**1.3 Active packaging**

Food packaging materials are very important in the food industry. The packaging material should isolate and preserve food from the external environment. It must be a nontoxic and impermeable physical barrier [4]. An important fact is that food packaging material should have very good mechanical, physical, chemical, water, and light barrier properties, and should be thermally stable and specifically processable during whole production and food packaging process. Additionally, it should be effective against microorganisms responsible for food spoilage [4, 51]. Various materials, such as glass, paper, paperboard, and metal have been used as packaging materials for many years because they are cheap, lightweight, and ecologically friendly [4]. Over the last decades synthetic, plastic materials have become an effective and dominant packaging material due to their light weight, high transparency, cost-effectiveness, and versatility. Moreover, these synthetic polymers have good mechanical, thermal, and barrier properties [4, 52–54]. Conventional food packaging is a passive, inert barrier that should protect food from the external environment. To extend the shelf-life of food products, food packaging has begun to evolve from simply passive to innovative interactive strategies, including intelligent, active, and smart packaging. This preservation is aided by active packaging that can even improve the quality of food through interactions between food products, surroundings, and the covering materials. Active packaging (AP) is defined as packaging that interacts with the packed food and environment dynamically to maintain both food product quality and extend shelf-life [1]. AP materials include additional ingredients in the packaging material or the package headspace to enhance system performance. **Figure 1** shows the forms of active packaging which may be used for food product preservation [55].

Form A (**Figure 1**) shows active packaging materials covered with coatings containing active compounds that are heat-sensitive or incompatible with the polymer matrix. One of the most common types of food AP systems is form B. The uniform distribution of the active agents makes the incorporation of active compounds into the polymer matrix

#### **Figure 1.**

*Typical forms of active food packaging systems according to Almasi et al. [55] (A) polymer covered with a coating containing active agents, (B) active agents incorporated into the polymer matrix, (C) active agents immobilized on the polymer surface, and (D) active sachets inside the food packaging.*

possible. It should be noted that active substances must have good compatibility and high resistance to polymer processing conditions and no adverse effects on polymer properties. It is also very important that the matrix should release these compounds gradually in control way to the food. The immobilization of active substances on the polymer surface by ion or covalent linkages requires the presence of functional groups on both the active compound and the polymer. This form (C) makes the strong bonding of active agents onto the polymers possible. It also allows the slow release of these substances into the food. The third form of active packaging is the introduction of pads or sachets containing active substances into the package [55]. Pads or sachets may be put in the packaging headspace in a free form, that alters the interior environment of the packaging by various mechanisms like an absorption and/or evaporation process, that in turn inhibit the growth of microorganisms or other food and hinder the deterioration processes. This form of active packaging, as well as the forms described above, are classed as "releasing systems" or "absorbing systems" based on their mode of action [1, 54]. "Absorbing system" means that the active components absorb unwanted substances from the food surface or the packaging during storage. Ethylene and oxygen scavengers were found to be very important examples of absorption systems. In the releasing system, antioxidants or antimicrobial agents migrate to the surface of food products, preventing food spoilage. They may be released by direct contact between the food and packaging material (leaching systems) or through gas-phase diffusion from the packaging layer to the food surface (volatile systems) [1, 55]. The type of food product, the gas composition, the particular packing machinery, type of package, headspace, additives (including antimicrobial additives), and storage temperature are significant factors that influence the shelf-life and the quality of packaged fresh or processed food. Considering both the type of packaging material and the type of food, the product can be stored under aerobic conditions in a modified atmosphere, or in a vacuum [1, 54, 55]. The selection of the proper storage conditions depends on the barrier against gasses and water vapor, hardness, stability, heat resistance capabilities, market requirements, and most important the cost of the packaging materials [1, 55]. The modified atmosphere packaging (MAP) preservation method has been used to protect food products against deterioration to extend their shelf-life [56]. MAP refers to the use of high-barrier packaging materials to package products in which gases, such as CO2, N2, and O2, are mixed in selected ratios and introduced into the packaging material to inhibit the growth of microorganisms to reduce the enzymatic reaction and decrease the rate of lipid oxidation [56–59]. Nitrogen, oxygen, and carbon dioxide are the most commonly used gases in MAP. The ideal CO2 and N2 concentration in MAP depends on the food products and the different gas atmospheres that have been used for specific food products. The shelf life of many food products is limited by microbiological growth in the presence of O2 [56, 57]. Generally, the effect of MAP is conditioned by the concentrations of CO2 available in the packaging. Carbon dioxide inhibits the growth of the microorganisms responsible for food spoilage growing in normal air conditions, such as *P.* sp. and *Shewanella putrefaciens* [56–59]. The reduction in the amount of oxygen in the package may be a possible solution to preserve food and maintain its quality [57, 59]. Another option for extending the shelf life of food is to use vacuum packaging. As an example, a) the mixture of gases containing 40% CO2, 30% N2, and 30% O2 was recommended for low-fat fish [57], b) the atmosphere of 40% CO2, 10% N2, and 50% O2 was recommended for chilled beef steak [60], c) mixtures such as 80% O2, 20% CO2; 50%O2, 20% CO2, and 30% N2, or 20% O2, 20% CO2, and 60% N2 was recommended for darkcutting beef stored under chilled and superchilled conditions [61], d) vacuum packaging and the mixture of gases for MAP containing 40% CO2, 20% N2, and 40% O2 was recommended for pork steaks [62]. Packaging functionality depends on the maintenance of the
