While the food supply in the USA is one of the safest in the world, food safety is still a big concern across the nation. According to the CDC (Centers for Disease Control and Prevention), about 48 million people get sick due to foodborne illness each year, among them 128,000 are hospitalized, and 3,000 die (CDC, 2011). It was also estimated that the annual health-related cost due to pathogenic foodborne outbreaks totaled $77.7 billion (Scharff, 2012). Foodborne illness is usually caused by food pathogens. There are 31 food pathogens confirmed to cause foodborne illness (FDA, 2014). Recent years, most of the multistate foodborne outbreaks investigated by the CDC are associated with Salmonella, Listeria monocytogenes, and Escherichia coli, and foods contaminated by these pathogens are linked to meat (fresh meat, deli meats etc.), fresh or fresh-cut produce (cantaloupe, alfalfa sprouts, cucumbers, etc.), and dairy products (cheese, raw milk, butter, etc.) (CDC, 2017b). Control of food pathogens, especially those frequently associated with foodborne outbreaks, is a key step to keep our food safe.
Another concern for the food industry is food waste or food loss. According to the USDA (US Department of Agriculture), an estimated 133 billion pounds, or 31% of the 430 billion pounds of food available for human consumption were wasted in the USA in 2010, which was worth about $161.6 billion (Buzby, Farah-Wells and Hyman, 2014). The top three food groups in terms of share of total value of food loss were meat, poultry, and fish (30%, $48 billion); vegetables (19%, $30 billion); and dairy products (17%, $27 billion) (Buzby, Farah-Wells and Hyman, 2014). Causes of food waste vary depending on the stage of the food supply chain (i.e. processing, distribution, retail, household). Some examples of causes can include improper storage, physical damage through distribution, insect contamination, spoilage microorganisms, oxidation or even confusion understanding date code. At retail or consumer level, the presence of spoilage microorganisms on raw materials and on processed foodstuffs due to cross contamination may be pointed out as a major reason for food waste (Parfitt, Barthel and Macnaughton, 2010). In order to reduce food waste or extend the food shelf-life, it’s necessary to inhibit the growth of spoilage microorganisms.
To keep food safety or reduce food waste, various methods have been developed to preserve foods, such as heating, freezing, addition of preservatives, high pressure processing, irradiation, pulsed electric field processing, packaging, etc. Among those preservative methods, antimicrobial food packaging attracts considerable attentions. Antimicrobial food packaging is the utilization of “food packaging systems that inhibit spoilage and reduce pathogenic microorganisms” (Barros-Velazquez, 2015). It can separate the food from the detrimental environment and inhibit the growth of microorganisms without altering the food itself (Valencia-Chamorro et al., 2011). Besides, antimicrobial packaging also allows the incorporated antimicrobial agent to release in controlled or sustained manners, thus exerting microbial inhibition along the food production chain (Mastromatteo et al., 2010). Antimicrobial food packaging includes several systems, such as (1) addition of sachets or pads containing volatile antimicrobial agents into packages; (2) incorporation of antimicrobial agents directly into polymers; (3) coating antimicrobial substances onto polymer surfaces, (4) immobilization of antimicrobials to polymers by ion or covalent linkages, or (5) use of antimicrobial macromolecules with film-forming properties (Véronique, 2008). Common methods to incorporate antimicrobials into packaging materials include compression molding and blown film extrusion for heat-stable antimicrobials, solvent compounding, surface coating for heat-sensitive antimicrobial, and cast film method (Sung et al., 2013).
In the past decades, large numbers of antimicrobial food packaging products were developed. Most of the products are proven able to control the growth of microbial and prolong food shelf-life effectively. However, there are only a few commercialized products found in the market; they are either volatile antimicrobial agents such as chlorine dioxide and sulfur dioxide packed in sachets/pads, or metal oxides such as silver compound incorporated into plastic films (Sung et al., 2013). Several reasons such as production technology, economic impact, consumer acceptance and unavailability of specific regulations about active packaging limits the application of antimicrobial packaging.
As aforementioned, only a few antimicrobial packaging products are available in the market. One major type of commercial antimicrobial packaging is based on plastic films, in which an antimicrobial is incorporated directly into a packaging material during production processes of plastic films. Currently, most of the antimicrobial plastic films in the market are incorporated with synthetic antimicrobials by compression molding or blown film extrusion (Appendini and Hotchkiss, 2002; Sung et al., 2013). For most consumers and the public, the utilization of natural antimicrobials is highly desirable over synthetic ones. However, if natural antimicrobials are incorporated into plastic films via molding and extrusion, rapid loss of these compounds may occur due to the high temperature involved. For the food industry, the most important consideration for manufacturing antimicrobial plastic films is cost-effective and high production capacity. For molding and extrusion processing, incorporating antimicrobials into polymers is not only costly but also difficult to realize high-efficiency mass production.
In order to reduce the loss of antimicrobial substances under high processing pressure and temperature, it is crucial to employ alternative approaches for rapid and efficient manufacturing. As a non-thermal approach, surface coating can be an ideal alternative due to its simplicity of the process. Various methods have been used in developing antimicrobial coatings at lab levels, such as thin layer chromatography, spin coating, Mayer rod drawdowns, film casting on a substrate (e.g. glass or Teflon coated plates) (Tiwari, 2017). Examples of antimicrobial-coated films include nisin-coated LDPE film (Neetoo et al., 2008), essential oil such as oregano oil-coated polypropylene film (Muriel-Galet et al., 2013), etc. While showing various levels of success in a lab scale, major challenges remain for scaling up these methods from laboratory concept to mass production: (1) properties and performance of coating material not suitable for large-scale equipment (e.g. solid content, viscosity), (2) coating methods not achieving consistent qualities with large facilities (e.g. uneven coating, low efficiency), (3) reduced transparency of films due to the crystalline nature of coating materials (e.g. salts or cellulose), and (4) loss of antimicrobial efficacy associated with processing and storage. There are three basic components for antimicrobial plastic films: (1) Base materials to form film or coating; (2) Antimicrobials to reduce or inhibit microorganisms; and (3) Methods to making and applying films or coatings (Jin, 2017).
Although biopolymers such as polysaccharides and proteins have been studied as film materials, the poor mechanical properties limit their commercial application as packaging materials. Currently, most of the antimicrobial films are based on plastic films. Plastic films are extensively used for their low cost, fine barrier property, excellent mechanical strength, high transparency, ability to be heat-sealed and easy to be printed on (Gemili, Yemenicioğlu and Altınkaya, 2009). The most widely used plastic films in packaging included low density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), ethylene vinyl acetate (EVA), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) (Sung et al., 2013).
Roll-to-roll (RTR) is a family of manufacturing techniques involving continuous processing of a flexible substrate as it is transferred between two rotating rolls of material. It is an important class of substrate-based manufacturing processes in which additive processes can be used to build structures. RTR is a “process” comprising many technologies combined that can produce rolls of finished material in an efficient and cost-effective manner with the benefits of high production rates and in mass quantities. RTR manufacturing is superior to conventional manufacturing due to RTR’s great advantage in terms of high throughput and low cost. Usually, for conventional manufacturing, it is slower and costs more due to the multiple steps involved in batch processing. Nowadays, RTR processing is applied in numerous manufacturing field such as flexible and large-area electronic devices, flexible solar panels, printed/flexible thin-film batteries, fibers and textiles, metal foil and sheet manufacturing, medical products, energy products in buildings, and membranes. In the field of food package, RTR is used to develop wrapping film with better barrier properties. Struller et al (2014) used a General K4000 vacuum metallizer, a roll-to-roll metalilizer, to depose aluminum oxide on BOPP and PET film. While, the film used in this study is pretreated with corona treatment before loaded in R2R system.
Gravure printing is a type of intaglio printing process, which involves engraving the image onto an image carrier. As shown in Figure 1, when the gravure image cylinder rotates in the coating solution, the engraved cells of the image or pattern is filled with solution. The doctor blade controls the coating solution by removing the excess from cylinder surface, leaving coating solution only within engraved cells. The gravure cylinder then comes in contact with the plastic substrate, transferring the printed image with the help of the rubber impression cylinder, which pulls the coating solution from the engraved cells. The design of gravure printing allows the adoption of such coating set into roll-to-roll system. And the two-dimension designable printing or coating pattern promises the feasibility of dot-pattern for this project. Moreover, according to Krebs, comparing to other printing method, gravure printing assures faster processing speed, reduction of the amount of coating solution wasted.
