Commercially sterilized food products are filled into commercially sterile containers under aseptic condi-tions and sealed hermetically during aseptic process-ing (Fig. 5.15). The terms “commercially sterile” and
“sterile” are often interchanged in discussions of aseptic processing. Aseptic systems allow the use of temperature short-time (HTST) and ultra high-temperature (UHT) processes because the product and the package are sterilized separately. Due to the shortened exposure to high temperatures in compar-ison with more traditional canning/retorting proc-esses, aseptically processed products have excellent sensory qualities and better retention of nutritional components (heat labile vitamins). Aseptic process-ing also provides flexibility in the selection of con-tainers to be used in packaging the product since the packaging materials do not need to withstand the harsh temperature and pressure conditions of con-ventional thermal processes.
Properties of the product, desired shelf life, and storage temperature determine the required
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Figure 5.15. Diagram of an aseptic process in which a commercially sterilized food is filled and sealed into a commercially sterilized package in a commercially sterile environment..
tion in microbial count for the sterilization of food-contact packaging materials. While a minimum 4D reduction in bacterial spores is required for pack-ages used with nonsterile acidic products (pH < 4.5), a 6D reduction is necessary for packages used with sterile, neutral, low-acid (pH > 4.5) food products (Robertson, 1993). The main sterilization tech-niques used for the sterilization of packaging mate-rials for aseptic processes include irradiation (ultra-violet rays, infrared rays, and ionizing radiation), heat (saturated steam, superheated steam, hot air, hot air plus steam, and extrusion), and chemical treatments (hydrogen peroxide, peracetic acid, eth-ylene oxide, ozone, and chlorine) (Floros 1993).
These techniques are used individually as well as in combination. For the verification of a sterilization process, the contact surfaces of the package are in-oculated with an indicator organism and passed through the package sterilization operation on an aseptic processing line. The package is then filled with growth medium and incubated, and a microbial count is obtained to determine the D value for the process.
Due to the separate package and food product sterilization, a wider variety of package designs and materials can be used than in traditional thermal processes (canning/retorting, hot-filling). The thick-ness and amount of PET used in a bottle for aseptic products is significantly less than the amount of PET necessary for a hot-filled product; thus there is sig-nificant cost savings in packaging materials used for aseptic processes. The following factors influence the choice of packaging material for aseptically processed products (Carlson 1996):
• Functional properties of the plastic polymer [gas and water vapor barrier properties, chem-ical inertness, and flavor and odor absorption (scalping)],
• Potential interactions between the plastic poly-mer and the food product,
• Desired shelf life,
• Cost,
• Mechanical characteristics of the packaging material [molding properties, material handling characteristics, and compatibility with packaging machinery and sterilization methods],
• Shipping and handling conditions [toughness, type of overwrap or cases required, vibration, and compression],
• Compliance with regulations, and
• Targeted consumer group.
MODIFIED ANDCONTROLLEDATMOSPHERE
PACKAGING
Both modified atmosphere packaging (MAP) and controlled atmosphere packaging (CAP) are de-signed to extend the shelf life of foods held at ambi-ent and refrigerated temperatures by modifying the gaseous environment in which the foods are stored.
MAP is accomplished by modifying the gaseous en-vironment in a package by gas flush packaging or vacuum packaging when the food is placed into the package, and no further control is exercised (Brody 1989). In gas flush packaging, air is replaced with a controlled mixture of gases (usually O2, CO2, and N2); in vacuum packaging all air is removed. CAP systems, on the other hand, first alter and then selec-tively control the gaseous environment in a package in order to maintain a precisely defined gaseous at-mosphere. True CAP systems are impractical, how-ever, due to the chemical and microbial nature of foods and the physical characteristics, including permeability, of packages (Ooraikul and Stiles 1991). Fresh food products (such as lettuce, carrots, and apples) and microorganisms continue to respire after they are packaged, and the CO2produced and O2 consumed change the gas concentration inside the package. In theory, a CAP system would respond to these changes by scavenging excess CO2and re-leasing O2to replace what was consumed in order to maintain the desired gaseous environment.
The important factors for CAP/MAP preservation of foods include the composition of the gas atmos-phere in the package; the type of food; the type of mi-croorganisms present; and the temperature, moisture, and pressure. Control of the concentration of the gases, particularly CO2, inside the package is the fun-damental concept of MAP and CAP food preserva-tion. An increase in CO2or a decrease in O2 concen-tration can slow the respiration rate of foods and the growth of microorganisms, thereby extending the shelf life of the foods. The gas atmosphere in a pack-age is a function of the gas transmission rate of the packaging material, the respiration rate of food and bacteria in the package, the initial atmospheric com-position in the package, and any control mechanisms added to the package to respond to changes in gas concentrations (Stiles 1991a). Optimum MAP/CAP gaseous atmospheric conditions depend on the type of food and the type of microorganism. For fresh pro-duce, an increase in CO2 concentration may have beneficial effects, but the total absence of O2will re-sult in the development of off flavors. Different types
of fruits and vegetables have specific gas concentra-tion requirements for optimum storage life. For ex-ample, recommended MAP conditions for apples are 0–5°C, 2–3% O2, 1–2% CO2, and 95–98% N2; for bananas, 12–15°C, 2–5% O2, 2–5% CO2, and 90–96% N2; and for lettuce, 0–5°C, 2–5% O2, 0%
CO2, and 95–98% N2(Kader et al. 1989). Optimum MAP conditions will reduce the respiration rate, de-crease ethylene production, delay initiation of ripen-ing, retard senescence, inhibit microbial growth and spoilage, and reduce some physiological disorders such as chilling injury (Powrie and Skura 1991).
Although increased CO2 levels will retard the growth of some microorganisms (including Pseudomonas, which causes off flavor to develop in meats), elevated levels of CO2 have less effect on other microorganisms, for example, fermentative bacteria such as lactic acid bacteria. The minimum effective CO2concentration for extending the shelf life of meat is 20–30% (Stiles 1991b). Packaging low-acid foods (pH > 4.6) in anaerobic conditions could allow Clostridium botulinum growth and toxin production. Therefore, understanding the microor-ganisms present in the packaged food is extremely important for designing appropriate MAP/CAP sys-tems. In addition to modifying gas concentrations, a decrease in temperature will slow the respiration rate and spoilage of foods, and low pressure can be used to remove ethylene (a ripening hormone) to ex-tend the shelf life of fresh produce (Stiles 1991a).
ACTIVEPACKAGING
Active packaging, also known as interactive or smart packaging, involves an interaction between the packaging components and the food product (Labuza and Breene 1989). Active packages respond to changes in the internal or external environment by changing their own properties or attributes to enhance the preservation of food products while maintaining nutritional quality (Brody et al. 2001).
Active substances are contained in sachets or incor-porated directly into the packaging component. The major active packaging technologies include oxygen scavengers, ethylene scavengers, moisture regula-tors, and antimicrobial agents (Rooney 1995, Ver-meiren et al. 1999).
Oxygen Scavengers
The majority of commercially available O2 scav-engers work on the principle of oxidation of iron
powder by chemical means or enzymes to prevent the deterioration of food constituents by oxidation or spoilage. In the first case, iron kept in a small sa-chet that is highly permeable to O2is placed inside a food package and is oxidized to iron oxide. This oxidation of the iron removes oxygen from the pack-age and limits O2interaction with the food product.
In enzyme systems, an enzyme such as glucose oxi-dase reacts with a substrate to scavenge oxygen.
Ethylene Scavengers
Ethylene acts as a growth hormone and accelerates ripening and senescence of fruits. Removing ethyl-ene from the environment surrounding a fruit can extend the shelf life of the fruit. Most ethylene scav-engers are based on potassium permanganate (KMnO4), which oxidizes ethylene to acetate and ethanol. Charcoal or finely dispersed minerals such as zeolites are also used as ethylene scavengers, but they are less effective than the KMnO4scavengers.
Moisture Regulators
Several desiccants such as silicates and humidity-controlling substances are used in food packaging to control the moisture content inside the package of very dry foods or of respiring, wet, and high relative humidity fresh/minimally processed foods.
Antimicrobial Agents
Antimicrobial agents such as sorbates, benzoates, ethanol, and bacteriocins are incorporated into or onto polymeric packaging materials to reduce the microbial growth on the surface of food products. In some packaging systems, these antimicrobial agents are released from the packaging film into the food product over time. In other systems, the antimicro-bial agent is immobilized in the packaging material.
EDIBLECOATINGS ANDFILMS
Edible films and coatings have the same functions as other packaging materials (e.g., preventing moisture loss, acting as a barrier to oxygen, and reducing fla-vor and aroma loss). In addition, they provide the further benefits of (1) being formed from natural substances and reducing waste and environmental pollution; (2) enhancing the organoleptic, physical, and nutritional properties of the foods; (3) continu-ing to offer protection after the package (often a
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plastic) has been opened; and (4) providing protec-tion for small pieces (such as raisins, nuts, etc.) (Labuza and Breene 1989).
Edible films and coatings can be classified as polysaccharide-based, protein-based, lipid-based, and multiconstituent films and coatings (Krochta et al. 1994). Polysaccharide-based films generally are poor water vapor barriers due to their hydrophilic nature. They also have poor oxygen barrier proper-ties at high relative humidiproper-ties. Polysaccharide-based films are used to retard the ripening of climac-teric fruits without creating severe anaerobic conditions. Protein-based films and coatings are generally formed from gelatin, whey protein, casein, corn zein, wheat gluten, and soy protein. They have much better oxygen barrier properties than polysac-charide-based films and also add nutritional value to the product. Lipid-based films are generally used to prevent weight loss in fruit and vegetables; however, anaerobic respiration and off-flavor development are possible. Since most lipid-based films lack sufficient structural integrity and durability to form freestand-ing films, they are used in combination with polysac-charide- and protein-based films. These multicon-stituent films are formed to combine the desirable properties of each component (barrier properties) while minimizing individual component weaknesses (structural integrity). Antimicrobial agents, antioxi-dant vitamins, and flavors can be added to modify the functionality of the films and coatings. Films and coatings are applied to food products by dipping the product into the film solution, spraying the film so-lution onto the surface of the product, or casting freestanding films and applying these to the product.