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Chemical and Physical Properties of Gases Used in Modified Atmosphere Packaging (MAP)

1.1 Scope of Food MAP

Modified atmosphere packaging (MAP) can be defined as the packaging of food under headspace different from normal air (Figure 1.1). It may also be termed as packaging that exposes food to a spacious environmental background which is different from normal air. Common examples of MAP are packages flushed with nitrogen or a gas mixture containing carbon dioxide. Gas composition of the package is the main variable considered in selection or design of MAP. However, the category of MAP includes air-pressurized packaging and vacuum packaging without a gas-phase. Pressure in the package may work as another variable in MAP: normal atmospheric or hypobaric pressure is usually assumed or covered when mentioning MAP even though the basic definition of MAP may include hyperbaric pressure; though aerosol cans of whipping cream and emulsions which are pressurized under nitrous oxide and/or carbon dioxide for propellent spraying use, are included in the definition of MAP in principle, in the usual review or discussion, they are mostly excluded; pressurization is rather used more as a short-term treatment than the stabilized state consistent with food packages traveling along the distribution channel. The volume of headspace should be selected with consideration of package stability and the desired interaction with food. Proper high ratio of gas to product is often needed to achieve the most beneficial MA effect. On the other hand, an extreme case of flexible vacuum package maintains minimum or negligible headspace with package film coming into direct contact with the food surface to attain the preferred visual appearance or end use. The level or extent of food MAP ranges from simple exclusion of oxygen to maintaining the specific desired modified atmosphere (MA). Intelligent combinations of different gases and/or partial pressures may be applied for maximum effectiveness. In some innovative applications of MAP, gas composition and/or pressure may be controlled in time-dependent mode responding to the needs of each food product traveling along the food supply chain. Food product may be reactive in its gaseous reaction in order to modify the package headspace by itself or be non-respiring little influencing the internal atmosphere. The package layer may be a high gas barrier or gas permeable depending on the purpose of maintaining the desired MA. Usually, shelf-stable products are packaged in high-barrier material under the intended atmospheric gas composition assuming little interaction between the package atmosphere and food. As for non-respiring foods, of relatively static nature, shelf-stable products are often packaged in high barrier, but some of them may be perishable with microbial or chemical activities causing modification of the internal atmosphere of the package. Respiring fresh fruits and vegetables should be packaged in gas permeable material to balance the gas permeation or transfer to their respiration activity. For providing very high gas transport, a package layer may be perforated or installed with gas diffusion tube. Figure 1.2 shows ramification of MAP depending on the needs and restrictions met in food packaging.

Figure 1.1 Food MAP system with components involved as design variables.

Figure 1.2 Ramification of food MAP according to possible positions or situations.

In the simplest case of MAP, food product may be wrapped with gas-barrier film under a gas mixture set beforehand as mentioned above. However, good MAP design often takes care of food deterioration mode, physiology or reactive characteristics of food components, packaging material properties and interactive response of food package to the environment. Therefore, the study and development of MAP should be an integrated one looking into all the relevant variables or components interactively. For example, perishable flesh foods packaged in high barrier may experience increase in carbon dioxide concentration and decrease in oxygen concentration due to microbial growth and spoilage. MA conditions may alter the dynamics of microbial ecology and atmospheric composition. On the other hand, fresh produce packages are fabricated to keep the desired MA through the shelf life by applying gas permeable film giving the transfer of oxygen and carbon dioxide at the correct levels. Any of these food packages require harmonization among the variables responding to the environment. Even shelf-stable products with negligible impact on package atmosphere may differ in their deterioration mode and rate with different package atmosphere, and thus shelf life would depend on MA condition.

The purposes or benefits of food MAP include antioxidative protection, freshness preservation, retardation of microbial spoilage, product color development, control of package volume or pressure, etc. A variety of benefits for different foods can be found in several monographs (Arvanitoyannis, 2012; Brody, 1989; Brody et al., 2011; Farber & Dodds, 1995; Ooraikul & Stiles, 1991; Thompson, 1998; Yahia, 2009). Properties of gases are exploited for attaining the desired purpose. However, the wanted properties can be realized only under the premise of a well-organized way of package construction and delivery. Thus, a critical ground of successful MAP is temperature control. Any construction or design of MAP should be matched to the optimal temperature for the storage of commodity or product. If the temperature is abused or deviated from the proper level or range, benefits of MAP disappear or unexpected situations of the worst food-safety crisis may occur. From this perspective, regulatory issues arise for protecting consumers from their mishandling and safety hazards of MAP in the food supply chain. Humidity is also an important requisite for successful MAP as a factor to constitute gas phase composition of humid air.

The first step of studying MAP starts with understanding the nature and properties of gases used in MAP. Each gas is different in its property and thus is employed for a different purpose. Common gases used in MAP are carbon dioxide (CO2), oxygen (O2) and nitrogen (N2). Other gases used occasionally in high concentrations include argon (Ar), helium (He) and nitrous oxide (N2O). Most of the commercial MAP products employ mixture of nitrogen, oxygen and carbon dioxide for reasons of consumer acceptance, cost, safety issue, etc. Uses of argon, helium, hydrogen (H2) and nitrous oxide often appear in research articles for innovative packaging developments. Carbon monoxide (CO), sulfur dioxide (SO2) and choline oxide (ClO2) may be applied in concentrations below 1% for microbial inactivation or food color development. Other volatiles in very low concentration may also be added or designed to be produced from package devices for the purposes of antimicrobial and/or antioxidant activities. In general terms, design of MAP considers optimal mixture of gases in concentrations of each greater than 1%. Therefore, application of minor gases such as carbon monoxide, sulfur dioxide, choline oxide and other volatile components applied less than 1% may be regarded or managed as tools of active packaging. Water vapor (H2O) should also be considered in the gas-phase dynamics of MAP because humidity control is sometimes very critical for its successful application in the product distribution channel. Theoretically, different gases alone or in combinations are selected utilizing their properties for maximum effectiveness and benefits of MAP. Basically, gases are negligible in mass (very low density) compared to food, liquid or solid phase, readily disperse homogenously into the headspace or void space of the package, may be adsorbed onto the food matrix and may be compressed or expanded in response to the outside atmosphere pressure.

1.2 Chemical and Physical Properties of Gases and Water Vapor Used in MAP

As discussed before, major gases play proper functions or roles in food MAP. Chemical and physical properties are responsible for the required functions and thus reviewed in a summarized form in Table 1.1. Water vapor was included in the table because humidity is important in providing the desired headspace environment for food quality preservation. Most gases except water vapor and nitrous oxide are non-polar in chemical polarity. They are colorless and odorless except nitrous oxide which has a slight metallic sweetish scent. Generally, most of the gases except water vapor, hydrogen and oxygen are inert in their reactivity at ambient temperature. Oxygen is key player in oxidative quality changes and reactions. It is also essential for respiring activity of fresh produce. Water vapor governing headspace humidity may be adsorbed to affect various chemical reaction in the foods. It may hydrate several compounds in foods influencing adsorption of other gases in the headspace. Hydrogen bonding would be induced among components through the adsorbed moisture. Humidity control utilizing properties of water vapor is essential in practicing effective MAP. While most gases such as nitrogen, carbon dioxide, argon, helium and nitrous oxide are inert protecting foods from oxidation, carbon dioxide is unique in dissolving into aqueous food systems to form carbonic acid, which reacts further with alkaline compounds.

Table 1.1 Chemical and physical properties of gases used for MAP.