Recyclable mono materials for packaging of fresh chicken fillets: New design for recycling in circular economy

The focus on sustainability and circular economy is leading to a need for development of new food packaging concepts, including recyclable materials that ideally consist of a single material in a monolayer system. This research was focused on the possibility of replacing complex multilayered material [amorphous polyethylene terephthalate/polyethylene (APET/PE)] with simple recyclable mono material [high‐density polyethylene (HDPE)] for packaging of chicken fillets in modified atmosphere packaging (CO2/N2: 60%/40%). Bacterial growth measured as total viable count (TVC), lactic acid bacteria and Enterobacteriaceae, Brochothrix thermosphacta and Escherichia coli for chicken fillets packed in HDPE mono materials was compared with chicken fillets packed in APET/PE.

are recycled. 5 In this manner, it is very challenging to find optimal food packaging solutions for reduction of food waste. 6 Maintaining the shelf life and the quality and safety of food substrate while reducing food packaging waste is a task to be solved. [6][7][8] Packaging innovations moved towards development of advanced materials with good mechanical, thermophysical and barrier properties. 9 In the last decades, the industry has focused on optimization in packaging processes and reducing thickness, but this has often resulted in increased use of complex structures including polyamide (PA). PA has significantly higher environmental impact per kilogramme than polyolefins. 10 In most cases, these materials were developed as complex structures to provide specific requirements of diverse food substrates, for example, laminated structures with multiple layers containing different materials types. 11 One of the main reasons for using complex structure is their gas barrier properties, which are needed in combination with modified atmosphere packaging (MAP) and of importance related to product shelf life (specific for long storage time). Recycling of these multilayered materials is very often not practical or possible; thus, the materials are ending in landfill and/or being incinerated.
Research and development nowadays are moving to packaging solutions for sustainable development like recyclable mono materials. However, these materials are rarely suitable for packaging of perishable and very sensitive food substrates. 12 Improvements in material properties for simpler mono materials can be compensated by design of packaging itself [e.g., by introducing packaging conditions through a modified atmosphere or by using active packaging (AP) solutions]. AP is an innovative way for shelf life extension and/or improvement of food packaging conditions. These systems are roughly divided in two groups, scavengers and emitters, and their activity is designed in accordance with specific food substrate and its properties. 13 Chicken meat is consumed worldwide because of its good nutritional quality, high-quality proteins and low total fat content. 14,15 However, chicken has a relatively short shelf life under refrigerator conditions and thus considered as a highly perishable food product. 14,[16][17][18] New techniques in prolonging fresh chicken shelf life under refrigerating conditions have been extensively researched. MAP has been recognized as a very efficient nonthermal technique for preservation of fresh products. Optimal combination of gases inside of packaging is preventing spoilage and extending the product shelf life. 14,15,[17][18][19][20][21][22][23] Common gas mixture for packaging of fresh chicken fillets in Norway consists of CO 2 (50%-70%) and N 2 (50%-30%), 23 whereas high content of oxygen combined with CO 2 (50%-75% O 2 /50%-25% CO 2 ) is often applied for meat and also poultry in many countries in Europe. However, it is very common to have some amount of residual oxygen related to vacuum time/flushing/production speed, as well as to the size/volume of the packages and flexibility of the material. One possibility to exclude residual oxygen from the package is addition of oxygen scavengers resulting in extended shelf life. 14 High amounts of CO 2 can, on the other hand, be dissolved in the meat product and cause collapse of the packaging. Minimal concentration of CO 2 should be 25%; however, it is well known that higher amounts of CO 2 will result in shelf life extension of chicken meat. 21,24 Besides concentration of CO 2 and food substrate, dissolution of CO 2 depends on the quantity of the gas introduced into the package. For that reason, gas volume to product volume (G/P) ratio in the package is also very important for the effective MAP. 25 Due to this fact, besides oxygen scavengers, CO 2 emitters can be used as AP solution for extended shelf life of packed chicken fillets.
The aim of this research was to identify possible substitution of complex multilayered amorphous polyethylene terephthalate/polyethylene (APET/PE) (laminate) with high-density polyethylene (HDPE) mono material without jeopardizing the quality of packed food. Evaluation of the effect of mono materials on food quality and shelf life was conducted through packaging of fresh chicken fillets. MAP and AP solutions (O 2 absorbers and CO 2 emitters) were used to overcome withdrawals in material characteristic to keep initial food quality and safety. were obtained using trays with different volumes (600 or 1200 ml) and/or different product weight (320-500 g). The G/P ratio 1.5 was selected based on the G/P ratio currently used by the producer. All samples were stored in the dark at 4 ± 0.8 C with sampling time after 8, 13, 17, 21 and 24 days. Triplicate analyses were carried out.

| Packaging
Tray material, tray volume, G/P ratio, product weight, gas composition and the use of liquid absorber and AP [O 2 scavenger (scav.) and/or CO 2 emitter (emit)] applied in the experiment are given in Table 2.
T A B L E 1 List of selected packaging materials with relevant information: Polymer type, tray volume, dimensions, weight (n = 5) and thickness (n = 5) and OTR (n = 4)

| Headspace gas composition
The headspace atmosphere of packages was analysed for CO 2

| Off-odour evaluation
Off-odour evaluation of the chicken fillets was performed by a lab panel of three to six experienced assessors at sampling days 8, 13, 17, 21 and 24 of storage. The panel was trained in using the actual method. Prior to the experiment and evaluation, the assessors were trained and agreement in freshness was discussed (meaning highest intensity of fresh odour). The samples were ranked on a scale of 1 to 5: 5 indicates a fresh product, 3 indicates alterations of the product, but still acceptable, whereas below 3 indicates an unacceptable product (similarly as performed at the producer). Fresh chicken fillet (stored at −20 C and thawed for 24 h at 4 C the day before off-odour evaluation) was used as a reference. The samples, including two fresh fillets, were served in random order, coded with a three-digit number.

| Appearance of package
At the time of packaging, the top web in packages with CO 2 is often almost flat or sometimes more gas is added, resulting in slightly convex packages. CO 2 is dissolved into the product and followed by a reduction in the CO 2 content in the headspace, leading to underpressure in rigid packages. 26 The appearance of the packages and top web of the samples stored in the trays was evaluated by using a scale of 0 to 6 where 0 was defined as packages with extremely underpressure (concave), 3 was neutral and 6 was defined as packages with extremely overpressure (convex). 24

| Microbiological analyses
Microbiological analyses were performed on chicken breast fillets at the time of packaging (Day 0) and at each sampling time. A piece of 3 × 3 cm 2 and 1-cm depth (approximately 10 g) was cut, weighed, diluted 1:10 with peptone water and macerated for 1 min. Around 100 μl of an appropriate 10-fold dilution was spread out on an agar plate either by using a sterile L-shaped rod (when expected bacterial number to be between log 2 and log 5 CFU/g) or by using Whitley automatic spiral plater (WASP) (Don Whitley Scientific Ltd., West Yorkshire, UK; when expected bacterial number to be more than log 5 CFU/g) on the following agar plates:    27 where 7 log 10 CFU/g of TVC was reached after 10-12 days in CO 2 /N 2 atmosphere. However, in their study, the initial level of TVC was 4.3-4.6 compared with our 2.3 log 10 CFU/g, in addition to only 30% CO 2 in the gas, compared with the presented study.

| Statistical analyses
On the 13th day of storage, significant difference was recorded between chicken stored in APET/PE in CO 2 /N 2 and chicken stored in high oxygen (high TVC count on high oxygen). Within the 17th and 21st day of storage, no significant difference between samples was recorded, whereas on the last storage day, significantly lower level of TVC was recorded for chicken stored in APET/PE in CO 2 /N 2 compared with chicken stored in HDPE in high oxygen. As can be seen, the differences were more related to the differences in gas composition than material type and will not be discussed further. However, no significant difference between samples packed in different material types and same gas composition was recorded.
TVC (anaerobic) initial value was 2.1 ± 0.1 log 10 CFU/g. As given in Table 4, the growth of TVC in anaerobic incubation condition was similar to growth of TVC at aerobic incubation. No significant difference between listed samples was recorded (p > 0.05) ( LAB and Enterobacteriaceae constituted a major part of the spoilage bacteria, but as described above, counts of B. thermosphacta were under the limit of detection (<2) also for chicken stored under aerobic conditions in oxygen-rich gas atmosphere.
In relation to microbiology, off-odour is also of great importance, as many volatile substances are produced by microbiological metabolism, causing unpleasant off-odour. MAP can have positive impact in this manner, depending on gas composition. In particular, CO 2 inhibits growth of bacteria that can make off-flavours in chicken breast fillets. 23 Influence of different materials and gas compositions on off-odour of packed chicken breast fillets is presented in Table 5.
The off-odour was evaluated with decreasing score during storage. It must be noted that there were no significant differences in off-odour between chicken stored in APET/PE and in HDPE in However, according to one-way ANOVA, no significant differences between samples stored in different materials or different gas composition were recorded. The drip loss in our study was in the same range as reported by others. 24,32 It has been suggested by Patsias et al. 27 that the water-holding capacity is decreased with CO 2 in the headspace due to CO 2 dissolution. The effect of gas composition/available CO 2 on drip loss of chicken has also reported by Holck et al., 24 showing that underpressure due to the solubility of CO 2 in the meat is also of importance to the drip loss and not only the amount of CO 2 .
In addition, appearance of the package is very important for the consumers. At the packaging day (Day 0), the appearance of packed chicken fillets was defined as neutral (scored with 3). The appearance of APET/PE (score 1.5) trays was significantly different than HDPE trays (score 2.0) for chicken stored in CO 2 /N 2 atmosphere in the beginning of the experiment (storage days 8 and 13), implying that samples packed in APET/PE had more underpressure than HDPE samples. At the end of the storage time (Day 24), no significant differences were recorded. However, values were 1.5 for APET/PE and 1.8 for HDPE samples, implying underpressure in packages to certain extent due to dissolved CO 2 in product, which is also in relation to the measured drip loss. Increased CO 2 content can result in increased CO 2 dissolved in the product.
All presented data are showing that the similar quality and shelf life can be obtained with mono material (HDPE) as APET/PE and AP is not needed to prolong the shelf life for the selected G/P ratio.

| Evaluation of the impact of G/P ratio on the effect of AP and mono material
Further, this research study was also focused on different G/P ratios and their potential effect on the shelf life. Possible reduction in G/P ratio can potentially increase transport efficacy, whereas increase of G/P ratio can increase product shelf life. Additionally, influence of added AP on product shelf life was studied in relation to different G/P ratio and material type.
Selection of AP solutions was in accordance with their activity. Oxygen scavengers are often used to remove residual oxygen to maintain oxygen-free atmosphere during storage, thus preventing the growth of aerobic microorganisms, discoloration and off-flavour of the product, 15 whereas CO 2 emitters are usually used to increase CO 2 level and inhibit bacterial growth. Influence of different G/P ratios and AP on TVC count (aerobic and anaerobic), LAB and Enterobacteriaceae count is presented in Figures 1, 2, 3  According to one-way ANOVA, no significant differences between the samples were observed in the first part of storage time.
Significant higher level of TVC (aerobic) was measured for chicken stored in HDPE with G/P ratio 1.0 with O 2 scav. (7.51 log 10 CFU/g) compared with G/P ratio 1.5 (6.26 log 10 CFU/g). Similar results and significance between G/P 1.0 and G/P 1.5 were observed for TVC (anaerobic). On the 21st day of storage, chicken stored in HDPE with G/P 1.0 reached high level of TVC (7.29 log 10 CFU/g for aerobic and 7.59 log 10 CFU/g for anaerobic), whereas TVC count for G/P 2.6 was below 7 log 10 CFU/g (6.61 for aerobic and 6.06 for anaerobic).
Despite the one-way ANOVA recording no significant difference, these results are implying that higher G/P ratio is resulting in lower   As can be seen in Figure 5 (Table 3). This is followed by a decrease in the headspace volume and underpressure in the packages, specific in rigid packaging. 21,26 Adding a CO 2 emitter can compensate for the reduced headspace and underpressure and thus reduce the drip loss in the product, which is also shown by others. 24 These results show that using CO 2 emitter will contribute in reducing the drip loss. Significant  F I G U R E 5 Influence of different G/P ratios and addition of active packaging (oxygen scavengers and CO 2 emitters) on drip loss difference in drip loss between chicken fillets packed in trays with G/P 1.0 and all other samples has also been detected (p < 0.05), implying that lowest G/P ratio is causing higher drip loss. Addition of oxygen scavengers had no influence on drip loss within the same G/P ratio; nevertheless, drip loss has been reduced to some extent for all samples with addition of CO 2 emitters.
The off-odour of the packed chicken fillets is affected by initial gas composition ( Figure 6). For all samples packed with gas composition CO 2 /N 2 , unacceptable off-odour level was reached at the end of the storage period (between 21-24 days). For samples with G/P ratio Addition of AP affected off-odour of samples with the lowest G/P ratio (1.0), whereas other samples were not affected by the addition of the AP. This is implying that in the case of lower G/P ratio, shelf life can be maintained and/or prolonged with addition of AP solutions. However, for higher G/P ratios (in our case, 1.5 and 2.6), addition of AP did not have any influence on the chicken fillet shelf life.
Both G/P ratio and AP solutions are influencing the appearance of the package (Figure 7). Increase in G/P ratio is causing decrease in appearance score (on 8th day of storage, the scores were 1.5 for 2.6 G/P, 2 for 1.5 G/P and 2.5 for 1 G/P). Similar trend is followed during storage time. Addition of oxygen scavengers had no influence on appearance, whereas systems with both oxygen scavengers and CO 2 emitters had positive influence on the appearance (scores were above 2). Appearance of the package is related to the drip loss, as packages with scores less than 2 are undepressed and can cause additional drip loss (Figure 7). If we take this into account, it is obvious that addition of emitters has positive influence on both appearance and drip loss (less drip loss and higher score for the appearance) within the same G/P ratio for samples ( Figure 7).

| CONCLUSION
The possibility of packaging chicken fillets in recyclable mono materials (HDPE) instead of complex multilayered materials (APET/PE) as a replacement for more sustainable packaging system was studied. All F I G U R E 6 Influence of different G/P ratios and addition of active packaging (oxygen scavengers and CO 2 emitters) on off-odour F I G U R E 7 Influence of different G/P ratios and addition of active packaging (oxygen scavengers and CO 2 emitters) on appearance of the package samples packed in HDPE showed acceptable level of bacteria and acceptable off-odour for chicken fillets up to 19 days without addition of any AP solutions. For some attributes, storage in HDPE was even a better choice than APET/PE (appearance). Chicken fillets packed in HDPE with lowest G/P (1.0) ratio showed higher drip loss compared with higher G/P ratios without any AP. Addition of oxygen scavengers had no influence on drip loss within same G/P ratio; nevertheless, drip loss has been reduced to some extent for all samples with addition of CO 2 emitters. Although the influence of AP was not so pronounced within same G/P ratios, it is obvious that AP solutions are of interest in combinations with G/P ratios, gas compositions and selection of materials.
This study showed that the recyclable mono materials can be used for packaging of fresh chicken fillets without jeopardizing the shelf life.
Outcome of this research presents a step forward in design for recycling, increase in recycling rates and less food packaging waste.
However, as food systems are very complex and diverse, this applies only to chicken fillets, and further research should be spreads on selection of diverse food systems to be packed in appropriate materials and selection of AP solutions. Moreover, further research should be also focused on recyclable top foil as well. Nevertheless, outcomes of this research are encouraging and shifting one step forward to EU sustainable goals and circular economy. Hopefully, this research will influence on use of more recyclable materials on Norwegian market, improve recyclability and use of recycled materials in diverse applications.