3.1. Optical Properties
Figure 3 shows the spectral distribution curves, which represent the internal transmittance (T
i) of the sheet, film, and tray samples as a function of the wavelength (λ). It can be seen that the paper-based samples were opaque since these sheets showed low T
i values, which are related to low light transmittance. In this regard, the different refractive indices of cellulose and air (1.5 and 1.0, respectively) cause the light scattering at the fiber surfaces and manifest as the opacity of paper [
38]. Moreover, the opacity of paper is habitually increased by the addition of titanium dioxide (TiO
2), and it is also affected by the level of pulp hydration and grammage of the paper [
4]. Similar results were also observed for paper double-coated with PHBV films [
15]. Thus, the paper-based trays had a very low transmittance percentage of light, which may be advantageous for protection against the oxidative processes of certain foods, such as oils or meat products [
39]. In this regard, the micrometer-sized fibers of paper also partly reduced the light passage transmittance of the ultraviolet type A (UVA) region, seen below 400 nm. However, one should consider that, under UV light, the C−C bonds in the paper fiber structure will break, which can lead to a notable decrease in its mechanical properties [
40].
Table 1 summarizes the results of the optical evaluation carried out on the film, sheet, and tray samples. It can be observed that the paper sheet showed the highest value of luminosity, whereas both the biopolyester blend and the PET films presented similar luminosities, being significantly higher (
p < 0.05) in the case of the petrochemical polymer. For the bilayer paper/PBS–PBSA structures, used in the trays, the incorporation of the biopolyester blend film significantly reduced (
p < 0.05) the luminosity of paper. However, this was not significantly different (
p > 0.05) in the case of the paper/PET trays. The latter effect can be ascribed to lower luminosity of the biopolyester film since it is based on a blend of biopolymers with different refraction indices [
41]. Furthermore, both PBS–PBSA and PET films showed values of −0.44 and 1.44 (a*) and −1.54 and 4.25 (b*), respectively, while the paper sheet presented values of approximately a* = 1.56 and b* = −5.83. In terms of the chroma (C
ab*), values of 6.03, 1.52, and 4.52 were observed for the paper sheet, PBS–PBSA, and PET films, respectively, while the bilayer paper/PBS–PBSA and paper/PET trays showed values equal to 0.63 and 3.58, all being significantly different (
p < 0.05). The hue or shade (h
ab*) of the PBS–PBSA and PET films was located in the 100–110 range, corresponding to a slight yellow-to-orange hue, whereas the paper and bilayer trays had a similar angle, in the 280–290 range, which corresponds to a blue hue. This result agrees with our previous findings for paper double-coated with PHBV and PET films, which exhibited more bluish (lower h
ab* values) but slightly less saturated (lower C
ab* values) color in comparison with uncoated paper [
15]. Lastly, the color difference (∆E
ab*) values between the paper sheet and the bilayer trays of paper/PBS–PBSA and paper/PET were 5.42 and 2.48, respectively. This suggests that the optical properties of the paper were modified, particularly for the paper/PBS–PBSA bilayer structure, where ∆E*
ab > 5, thus an unexperienced observer would notice different colors [
42]. The high color difference attained in the biopolymer-containing structure can be related to the use of a biopolymer blend that generally results in a hazy film with low transparency [
43].
Figure 4 shows the cross-sectional morphologies of the polyester films, paper sheet, and bilayer tray samples obtained by FESEM. One can observe in
Figure 4a,b that the PBS–PBSA and PET films showed a continuous section characteristic of a plastic material. Whereas the PBS–PBSA film presented a continuous structure that correspond to a single layer, the petrochemical film was composed of several layers, where the thickest layers, shown on both sides, would correspond to the polyester. Similar morphologies, based on multilayer structures containing EVOH inner layers, have been reported for high-barrier films [
34]. In contrast,
Figure 4c shows that the uncoated paper sheet presented an average thickness of approximately 290 µm, and it was composed of micrometer-sized fibers with an average diameter of nearly 20 µm [
15]. In the case of the paper/PBS–PBSA and paper/PET trays, shown in
Figure 4d,e, it was confirmed that their cross-sectional morphologies were based on a bilayer structure. Furthermore, both were seen to present good adhesion between the polyester layers and paper substrate, suggesting adequate mechanical resistance for handling and transport in food packaging.
3.2. Thermal Properties
Figure 5 gathers the TGA curves of all developed samples, that is, the paper sheet, PBS–PBSA and PET films, and paper/PBS–BSA and paper/PET trays. From these curves, the corresponding values of T
onset (temperature of initial degradation), T
deg (temperature at maximum degradation rate), and residual mass at 700 °C were determined, and the results are included in
Table 2. One can observe that the paper sample was thermally stable up to approximately 280 °C, while the thermal stability of the biopolyester blend and PET films was slightly higher (~300 °C). This result agrees with the thermal stability previously reported for paper, showing that decomposition occurs between 220 and 390 °C [
40]. Thus, the thermal stability of paper was improved in both bilayer structures used to form the trays, increasing the thermal resistance by approximately 5–10 °C. In this context, Seoane et al. [
44] reported similar results for paperboard/poly(3-hydroxybutyrate) (PHB) structures that were prepared by compression molding, showing that thermal degradation of paper occurs at 280 °C. Therefore, both thermoplastic layers offered to paper a slight improvement in thermal stability, and the bilayer trays were able to withstand temperatures close to ~290 °C. Thus, they can be adequate for most packaging applications that do not make use of temperatures above 250 °C, such as microwave heating or low-temperature cooking in oven [
4].
In terms of the thermal degradation profile of the trays, three main mass losses were seen to occur in the samples. These took place at temperatures of approximately 100
°C, 335
°C, and 472
°C, which have been described for lignocellulosic materials [
45]. Briefly, the first mass loss is related to moisture evaporation. The second one, which took place in the 310–350 °C range, is referred to as the “active pyrolysis zone” since the mass loss rate is high. The third one, seen to occur from 350 °C to above 520 °C, represents the “passive pyrolysis zone” since the mass loss rate is much lower. The second mass loss corresponds to the decomposition of hemicellulose and cellulose, whereas the third mass loss is ascribed to the lignin thermal degradation [
46]. Moreover, the paper sample showed a remaining mass of approximately 16% at 700
°C that corresponds to inorganic materials and ashes generated from the organic material decomposition in an inert atmosphere. In the case of the polyester films, an additional thermal loss in the 500–600
°C range was observed, which has been ascribed to the thermal decomposition of the organic mass produced during the previous steps [
47]. This thermal degradation step was not seen in the case of the paper-based samples since it overlapped with the decomposition of lignin.
3.3. Mechanical Properties
Figure 6 shows the tensile stress–strain curves of the paper sheet, PBS–PBSA and PET films, and paper/PBS–BSA paper/PET trays. These curves allowed obtaining the mechanical parameters of tensile modulus (E
tensile), tensile strength at yield (σ
y tensile), and deformation at break (ε
b), which are presented in
Table 3.
Figure 6a shows the mechanical curves of the samples prior to storage. On the basis of the presented results, paper can be considered a brittle and rigid material, characterized by high E
tensile (1787 MPa) and σ
y tensile (31.9 MPa) values, but with low ductility (ε
b ≈ 7%). In this sense, the mechanical performance of paper is dependent on the strength of its cellulose fibers, their surface area, length, and bonding strength [
48]. Both monolayer films of PBS–PBSA and PET presented significantly lower (
p < 0.05) values of E
tensile and σ
y tensile, but broke after deformations of approximately 150% and 300%, respectively. The mechanical properties of the bilayer trays were still in the range of those of the monolayer paper, with moderate improvements in the mechanical resistance and no significant differences (
p > 0.05) in ductility. In this regard, a slight mechanical enhancement in terms of ductility was also observed by Zhu et al. [
38], where the elongation of paper increased from 1.65% to 2.75% after coating with cross-linked copolymers of chitosan and tannin extract-based epoxy. Similarly, other previous studies have demonstrated that the mechanical properties of Kraft paper coated with biopolymers, such as chitosan or starch, are still controlled by the cellulose fiber matrix, having a slight decrease in mechanical strength and increase in ductility [
49,
50,
51,
52].
Figure 6b shows the mechanical curves obtained after keeping the samples for 21 days in refrigeration conditions (5
°C and 85% RH). Interestingly, one can further observe that the paper-based samples presented a dissimilar performance after storage for 21 days. Thus, the neat paper sheet showed a significant reduction (
p < 0.05) in E
tensile, reaching a value of 1124 MPa, whereas the sample was slightly more ductile (ε
b ≈ 9%). This reduction in terms of elasticity, by approximately 37% when compared with the dry paper, can be ascribed to the plasticizing effect of water during storage. However, both polyester films presented similar mechanical parameters, showing no significant differences (
p > 0.05) due to storage time. Nevertheless, both bilayers presented lower mechanical performance after storage due to the presence of paper. For the paper/PBS–PBSA tray, E
tensile was reduced from 1081 MPa to 859 MPa, which represents a percentage reduction of nearly 20%. In the case of the paper/PET tray, it was also reduced from 946 MPa to 924 MPa, that is, 2–3%, showing no significant differences (
p > 0.05). Therefore, the paper substrate was effectively protected from moisture by the films. However, in the case of the biopolymer blend film, the mechanical performance was slightly impaired during storage in humid conditions. Furthermore, both resultant trays would still be restricted to rigid applications that can withstand certain stresses but do not require high deformations.
The effect of the polyester films on the mechanical properties of the paper substrate was also evaluated by means of flexural and puncture tests. The curves of these mechanical analyses are included in
Figure 7. In this regard, flexural properties play an important role in defining the performance of paper packaging materials. In particular, they define the tendency of a material to bend, and one of the main advantages of paper is its high bending stiffness in relation to its relatively low weight.
Figure 7a shows the flexural curves of the trays of paper, paper/PBS–PBSA, and paper/PET. Moreover,
Table 4 provides the values of flexural modulus (E
flexural), flexural strength at yield (σ
y flexural), and elongation at yield (ε
y flexural) obtained from these curves. Thus, one can observe that the mechanical performance of paper was notably modified, in a similar way previouly observed during the analysis of the flexural properties. Thus, for neat paper, E
flexural, also known as the bending modulus, was significantly (
p < 0.05) reduced by the incorporation of the polyester films. In particular, E
flexural decreased from nearly 1500 MPa, for the neat paper tray, to 1235 MPa and 912 MPa, for the paper/PBS–PBSA and paper/PET trays, respectively. Interestingly, the presence of the PBS–PBSA blend film increased σ
y flexural, from 30.4 MPa to 35.9 MPa, as well as ε
y flexural, from 0.69% to 1.14%. This improvement in the bending response of paper can be ascribed to the combination of the inherently high strength of PBS and high flexibility of PBSA, which show values of E
flexural and σ
y flexural for the homo- and copolyesters of 500–600 MPa and 30–40 MPa [
53,
54] and 300–450 MPa and 10–20 MPa [
55,
56], respectively. In the case of the paper/PET tray, this sample was able to deform to a higher extend, reaching a value of ε
y flexural of 1.36%, but the σ
y flexural was lower than in the other paper samples, that is, 20.3 MPa. Furthermore, the mechanical improvement attained in the paper trays with the incorporation of the biopolyester blend was confirmed by puncture resistance tests.
Figure 7b shows the puncture curves for the paper, paper/PBS–PBSA, and paper/PET trays. The puncture results showed that the paper tray presented moderate force but low displacement and energy. Interestingly, the values of puncture resistance force and energy of the paper/PBS–PBSA trays were high, and displacement was moderate. Therefore, F
max, d
total, and E
puncture respectively increased from 53 MPa, 1.4 mm, and 25 mJ, for the neat paper tray, to 64 MPa, 5.4 mm, and 130 mJ, for the the paper/PBS–PBSA tray. Moreover, a moderate puncture force and energy with a high displacement in the puncture test of the paper/PET trays was achieved. Thus, the energy was significantly lower (
p < 0.05) when compared to the bilayer trays of paper and renewable succinic acid derived biopolyester blends. This can be ascribed to the fact that the PBS–PBSA blends are mechanically strong and tough at the same time [
57]. For instance, a value of E
puncture as high as 194 mJ was reported for extruded PBS films, however, caution must be taken in the comparison of the puncture resistance results because of the different thickness among the different samples [
58].
Thermo-sealing of polymer films on paper substrate is a complex process that is influenced by different factors, e.g., polymer–paper compatibility, interfacial energy between both surfaces, and structural changes occurring during storage [
59]. Thus, the analysis of the seal strength in the bilayer structures is of relevance to ensure the material’s functionality in food packaging. In this test, the bilayer samples were unsealed at the edges to produce the peel arms, where one edge was paper and the other edge was film. Then, the peel arms were clamped in the grips of the tensile tester and pulled apart at a constant speed.
Figure 8 shows the force–distance curves of the bilayer structures of paper/PBS–PBSA (
Figure 8a) and paper/PET (
Figure 8b). The images included below the mechanical curves are representative pictures of the bilayer samples obtained after the mechanical test. As can be seen in the
Videos S1 and S2 (see Supplementary Materials), both test strips peeled apart in the seal area. Therefore, both bilayer structures exhibited delamination detachment rather than cohesive failure, breaking, tearing, or elongation of the substrate paper. The average peel force, measured by the testing machine as a part of the test cycle, was also gathered in the graphs, showing values of 97.4 and 127 N/m for the paper/PBS–PBSA and paper/PET samples, respectively. In general terms, although both bilayer structures presented delamination, they exhibited high sealing, particularly in the case of the petrochemical film, thus indicating that strong adhesion forces were established between the paper substrates and polyesters. Adhesion tests were similarly carried out by Seoane et al. [
44] on paperboard/PHB structures. The authors showed that the PHB-based layer was also peeled off with torn paperboard fibers, up to the final failure of the PHB layer, suggesting that the interfacial adhesion also presented greater resistance to tear.
3.4. Barrier Properties
Table 5 gathers the permeance and permeability values of water and limonene vapors and oxygen gas of the paper sheet, PBS–PBSA and PET films, and paper/PBS–PBSA paper/PET trays. The barrier performance to these vapors and gases is, in fact, of main interest for food packaging. The barrier properties were expressed in terms of permeance since it represents the actual amount (mass or volume) of permeant per unit of time, area, and difference of partial pressure passing through a multilayer structure that is formed by materials of different permeabilities at the tested temperature and %RH conditions. In the case of the monolayers, that is, the paper substrate and PBS–PBSA and PET films, the permeability was also determined by correcting permeance with the thickness sample.
Water vapor barrier is of great importance for shelf-life extension since most of the physical and chemical deteriorations are related to equilibrium moisture content [
60]. This is of high relevance in the case of paper packaging since it is composed of hygroscopic cellulose fibers forming a network with high porosity. As reported in
Table 5, the water vapor barrier of the paper sheet is very limited with a permeance value of 1.10 × 10
−8 kg/m
2·Pa·s, resulting in a permeability value of 3.21 × 10
−12 kg·m/m
2·Pa·s, which agrees with the value reported previously [
15]. In contrast, the biopolyester blend film and, more notably, the PET film presented significantly lower (
p < 0.05) permeability values to water vapor. In particular, the 200-µm PBS–PBSA film resulted in a permeance of 1.47 × 10
−10 kg/m
2·Pa·s, yielding a permeability of 3.16 × 10
−14 kg·m/m
2·Pa·s. This barrier performance is in the range of the permeability reported for other biopolyesters, such as PHBV, PLA, or PBAT, which are adequate for medium-water-barrier packaging [
24]. In contrast, the 100-µm PET film yielded a permeance value of 5.50 × 10
−11 kg/m
2·Pa·s, which corresponds to a permeability of 5.58 × 10
−15 kg·m/m
2·Pa·s, considering it as a monolayer material. This water vapor permeability is one order of magniture lower than that of the biopolyester blend since it is based on a multilayer structure containing polyolefins, such as LDPE (1.2 × 10
−15 kg·m/m
2·Pa·s at 38 °C and 90% RH) [
61]. In any case, the two polyester-based films significantly increased (
p < 0.05) the barrier properties of paper, resulting in bilayer structures with water vapor permeances of 1.77 × 10
−10 and 1.90 × 10
−11 kg/m
2·Pa·s for the paper/PBS–PBSA and paper/PET trays, respectively. These values of water vapor permeance were, respectively, two and three orders of magnitude lower than the permeance of the uncoated paper. In this regard, the water barrier enhancement attained herein is notably superior to that reported, for instance, for paper coated with PLA [
48] or chitosan [
62], with barrier improvements of approximately ten and five times, respectively. Therefore, both trays can be suitable for food packaging applications in humid conditions or even for storing food with high-to-moderate values of a
w.
The transport properties of limonene vapor are also important in packaging applications because it is often used as a standard system for predicting the aroma barrier of a packaging material. As with water vapor, both polyester films offered a very noticeable improvement over the uncoated paper. In particular, the paper presented a permeance of 2.23 × 10
−9 kg/m
2·Pa·s, corresponding to a permeability value of 6.50 × 10
−13 kg·m/m
2·Pa·s [
15]. Thus, the 200-µm PBS–PBSA film yielded a permeance to limonene vapor of 2.63 × 10
−10 kg/m
2·Pa·s, equivalent to a permeability of 5.58 × 10
−14 kg·m/m
2·Pa·s, whereas the 100-µm PET film showed a permeance value of 5.10 × 10
−11 kg/m
2·Pa·s, resulting in a permeability of 5.15 × 10
−15 kg·m/m
2·Pa·s (assuming a monolayer material). One can further observe that both the biopolyester blend and petrochemical polyester films offered a significant reduction (
p < 0.05) in the aroma permeability of paper, of one and two orders of magnitude, respectively. Thus, the incorporation of the films improved the aroma barrier performance of the paper, resulting in bilayer trays of paper/PBS–PBSA and paper/PET trays with respective permeance values of 1.70 and 1.20 × 10
−10 kg/m
2·Pa·s. Similar to water vapor, this represents a reduction in the permeance of aroma vapor by two and three orders of magnitude, respectively, thus being also adequate to preserve aroma in food.
Oxygen barrier properties are relevant to fresh product preservation, especially when they are susceptible to oxidation processes (e.g., meat, fish, or high-lipid-content products). In the case of the uncoated sheet paper, it was not possible to determine the permeability since its permeance was above the detection limit (D.L.) of the equipment (5 × 10
−11 m
3/m
3.Pa·s). The results showed, on the one hand, that the permeance of the biopolyester blend film was nearly three times higher than that of the petrochemical one, with respective values of 6.17 and 2.15 × 10
−15 m
3/m
2·Pa·s. This resulted in permeabilities of 1.27 × 10
−18 and 2.17 × 10
−19 m
3.m/m
2·Pa·s, that is, one order of magnitude lower for the petrochemical polyester when considered as a monolayer material. However, it should be noted that this commercial film is based on interlayers of EVOH, a high-oxygen-barrier copolymer in low-humidity conditions (0.77 × 10
−21 m
3.m/m
2·Pa·s) [
61], which can be achieved in packaging structures using hydrophobic external layers (e.g., LDPE and PET) [
63]. As a result, the bilayer structures presented values of permeance of 5.15 and 2.34 × 10
−15 m
3/m
2·Pa·s for the paper/PBS–PBSA and paper/PET trays, respectively. Therefore, the oxygen permeance of the trays developed herein have a performance suitable for foods requiring a low or intermediate oxygen barrier. This fact, together with the high barrier to water and aroma vapors, makes the trays very suitable as packaging materials for preserving food products with high humidity but low susceptibility to oxidation.
3.5. Preservation of Fresh Pasta
The application of the newly developed paper-based trays in food packaging was carried out by analyzing the preservation of fresh pasta, a “fusilli calabresi” type. This food was selected due to its high a
w and the fact that it is locally produced in Southern Italy. Thus, it was easily accessible and is of high interest in the region. According to Italian law, “fresh pasta” can be defined as the product obtained by extrusion or lamination of a dough made of durum wheat semolina or alternative flours and water, having a moisture content >24% and a
w ranging between 0.92 and 0.97, whereas it requires storage at 4 ± 2 °C [
64]. In this regard, the shelf life of fresh pasta depends on several factors, such as heat treatment, storage temperature, proper preservatives, and type of packaging. Industrial fresh pasta is habitually subjected to heat treatment, equivalent to pasteurization, which confers a shelf life of 30–90 days [
65]. However, non-thermally treated artisanal fresh pasta has, on average, a shelf-life of only 2–3 days under refrigerated temperatures [
66]. Additionally, it can be prolonged for up to 30 days with the use of preservatives and modified atmosphere packaging (MAP) [
67].
Shelf-life evaluation consisted of a visual observation of the surface appearance of the packaged pasta, as well as a quantification of the color parameters, weight loss, and a
w throughout a whole storage period of 3 weeks at 5 °C and 85% RH. This analysis was carried out in both an unopened container and the sealed trays.
Figure 9 shows the visual appearance of the packaged pasta in the trays, closed and open, at the different storage times. Images of the unpackaged pasta were also taken as a reference or control to show the effect of packaging. It can be observed that, in all cases, the visual appearance of the packaged pasta was very similar during the first 4 days. However, the unpackaged pasta was slightly more yellow, suggesting loss of water and/or oxidation. After 9 days of storage, only the pasta packaged in the bilayer trays with the polyester films preserved its original brown color, while the unpackaged and packaged pasta in the monolayer paper developed a yellowish hue. For the latter samples, after 13 days of storage, visible molds were seen, which became more evident after 17 days. For the pasta stored in the paper trays with the PBS–PBSA and PET films, these acquired a yellowish hue after 13 and 17 days of storage, respectively, but both successfully harbored mold growth during this period. Lastly, after 21 days, all pasta samples presented mold overgrowth on the surface. In this regard, the spoilage of fresh pasta is generally related to mold growth [
68]. Furthermore, mold spoilage is often visible to the naked eye as colonies once they reach a diameter of 3 mm [
69]. For instance, cooked pasta stored in an uncoated tray was covered by approximately 80% of mold on the surface after 30 days of storage [
1]. Thus, in terms of spoilage due to molds, the use of the biopolyester films on the paper substrate successfully extended the shelf life of pasta by nearly 1 week.
Fresh pasta color is a very important quality attribute since it greatly influences consumer acceptance and is the main property the consumer can evaluate when selecting a product in the market. To quantify the visual changes of the pasta during storage, the color parameters of the samples were analyzed by means of a colorimeter on their surface after opening. Thus,
Table 6 shows the values of the color coordinates L*a*b* (CIELAB), where L* indicates the lightness (L* = 0 black, L* = 100 white), a* indicates the color between red (+) and green (−), and b* indicates the color between yellow (+) and blue (−). As can be observed, fresh pasta was found to have coordinates of L* = 81.3, a* = −2.2, and b* = 16.6. These color parameters are very similar to those reported by Carrini et al. [
70] for fresh pasta produced using durum wheat semolina. Color evolution of the packaged pasta showed an intense reduction in brightness, decreasing from an initial L* value of 81 to values in the 77–69 range. Furthermore, the a*b* coordinates confirmed the development of a more yellowish hue, mainly due to an increase of the b* value. In particular, in terms of color variation, the unpackaged pasta showed increases in C
ab* and h
ab* from 16.78 and 82.57 to 21.36 and 86.40, respectively. The increase observed for the color parameter a* was previously ascribed to oxidative reactions occurring in food pasta [
71]. Similarly, Zardetto and Dalla Rosa [
72] indicated that, during physicochemical deterioration, fresh pasta tends to evolve to lower L* and b* parameters and higher values of a*, which are representative of a yellowish process. These color changes were mainly observed for the unpackaged pasta and, more notably, for the pasta packaged in the monolayer paper trays. Therefore, in the latter samples, although color variations were observed to occur later, the drying process was more intense. This may be ascribed to the tendency of paper to absorb moisture due to its hydrophilic nature and fibrillar structure, which could promote and favor food drying. In the case of the unpackaged pasta sample, slight but still statistically significant lower (
p < 0.05) values than in the paper-packaged pasta were observed. This fact can be related to the high humidity of the chamber (∼85%), although color changes occurred faster. Lastly, it was confirmed that the pasta packaged in the bilayer paper/PBS–PBSA and paper/PET trays nearly maintained the original color for 13 and 17 days, respectively.
The initial moisture content of the fresh pasta was approximately 32 g of water per 100 g of product. Then, the weight changes that occurred in the different packaged pasta samples over time were monitored.
Figure 10, which shows the evolution of mass loss with storage time, confirmed that a drying process occurred in the fresh pasta since the samples showed a continuous increase in mass loss over time. Mass loss evolution also confirmed that this drying process was more intense in the unpackaged and paper-packaged pasta samples. As previously suggested during the color analysis, the unpackaged pasta samples showed lower mass loss due to drying than the pasta packaged in the monolayer paper trays, but the mass of these samples stabilized faster, after 13 days of storage. However, all the food samples packaged in the paper trays, both monolayer and bilayers, showed a continuous and progressive loss of mass during the whole storage period. It can be observed that all mass losses were significantly different (
p < 0.05) among the pasta samples packaged in the different paper trays, which correlated well with the water barrier properties determined above for each material. Thus, the lowest mass loss was attained in the case of the pasta packaged in the paper/PET trays, reaching values of 2.7% after 3 weeks of storage, whereas the food samples packaged in the paper/PBS–PBSA trays yielded values of 8.4%. However, compared to the monolayer paper tray sample, in which the mass loss reached a value of nearly 20%, the mass loss in the bilayer trays with the biopolymer blend was approximately three times lower. In this context, Sousa et al. [
72] reported that food pastas intercalated with biodegradable films made from rice flour, PBAT, and glycerol containing different amounts of potassium sorbate suffered a gradual reduction in moisture content, ranging from 16% to 28%, after 2 weeks of storage. This result confirms the relatively good protection that the biopolyester film can offer to the paper trays against moisture, with a notable capacity to reduce the drying process in the food, particularly during the first 13 days of storage.
To conclude the shelf-life analysis, the a
w of the pasta packaged in the paper trays was evaluated as function of the storage time. This physicochemical property refers to the free and available water content in the food. It is, therefore, a very useful parameter for determining the drying process extension and, consequently, the loss of quality in humid foods [
73]. Moreover, a
w has been shown to be a determinant factor for the growth of microorganisms and is well related to most degradation reactions of chemical, enzymatic, and physical nature observed in pasta [
74]. As shown in
Figure 11, the as-received fresh pasta presented a value of a
w of 0.962, which provides sufficient moisture to support the growth of bacteria, yeasts, and mold, thus making this product vulnerable to fast spoilage [
75]. This value was very close to the legal limit, 0.97 [
76], but it must be taken into consideration that this pasta did not undergo pasteurization, which is known to slightly reduce a
w. Very similar a
w values, in the 0.97–0.95 range, have been reported for Italian fresh pasta [
77]. It can be observed that a
w was sharply reduced during storage in the fresh pasta packaged in the monolayer paper trays, reaching a value as low as 0.663 after 3 weeks. The reduction in a
w in the unpackaged pasta was significantly lower (
p < 0.05), which also stabilized to a value of 0.854 after 3 weeks due to the fact that the sample was able to reach equilibrium with the relative humidity of the chamber (~85%). It is worth noting that a
w values of 0.8 or even lower can support the growth of molds, which would explain the presence of these organisms in some of the dried food samples. For instance, Xiong et al. [
78] showed that the decrease in a
w from approximately 0.955 to below 0.925 by applying thermal treatments reduced microbial growth in fresh noodles, but other factors such as secondary contamination and air exposure also had adverse effects on the quality during storage. Furthermore, the a
w values for the pasta samples packaged in the bilayer paper/PBS–PBSA and paper/PET trays were very similar, reaching values in the 0.94–0.95 range after 3 weeks of storage, being slightly lower for the biopolyester-containing structure but, interestingly, with no significant differences (
p > 0.05). In this regard, Sanguinetti et al. [
79], who studied the evolution of a
w with storage time, showed that MAP packaging can keep gluten-free fresh pasta at the 0.96–0.95 range for 42 days. The latter result, which was nearly in the same range of a
w, confirms the high potential of the newly developed trays of paper and bio-based polyesters for preserving fresh pasta.
3.6. Overall Migration
The migration of the packaging constituents into food is an important issue in food contact materials from the point of view of food safety. However, it has scarcely been investigated in biopolymers developed for food packaging applications. Thus, ethanol 10%
v/
v (simulant A) and Tenax (simulant E) were chosen to simulate migration into fresh pasta via direct contact, without immersion, at 40 °C for 10 days. Food simulant A was chosen due to the high a
w of fresh pasta, whereas the selection of simulant E was based on the fact that it is habitually employed for paper-based packaging analysis due to the poor moisture resistance of paper. The results, shown in
Table 7, indicate that all tested materials successfully complied with the overall migration limit (OML) set in Commission Regulation (EU) No. 10/2011 [
21] on plastic materials and articles intended to come into contact with food. This is also valid for its subsequent amendment for each tested simulant under the evaluated exposure conditions. In particular, for both tests, the regulation sets a threshold value of 10 mg/dm
2 (maximum OML as sum of all substances that can migrate from the food contact material to the food simulant). In the case of the simulant A, the biopolyester blend film yielded a migration value of 3.6 mg/dm
2, whereas the bilayer tray showed a significantly lower (
p < 0.05) value, equal to 1.9 mg/dm
2. Since, in both cases, the food simulant A was in contact with the PBS–PBSA layer, the lower value observed in the bilayer tray sample can be ascribed to the lamination process of the film onto paper during thermoforming or potential absorption of hydrophilic substances in the paper layer. Similar results were recently attained for films of PHBV, a microbial copolyester, showing values in the 1–3 mg/dm
2 range [
80]. In the case of the food simulant E, both samples yielded values lower than 2 mg/dm
2, showing no significant differences (
p > 0.05) and being well below the legal threshold. Therefore, it can be concluded that the newly developed trays can be safely applied to preserve fresh pasta and other types of foods with high a
w. Furthermore, according to the legislation, the particular conditions selected herein also cover the packaging material being in contact for longer periods at room temperature or lower temperatures. Lastly, they also cover high-temperature conditions and/or packaging subjected to heating processes (from 70 °C for 2 h up to 100 °C for 15 min).