and outer medium effects. Polymer Testing, 24, 483–489. Charara, Z. N., Williams, J. W., Schmidt, R. H., & Marshall, M. R. (1992). Orange flavor absorption into various polymeric packaging materials. Journal of Food Science, 57(963-966), 972. Chemspider, Royal Society of Chemistry, http://www.chemspider.com/ Accessed July 8th, 2019. Ikegami, T., Nagashima, K., Shimoda, M., Tanaka, Y., & Osajima, Y. (1991). Sorption of volatile compounds in aqueous solution by ethylene-vinyl alcohol copolymer films. Journal of Food Science, 56, 500. Keršiene, M., Adams, A., Dubra, A., De Kimpe, N., & Leskauskaite, D. (2008). Interactions between flavour release and rheological properties in model custard desserts: Effect of starch concentration and milk fat. Food Chemistry, 108, 1183–1191. Nielsen, T. (1992). Study of factors affecting the absorption of aroma compounds into low-density polyethylene. Journal of the Science of Food and Agriculture, 60, 377–381. Nielsen, T., & Jägerstad, M. (1994). Flavour scalping by food packaging. Trends in Food Science & Technology, 5, 353–356.
11