Emerging packaging technologies for fresh produce
Abstract:
The nature of fresh produce is that of a live respiring body, whose freshness is maintained only when its cells are live and active. Packaging technologies employed to keep produce fresh should be based on the principle that any changes in physical, chemical or biological quality should be suppressed, while the minimum level of metabolic activity in live plant cells should be maintained. Modified atmosphere packaging, usually with reduced O2 and elevated CO2 concentrations, can lower the rate of both respiration and ethylene production, thereby delaying produce senescence. Packaging for produce that absorbs deleterious volatile compounds and releases beneficial compounds can reduce physiological injury, delay the ripening process, and inhibit microbial spoilage. Such freshness-enhancing active packaging makes use of sachets, films, coatings, or a combination of these methods. To find application in the field of fresh produce packaging, innovative technologies need to be combined, or fine-tuned for specific commodities, storage/distribution conditions and shelf life requirements, and to meet specific consumer preferences.
7.1 Introduction
Packaging technologies remain a key area of interest for the fresh produce industry. When they are used successfully, the safety and shelf life of produce can be greatly improved. Today one of the main goals in the field is the development of innovative packaging with enhanced functions: this is in response to both increased consumer demand for minimally processed fresh produce, for reasons of health and convenience, and to the ongoing difficulties in maintaining fresh characteristics during postharvest storage and distribution. Changes in retail and distribution practices associated with market globalization, new consumer product logistics, and more stringent regulatory requirements have also increased the demand for emerging technologies able to reduce losses while improving the quality and safety of minimally processed fresh produce. Active and intelligent packaging technology is being developed as a result of these driving forces. Several emerging technologies have been of particular interest, including antimicrobial packaging and coating, chlorine dioxide package systems, 1-MCP package delivery systems and combinations of MAP and active packaging. Each part of this chapter focuses on one of these emerging technologies, describing the principles and mode of action of key developments in these areas.
7.2 Modified atmosphere packaging (MAP)
Modified atmosphere packaging (MAP) is a widely used modern packaging technology used to extend the shelf life of minimally processed fresh produce. As the name suggests, MAP of fresh produce involves packaging produce in polymer films that maintain a commodity-specific modified atmosphere such as reduced oxygen levels and elevated carbon dioxide levels (Kader & Saltveit, 2003; Yam & Lee, 1995).
The mixture of gases inside a package for fresh produce can be different from that in the normal atmosphere outside the package, so long as the total pressure inside and outside the package is equilibrated. In order to maintain this equilibrium, a number of parameters must be taken into account, including the respiration rate and mass of the product in question, the thickness and surface area of the polymer film, the gas transmission rates through the film, the initial free volume and atmospheric composition within the package, and external environmental factors such as temperature and relative humidity (Fig. 7.1) (Yam & Lee, 1995; Kader & Saltveit, 2003; Mannapperuma & Singh, 1994). Metallocene polymerization, blending, lamination, and coextrusion are examples of emerging technologies which are used to deal with the permeability of the film and its physical properties such as optical properties and seal bonding (Brandenburg & Zagory, 2009; Acosta et al., 2011).
Modified atmosphere can be created in two ways:
1. Passive MAP. The produce respiration rate is matched with films of appropriate permeability to generate an atmosphere that passively evolves as a result of consumption of oxygen and evolution of carbon dioxide due to the respiration. Films of adequate gas permeability must be chosen such that oxygen enters the package at a rate offset by the consumption of oxygen by respiration of the fruit or vegetable in question. Similarly, carbon dioxide must be vented from the package at a rate offset by the production of carbon dioxide as a result of produce respiration. This modified atmosphere should ideally be established rapidly, without creating injuriously high levels of carbon dioxide or low levels of oxygen. A proper balance between gas permeation and respiration is important in attaining the target atmosphere in the package.
2. Active MAP. Active MAP is performed in two ways. In the first method, the package is evacuated and then flushed with the desired gas mixture. In the second method, the desired gas mixture is continuously flowed into the package through a lance, replacing the existing air. Absorbers or adsorbers may be used within the package to scavenge oxygen, carbon dioxide or even ethylene, helping to maintain an atmosphere that promotes extended shelf life. Active MAP for fresh produce is an essential method for high oxygen MAP, which will be described later in the chapter.
7.2.1 Low oxygen MAP
The most commonly applied MAP for fresh produce is low oxygen MAP, where the oxygen concentration inside the package is lowered to 1–10% instead of the usual 21% in air. These low concentrations of oxygen in the package act to suppress produce respiration rates. Low oxygen MAP also involves increasing the carbon dioxide concentration inside the package to above the usual 0.03% in air. Elevated carbon dioxide levels also help to reduce produce respiration rates. The combination of reduced oxygen and elevated carbon dioxide levels reduces the respiration rates of fresh produce and decreases ethylene production, thereby delaying ripening, preventing softening of texture, maintaining freshness, and ultimately extending the shelf life of the product.
The use of low O2 MAP for the packaging of various fresh produce commodities is already well established. For most fresh produce a typical gas mixture for MAP contains oxygen concentrations in the range of 1–10% and carbon dioxide concentrations in the range of 1–20%. For example, a packet of mixed salad has a typical equilibrium modified atmosphere consisting of 5% oxygen, 15% carbon dioxide, and 80% nitrogen. Specific gas mixture compositions for a variety of fruits and vegetables can be found in the literature (Yam & Lee, 1995; Sandhya, 2010).
7.2.2 High oxygen MAP
Research has also been carried out on the application of high oxygen MAP to the packaging of fresh produce. High oxygen MAP involves using oxygen concentrations greater than 40%, and usually between 70% and 95%. High oxygen MAP has been found to be effective in inhibiting enzymatic discoloration, preventing anaerobic fermentation reactions and inhibiting micro-bial growth. Studies have also shown that exposure to high oxygen alone (80–90% oxygen, balance nitrogen) did not inhibit microbial growth but caused a variable reduction in growth rate of some tested microorganisms at low temperature and prolonged the lag phase of growth (Jacxsens et al, 2001; Conesa et al., 2007). It has been suggested that high oxygen levels lead to intracellular generation of reactive oxygen species (ROS), damaging cell components and thereby reducing cell viability. When high oxygen levels are combined with an increased carbon dioxide concentration (10–20%), a more effective inhibitory effect on the growth of microorganisms has been observed. Exposure to high oxygen levels may stimulate or reduce produce respiration rates, or may have no effect, depending on produce type, maturity and ripeness, oxygen concentration, storage time and temperature, and the carbon dioxide and ethylene concentrations. This respiration rate is correlated to the shelf life of the produce commodities.
Several factors have so far prevented the commercialization of high oxygen MAP for fresh produce: a lack of understanding of the biological mechanisms involved in inhibiting microbial growth and enzymatic browning; the effect of the MAP on the respiratory activity and nutritional quality of produce; and concerns about the safety of packaging produce in high oxygen MAP. Concentrations of oxygen above 25% are considered to be explosive and hence special precautions need to be taken when packaging produce in high oxygen MAP. A study on shredded chicory endives, grated celeriac and sliced mushroom have shown that high oxygen atmospheres can be used to extend shelf life as an alternative to low oxygen atmospheres for some specific ready-to-eat produce commodities that are sensitive to enzymatic browning and spoilage by yeasts (Fig. 7.2) (Jacxsens et al., 2001). However, it should be mentioned that the effectiveness of high O2 MAP differs with commodities, types of packaging film, and storage temperatures. Further research into high oxygen MAP in various types of fresh produce and its relation to shelf life is currently being carried out in order to determine whether this method can be used to extend the shelf life of packaged fresh produce.
Fig. 7.2 Comparison between high O2 MAP (HOA, filled with 95% O2 with balance N in barrier film bag) and low O2 MAP (EMA, typically 3% O2/5% CO2) in shelf life limited by yeast growth (m) or sensory color change (s) at 4 °C. Constructed from the data of Jacxsens et al. (2001).
7.2.3 Development in MAP of fresh produce and increasing its applications
In order for the application of MAP technology to be extended to a wide variety of commodities, plastic films with a wide range of gas permeability properties are required. Many studies have been devoted to the search for polymer materials able to enlarge the available range of O2 and CO2 permeabilities for MAP of fresh produce; however, for certain commodities and in conditions of temperature abuse, optimal modified atmospheres remain very difficult to achieve. Even though polyolefin films, with a CO2 to O2 permeability ratio in the range of 3–7, work well for some commodities whose optimal atmosphere has low concentrations of O2 and CO2 (such as apples and tomatoes), it is hard to find an appropriate polymer film that will provide benefits for many fruits and vegetables. Specialized films, windows, and permeation tools have been tried as means of attaining high gas permeability and a wide range of CO2 to O2 permeability ratio. Some typical examples include a window of silicon gum film (Li et al., 2007), hydrophilic films (Barron et al., 2001), and a microporous earthenware window (Yun et al., 2006). Microperforations on the plastic film have recently been widely adopted for fresh-cut products with high respiration rates in order to increase the gas permeation: this technology will be discussed in detail below.
The benefits offered by microperforated films are a high rate of gas transmission and an almost equal ratio of CO2 to O2 transmission, which cannot achieved by synthetic polymeric films. Although the incorporation of inert fillers such CaCO3 and subsequent biaxial orientation can create small perforations of 0.14–1.4 μm, this is not sufficient to meet the requirements for fresh produce with a high respiration rate. Drilling holes in the plastic films can achieve the required degree: macroperforation greater than 300 μm in diameter and microperforation of 5–300 μm in diameter (Mir, 2009). Another advantage of microperforation technology is its use of a thicker tray, which cannot provide a high transmission rate. The gas transmission depends on hole size and perforation number, and can be estimated using several mathematical models (Ozdemir et al., 2005; Gonzalez et al., 2008; Del-Valle et al., 2003; Allan-Wojtas et al., 2008). A large number of small holes is better able to provide a consistent O2 transmission rate than a small number of large holes (Allan-Wojtas et al., 2008). The O2 and CO2 transmission contributed by the plastic film layer and by air in microperforations can help to create the optimal modified atmosphere for produce with high respiration rates as shown in Fig. 7.3.
Microperforation can be achieved in a number of ways. Mechanical puncturing using a cold or hot needle has been practised for some time and is commonly used for making macroperforations. The electrostatic discharge process causes melting and vaporization by a series of electrical discharges onto the plastic material and is also known as spark machining or spark eroding. The laser method uses high energy laser radiation on the film and is a relatively new means of producing microperforations at high speed and with consistent quality (Mir, 2009). The laser energy generated by the CO2 laser system is absorbed by a plastic film at specified points, allowing the film spots to be heated and melted. Commercially available technologies are Albert Fisher’s FreshHold® and Print Pak’s P-Plus®.
Temperature abuse during transportation, storage, and marketing of fresh produce is a primary concern in MAP for fresh produce, because poor temperature control can lead to the deterioration of a packaged product as a result of an increase in product metabolism and the growth of food spoilage organisms. With an increase in temperature, the O2 level in the package decreases and CO2 level increases: this is because the temperature increase does not cause the permeability of the package film to the O2 and CO2 gases to increase to the same extent as the respiration rate of the produce. The low O2 concentration and high CO2 concentration are detrimental to fresh produce, causing physiological damage and off-flavors. The phase-transition of crystallizable polymer can cause reversible changes in gas permeability at the transition temperature, which can be tailored to respond to the temperature-dependent respiration of fresh produce. This concept can be used to develop a modified atmosphere packaging system irrespective of temperature increase and has been commercialized by Landec under the name of Intelimer® polymers. This type of temperature-switching polymer is sometimes used as a customized permeable membrane which is applied as a package label, and has also been combined with microporous polymeric film to create a more advanced variant in a recent patent (Clarke et al., 2002).
Hypobaric packaging has also been employed in attempts to attain low oxygen concentrations around produce (An et al., 2009; Knee & Aggarwal, 2000). Although it is not a commercial package, the produce compartment in household refrigerators has been tested for its ability to maintain high humidity and a passive modified atmosphere, which would be beneficial in keeping produce fresh (Kim et al., 2010). It was shown that the mechanical tightening of the sliding drawer door greatly reduced gas permeability, which in turn helped to maintain high humidity levels and modified atmosphere conditions (specifically, lower oxygen and higher carbon dioxide concentrations compared to outside air). Air-tight compartments were helpful in preserving the physical, chemical and sensory qualities of prepared or minimally processed fresh vegetables by maintaining high relative humidity and reducing gas permeability.
The complexity of fresh produce MAP and the number of different variables involved has led many researchers to search for a systematic method of MAP design by using mathematical modeling (Hayakawa et al., 1975; Hertog et al., 1999; Mannapperuma & Singh, 1994). Any model of fresh produce MAP design is based on mass balance equations of O2, CO2, and N2 consisting of respiration from the produce and transport through the packaging film:
where V is the headspace in the package (mL), yO2, yCO2, and yN2 are the respective gas concentrations for O2, CO2, and N2 (molar fraction), L is the thickness of polymeric film (μm), 
, 
, and 
are, respectively, O2, CO2, and N2 permeability of the package (mL μm h– 1 m– 2 atm– 1), Af is package film area (m2), RO2 and RCO2 are the respective respiration rate in consumed O2 and produced CO2 (mL h– 1 kg– 1), and W is weight of the produce (kg). Nitrogen balance (Eq. [7.3]) is often omitted in simplified treatments for equilibrated conditions due to the substantially lower permeability of N2.
While studies on modeling vary in the way that they deal with underlying assumptions and with the dependence of respiration on environmental conditions, their value lies in the convenience of their application in practical situations such as in produce packers and processors. By compiling the data on produce respiration data and plastic film permeability, a knowledge base has been established that allows a package to be designed specifically for a particular product, in the form of user-friendly software called PACKin-MAP® (Mahajan et al., 2007). Figure 7.4 shows the input window for designing MAP, which will output the parameters creating the right atmosphere for packaging a particular product. The software considers three types of packaging systems: polymeric films without perforations, macro-perforated polymer films and perforation-mediated packaging systems (microperforated). The software can determine the intrinsic properties of the produce, i.e. respiration rate, optimum oxygen and carbon dioxide gas concentrations, and film permeability characteristics, and can also determine a suitable film for the particular product, i.e. its area, thickness, filling weight, equilibrium time and equilibrium gas composition based on the mass balance equations for the package, built-in databases, predictive models and required input data from the user. For the perforations examined earlier in this chapter, Eqs [7.1–7.3] need to be modified to incorporate the diffusion of gas through air channel.
Fig. 7.4 Screenshots from the PACK-in-MAP software for MAP design for fresh produce (adopted from packinmap.com).
Recently there have been trials using argon, nitric oxide, nitrous oxide, helium, and xenon gases as an additive to low O2 or high O2 MAP of fresh produce. The possible beneficial effects are the inhibition of ethylene action, enzymatic browning, and/or respiration activity (Jiang et al., 2011; Rocculi et al., 2005; O’Beirne et al., 2011; Zhang et al., 2008). However, literature sources differ in their accounts of the response of the product to these gases, and thus the application of these gases in MAP is dependent on further research to determine the specific conditions of use (Tomas-Callejas et al., 2011).
7.3 Active packaging
The conventional function of a package is to protect the food from the external environment, inhibiting chemical and physical changes. Increased consumer demand for minimally processed foods and foods without additives, along with increased consumer concern about food safety, have led the food packaging industry to search for an innovative packaging technology where the package contributes to maintaining the quality and safety of the product in addition to simply acting as a protective barrier. Active packaging is an emerging area in which most of these requirements can be met. With the introduction of the concept of ‘active packaging’, the function of the package has changed from passive protection to active contribution to maintaining the quality and safety of the food. Traditionally, packages have been considered to offer passive protection, as they only provide a barrier to protect the product against moisture, oxygen and physical damage such as bruises and cuts. However, in active packaging of fresh produce, the package, the produce, and the environment interact in a positive way that helps to extend the shelf life while satisfying consumer demand for quality and convenience.
The workings of any active packaging technology are based on chemical, physical, and biological changes between the food, package, and product headspace. There are two main types of active packaging: absorbing systems and releasing systems. In an absorbing system, the package interacts with the product and package headspace and removes some of the undesirable compounds from the package headspace that may cause physiological damage to the product and shorten its shelf life. Ethylene, moisture, and carbon dioxide are examples of compounds that can be detrimental to the quality of fresh produce if present in excessive amounts for a longer period of time. On the other hand, a releasing system releases some compounds (antimicrobials, ethylene inhibitors) into the package headspace that are beneficial for the product and enhance food quality and safety for an extended period.
7.3.1 Absorbing system
Fresh produce continues to respire even after harvest, producing moisture and carbon dioxide:
The transpiration of moisture from respiration and the humidity differential between the surface of the produce and the environment leads to the accumulation of moisture vapor inside the package, which may saturate the headspace and condense on the internal surface of the package and on the surface of the produce. The condensed moisture may act as a site for micro-bial growth and decay of the product. The controlled absorption of moisture from the package headspace by a moisture absorber can maintain unsatu-rated water activity at the correct level inside the package. Different types of moisture absorbers are available in the market depending on the specific requirements of the application, for example, CaCl2, KCl, and sorbitol for fast absorption, and bentonite for slow absorption. A combination of different absorbers can be used to achieve a particular absorption rate (Mahajan et al., 2008). These absorbers can be used in the form of a sachet, whereby the absorber content in the sachet is determined by the size of the package, the type of produce, and the duration of storage. Salts, sugars, clays, and silica gel have been used in attempts to buffer humidity inside the package for blueberry, mushroom, tomato, etc. (Mahajan et al., 2008; Song et al., 2001). The use of a fiberboard box is another innovative approach that does not require a desiccant insert. An integral water vapor barrier is attached to the inner surface of the box; this acts as a wick and as an unwet-table layer, which is, however, highly permeable to water vapor, next to the fruits and vegetables inside the box. This multilayered box is able to absorb water vapor when humidity rises and temperature drops inside the package, and conversely can release water vapor when temperature rises and humidity drops. For proper control of relative humidity inside the package, the sorption isotherm of the absorbent material and sachet permeability needs to be characterized.
In addition to moisture absorber sachets, several companies manufacture drip absorbent pads, sheets and blankets for liquid water control in foods with high water activity such as fruits and vegetables. These consist of two layers of microporous non-woven plastic film such as PE or PP between which a superabsorbent polymer is inserted that is capable of absorbing 500 times its own weight in moisture. Polyacrylate salts, carboxymethyl cellulose (CMC), and starch copolymers are generally used in the absorbent layer because of their high affinity for water. Agricultural products are often covered with large sheets and blankets during transportation to reduce transpiration rates.
Because ethylene is a plant hormone produced by fruits and vegetables that accelerates respiration rate and increases the speed at which ripening occurs, its removal from the package can delay senescence and extend the shelf life. The most commonly-used ethylene absorber is potassium permanganate (KMnO4). Due to the presence of double bond, ethylene (H2C=CH2) is very reactive and is rapidly oxidized to CO2 by KMnO4. To provide a greater surface area, KMnO4 (purple powder) is generally adsorbed on an inert material like silica or activated carbon. However, it is toxic and should not therefore be in direct contact with the fruit or vegetable. Ethylene can also be removed from the package headspace by using polyethylene films into which ceramic powder has been incorporated. The incorporation of ceramic powder increases the permeability of the film and thus helps to reduce the ethylene concentration in the package.
Carbon dioxide, mainly produced by the respiration of fresh fruits and vegetables, can cause physiological stress in fresh produce if it is allowed to accumulate to excessive levels inside the package. The controlled removal of CO2 is therefore helpful in maintaining the quality of the product. The most commonly used CO2 absorbers are calcium hydroxide, activated charcoal, zeolite, and magnesium oxide. Active charcoal and zeolite only work well in conditions of low humidity, because they have a greater affinity towards moisture than towards CO2.
Because some level of oxygen is required for the normal metabolism of respiration in fresh produce, its removal by absorption is usually not attempted in a fresh produce package. In some cases, in order to achieve the optimal atmosphere as rapidly as possible, oxygen absorbers can be positioned inside the package as inserts in the form of a sachet, or as self-adhesive labels which are either placed inside the package or attached to the polymer film (Charles et al., 2003). Powdered iron, catechol, and ascorbic acid can all be used to scavenge oxygen. EMCO Fresh Technologies produces a sachet system (OxyFresh®) that supports high O2 MAP. It has the function of absorbing CO2 while at the same time emitting O2.
7.3.2 Releasing system
There are some gas-phase molecules which are beneficial in maintaining the quality of fruits and vegetables and are often deliberately delivered inside the package so that they are available in the package headspace, for example, 1-methylcyclopropene (MCP, C4H6), an ethylene antagonist and chlorine dioxide (ClO2), a microcidal agent. Volatile antimicrobials are helpful in the preservation of the microbial quality of produce when delivered from antimicrobial sachets or films. Most antimicrobial packaging relies on the release of antimicrobials; a separate section is devoted to this important topic. This section will discuss the release of MCP and ClO2 in typical release packaging systems.
Exposure of fresh produce to ethylene generally leads to over-ripening and facilitates microbial growth, thus limiting the shelf life. The discovery of the ethylene inhibitor MCP offers another method of delaying the ripening and senescence of fruits and vegetables, thereby extending the shelf life. At concentration of 2.5 nL/L to 1 μL/L, MCP inhibits the effects of ethylene in a wide range of fruits, vegetables, and floriculture crops (Blankenship & Dole, 2003). The response of fruits and vegetables to MCP and its commercial applications are discussed in a number of review papers (Blankenship & Dole, 2003; Watkins, 2006). Its effectiveness in inhibiting ripening and senescence of produce is dependent on the commodity, the concentration applied, duration of the treatment, temperature, use of multiple application modes, among other factors. Table 7.1 lists the commodities that can benefit from MCP treatment.
Table 7.1
Climacteric and non-climacteric fruit and vegetables for which positive responses to MCP have been reported
| Climacteric fruits | Non-climacteric fruits | Vegetables |
| Apple, Apricot, Avocado, Melon, Mountain papaya, Nectarine, Banana, Blueberry, Papaya, Peach, Chinese bayberry, Chinese jujube, Pear, Custard apple, Persimmon, Figs, Plums, Guava, Tomato, Kiwifruit, Lychee |
Cherry, Clementine, Cucumber, Grape, Grapefruit, Lime, Orange, Pepper, Strawberry, Pineapple, Watermelon |
Broccoli, Carrot, Chinese cabbage, Chinese mustard, Choy sum, Chrysanthemum, Coriander, Lettuce, Mibuna, Mizuna, Pak Choy, Parsley, Potato |
Summarized from Watkins (2006).
The most common commercial method used for MCP treatment is the release of MCP gas from cyclodextrin powder. Moisture should be supplied as a trigger to release the gas from cyclodextrin into the room containing the fresh produce. Sachet and film forms have been investigated for the release of MCP in the produce package (Lee et al., 2006; Hotchkiss et al., 2007). The major variables governing the release of MCP into the head-space are the film material, its moisture and MCP permeabilities, thickness and surface area, as well as the storage temperature and humidity. Optimizing the delivery mode and release profile for the commodity and package conditions is the main task for the successful application of MCP to the produce package; this requires in-depth research into the connection between plant physiology and mass transfer kinetics.
The delivery of chlorine dioxide gas to the package headspace is a very promising solution to the problem of microbial spoilage in the packaged product. Chlorine dioxide has bactericidal, fungicidal, and viricidal properties which are helpful for sanitizing and decontaminating fresh fruits and vegetables. Its antimicrobial effects are presumed to result from oxidative attack on the cell surface membrane proteins. The most common method of using chlorine dioxide has been to wash the produce with a chlorine dioxide solution before packaging. The aqueous solution used to be produced by dissolving chlorine dioxide gas produced from the generator. Now chlorine dioxide is available in powder form, which requires a moisture trigger to release chlorine dioxide gas. The composition of the powder is a mixture of chlorine-containing hygroscopic inorganic salts that absorbs moisture from the humid environment triggering the reaction. The powder form can be used as a sachet (powder enclosed in a pouch), label (incorporation of the powder during manufacturing of the label) or pad (entrapment of the powder inside water absorbing material) inside the package depending on the type and size of the product as well as the size of the package, in order to provide a controlled release of chlorine dioxide to the produce over time. The moisture from the respiration of the produce in the package acts as a trigger for the reaction. A powder form was tested for a punnet of blueberries, and at least three log reductions of Listeria monocytogenes, Salmonella, Escherichia coli O157 : H7, yeasts and molds inoculated at a level of about 106 cells/g were achieved with treatment of 4 mg/L ClO2 for 12 hours (Popa et al., 2007).
Chlorine dioxide can also be produced from the reaction of chlorinated salt and acid in the presence of moisture. The rate of chlorine dioxide production can be controlled based on the combination of salt and acid and on the ratio of salt to acid. This reactant mixture can also be used in a sachet or incorporated in the film to use as a label or packaging film. The salt and acid must be mixed homogeneously so that individual salt molecules are in contact with acid molecules to start the reaction when it is triggered by moisture. Single molecules of acid and/or salt that are not in contact with each other do not participate in the reaction. Uniform mixing of salt and acid is therefore another critical factor that controls the rate and amount of chlorine dioxide production (Fig. 7.5). A chlorine dioxide releasing sachet can be used either inside the primary package or in the secondary package. Due to consumer concern, use inside the secondary package is preferred, so that it is not directly visible.
One major disadvantage of using chlorine dioxide gas in a packaging system is the discoloration of the pigments and/or burning on the surface of the produce. Since chlorine dioxide is a strong oxidizing agent, its concentration must be controlled throughout the shelf life of a product, to ensure that fruits and vegetables do not suffer from overexposure.
7.4 Antimicrobial packaging
Antimicrobial packaging can usually be considered as a kind of active packaging involving the release of antimicrobial substances. Most available antimicrobial systems are based on migration of antimicrobials, although some systems exist which are based on the intrinsic antimicrobial properties of the contacting surface of the packaging material (Lee & Han, 2011). In a direct contact system, where the food surface should be in close contact with antimicrobial packaging, either a migratory or non-migratory system can act to inhibit microbial growth on the surface of the produce (Fig. 7.6(a)). The problem of a direct contact system is that, if the produce surface is irregular, close contact between the packaging layer and the product surface is not always possible. Wrapping the produce in cling film is one means of achieving contour contact with the product surface as shown in Fig. 7.6(a). Because an indirect contact system does not allow the delivery of antimicrobials in liquid or solid phase, only volatile antimicrobials can work in this system as shown in Fig. 7.6(b). The migration or delivery of antimicrobials occurs thorough desorption (or evaporation) and gas-phase diffusion. The advantage of volatile antimicrobials is that they can readily penetrate irregular food surfaces through void spaces or channels. These advantages offered by an indirect contact antimicrobial packaging system have led many researchers to look for an effective method of incorporating volatile antimicrobials into a polymer matrix. The ClO2-releasing packaging mentioned above is one useful indirect system. Gontard and Guillaume (2010) discussed one example of a nanocomposite paper that released car-vacrol, a volatile antimicrobial, when triggered by high humidity.
Fig. 7.6 Antimicrobial packaging systems workable for fresh produce: (a) direct contact and (b) indirect contact.
Attempts have been to incorporate a variety of antimicrobials into the polymer matrix by a number of different methods. Table 7.2 summarizes the combinations of incorporated antimicrobials and polymers. Depending on the properties of the antimicrobials and polymers and the potential application of antimicrobial packaging system in question, different methods of fabrication are employed:
Table 7.2
Antimicrobials incorporated directly into polymers for food packaging
| Antimicrobial | Polymer/carrier | Main target organism |
| Organic acids/anhydrides: Propionic, benzoic, sorbic, acetic, lactic, malic | Edible films, EVA, LLDPE |
Molds |
| Inorganic gases: Sulfur dioxide, chlorine dioxide | Various polyolefins | Molds, bacteria, yeast |
| Metals: Silver | Various polyolefins | Bacteria |
| Fungicide: Benomyl, imazalil | LDPE | Molds |
| Bacteriocins: Nisin, pediocins, lacticin | Edible films, cellulose, LDPE |
Gram-positive bacteria |
| Enzymes: Lysozyme, glucose oxidase | Cellulose acetate, PS Edible films | Gram-positive bacteria |
| Chelating agents: EDTA | Edible films | Gram negative bacteria |
| Spices: Cinnamic, caffeic | Nylon/PE, cellulose | Molds, yeast, bacteria |
| Essential oils (Plant extract): Grapefruit seed extract, bamboo powder | LDPE, cellulose | Molds, yeast, bacteria |
| Parabens: Propylparaben, ethylparaben | Clay-coated cellulose | Molds |
| Miscellaneous: Hexamethyl-enetetramine |
LDPE | Yeast, anaerobes, and aerobes |
Abbreviations: EVA (ethyl vinyl acetate), LLDPE (linear low density polyethylene), LDPE (low density polyethylene), PS (polystyrene), PE (polyethylene).
• incorporation of volatile and non-volatile antimicrobial agents directly into polymers,
• coating or adsorbing antimicrobials directly onto polymer surfaces,
• immobilization of antimicrobials to polymers by ion or covalent linkages,
Heat-resistant antimicrobials can be incorporated at 0.1–5% w/w of the packaging film by extrusion and injection molding. Heat-labile compounds can be achieved through solvent compounding. Antimicrobials that cannot tolerate the temperatures of polymer processing are often coated onto the material after forming or are added to cast films. Antimicrobials can be immobilized to polymers by ionic or covalent linkages: this type of immobilization requires the presence of functional groups on both the antimicrobial and the polymer; ionic bonding of antimicrobials onto polymers allows them to be released slowly into the food. Some polymers such as chitosan are inherently antimicrobial and have been used in films and coatings.
The use of antimicrobials in produce packaging is seen as a potential solution to microbial spoilage and safety problems in the fresh produce industry. It offers particular benefits in this area because the product is designed to be consumed fresh. For fresh-cut or minimally processed fruits and vegetables susceptible to rapid microbial growth and spoilage, the proper use of antimicrobial packaging in combination with chilling and modified atmosphere packaging will be able to provide safety and quality alongside freshness and desired shelf life. One study carried out examined an antimicrobial multilayer film structure consisting of linear low density polyethylene (LLDPE)/soy protein isolate (SPI) on the inside, followed by a volatile antimicrobial compound/oriented polypropylene (OPP) then a polyethylene (PE) laminate. This proved effective at delaying aerobic bacterial growth on the fresh sprouts, extending the shelf life and enhancing the microbial safety of the sprouts (Gamage et al., 2009). The volatile antimicrobials (trans-cinnamaldehyde, allyl isothiocyante (AIT), garlic oil active compound, rosemary active compound) sandwiched between film layers were able to permeate the LLDPE layer to reach the produce through the headspace. Figure 7.7 shows some results of the use of AIT-incorporated film structure on mesophilic microbes.
Fig. 7.7 Microbial count of fresh vegetable sprouts at 10 °C as affected by 1.0% AIT-incorporated SPI antimicrobial packaging of multilayer structure. Dotted lines are for control and solid lines for the antimicrobial packaging. 
: alfalfa, 
: broccoli, 
: radish. Summarized from data of Gamage et al. (2009).
The commercialization of antimicrobial packaging requires optimization of the packaging system so that it is able to respond to the specific requirements of the product, which will depend on storage temperature, microbial spoilage behavior, possible safety hazards, shelf life, etc. The current intensive research in this area should also take into account and be compatible with regulatory concerns and requirements in the field of food packaging.
7.5 Edible coatings
Traditionally, edible wax coatings have been applied to the fresh produce surface in order to improve appearance, reduce bruising during handling and shipping, reduce weight loss, and to provide a carrier for active compounds. Commodities commonly coated include apples, avocados, bell peppers, cantaloupes, cucumbers, eggplants, grapefruits, lemons, limes, melons, oranges, parsnips, passion fruit, peaches, pineapples, pumpkins, rutabagas, squash, sweet potatoes, tomatoes, turnips and yucca. The edible mac-romolecules used as the coating can be classified into polysaccharides, proteins, and lipids. Comprehensive information on the use of edible coatings on fresh produce can be found in Baldwin (1994). The physical properties of the coating and its permeability to moisture, oxygen, and carbon dioxide are important in realizing its beneficial effects. One recent trend is the development and use of edible coatings that can improve the safety and quality of fresh-cut produce. Edible coatings may contribute to extending the shelf life of fresh-cut produce by reducing moisture and solute migration, gas exchange, respiration and oxidative reaction rates, and by reducing or even suppressing physiological disorders (Rojas-Graü et al., 2009).
Edible or wax coatings are applied by dipping, brushing, or spraying wax onto the surface of the product. Commonly used coatings are oils, waxes, starch, alginate, methylcellulose, zein, whey protein, and fatty acid esters. It is usually assumed that these coatings will be consumed together with the product. After waxing, a thin layer of wax adheres tightly to the surface, reducing the respiration rate and sealing the moisture inside the produce, thereby helping to maintain the crispiness, firmness, and juiciness of the fruits and vegetables (Fig. 7.8). Another advantage of edible coating is that it enhances the appearance of the produce in such a way that it appears fresh to customers.
Recent innovations have led to the development of edible coatings that can act as carriers of active ingredients such as antimicrobial agents, antibrowning agents, colorants, flavors, nutrients, and spices that can extend the shelf life and reduce the risk of pathogen growth on produce surfaces (Rojas-Graü et al., 2009). There are a number of compounds that can be incorporated into the coating of fresh produce to delay their senescence, maintain their organoleptic properties, texture, and microbiological safety (Table 7.3).
Table 7.3
Active compound incorporating edible coating formulations to improve the quality and shelf life of fresh-cut fruit
Abbreviations: AA, ascorbic acid; MA, malic acid; CaCl2, calcium chloride; CaL, calcium lactate; CA, citric acid; GSH, glutathione; NAC, N acetyl cysteine; cys, cysteine; PS, potassium sorbate.
From Oms-Oliu et al. (2010) with permission.
The incorporation of antimicrobials into edible coatings can play the same role as direct contact antimicrobial packaging. Antimicrobial edible coatings are able to release antimicrobials in a controlled manner, maintaining effective concentrations on produce surfaces over a period of time in order to ensure the safety and quality of fresh produce. Antimicrobial incorporated coatings offer advantages over the direct application of antimicrobial agents because the coating can be designed to slow antimicrobial diffusion from the surface. This maintains the preservative activity at the surface of the food. Smaller amounts of antimicrobials would thus be needed in an edible coating.
Browning is a major problem in most stored fresh produce, and can strongly influence consumer acceptance of the product. Polyphenol oxidase (PPO) is the key enzyme in enzymatic browning. Dipping treatments with antibrowning compounds like citric acid, ascorbic acid, and sodium erythor-bate have traditionally been used. However, the use of edible coatings incorporating N-acetylcysteine and glutathione in alginate and gellan-based coatings has helped to prevent browning in fresh-cut fruits like apples, pears, and papayas. The incorporation of acids like ascorbic acid and sorbic acid into methylcellulose has also been studied to prevent browning in fresh-cut pears (Oms-Oliu et al., 2010). Chitosan, when incorporated with plant oleoresins from rosemary and olive, was found to generate significant antioxidant activity on PPO in butternut squash (Ponce et al., 2008). Besides contributing to preventing browning and extending the shelf life of fresh-cut fruit, these coatings also help to increase the antioxidant potential of the fruits (Oms-Oliu et al., 2010).
Enzymatic degradation of the cell wall of fruit by enzymes such as pectin methylesterase and polygalacturonase due to fruit cutting plays a key role in fruit softening. Calcium chloride has been incorporated into edible coatings for fresh-cut fruit in order to maintain the firm and crispy texture of the fruit. However, due to the bitter taste of calcium chloride, calcium lactate has also been used to prevent fruit softening. Calcium interacts with pectic acids in the fruit to form a cross linked polymer network that increases the firmness of the fruit. Calcium chloride has been incorporated into algi-nate and gellan coatings to minimize softening of fresh-cut apples and melons by preventing loss of moisture and turgor (Oms-Oliu et al., 2010).
Some studies have also investigated the possibility of incorporating minerals, vitamins and fatty acids into edible coating formulations to enhance the nutritional value of some fruits and vegetables that contain low quantities of these micronutrients (Rojas-Graü et al., 2009). As shown in Table 7.3, combinations of these additives may be incorporated together to provide integrated and enhanced effectiveness.
7.6 Combining different technologies
Combining the different technologies described above can have additive or synergistic benefits for the quality of produce, and the effectiveness of various combinations under different conditions has been studied. The combination of different packaging technologies such as MAP, active packaging, antimicrobial packaging, and edible coating has allowed the development of emerging technologies able to respond to consumer demands and provide solutions to concerns in the fresh produce industry. It is expected that the combined use of packaging technologies can provide effective solutions to minimize produce loss and enhance quality, thereby increasing shelf life. Possible combinations are the integration of MAP and active packaging systems or of MAP and edible coatings.
Two important factors that need to be controlled in order to extend the shelf life of fresh produce are temperature and headspace atmosphere. A modified atmosphere can be created by using tailored polymer film, micro-perforated film, or a microporous membrane. The package structure can have different designs based on the requirements of the fresh produce commodity in question in order to achieve the desired gas composition, and can consist of a combination of inert film area, microperforations, and porous polymer. The antimicrobial release device can be added to the MAP system by incorporating the antimicrobial into a label, sachet or coating which is attached to the package. This release system of volatile antimicrobials will be very effective for produce which is not in direct contact with the package. Figure 7.9 shows the effect of a combined system of MAP and volatile antimicrobial on the decay of table grapes. The concentration of antimicrobials and polymer composition can be manipulated to achieve a specific release profile able to offer optimum protection for the specific application. The combination of MAP with an antimicrobial release system offers an effective method of inhibiting microbial growth and limiting quality deterioration by creating an additional stress factor for the microorganisms, thereby extending the shelf life of fresh produce.
Fig. 7.9 Effect of MAP (10–14.5 kPa O2 and 1.4–2.0 kPa CO2) added inside with volatile antimicrobial absorbed in gauze on table grape spoilage at 1 °C. Constructed from data of Valverde et al. (2005).
The ethylene antagonist MCP can be combined with MAP to enhance the preserving function of MAP by blocking the ethylene activity. De Reuck et al. (2009) identified the potential use of MCP release in combination with MAP to extend the storage shelf life of litchi. This combination could reduce PPO activity and browning, preserve the pericarp color and maintain the anthocyanin content. Controlling the MCP concentration in the package seems to be an important factor in fruit preservation.
While MAP is a dynamic packaging tool able to suppress physiological changes, it may be supplemented with the use of edible coating as a tool to deliver active compounds such as antimicrobials and antioxidants. Mastromatteo et al. (2011) studied the combined effect of active coating and MAP on prolonging the shelf life of minimally processed kiwifruit. They observed that the combined use of active MAP and active (antimicrobial grapefruit seed extract) sodium alginate coating treatment prolonged shelf life up to 13 days, whereas when active MAP was used alone the product had a very short shelf life (2.7 days). For the samples packed under active MAP with the coating treatment, the coating treatment reduced the excessive dehydration of the produce that often occurs under MAP conditions. A combination of an active compound solution dip, carrageenan coating, and modiied atmosphere can inhibit weight loss and PPO activity in banana slices (Bico et al., 2009). This combination of MAP and active coating may be an effective solution for extending the shelf life of minimally processed fruits and vegetables.
Other combinations of MAP and active packaging (moisture absorbers, ethylene absorbers) would also be possible methods of preserving produce quality. MAP with optimal package atmosphere can help to reduce the senescence of the produce in the package, while the moisture absorber maintains the correct humidity level in the headspace and the ethylene absorber inhibits the ripening of fresh produce by absorbing the ethylene gas from the headspace. In order to optimize the design of the packaging system, the relationship between respiration, gas transmission of package structure, and the absorption or release kinetics of the active packaging components must be fully understood. The combination of technologies will be the most important means of enhancing the safety and quality of produce in the future.
7.7 Conclusion and future trends
Maintaining freshness is the central idea in the packaging of fresh produce. MAP, active packaging, and edible coatings are the technologies available for preserving freshness and extending the shelf life. The main consideration in MAP design is the manipulation of packaging variables such as ilm formulation, package dimension, and microperforations. Antimicrobials, ClO2, antioxidants, and MCP can all be released by active packaging systems, while moisture, CO2, and ethylene are ideally removed from the package. The correct combination of technologies can provide maximum effectiveness in preserving freshness. Innovative technologies would be more effective for fresh-cut produce whose quality deteriorates rapidly. Considering that minimally processed fruits and vegetables are favored by consumers for reasons of convenience and for their health beneits, the consequences of successful innovation would be much valued in this area.
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