Packaging materials for non-thermal processing of food and beverages
Abstract:
Non-thermal processing of foods (high pressure processing, pulsed electric field, irradiation, and pulsed light) offers an alternative to conventional thermal processing to meet consumer demands for convenient, high quality, and minimally processed food products. Non-thermal processes have different mechanisms of preserving foods compared to thermal processes. Thus, the development of packaging for non-thermal processing requires understanding of packaging material properties (mechanical and barrier) and interaction between packaging materials and food components during the non-thermal processes. Packaging materials for non-thermal processes should have proper resistance to the particular non-thermal processing mechanisms. The material and design of packaging should be changed accordingly to assist preservation of food products by these non-thermal processes. This chapter provides a brief introduction to non-thermal processing of foods and beverages. Subsequent sections then cover the design of packaging materials for non-thermal processing, and discuss the likely research focus for future developments.
16.1 Introduction
Thermal processing of foods and beverages is the most widely used method of food preservation. There are several methods of thermal processing with pasteurization and sterilization being the two most commonly used. Pasteurization refers to heat treatment of a food product to kill all present pathogenic vegetative microorganisms. Sterilization refers to killing all living microorganisms including spores in the food product. However, thermal treatment of the products in conventional thermal processing results in degradation of color, flavor, texture, and nutrients.
With consumers becoming more health conscious and educated, the demand for convenient, high quality, and minimally processed food products has increased over time. Non-thermal processing offers an alternative option to conventional thermal processing to meet these demands. Non-thermal processes include high pressure processing (HPP), pulsed electric field (PEF), irradiation, and pulsed light. Non-thermal processes have different mechanisms of preserving foods compared to thermal processes. For example, the mechanism of preservation by HPP is microbial inactivation due to ultra high pressure. The critical properties of packaging materials should be maintained during and after non-thermal processing. Therefore, the material and design of packaging should be changed accordingly to assist preservation of food products by these non-thermal processes.
This chapter provides background on the design of packaging materials for non-thermal processing. First, non-thermal processing (HPP, PEF, irradiation, and pulsed light) of foods and beverages is described briefly. Next, the design of packaging materials for non-thermal processing such as HPP, PEF, irradiation, and pulsed light is discussed. Finally, the chapter closes with focus on some of the future research directions in the area of packaging for non-thermal processing.
16.2 Non-thermal processing of foods and beverages
Non-thermal processing offers an alternative to conventional thermal processing to meet demands for convenient, high quality, and minimally processed food products. Non-thermal processes are considered to be more energy efficient and are better at preserving the quality attributes of the product (Morris et al., 2007). High pressure processing (HPP), pulsed electric field (PEF), irradiation, and pulsed light are described below.
16.2.1 High pressure processing (HPP)
High pressure processing is a non-thermal process in which a solid or liquid food product is subjected to pressures ranging from 100 to 800 MPa for a specified period of time (0.5–5 min.). HPP inactivates most of the pathogenic and spoilage microorganisms in foods with minimal changes in nutrients, texture, color, and flavor. The microorganisms are inactivated due to interruption of cellular functions at such high pressures (Torres and Velazquez, 2005). The extent of microbial inactivation depends on the type of microorganisms (Gram-positive bacteria are more resistant than Gram-negative bacteria), composition of the food, pH, water activity, package integrity, product temperature, vessel temperature, pressure applied, time to achieve pressure, treatment time, and time of decompression (Morris et al., 2007).
A typical HPP unit consists of a high pressure vessel (2 L to 360 L) with enclosure, a pressure generation system, a temperature sensing device, a material handling system, and a pressure transmitting medium (Barci and Wilbey, 1999). The process starts with loading of pre-packaged products in the pressure vessel, followed by filling the remainder of the vessel with pressure transmitting medium (usually water), closing the vessel, achieving the desired process pressure, holding the product at the desired pressure for a specified period of time, depressurizing the vessel, and finally unloading the product. HPP can also be used to process the product in bulk with a piston imparting the desired pressured and then aseptically filling the treated product into pre-sterilized packages.
Some of the current applications of high pressure processing include pasteurization of jams, oysters, ready-to-eat meats, guacamole, liquid whole eggs, and fruit juice (Balasubramaniam and Farkas, 2008). HPP can also be used as a post-packaging lethality step for inactivation of Listeria monocytogenes in ready-to-eat meats such as sliced ham and deli meat. Some of the disadvantages of HPP include higher cost, batch or semi-continuous operation, and limited effectiveness against spores and enzymes (Balasub-ramaniam and Farkas, 2008). Some of the future developments in HPP include increasing the pressure to 800 MPa for commercial units and combining HPP with thermal processing to possibly achieve ambient temperature shelf stable low acid food products (Morris et al., 2007).
16.2.2 Pulsed electric field (PEF)
Pulsed electric field utilizes high intensity electric pulses to inactivate microorganisms in a fluid food product. The field intensity is in the range of 15–80 kV/cm and is applied only for a couple of microseconds. The applied electric field inactivates microorganisms by creating a potential difference which is high enough to break down the lipid membrane of microorganisms. The extent of microbial inactivation depends on the type of microorganisms, composition of the food, pH, ionic strength, ionic conductivity, presence of air bubbles and particles, field strength, treatment time, treatment temperature, and pulse shape.
A typical PEF unit consists of a pulse moderator and a set of PEF treatment chambers. The pulse moderator can turn on and off to generate a nearly square waveform. The food product is exposed to the electric pulses in the treatment chamber. It consists of at least two electrodes (one on high voltage and the other on ground potential) separated by an insulating material. The geometry of treatment chambers can be parallel plate, co-field flow, or coaxial cylinders (Ravishankar et al., 2008). The low amount of energy required for microbial inactivation during PEF raises the product temperature only by a few degrees. This results in product with high quality compared to conventional thermal processing.
Some of the disadvantages of PEF include higher cost, applicability to fluid products without air bubbles and products with low electrical conductivity, and limited effects on spores (Morris et al., 2007). Some of the current applications of PEF include pasteurization of juices, milk, soups, and liquid egg products (Rastogi, 2003).
16.2.3 Irradiation
Irradiation of foods involves exposure of pre-packaged or bulk foods to ionizing radiations such as-rays from the radioactive isotopes of 60Co and 137Cs, X-rays, or electron beams. The applied radiation inactivates microorganisms by damaging their genetic materials. The dose of radiation is expressed in kiloGray (kGy), where 1 kGy is equal to 1 kJ/kg. Food products could either be pasteurized (1–5 kGy) or sterilized (10–74 kGy) depending on the dose of radiation (Morris et al., 2007). All radiation processes, including their packaging materials, should be approved by the US Food and Drug Administration (FDA) because the FDA considers the radiation sources as food additives. The FDA also requires all irradiated foods to carry a 'Radura' symbol along with the statement 'treated with radiation' or 'treated by irradiation'. The primary regulation governing irradiation of food is covered in 21 CFR 179.
Some of the FDA-approved uses of irradiation include insect disinfesta-tion (0.2–0.5 kGy), sprout inhibition in potatoes (0.05–0.1 kGy), food inspection (0.01 kGy), microbial disinfection of dried spices (< 30 kGy), irradiation of fresh or frozen poultry products (1.5–3 kGy), irradiation of refrigerated and frozen meats (4.5–7 kGy), and sterilization of aseptic packages (Paul and Takeguchi, 1986; Komolprasert and Morehouse, 2004; Morris et al., 2007). Some of the disadvantages of irradiation include undesirable changes (lipid oxidation, off-flavor, discoloration, etc.) in some of the products and adverse consumer perception (Morris et al., 2007).
16.2.4 Pulsed light
Pulsed light involves exposure of foods to intense (0.01–2500 J/m2) and short duration (1 μs to 0.1 s) pulses of broad spectrum white light (Morris et al., 2007). The spectrum of light used in pulsed light includes ultraviolet (200–400 nm), visible (400–700 nm), and infrared (800–1100 nm). UV radiation with shorter wavelengths yields a higher microbial inactivation because they have higher energy levels. Pulsed light is very effective for inactivation of microorganisms on food surfaces, processing equipment, and food packaging materials (Oms-Oliu et al., 2010). The microorganisms are inactivated due to the chemical modification and cleavage of DNA. The extent of microbial inactivation depends on the transmissivity of the product, geometric configuration of the reactor, duration and number of pulses, intensity and wavelength of light, distance of the product from the light source, thickness of the product, and type of the package. Pulsed light is better for package sterilization than chemical sterilization by H2O2 or peracetic acid because it does not leave any chemical residue.
A typical pulsed light unit consists of a high energy capacitor, a trigger, a flash lamp filled with an inert gas, and a pulse forming network. The trigger signals the discharge of high electrical energy into the flash lamp. The energy released into the lamp produces an intense light pulse which can be directed towards the target (Uesugi, 2010). Some of the current applications of pulsed light include mold inactivation in baked products and shelf life extension of fish (Morris et al., 2007). Some of the disadvantages of pulsed light include limitation to inactivate microorganisms only on the surfaces and excessive temperature increase at the surface of the food (Morris et al., 2007).
16.3 Selection of packaging materials for non-thermal processing
Foods are packaged to protect them from the environment and to maintain them in proper condition during shipping, distribution, retailing, and home storage. Food packaging enables foods to be safe and wholesome from the time of production until the food is consumed. Food packaging can extend the shelf life of a product by retarding deterioration of the product and retaining the beneficial effects of processing. A package has to provide protection from chemical, biological, and physical agents. Chemical agents lead to compositional changes by factors such as exposure to gases, moisture, or light (visible, infrared, or ultraviolet). Biological agents such as microorganisms, insects, and rodents lead to spoilage of food products. Physical agents such as mechanical forces (impact, vibration, and compression) damage the product (Marsh and Bugusu, 2007).
Properties of interest for food packaging materials are mechanical, barrier, thermal, and rheological properties. Mechanical properties of interest are tensile modulus (TM), tensile strength (TS), percent elongation (%E) at break, and seal strength. Tensile modulus is a measure of the resistance of a material to deformation. Tensile strength is the maximum tensile stress a material can sustain whereas %E is an indication of flexibility of a packaging material. Seal strength is a measure of the ability of a package seal to resist separation. Barrier properties of a packaging material play an important role in determining the shelf life of a food product. Barrier properties of a material indicate their resistance to sorption and diffusion of moisture and gases across the packaging material. Barrier properties of interest in food packaging are water vapor permeability (WVP) and oxygen permeability (OP). Thermal properties of interest for packaging materials are glass transition temperature (Tg), thermal stability, and heat deflection temperature (HDT). Rheological properties of a packaging material are important to understand the processability of the material. Rheological measurements indicate melt processing behavior of packaging materials during unit operations such as injection molding and blown film processing.
Most food packaging materials have been designed for thermally processed foods. When designing food packaging materials for thermally processed foods, one considers the properties of packaging materials at the treatment temperature. Non-thermal processes have different mechanisms of preserving foods compared to the thermal process (pressure in HPP, electric field in PEF, etc.). Thus, the development of packaging for non-thermal processing requires an understanding of the packaging material properties and interaction between packaging materials and food components during the non-thermal processes. Packaging materials for non-thermal processes should have proper resistance to the particular non-thermal processing mechanisms. The material and design of packaging should be changed accordingly to assist preservation of food products by these non-thermal processes.
16.3.1 Packaging materials for HPP
Food products are processed using HPP either in bulk along with aseptic packaging or prepackaged in flexible or semi-rigid packaging materials. HPP in bulk along with aseptic packaging is not very common because it requires a costly aseptic packaging installation to package HPP-treated foods. For prepackaged foods, selection of the packaging material becomes crucial because the packaging material needs to be flexible enough to withstand such high pressures while maintaining sealability and physical integrity. HPP causes the product volume to compress by as much as 12% depending on the treatment pressure. The package should be able to withstand this reduction in volume and restore itself to the original volume without any significant change in mechanical and barrier properties. Therefore, metal cans, glass bottles, and paperboard-based packages are not well suited for HPP because they tend to deform irreversibly or fracture (Caner et al., 2004; Han, 2007). The packaging materials should also prevent any migration (global migration limit of 10 mg/dm2) of package components to the food product under the high pressure treatment. Another important aspect of packaging for HPP is the requirement for minimum headspace. The headspace in the package should be kept as small as possible because air, being very compressible, can exert higher deformation strains on the packaging material during pressure treatment (Lambert et al., 2000).
HPP is a non-thermal process. However, the temperature of the food rises as a result of adiabatic heating. The degree of adiabatic heating depends on the composition of the food product. For water-based foods, temperature increase due to adiabatic heating is 2–3°C for every 100 MPa increase in pressure. The temperature drops back to or below the initial temperature after the pressure is released. The effect of increased temperature on the properties (mechanical, barrier, and thermal) of packaging material during pressure treatment should also be considered while designing the packaging material (Caner et al., 2004).
Mechanical properties
Lambert et al. (2000) studied mechanical (tensile strength, seal strength, and delamination) properties of six multilayered plastics with different combinations of polyamide (PA), polyethylene (PE), polyethylene terephthalate (PET), and polyvinylidene chloride (PVDC). The tensile strength of the laminates increased after high pressure treatment. Increased tensile strength indicates that the laminates became more rigid and less flexible. However, for five of the six packages, the increase was within the 25% allowable deviation used as an industry norm. The change in tensile strength was not influenced by the initial rigidity or thickness of the packaging material. The change in seal strength for five of the six packages was also within the 25% allowable deviation. Delamination occurred in the package which had a significant (> 25%) change in tensile strength and seal strength. Delamina-tion can also occur due to the presence of air in the package or product (Lambert et al., 2000).
Galotto et al. (2009) studied the effect of high pressure processing on the mechanical properties of a biopolymer (polylactic acid (PLA) coated with silicon oxide) and a synthetic polymer (PET coated with aluminum oxide). The TS of the PET films increased after HPP treatment whereas %E decreased. The decrease in % E can be attributed to the formation of pin-holes and cracks in the film during HPP. For the PLA films, there was a decrease in both TS and %E after HPP treatment.
Koutchma et al. (2010) evaluated the effects of preheating and subsequent high pressure-high temperature processing at 688 MPa and 121°C on seal strength of six selected commercially available packaging materials (biaxial nylon/coextruded ethylene vinyl alcohol (EVOH), nylon/polypropylene (PP), PET/aluminum oxide (AlOx)/casted polypropylene (CPP), PET/polyethylene (PE), PET/Al/CPP, and nylon/Al/CPP). Preheating to 90°C affected the seal strength of the EVOH pouch. However, the effect of preheating on the seal strength of PET/AlOx and PET-Al pouches was not significant. The PET/AlOx pouch showed the lowest seal strength among the pouches tested. The seal strengths of PET-Al and nylon/Al pouches increased significantly after the high pressure-high temperature process. There were no significant changes in seal strength for the other materials.
Barrier properties
Lambert et al. (2000) studied barrier (WVP and OP) properties of five multilayered plastics with different combinations of polyamide (PA), polyethylene (PE), polyethylene terephthalate (PET), and polyvinylidene chloride (PVDC). The industry norm for the deviation in barrier properties after HPP is 12%. There was significant change in the value of WVP after HPP for all the packaging materials. WVP values increased or decreased depending on the constituents of packaging materials. There was significant change in the value of OP after HPP for two of the five packaging materials. The increase or decrease in the value of OP also depended on the constituents of packaging materials (Lambert et al., 2000).
Galotto et al. (2009) studied the effect of high pressure processing on the barrier properties of a biopolymer (polylactic acid (PLA) coated with silicon oxide) and a synthetic polymer (PET coated with aluminum oxide). There was a significant increase in the OP and WVP values of HPP-treated films compared to the untreated films. For the PLA films, a large change in the properties occurred when the film was in contact with water. This was attributed to the plasticization effect of water on PLA. Thus, the effect of HPP on the barrier properties of selected packaging materials should be considered when designing packaging materials for HPP.
Koutchma et al. (2010) evaluated the effects of preheating and subsequent high pressure-high temperature processing at 688 MPa and 121°C on oxygen permeability (OP) of six selected commercially available packaging materials (biaxial nylon/coextruded ethylene vinyl alcohol (EVOH), nylon/ polypropylene (PP), PET/aluminum oxide (AlOx)/casted polypropylene (CPP), PET/polyethylene (PE), PET/Al/CPP, and nylon/Al/CPP). Preheating significantly increased OP values for all the materials tested. The PET-AlOx pouch showed the greatest loss of oxygen barrier followed by PP, EVOH, PET-Al, nylon-Al, and PE. After subsequent high pressure-high temperature processing, there were no significant changes in OP values for the EVOH, PET-AlOx, and PET-Al pouches. However, a slight increase in OP value was observed for the nylon-Al pouch. The PE pouch had the highest OP values among all the materials tested.
16.3.2 Packaging materials for PEF
Aseptic packaging is considered the most appropriate packaging format for PEF-treated food products. Selection of proper packaging materials is important to retain the quality of PEF-treated foods during storage because the packaging materials can absorb the flavor compounds or degrade flavor, color, and nutrients through transmission of oxygen (Ayhan et al., 2001). Plastic containers and paper laminated materials are commonly used packaging materials for aseptic food packaging. The shelf life of products packaged in plastics and paper laminated materials depends on the barrier properties (OP and WVP) of the packaging material. Packaging materials with very low values of OP should be selected for PEF-treated foods which are prone to oxidation. For oxygen-sensitive food products, oxygen in the food and in the headspace of the package should also be minimized. For thermoformed containers, thermoforming conditions should be optimized so that the packaging material retains sufficient barrier and mechanical properties (Han, 2007).
Ayhan et al. (2001) studied the effects of packaging materials (glass, PET, high density polyethylene (HDPE), and low density polyethylene (LDPE)) on flavor, color, and nutrient quality of single strength orange juice treated in a pilot plant scale PEF (35 kV/cm, 59 μs) system. Glass and PET were effective in retaining the flavor compounds in the juice. However, significant absorption of flavor compounds by HDPE and LDPE occurred within 2 weeks of storage at 4 and 22°C. There was no significant color change for glass and PET during 112 days of storage at 4 and 22°C. However, the color of the juice packaged in HDPE and LDPE bottles changed significantly after 28 days of storage at 22°C. This could be attributed to higher OP values of polyethylene packages at 22°C. The results also showed that the concentration of ascorbic acid in glass and PET bottles was significantly higher than that in HDPE and LDPE bottles during storage at 4°C.
16.3.3 Packaging materials for irradiation
Foods are usually prepackaged for irradiation to prevent recontamination. Any packaging material must be approved by the FDA for use in food irradiation because gases and low molecular weight polymers formed during irradiation could potentially migrate to foods. Packaging materials used in irradiation should not transmit any radiolysis product (RP) to foods. Formation of RPs depends on the absorbed dose, dose rate, atmosphere, temperature, time after irradiation, and food stimulant. The RPs from the polymers usually consist of low molecular weight aldehydes, acids, and olefins. Therefore, a pre-market safety assessment of packaging materials and their RPs after exposure to irradiation is carried out to evaluate new packaging materials for irradiation. Packaging materials approved for use during irradiation are listed in 21 CFR 179.45 (Han, 2007).
Packaging materials for irradiation should have chemical and physical stability under radiation without depolymerization or significant change in mechanical and barrier properties. The two main effects of ionizing radiation on polymers are crosslinking (polymerization) and chain scission (degradation). These two competing effects occur simultaneously and which effect dominates depends on the composition of the polymer, irradiation condition, and irradiation dose. Crosslinking of polymers during irradiation dominates under vacuum or an inert atmosphere, whereas chain scission during irradiation dominates in the presence of oxygen or air. Crosslinking can increase TS and decrease %E. Chain scission involves random rupturing of molecular bonds and results in decreased chain length and production of gases (Chuaqui-Offermanns, 1989; Ozen and Floros, 2001; Komolprasert and Morehouse, 2004; Han, 2007). Irradiation at higher dose can also result in significant loss of seal strength. Therefore, physical testing should be conducted to determine the package integrity of packaged foods irradiated at a very high dose (Han, 2007).
Mechanical properties
Goulas et al. (2004) studied the effect of gamma radiation on the mechanical and physico-chemical properties of different monolayer and multilayer semi-rigid plastic packaging materials. The packaging materials evaluated included polystyrene (PS), polypropylene (PP), PET, HDPE, polyvinyl chloride (PVC)/HDPE, and HDPE/PA-6. For all polymers, radiation up to dose levels of 10 kGy had no significant effect on mechanical properties. PET was the most stable polymer with no effect of irradiation on mechanical properties until a dose level of 60 kGy. The remaining polymers showed moderate to severe degradation at 60 kGy. The degradation in mechanical properties can be attributed to the radiation induced oxidative degradation of polymers in the presence of air. The mechanical property most affected by irradiation was % E. Thus, % E could be used as a parameter to assess radiation stability of polymers. Radiation doses of up to 10 kGy induced no significant changes in the overall migration from the packaging materials. Higher dose levels (30 and 60 kGy) induced differences in overall migration from PP, HDPE, and PVC/HDPE. The highest migration value (3.3 mg/dm2) obtained was from PP at 60 kGy in contact with iso-octane. This migration value was still much lower than the overall migration limit of 10 mg/dm2 set by the European Union (Goulas et al., 2004).
Barrier properties
Goulas et al. (2003) reported that there was no significant difference in the barrier properties (oxygen, CO2, and water vapor) of irradiated and control samples for different multilayer commercial coextruded packaging films (PP/ethylene vinyl alcohol (EVOH)/LDPE/linear low density polyethylene (LLDPE), LDPE/EVOH/LDPE, ionomer/EVOH/LDPE, PA/LDPE, LDPE/PA/ionomer). Similar results on the barrier properties of LDPE, HDPE, PET, and PS have also been reported (Han, 2007).
16.3.4 Packaging materials for pulsed light
Packaging materials for pulsed light treatment should be transparent because light cannot penetrate opaque surfaces (Han, 2007). In addition, packaging materials should have resistance to heat because pulsed light treatment can result in excessive heating at the surface of the food. The surface topography of the packaging materials affects microbial inactiva-tion by pulsed light treatment. The surface with smooth finish reduces the efficacy of pulsed light treatment because the hydrophobic and reflective nature leads to clustering of the microbial cells. On the other hand, rough surfaces with pores are not suitable for pulsed light treatment because the microorganisms can hide in those small openings (Oms-Oliu et al., 2010).
16.4 Future trends
Future developments in the design of packaging materials for non-thermal processing will focus on determining physical and chemical changes of packaging materials subjected to non-thermal processes, understanding the interactions between packaging materials and the food components during the non-thermal processes, development of better testing protocols, and understanding the mechanism of microbial inactivation by non-thermal processes. There is a need for better collaboration between industry, academia, and government agencies to work towards commercialization of these non-thermal processes. Future research will also focus on expanding the use of non-thermal processes by combining these processes with other thermal or non-thermal processes. The use of HPP in combination with moderate thermal processing is already an active research area to produce shelf stable low acid food products. Research is also geared towards combining HPP with active packaging technologies such as antimicrobial packaging to achieve the desired level of microbial inactivation.
As with any new technology, consumer acceptance and regulatory requirements for these non-thermal processes will govern the commercialization of these processes in the future. Currently, regulations are in place only for packaging materials required for irradiation. There is a need to develop similar regulations for packaging of other non-thermal processing such as high pressure processing. Non-thermal processing will become more widespread because of the increasing consumer demand for convenient, high quality, and minimally processed food products. However, the selection of the appropriate packaging material will be very critical to maintain the high quality throughout the shelf life of the food products.
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