Food products can be subjected to microbial contamination that is mainly caused by bacteria, yeasts, and fungi. Many of these microorganisms can cause undesirable reactions that deteriorate the flavor, odor, color, sensory, and textual properties of foods (Appendini and Hotchkiss 1997; Vermeiren and others 1999; Weng and others 1999; Appendini and Hotchkiss 2002; Vermeiren and others 2002; Devlieghere and others 2004a; Han 2005; Rupika and others 2005; Davidson and Taylor 2007; Gutierrez and others 2008). Microbial growth in food products is a major concern because some microorganisms can potentially cause food-borne illness (Padgett and others 1998; Natrajan and Sheldon 2000; Cha and Chinnan 2004; Davidson and others 2005; de Oliveira and others 2007). In packaged foods, the growth and survival of common spoilage and pathogenic microorganisms such as Listeria monocytogenes, Escherichia coli O157, Salmonella, Staphylococcus aureus, Bacillus cereus, Campylobacter, Clostridium perfringens, Aspergillus niger, and Saccharomyces cerevisiae are affected by a variety of intrinsic factors such as pH, water activity, and the presence of oxygen or by extrinsic factors associated with storage conditions including temperature, time, and relative humidity (Singh and others 2003; López-Malo and others 2005; Rydlo and others 2006). Many food products including various types of cheeses, meats, poultry, and baked products are highly susceptible to microbial spoilage (Weng and Hotchkiss 1993; Suppakul 2004; Limjaroen and others 2005; Schelz and others 2006; Silveira and others 2007).
To prevent the growth of spoilage and pathogenic microorganisms on foods, various traditional preservation techniques such as heat treatment, salting, acidification, and drying are used in the food industry (Quintavalla and Vicini 2002; Ozdemir and Floros 2004; Davidson and Taylor 2007; Farkas 2007). In recent years, a rise in consumer demand for safe, fresh, and minimally processed foods has led to the development of new preservation techniques. Active packaging (AP) technologies, for example, can provide safe food products with longer shelf lives (Rooney 1995; Lau and Wong 2000; Vermeiren and others 2002; Fitzgerald and others 2003; Ozdemir and Floros 2004; Gutierrez and others 2008). In the food industry, spoilage of food products, including spoilage that is caused by microorganisms, is a major concern. The AP technologies designed primarily to protect food products from deterioration and from the growth of microorganisms can involve the use of synthetic or natural antimicrobial (AM) agents (Juneja and Sofos 2005). To diminish food spoilage by microorganisms, different AM agents (primarily synthetic) are commonly incorporated directly into the food. This method has many disadvantages: (i) consumers prefer foods with no or minimal synthetic additives because of concerns about side-effects; (ii) since food spoilage occurs primarily on the surface, incorporation of relatively large quantities of the agents in the bulk of the food is not justified; (iii) some of the synthetic agents possess a distinct flavor that may be rendered to the food flavor, and (iv) synthetic additives have to be declared on the package. Therefore, packaging materials that incorporate in them the AM agent as an additional protective barrier are emerging as the preferred preservation method. Several authors have reported AP technologies that involve the use of films produced from synthetic polymers (Miltz and others 1995; Rooney 1995; Smith and others 1995). These materials can act as carriers for active agents, including AM compounds, in order to maintain high concentrations of the agent on or near the food surface to control or prevent the growth of spoilage and pathogenic microorganisms (Krochta and De Mulder-Johnston 1997; Joerger 2007; Raybaudi-Massilia and others 2009; Rojas-Graü and others 2009; Suppakul and others 2011a). Thus, a packaging film impregnated or coated with an AM agent could potentially extend the shelf life and improve the microbial safety of food products (Appendini and Hotchkiss 2002; Suppakul and others 2003b; Burt 2004; Kuorwel and others 2011b).
Although AM agents such as essential oils (EOs) and/or their principal components may exhibit AM activity against various microorganisms when incorporated into packaging materials, the organoleptic properties of the packaged food products are one of the important factors that must also be taken into consideration. According to Davison and Zivanovic (2003), the concentration of AM agents required to demonstrate AM activity against various microorganisms on food products might be higher than the concentration applied for flavoring purposes. As a result, this might cause food tainting and/or adverse sensorial effects to food products (Smith-Palmer and others 2001; Bagamboula and others 2004). The adverse sensorial effects of AM agents to food products can be overcome by masking the odor of AM agents with other approved aroma compounds as suggested by Gutiérrez and others (2009). An understanding of the relationship between minimum inhibitory concentration and acceptable organoleptic properties of AM agents such as EOs and/or their constituents is also important (Lambert and others 2001). In some cases, the replacement of EOs with one or a number of their principal constituents may provide equal AM effectiveness but with milder flavoring attributes (Lambert and others 2001; Smith-Palmer and others 2001).
In a recent review, the current authors presented an evaluation of the AM activity of biodegradable polysaccharide and protein-based films containing natural agents (Kuorwel and others 2011b). These films showed the potential for a wide range of applications in food packaging where undesirable microbial growth is a concern. Moreover, these films degrade readily in the environment but the acquisition of this attribute may harm the processability and mechanical stability of the film. Thus, in spite of the increasing concern in recent years about the use of synthetic polymers due to their poor biodegradability, these materials have several advantages including low cost, good processability, and sound mechanical and physical properties. Therefore, development of AM packaging materials manufactured from synthetic polymers such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polystyrene (PS), polyethylene terephthalate (PET), and polypropylene (PP) is still important in offering commercial benefits for packaging food products. In the current review, a detailed summary of synthetic films utilizing common synthetic and natural AM agents is presented with an emphasis on the principal components of basil, oregano, and thyme EOs, namely, linalool, carvacrol, and thymol, respectively. This is followed by a list of other natural AM agents that have the potential for controlling microbial growth on foods.