Active packages are the best advancement in packaging technology for extending food shelf life.

To maintain the freshness of packed food products like fresh fruits, vegetables, dairy and meat products, there are several solutions available, active packaging being the most technologically advanced. What is meant by ‘active packaging’? Unlike traditional packages, which simply act as a passive barrier protecting food from external environment with the use of preservatives, active packaging contains “active” agents designed to control or change conditions inside the package.

These agents can modify microbiological parameters, physiological processes, chemical reactions and sensory properties, aiming to extend shelf-life and improve food appearance. The active agents are antimicrobials, antioxidants, carbon dioxide absorbers, etc., incorporated into coatings or embedded within packaging material or placed in small bags inside the package. An extensive review published by Indian researchers (N. Yadav et al., 2024) gathered the most advanced technologies in the field of active packaging.

These technologies are classified into two main categories by their mechanism of action: absorbing systems that remove undesirable substances for food quality like humidity, oxygen and ethylene, and releasing systems that prolong shelf-life introducing beneficial substances such as carbon dioxide, antimicrobials or antioxidants.

Active Packaging with Absorbent Properties

The reduction of moisture, oxygen and ethylene in food packaging is crucial to prolong food quality.

Moisture absorption

The simplest moisture absorber is table salt. It is known that high-moisture foods are more prone to spoilage. Fresh fruits, vegetables, fish are at the top of the list, followed by baked goods, bread, cakes, biscuits, pizza, pasta and cheese, to finish with all dry foods (nuts, coffee, etc.) that have a low moisture content. The active agents used for moisture absorption are substances with drying properties, from common table salt (sodium chloride) to calcium oxide, calcium chloride and sodium salt of polyacrylic acid.

• Sodium chloride: as is known, table salt is hygroscopic, that is, it easily absorbs moisture from the surrounding environment, so much so that it has always been used for food preservation. Sodium chloride is used for the active packaging of tomatoes in thermoformed polyethylene trays containing 12% sodium chloride. This type of packaging absorbs almost twice as much moisture as polyethylene trays without sodium chloride;
• Mixture of calcium oxide, calcium chloride and sorbitol: plastic containers containing this mixture in a 0.5:0.26:0.24 proportion (relative ratio of calcium oxide, calcium chloride and sorbitol) are effective for packaging mushrooms, which contain 96% relative humidity;
• Polyacrylic Acid Sodium Salt (SAP): This desiccant in the form of fine particles placed in breathable bags is used to keep fresh corn dry, which has a moisture content of 32%. SAP is known for its cost-effectiveness and its high absorbency so that a small amount is enough to keep the corn dry.

Oxygen Absorption

Oxygen present in food packaging can promote undesirable chemical reactions, such as oxidation and microbial growth, which are particularly severe in the presence of fatty foods such as meat, with considerable alterations in nutritional value and taste, as well as in colour and visual appeal. Moreover, the presence of both oxygen and moisture significantly accelerates and enhances oxidative processes; therefore, it is important to keep both the oxygen and humidity levels low. Common oxygen scavengers are substances that react with this gas reducing its activity, typically ferrous iron or ascorbic acid:

• Iron-based substances: they can function similarly to haemoglobin by absorbing oxygen and using it to oxidise iron. These absorbers or scavengers are small sachets typically used for meat storage. They come in different sizes and capacities with different oxygen-binding properties and are easy to use;
• ascorbic acid, which is a strong antioxidant, 2.2 times stronger than iron-based substances and is typically used for preserving raw meat. Next to its antioxidant properties, ascorbic acid possesses significant antimicrobial properties. Ascorbic acid is used in sealed bags of linear low-density polyethylene (LLDPE) for the storage of baked goods such as rolls and bread slices to extended shelf-life for 5-6 days. By comparing LLDPE packages with and without ascorbic acid, it results that LLDPE packages with ascorbic acid are more effective than standard packages.

Ethylene Absorption

Ethylene triggers ripening and softening in fruits, thus reducing the shelf-life. Fruits that produce ethylene include bananas, peaches, apples, pears and kiwis. Ethylene absorbers are generally substances that react with ethylene, thereby reducing its effect on foods; examples include nanofibers of titanium dioxide or zinc oxide, and potassium permanganate:

• nanofibers of various substances: for example, nanofibers of titanium dioxide almost completely degrade ethylene, as do many other organic substances. Incorporated in polyethylene, these nanoparticles are used for the storage at 4 °C of strawberries for 12 days and kiwifruit for 42 days. When incorporated into chitosan films, nanoparticles of titanium dioxide delay the ripening of tomatoes and significantly extend their shelf-life. Zinc oxide nanofibers effectively degrade ethylene: Coated with polyvinyl chloride, they slow the respiration rate of fresh-cut Fuji apples, which in these packages can be kept at a temperature of 4°C for 12 days;
• potassium permanganate in sachets is useful for storing apples, bananas, mango, carrots, tomatoes and onions. However, direct contact of permanganate with food should be avoided, as permanganate is unsafe for food-contact.

Active Packaging with Release Properties

The release of carbon dioxide, antimicrobial agents and antioxidants significantly aids food preservation.

Carbon Dioxide Release

Modified atmosphere (MAP) packaging is an established food preservation technique that creates an internal atmosphere in the package that inhibits the growth of pathogenic micro-organisms and maintains food freshness for longer. Gas mixtures with 60% carbon dioxide and 40% nitrogen or 100% carbon dioxide are typically used in MAP. For example, storage in 100% carbon dioxide effectively inhibits microbial growth in chicken meat.

Release of Antimicrobial Substances

The release of antimicrobial substances can prolong the latency phase of micro-organisms and increase their inactivation time; common examples of such substances are essential oils, enzymes, bacteriocins and polymers with antimicrobial properties:

• essential oils, used both as film coatings and incorporated into polymers. For example, thyme essential oil incorporated as coating into chitosan films is uses for the preservation of cooked pork meat, where it inhibits the growth of bacteria for 21 days. Oregano essential oil used when integrated with pectin films is used for the preservation of tomatoes, where it effectively inhibits the growth of Alternaria alternata;
• enzymes and bacteriocins: lactoferrin is used in paper coatings for wrapping sliced meat. Nisin is also used as a paper coating for the refrigerated storage of hot dogs and significantly reduces Listeria monocytogenes for up to 60 days of refrigerated storage;
• polymers with antimicrobial properties: chitosan itself has antimicrobial properties; however, its effectiveness is not universal against all microorganisms. For example, chitosan films coated with antimicrobial agents (nisin, sodium lactate, sodium acetate, sodium sorbate, etc.) combined with sodium lactate are used to inhibit L. monocytogenes; they are used for the packaging of ham and smoked salmon.

Release of Antioxidants

Fat oxidation has negative effects on the quality of food, so, in addition to the aforementioned oxygen absorption in the package, another effective strategy is available to reduce it: the release of antioxidants, which react with free radicals that cause fat oxidation. Antioxidants are a very broad family of substances, including both natural products such as vitamin E and synthetic products such as butylated hydroxytoluene:

• Vitamin E (α-tocopherol) is naturally present in various foods, including sunflower seeds, almonds, hazelnuts, etc. Used in polylactic acid (PLA) films in very low concentrations (2.58%), it is used for the preservation of soybean oil, where it slows the lipid oxidation between 20 and 40°C. Vitamin E incorporated in low-density polyethylene (LDPE) works synergistically with MAP to significantly reduce oxidation in tuna fish and salmon fillets.
• butylated hydroxytoluene (BHT): Commonly used as a food additive E321, although subject to dose limits by the Food and Drug Administration (FDA). BHT in LDPE reduces lipid oxidation and increases fish storage time.

In conclusion, active packaging can be used to delay the deterioration of packaged foods. However, despite the many advantages offered, the commercialization of active packaging systems is indeed limited by high production costs and stringent safety standards. Therefore, further research is needed to create cost-effective packaging systems that are suitable for food storage.

References: Yadav et al., Journal of Packaging Technology and Research 8, 2024, pp. 15-50.