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Hoque, M.;  Mcdonagh, C.;  Tiwari, B.K.;  Kerry, J.P.;  Pathania, S. High-Pressure Processing on Biopolymer-Based Films. Encyclopedia. Available online: https://encyclopedia.pub/entry/26021 (accessed on 27 July 2024).
Hoque M,  Mcdonagh C,  Tiwari BK,  Kerry JP,  Pathania S. High-Pressure Processing on Biopolymer-Based Films. Encyclopedia. Available at: https://encyclopedia.pub/entry/26021. Accessed July 27, 2024.
Hoque, Monjurul, Ciara Mcdonagh, Brijesh K. Tiwari, Joseph P. Kerry, Shivani Pathania. "High-Pressure Processing on Biopolymer-Based Films" Encyclopedia, https://encyclopedia.pub/entry/26021 (accessed July 27, 2024).
Hoque, M.,  Mcdonagh, C.,  Tiwari, B.K.,  Kerry, J.P., & Pathania, S. (2022, August 10). High-Pressure Processing on Biopolymer-Based Films. In Encyclopedia. https://encyclopedia.pub/entry/26021
Hoque, Monjurul, et al. "High-Pressure Processing on Biopolymer-Based Films." Encyclopedia. Web. 10 August, 2022.
High-Pressure Processing on Biopolymer-Based Films
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This entry is adapted from the peer-reviewed paper 10.3390/polym14153009

Suitable packaging material in combination with high-pressure processing (HPP) can retain nutritional and organoleptic qualities besides extending the product’s shelf life of food products. However, the selection of appropriate packaging materials suitable for HPP is tremendously important because harsh environments like high pressure and high temperature during the processing can result in deviation in the visual and functional properties of the packaging materials. Traditionally, fossil-based plastic packaging is preferred for the HPP of food products, but these materials are of serious concern to the environment. Therefore, bio-based packaging systems are proposed to be a promising alternative to fossil-based plastic packaging. Some studies have scrutinized the impact of HPP on the functional properties of biopolymer-based packaging materials. 

high-pressure processing film-forming solution biopolymer-based packaging morphological properties mechanical properties thermal properties barrier properties migration potential

1. Introduction

Currently, the consumer trend is towards a growing demand for minimally processed food with improved food safety, nutritional value, freshness, and natural flavors. In order to meet these demands, food industries have been using different processing technologies allowing reduced additives without compromising the sensory and nutritional qualities of the food material. Among different technologies, high-pressure processing (HPP) is one of the emerging noble technologies in the food industry that produce clean label foods free from chemical additives and causes minimal product quality loss along with extending shelf-life by inactivating microbes and enzymes [1][2][3][4]. HPP technology accomplishes these by applying high hydrostatic pressures ranging from 100 -1000 MPa to food products. The United States Department of Agriculture Food Safety and Inspection Service (USDA-FSIS) has approved HPP to be applied on both raw and ready-to-eat products [5]. Globally, in 2015, more than 500,000 tons of food products were produced using HPP technology mostly in the area of meat (27%), dairy and egg products (20%), aquatic (12 %), fruits and vegetables (27%), and beverage (14%). In 2015, the global market for HPP food products was USD 9.8 billion and is expected to be USD 54.77 billion by 2025 [1][6].
Commercially, HPP treatment is carried out in batch, semi-continuous, or continuous mode. Liquid products can be treated in a semi-continuous or continuous process whereas solid products are only treated in batch mode [7]. It has been documented that almost 90% of commercial HPP food products are processed in batch systems, where flexible or partially rigid materials are used as the packaging material prior to processing [8][9]. The selected packaging system must be adequate to withstand volume changes (compression up to 15%) and be able to return to its original shape (decompression) without leaching undesirable packaging chemicals into the product [10]. Also, the packaging materials should be able to withstand the operating conditions of HPP and should have sufficient mechanical, heat sealing integrity, and barrier properties to avoid damage to the product during the processing and distribution in the market [1][11].
In the food industry, fossil-based plastic is the ultimate choice for HPP of packaged food materials due to its excellent thermo-mechanical properties with high strength and flexibility, low cost, lightweight, shape versatility, high performance, and easy transportation. Among, others, polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polyamide (PA), and ethylene-vinyl alcohol (EVOH) are used in food industries [12][13][14]. López-Rubio, Lagarón [15] reported that ethylene-vinyl alcohol-based food packaging materials subjected to 400 and 800 MPa showed no detrimental effect on the barrier and morphological properties. Marangoni Junior, Alves [16] applied 600 MPa at different time-temperature combinations and observed no significant change in water barrier and light barrier properties of multilayer films such as LDPE/PA/LDPE, LDPE/EVOH/LDPE, PET/LDPE/PA/EVOH/PA/LDPE, and PET/Al/PA/PP. Although conventional plastics demonstrate excellent suitability for HPP, these are non-biodegradable and non-renewable. The large application has raised plastic production and its worldwide production was 368 million metric tons in 2019 and has been continuously growing at 4% every year [17][18]. The critical point of high plastic production is the manufacturing of single-use plastics (almost 50%) that cause the issue of plastic pollution and its waste management. Only 9% of all the plastics get recycled and the rest ends in the land fields and water bodies. This high production of plastics is not only a serious threat to individuals, or communities but to the whole ecosystem, especially the marine ecosystem [17][19].
Biopolymer-based compostable packaging is one of the alternatives to plastic packaging. Biopolymers like polysaccharides and proteins from plant or animal origins are being explored to develop packaging materials. Previous studies showed that biopolymer-based films have good film-forming capacity but this packaging system exerts certain shortcomings like low mechanical strength, high hydrophilicity, and poor barrier property as compared to synthetic plastics [20][21][22][23]. These limitations can be overcome by the application of HPP. As stated by Pascal’s principle, the application of high pressure reduces volume, and thus biopolymer-based packaging materials when subjected to HPP produce a compact network microstructure that enhances mechanical strength and barrier properties [1][24]. The HPP also promotes modification of macromolecular arrangements like starch gelatinization, and protein denaturation and increases interaction between different components [25][26][27][28]. Therefore HPP has been proposed as an effective technique to overcome the shortcomings of biopolymer-based materials and several studies have reported forming much denser and more uniform packaging with desired properties [22][29][30].
Until now, HPP has been found to be effective in the modification of biopolymers like polysaccharides [30], proteins [29], and bioplastics like PLA [31]. The HPP technique can be used to modify the properties of biopolymer in two forms as shown in Figure 1. HPP can be applied to the film-forming solution (FFS) before drying (Figure 1a) or can be applied to the pre-formed packaging films (Figure 1b). The sequence of application and processing conditions (pressure, time, and temperature) results in different properties. In this entry, the authors systematically analyzed the performance change in the morphological, mechanical, barrier properties, and thermal properties of the biopolymer-based packaging material when subjected to HPP treatment (with different pressure levels, time, and temperature) before and after film formation.
Figure 1. A graphical representation of the application of HPP on the films: (a) HPP applied to film-forming solution; (b) HPP applied on the dried film.

2. Impact of HPP When Applied to the Film-Forming Solution (FFS)

Bio-based polymers such as polysaccharides, proteins, or their derivatives show good film-forming capacity and are believed to be futuristic replacements for synthetic plastics. Although these polymers form films, due to certain limitations in their pristine forms, some modifications are reported to enhance their packaging properties. Different physical and chemical modifications of biopolymers are being carried out. HPP is a novel technique that utilizes high pressure of more than 100 MPa and is primarily used for the preservation of food products in food processing industries. Recently, it has also been applied in the physical modification of biopolymer-based films [31][32]. For the modification of the packaging properties of the film, high pressure can either be applied to the film-forming solution or the prepared film. HPP when efficiently applied can induce macromolecular changes like protein denaturation or starch gelatinization that may influence the packaging properties of the biopolymer-based films [1][28][33][34]. Different biopolymers interact differently when subjected to pressure and this section elaborates on the impact of HPP on the different properties of the films when applied to the film-forming solution (FFS).

2.1. Mechanical Attributes

Tensile strength (TS) and elongation at break (EAB) are critical mechanical properties associated with the performance of automatic packaging lines and also maintain food quality and integrity during handling, processing, shipping, and storage [4][17]. HPP of FFS can cause the volume-driven transition, promotes cross-linking, and induces phenomena such as protein denaturation or starch gelatinization that can affect the mechanical properties of the films [28][29][30][33][35]. The impact of HPP on the mechanical properties of the film depends on several factors like the level of pressure used, time of exposure, types of bonds in the matrix, and the presence of cross-linkers. The mechanical results of HPP on the preparation of biopolymer-based films are listed in Table 1.
Table 1. Effect of HPP on the film forming solution (FFS).
Film Matrix Processing Conditions Water Solubility (WS) Barrier Property (WVP/OP) Mechanical Property Thermal Properties R
TS EAB
Buckwheat starch (BS) 600 MPa at 20 °C for 20 min WS of the thermally processed BS film was 19.85 ± 0.33% significantly decreased to 11.67 ± 0.69% upon application of 600 MPa WVP of the thermally processed BS film 3.10 × 10−9 g/m s Pa significantly decreased to 2.10 × 10−9 g/m s Pa, upon application of 600 MPa TS of the thermally processed BS film 13.61 ± 1.06 MPa significantly increased to 18.29 ± 1.05 MPa upon application of 600 MPa EAB of the thermally processed BS film 5.65 ± 0.23% significantly increased to 7.92 ± 0.58% upon application of 600 MPa To, Tm, and ΔH of thermally processed BS film 70.52 °C, 112.75 °C, and 78.64 J/g increased to 76.16 °C, 120.64 °C, and 79.30 J/g, respectively; upon application of 600 MPa [30]
Tapioca-starch (TPS) 600 MPa at 20 °C for 20 min WS of the thermally processed TPS film 28.53 ± 0.68% significantly decreased to 17.53 ± 0.51% upon application of 600 MPa No significant variation in WVP for TPS film when treated with HPP TS of the thermally processed TPS film 24.67 ± 1.03 MPa significantly increased to 26.92 ± 0.43 MPa upon application of 600 MPa EAB of the thermally processed TPS film 5.04 ± 0.56% significantly increased to 5.71 ± 0.20% when subjected to 600 MPa To, and ΔH of thermally processed TPS increased from 70.92 °C and 56.92 J/g to 84.32 °C and 78.40 J/g, respectively but Tm decreased from 124.62 to 122.07 °C; upon application of 600 Mpa
PVA, chitosan (CHI), and nano-TiO2 200, 400, and 600 MPa at 23 ± 2 °C for 15 min -- WVP of PVA–CHI–TiO2 (0.10%) (4.36 ± 0.308) × 10−12 g·cm/cm2·s·Pa significantly decreased to (3.60 ± 0.137) × 10−12, (3.47 ± 0.139) × 10−12, and (3.92 ± 0.0433) × 10−12 g·cm/cm2·s·Pa when subjected to 200, 400, and 600 MPa, respectively; OP of the film 1.34 ± 0.05 cm3 m−2·s−1·Pa−1 showed no significant variation when treated with 200 MPa but OP significantly decreased to 1.30 ± 0.05 and 1.25 ± 0.05 cm3 m−2·s−1·Pa−1 when treated with 400 and 600 MPa TS of PVA–CHI–TiO2 (0.10%) 8.24 ± 0.27 MPa significantly increased to 13.67 ± 0.41, 13.98 ± 0.33, and 17.15 ± 0.97 when subjected to 200, 400, and 600 MPa, respectively EAB of PVA–CHI–TiO2 (0.10%) 64.82 ± 1.10% significantly increased to 68.48 ± 1.66, 68.12 ± 1.94, and 67.92 ± 2.73% when subjected to 200, 400, and 600 MPa, respectively -- [36]
Chitosan 100, 200, 300, 400, and 500 MPa for 15 min -- WVP and OP of the chitosan film decreased continuously when the pressure increased from 100 to 500 MPa TS of film increased 35.2% as compared to the untreated film when treated at 400 MPa for 15 min but further increase in the pressure decreased the TS EAB of the chitosan film decreased continuously as the pressure increased from 100 to 500 MPa -- [37]
Pigskin gelatin 0.1, 300, and 600 MPa at 20, 40, and 60 °C for 5, 17.5, and 30 min -- WVTR of the untreated film 65.56 ± 1.2 g/(day m2) significantly decreased to 63.47 ± 0.9 g/(day m2), when subjected to 600 MPa for 30 min at 20.5 °C TS of the untreated film 25.7 ± 2.2 MPa significantly increased to 28.7 ± 2.5 MPa when subjected to 600 MPa for 30 min at 20.5 °C EAB of the untreated film 8.6 ± 0.6% insignificantly increased to 10.1 ± 1.5% when subjected to 600 MPa for 30 min at 20.5 °C Tg, and Tm of the untreated film 58.8 ± 0.4, and 131.5 ± 0.7 °C increased to 60.7 ± 4.5, and 138.2 ± 0.5 °C, respectively, but ∆Hm decreased from 46.4 ± 0.8 to 36.5 ± 3.3 J/g, subjected to 600 MPa for 30 min at 20.5 °C [38]
Amaranth protein 200, 400, and 600 for 5 min WS of the untreated film 79.9 ± 2.1% significantly decreased to 56.4 ± 5.5, 46.1 ± 0.5, and 46.1 ± 2.5% when treated with 200, 400, and 600 for 5 min, respectively WVP of the untreated film (5.6 ± 0.5) × 10−12 g H2O/Pa m s significantly decreased to (4.8 ± 0.4) × 10−12, (4.6 ± 0.1) × 10−12, and (3.2 ± 0.6) × 10−12 g H2O/Pa m, when treated with 200, 400, and 600 for 5 min, respectively TS of the control film increased by 26%, 101%, and 165% when subjected to 200, 400, and 600 for 5 min, respectively No significant variation in EAB under high-pressure treatment -- [35]
Nisin-soy-protein-isolate 100, 200, 300, 400, and 500 MPa at 20 °C for 10 min -- WVP of the untreated film significantly decreased as the pressure level increased from 100 to 500 MPa TS of the untreated film significantly increased as the pressure level increased from 100 to 500 MPa EAB of the untreated film significantly decreased as the pressure level increased from 100 to 500 MPa -- [29]
Whey protein concentrate, thyme (TEO) 600 MPa at 70 °C, for 20 min -- WVP of thermally treated WPC-TEO film was (24.867 ± 2.855) × 10−10 g/s.m.Pa significantly decreased to (10.178 ± 1.690) × 10−10 g/s.m.Pa, when subjected to 600 MPa at 70 °C, for 20 min -- -- -- [39]
Poly (lactic acid) and Ag (5%) 0, 200, and 400 MPa for 15 min at 25 °C -- WVP of untreated PLA/Ag-5% film (4.3 ± 0.3) × 10−10 (g·m/m2·s·Pa) significantly decrease to (2.8 ± 0.1) × 10−10 and (3.2 ± 0.2) × 10−10 (g·m/m2·s·Pa), when subjected to 200 and 400 MPa for 15 min TS of untreated PLA/Ag-5% film 34 ± 2 MPa significantly increased to 36 ± 2 MPa at 400 MPa for 15 min EAB of untreated PLA/Ag-5% film 170 ± 8% significantly decreased to 161 ± 14 and 119 ± 14%, when subjected to 400 MPa for 15 min Tg, and Tc of PLA/Ag-5% film 50.1 ± 0.2, and 110.4 ± 0.4 °C significantly decreased to 51.9 ± 0.2, and 112.9 ± 0.5 °C, respectively, when treated with 400 MPa for 15 min; Tm showed no significant variation between treated and untreated film [31]
Poly (lactic acid) and ZnO (0, 2.5, 5.0 and 10.0 % of PLA) 0, 200 and 400 MPa for 10 min -- OP of the untreated PLA/ZnO-5% film 4.83 ± 0.13 (cm3 24 h−1 m−2) × (cm bar−1) slightly decreased to 3.02 ± 0.29 (cm3 24 h−1 m−2) ×  (cmbar−1) when subjected to 400 MPa for 10 min.;
WVP of the PLA/ZnO-5% film decreased significantly when subjected to 400 MPa for 10 min.		 TS of untreated PLA/ZnO-5% film 35.8 ± 1.48 MPa, increased to 41.9 ± 1.43, and 42.9 ± 1.08 MPa when subjected to 200, and 400 MPa for 10 min, respectively EAB of untreated PLA/ZnO-5% film 8.19 ± 0.17% decreased to 7.90 ± 0.34, and 7.61 ± 0.58% when treated with 200, and 400 MPa for 10 min, respectively Tg and Tc of untreated PLA/ZnO-5% film 46.7 ± 1.82 and 95.9 ± 0.30 °C significantly increased to 49.8 ± 1.50 and 100.9 ± 0.70 °C and showed no significant variation in Tc when subjected to 400 MPa for 10 min [40]
TS: Tensile strength; EAB: Elongation at break; R: Reference.
In the case of starch, HPP treatment induces volume-driven transition like the transformation of starch crystalline structure from A- to B-type. Films prepared from HPP-treated FFS possess higher moisture content that helps in stabilizing the scattered amylopectin structure of A-type starches via van der Waals forces, resulting in the rearrangement of the helix structure leading to the formation of compact B-type structure [30][41][42][43]. In this regard, Kim, Yang [30] stated that the TS value of HP-treated buckwheat starch (BS) was higher (18.29 ± 1.05 MPa) as compared to the untreated film (13.61 ± 1.06 MPa); similarly, HP-treated tapioca starch (TPS) films showed higher TS of 26.92 ± 0.43 MPa as compared to untreated film 24.67 ± 1.03 MPa. It was also found that increment in the TS values of BS was higher than that of TPS and this was ascribed to the fact that BS has A-type starch which is more sensitive to HPP compared to C-type starch present in the TPS starch [30]. Also, the application of HPP (600 MPa at 20 °C for 20 min) increased the EAB of BS film from 5.65 ± 0.23% to 7.92 ± 0.58%, and the EAB of TPS film increased from 5.04 ± 0.56% to 5.71 ± 0.20%. This enhancement in the extensibility of the film could be due to the plasticizing effect of the higher moisture present in the HPP-treated film as compared to the untreated film [30].
In the case of protein-based film, HPP is an effective factor that can induce the formation of hydrogen bonds, and disulfide bonds or may increase hydrophobic interaction between the components of the film, resulting in the formation of a more compact structure that leads to the considerable enhancement in the mechanical properties. Molinaro, Cruz-Romero [38] reported that with an application of high pressure (600 MPa at 20.5 °C for 30 min) to the FSS from pigskin gelation-(PSG), the TS value significantly increased from 25.7 ± 2.2 to 28.7 ± 2.5 MPa; and EAB increased from 8.6 ± 0.6 to 10.1 ± 1.5%. This enhancement could be due to the increase in the stiffness in the gelatin matrix due to the formation of H-bonds. It was also observed that the different pressure level applied to the FFS influences the mechanical properties. As in the case of soy protein isolate (SPI) based FFS, application of HP from 100 to 500 MPa at 20 °C for 10 min progressively increased hydrophobic interactions between the SPI along with the formation of disulfide bonds resulting in the enhancement in the TS values but decreased EAB [29]. Similarly, an increase in the pressure level from 200 to 400 and then 600 MPa in amaranth protein-based FFS, increased the TS of the film by 26, 101, and 165%, respectively, as compared to untreated film, but the EAB was unaltered [35].
HPP processing of FFS, containing nanoparticles also significantly influences the mechanical properties of the film. Lian, Zhang [36] showed that polyvinyl alcohol-chitosan-TiO2 (0.10%) FFS when subjected to 200, 400, and 600 MPa increased the TS from 8.24 ± 0.27 MPa to 13.67 ± 0.41, 13.98 ± 0.33, and 17.15 ± 0.97 Mpa, respectively, and EAB also increased from 64.82 ± 1.10% to 68.48 ± 1.66, 68.12 ± 1.94, and 67.92 ± 2.73%, respectively, although the pressure level showed no significant impact. Similarly, TS of PLA-Ag-3% or PLA-Ag-5% when subjected to 200 and 400 MPa increased the TS but decreased the EAB as shown in Table 1 [31]. Such a trend in mechanical properties where TS increased and EAB decreased with the increased pressure level (0, 200, and 400) also was reported for PLA film loaded with ZnO NPs at 0, 2.5, 5.0, and 10.0 % of PLA [40]. Such variation in the mechanical properties of the nanocomposite film exposed to HPP can be ascribed to the enhancement in the crystallinity, development of ordered molecular chain arrangement, and reduction in porosity as well as the increase in the intermolecular interaction resulting in the restricted chain mobility [31][40].

2.2. Water Solubility (WS)

Solubility in water is one of the key features of biopolymer-based film. In food packaging applications, packaging material should be water-insoluble to protect the integrity of the packaged product, prevent moisture transport, and enhance shelf-life [20][21]. The application of HPP is expected to reduce the WS of the biopolymer-based films. Table 1 summarizes the WS results of HPP on the preparation of biopolymer-based films. In one of the studies, Kim, Yang [30] reported that application of 600 MPa at 20 °C for 20 min to the FFS buckwheat starch reduced the WS of the film from 19.85 ± 0.33 to 11.67 ± 0.69%; and tapioca starch from 28.53 ± 0.68 to 17.53 ± 0.51%. This decrease in WS could be due to the formation of a compact helix structure in the starch-based matrix through hydrogen bonding that might have reduced the hydroxyl site for the interaction with water molecules and thus reduced water solubility. Similarly, Kim, Choi [44] reported that corn starch exposed to 400 MPa, at 25 °C, for 15 min showed lower water solubility compared to the conventional thermal processing of starch. Such decrease in WS of the protein-based film was reported for amaranth protein isolate films and it decreased progressively with the increase in the intensity of the pressure. It was reported that WS of the untreated film 79.9 ± 2.1% decreased to 56.4 ± 5.5, 46.1 ± 0.5, and 46.1 ± 2.5% for the film prepared from FSS subjected to 200, 400, and 600 MPa for 5 min, respectively. This decrease in WS could be due to the higher crosslinking of the film developed from the protein, unfolded by HPP. Also, the unfolded protein might have facilitated different interactions resulting in a stable and compact protein matrix with higher surface hydrophobicity that might have retarded the passage of water molecules through it and retained the film structure when in contact with water [35].

3. Effect of HPP on the Properties of Flexible Biopolymer-Based Packaging Materials

While the previous section discussed the physical properties of the film obtained from HPP of film forming solution (FFS). This section elaborates on the impact of HPP on the properties of the preformed flexible biopolymer-based packaging materials. Currently, HPP has been extensively used in food processing and preservation of aquatic food, meat, dairy, fruits, and vegetables. It is mostly focused on the sterilization effect of food microbes and the quality change of food products [32][45]. For the packaging materials to be used in the HPP environment, several factors like pressure resistance, water, and oxygen barrier property, restriction to the leaching, seal integrity, and clarity have to be considered. When exposed to HPP, volume compression (approximately 19%) and equal expansion upon decompression occur and thus packaging materials should be able to withstand the tremendous pressure variation retaining the seal integrity, mechanical strength, and sufficient barrier properties [45]. Generally, flexible packaging materials experience reversible changes during HPP [4][46]. However, the irreversible changes cause visible deformation and variations in the functional properties of the packaging materials. A probable elucidation for irreversible deformation could be due to the fact that during HPP, gases are adsorbed within the layers of the film matrix due to compression and instantly released in the course of depressurization. During compression, gas may act as a plasticizer and dissolve easily in the inner film resulting in the change in crystallinity of the polymers. Also, an instantaneous increase in specific volume during the rapid depressurization may cause the gas bubbles to expand fast and burst leading to the development of cavities [4][45][47]. This compression and decompression leads to the deviation in the properties of the HPP-treated packaging materials.

3.1. Surface Attributes (Morphological Characteristics)

Food packages when subjected to HPP should be able to retain properties similar to those manufactured, evading defects such as delamination, formation of wrinkles, holes, or other defects, and should maintain their visual integrity. Such defects, and the deviation of built-in properties of the materials, may result in changes in aesthetic qualities including their design or dimension, compromising the safety and shelf life of food products that can even result in the rejection of the product [4]. Besides visual defects that are detected with naked eyes, the biopolymer-based film may also experience microscopic defects and should be assessed. The most widely used techniques for microscopic analysis include SEM or atomic force microscopy (AFM).
The visual defects, such as bubbles, embrittlement, and opacity are observed in the HPP of biopolymer-based packaging materials. Sansone, Aldi [48] investigated the suitability of the commercial PLA-based flexible packaging (Biophan 121) for HPP (pasteurization and sterilization of carrot-based products). The pasteurization (25–40 °C), and sterilization (90–115 °C) were carried out at different pressure levels of 200, 500, and 700 MPa for 5 min. HP pasteurization showed no significant variation in the structural or functional properties of the Biophan 121. 

3.2. Barrier Properties

In food packaging, barrier properties (importantly water vapor and oxygen barrier properties) are key parameters in the selection of packaging materials as they play a significant role in maintaining the quality attributes of foods. Packaging materials subjected to HPP should be able to withstand the changes caused due to compression and instant depressurization during processing. As per industrial norms, a deviation up to 12% in barrier property is acceptable without compromising their integrity and performance [1]. In general, biopolymer-based packaging materials have lower barrier properties as compared to conventional packaging materials [49]. Therefore, the selection of biopolymer-based packaging material should be in such a way that when exposed to HPP, barrier properties are enhanced or at least do not change. Thus, some studies have examined the effect of different conditions of HPP on the barrier properties of biopolymer-based films.
Currently, PLA is the most frequently used commercial biopolymer-based packaging material for HPP. Some of the studies have reported that HPP increases the WVP and OP. The extent of modification of barrier properties of biopolymer-based film subjected to HPP depend on the pressure level applied. It can be observed that with the enhancement in the pressure levels from 0 to 600 MPa, the WVP of PLA-PEG-GO-2% nanocomposite film increased from (1.05 ± 0.11) × 10−14 (kg m) (m² s Pa) to (1.68 ± 0.21 × 10−14) (kg m) (m² s Pa). This increase in the WVP can be attributed to the plasticizing effect of water on polyethylene glycol in an aqueous HPP environment. This enhancement in WVP could also be due to the formation of holes and cavities on the film structure during HPP. Such an increasing trend in WVP with the increase in pressure level was observed in the case of co-extruded PLA films, where, the WVP of the untreated film (1.55 ± 0.12) × 10−14 (kg·m/[m2·s·Pa]) increased to (1.59 ± 0.10) × 10−14 and (1.62 ± 0.13) × 10−14 (kg·m/[m2·s·Pa]) when subjected to 450 and 600 MPa [50]. The authors state that the reduction in the crystallization of the co-extruded film might have facilitated the transport of water vapor and thus enhanced the permeability. A similar increase in WVP was also noticed in the case of PLA-Ag NPs-based nanocomposite films when exposed to 400 MPa for 20 min [32].

3.3. Mechanical Properties

Among the biopolymers, PLA-based packaging is mostly used for the HPP of food materials at the commercial level. Only a few studies have shown the application of other biopolymers such as cellulose acetate and wheat gluten-based packaging systems for HPP. The mechanical properties of the biopolymer-based packaging system depend on the type of polymer matrix, different additives, and their interactions. It has been observed that HPP can lead to the modification of biopolymer-based packaging materials and is dependent on the processing conditions such as pressure level, temperature, and time of exposure. Only a few studies were found in the literature that reported the enhancement in mechanical properties of biopolymer-based packaging systems when subjected to HPP.
The extent of modification of mechanical properties of the biopolymer-based film subjected to HPP is dependent on the level of pressure. It was observed that TS of untreated PLA-PEG-GO-2% film 50.80 ± 0.75 MPa decreased to 43.13 ± 6.64, 40.69 ± 0.77, and 40.14 ± 1.00 MPa when subjected to 300, 450, and 600 MPa, respectively. Similarly, EAB of the untreated nanocomposite film 25.31 ± 0.27% decreased to 20.32 ± 1.35, 17.98 ± 0.92, and 11.72 ± 1.35% when subjected to 300, 450, and 600 MPa, respectively [45]. Such a decrease in TS and EAB might be attributed to the formation of cracks and cavities in the film structure during HPP [45]. Besides pressure level, the time of processing is also an important factor that can influence the mechanical properties of flexible packaging materials. The application of 200, 300, and 400 MPa for different periods such as 5 and 10 min had a different impact on the mechanical properties of the cellulose acetate film. For instance, TS of untreated cellulose acetate film was 40.9 ± 1.2 MPa when subjected to 300 MPa for 10 min, TS was 36.6 ± 1.3 MPa but the reduction in TS was higher (28.9 ± 1.3 MPa) when subjected to HPP for 5 min. This reduction could be due to changes in crystallinity, film delamination, or plastification caused by HHP [51]. It was also observed that the EAB of HP-treated PLA-Ag nanocomposite film decreased with the increase in migration time, although TS was not altered [32]. In contrast, Chi, Li [52] reported that TS increased with the increase in Ag NPs concentrations (from 0 to 20 wt%) when subjected to 300 MPa but EAB decreased. The increase in TS could be due to the increased stiffness of the composite film during HPP [52].
Literature also reports that the extent of modification of flexible packaging materials subjected to HPP also depends on the types of food being packed. The application of HP into the film containing liquid food can result in plasticization of the material that weekends the structure leading to a reduction of mechanical properties. Notably, plasticization of polymers occurs in the amorphous region rather than the crystalline region. Such effect was evident in the low crystalline PLASiOx-PLA film, where the film in contact with olive oil (fatty acid simulant) experienced higher degradation in mechanical properties as compared to film contact with distilled water (aqueous simulant). PLASiOx-PLA film when subjected to 500 MPa at 50 °C for 15 min decreased TS and EAB by almost 25% and 32%, respectively when in contact with fatty acid simulant and 22% and 29%, respectively when in contact with aqueous simulant [11].
Some studies have reported that HPP had a minor to no significant impact on the mechanical properties of the biopolymer-based films. Ahmed, Mulla [53] investigated the synergistic effect of HPP and PLA-PEG-based active film on refrigerated storage of the chicken sample. It was reported that the application of 300 MPa at 23 °C for 10 min had no significant impact on the TS and EAB of PLA-PEG film and PLA-PEG-CIN-17% film. Similarly, Fan, Cui [32] studied the effect of Ag nanoparticle content (1, 5, 10, 15, and 20 wt%), HPP (100/200/300/400 MPa for 10/20/30 min), and the migration time (0, 14, 28, 49 days) and reported that HPP had no significant impact on the TS, although EAB was decreased with the increase in migration time.

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Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Monjurul Hoque
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Ciara McDonagh
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Brijesh K. Tiwari
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Joseph P. Kerry
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Shivani Pathania
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Update Date: 11 Aug 2022
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