In many food products, microbial growth occurs primarily at the surface. More recently, the concept of incorporating antimicrobial agents directly into package films that contact the surface of the food has been developed. Active packaging systems based on the application of packaging materials with incorporated antimicrobial agents provide one of promising trends in food processing. Antimicrobial food packaging reduces, inhibits or retards the growth of pathogenic and/or spoilage microorganisms that may be present on packaged food surfaces due to releasing of antimicrobial components.
Active packaging, sometimes referred to as interactive or smart packaging, is intended to sense internal or external environmental change and to respond by changing its own properties or attributes and hence the internal package environment. The goal of active packaging, in conjunction with other food processing and packaging, is to enhance preservation of contained food and beverage products.
“Antimicrobial packaging” is one of many applications of active packaging. Antimicrobial packaging is the packaging system that is able to kill or inhibit spoilage and pathogenic microorganisms that are contaminating foods. The new antimicrobial function can be achieved by adding antimicrobial agents in the packaging system and/or using antimicrobial polymers that satisfy conventional packaging requirements. When the packaging system acquires antimicrobial activity, the packaging system (or material) limits or prevents microbial growth by extending the lag period and reducing the growth rate or decreases live counts of microorganisms.
What are Antimicrobials?
“Packaging antimicrobials” are materials that can be added to packaging materials and will prevent microbial growth. Use of antimicrobial packaging offers a degree of protection and extends the shelf life of perishable food products.
Types of Antimicrobial Packaging
Antimicrobial packaging can take several forms including:
Addition of sachets/pads containing antimicrobial agents into packages
The most successful commercial application of antimicrobial packaging has been sachets that are enclosed loose or attached to the interior of a package. Three forms have predominated: oxygen absorbers, moisture absorbers and ethanol vapour generators. Oxygen and moisture absorbers are used primarily in bakery, pasta, produce and meat packaging to prevent oxidation and water condensation. Although oxygen absorbers may not be intended to be antimicrobial, a reduction in oxygen inhibits the growth of aerobes, particularly moulds. Moisture absorbers can reduce water activity (aw), also indirectly affecting microbial growth.
Incorporation of antimicrobial agents directly into polymers
Incorporation of bioactive agents including antimicrobials into polymers has been commercially applied in drug and pesticide delivery, household goods, textiles, surgical implants and other biomedical devices. The number of recently published articles and patents suggest that research on the incorporation of antimicrobials into packaging for food applications has more than doubled in the past five years. Of all the antimicrobials, silver substituted zeolites are the most widely used as polymer additives for food applications, especially in Japan. Sodium ions present in zeolites are substituted by silver ions, which are antimicrobial against a wide range of bacteria and moulds. Commercial examples of silver substituted zeolites include Zeomic, Apacider, AgIon, Bactekiller and Novaron.
Coating or adsorbing antimicrobials to polymer surfaces
Early developments in antimicrobial packaging incorporated fungicides into waxes to coat fruits and vegetables and shrink films coated with quaternary ammonium salts to wrap potatoes. Other early developments included coating wax paper and cellulose casings with sorbic acid for wrapping sausages and cheeses. Antimicrobials that cannot tolerate the temperatures used in polymer processing are often coated onto the material after forming or are added to cast films.
Immobilisation of antimicrobials by ionic or covalent linkages to polymers
This type of immobilisation requires the presence of functional groups on both the antimicrobial and the polymer. Examples of antimicrobials with functional groups are peptides, enzymes, polyamines and organic acids. In addition to functional antimicrobials and polymer supports, immobilisation may require the use of spacer molecules that link the polymer surface to the bioactive agent. These spacers allow sufficient freedom of motion so the active portion of the agent can contact microorganisms on the food surface. Spacers that could potentially be used for antimicrobial packaging include dextrans, polyethylene glycol, ethylenediamine and polyethyleneimine, due to their low toxicity and common use in foods.
Use of polymers that are inherently antimicrobial
Some polymers are inherently antimicrobial and have been used in films and coatings. Cationic polymers such as chitosan and poly-L-lysine promote cell adhesion. Chitosan has been used as a coating and appears to protect fresh vegetables and fruits from fungal degradation. Although the antimicrobial effect is attributed to antifungal properties of chitosan, the chitosan acts as a barrier between nutrients contained in produce and microbes.
Bacteriocins
A group of antimicrobial substances known as bacteriocins - proteins derived from microorganisms, are effective against microorganisms such as Clostridium botulinum. One such compound, nisin, has been accepted by regulatory authorities in some countries, for food use. These peptidetype compounds can theoretically be attached to the surface of food-contact films. It has not been reported whether such bound bacteriocins would be effective.
Antimicrobial enzymes
Antimicrobial enzymes might also be bound to the inner surface of foodcontact films to produce microbial toxins. Several such enzymes exist, such as glucose oxidase, which forms hydrogen peroxide - a potent antimicrobial.
Radiation-Emitting Film
A third possibility for antimicrobial films is to incorporate radiation - emitting materials into food-packaging films. Japanese researchers have reportedly developed materials that emit long-wavelength infrared (IR) radiation on exposure to water or water vapour.
Antimicrobial activity of silver and copper ions
The antimicrobial agents with the greatest potential appear to be those containing releasable silver salts. Antimicrobial activity of metals is due to the minute quantity of ions formed from the metals. Copper ions can destroy microorganisms and viruses, and copper is indispensable for life as a constituent of metallic enzymes. Recently, water filters and daily utensils utilising antimicrobial activity of copper ion have been commercially available. Copper is regarded as toxic in contact with food, and generally it is no longer permitted as an additive by regulatory authorities.
Antimicrobial packaging is a promising form of active food packaging in which the antimicrobial substances incorporated into package materials can control microbial contamination by reducing the growth of the microorganisms. Antimicrobial packaging systems can inhibit the growth of spoilage and pathogenic microorganisms, and contribute to the improvement of food safety and the extension of shelf-life of the packaged food.
(The authors are Ph D scholar, HOD, ADP at Collegeof Food Technology, VNMKV, Parbhani)