Illnesses due to the consumption of foods contaminated with pathogens have a huge economic and public health impact worldwide.[1] The U.S. Centers for Disease Control and Prevention estimates that each year, 1 in 6 people in the U.S. gets sick, 128,000 are hospitalized, and 3,000 die of foodborne illnesses (FBIs).[2] The Economic Research Service of the U.S. Department of Agriculture reports that the annual cost of medical care, loss of productivity, and premature deaths due to FBIs is estimated to be $16.3 billion.[3] Estimates from 2011 show that the main pathogens contributing to domestically acquired FBIs resulting in death included Salmonella, Toxoplasma gondii, Listeria monocytogenes, norovirus, Campylobacter spp., and Escherichia coli O157:H7.[2] Despite advances in food processing technology, food handling, and food safety awareness, FBIs remain common in the United States.

In the U.S., illnesses and cases associated with fresh produce were larger than those involving seafood, poultry, beef, pork, or eggs. An analysis of outbreak data in the U.S. from 1988 to 2008 showed an average of 6.3–13.2 illness outbreaks each year caused by produce.[1] For U.S.-grown leafy vegetables alone, more than 20 foodborne outbreaks have been linked to contamination by E. coli O157:H7 since 1995, resulting in at least 600 reported illnesses and five deaths.

Since most fresh produce is eaten raw and receives minimal processing, contamination with pathogens represents a serious risk. The contamination can occur either pre- or postharvest. Preharvest sources include soil, irrigation water, insects, inadequately composted manure, and wild or domestic animals. Postharvest contamination relates to handling and preparation such as cutting, slicing, or peeling of produce that damages their tissues, releasing nutrients and facilitating growth of microorganisms. It is estimated that as much as 25 percent of all food produced is lost postharvest owing to microbial activity.[4]

Food Packaging Functions
Since food contamination occurs mostly at the surface of food, food packaging plays an important role in preventing the growth of microorganisms, thus enhancing safety and maintaining quality of the packaged foods.[5] The primary purpose of packaging is to protect foods from the environment and maintain them in proper condition during shipping, distribution, and storage. Food packaging also enables foods to travel safely from their point of origin and still be wholesome at the time of consumption.[6] However, the packaging must balance the protection of food with other issues such as impact on the environment. The main types of packaging that are currently used are derived from nonrenewable petroleum resources, including nonbiodegradable plastic materials. The volume of plastics discarded annually creates a substantial waste disposal issue. There was a 37 percent increase in municipal solid waste from 179.6 million tons in 1988 to 245.7 million tons in 2005 due to plastic packaging. Packaging made up the largest portion of waste generated at 31.2 percent. Food packaging alone accounts for almost two-thirds of total packaging wastes by volume. Therefore, food packaging has become a focus of waste reduction efforts because proper waste management is important to protect human health and the environment. Although some materials can be recovered and recycled, many synthetic polymer-based materials end up in landfills or in the environment. To address some of these problems, considerable research has been undertaken to obtain environmentally friendly food packaging materials.[7]

In the last few years, this research has aimed to develop materials derived from plants, whose main advantage is that they are fully biodegradable. Biodegradable films have been proposed as alternative food packaging to improve the quality and safety of food products. This technology protects foods from dehydration and acts as a gas barrier with the surrounding media, extending the shelf life of the foods.[8]

New Options for Food Packaging
Several studies showed the efficacy of starch for food packaging to preserve food quality against microorganisms. Starch is the most commonly used raw material in agriculture due to its relatively low cost, the benefits of natural polymerization, abundant availability of raw materials, and fast biodegradability.[9] Starches derived from potato, yam, corn, maize, wheat, rice, and cassava have been frequently investigated for the development and characterization of starch-based edible films.[10] Sweet potato (SP) (Ipomoea batatas Lam) is an inexpensive and readily available vegetable that is cultivated extensively for its nutritious value across many regions of the world. In the past 15 years, the U.S. sweet potato industry has grown significantly.[11] In 2014, North Carolina produced 53 percent of all SP grown in the country.[12] Technological applications of starches revealed their limited mechanical properties compared with conventional synthetic polymers due to starch’s hydrophilic characteristic. On its own, starch is not a viable wide-scale substitute for conventional packaging, but scientists are discovering ways to combine starch with novel materials to improve its potential as food packaging.[13]

Biopolymer-clay nanocomposites are a new class of materials with potentially improved mechanical properties. These composites are prepared by the addition of low amounts of nanoclay to the biopolymer matrix. The addition of nanoscale particles to starch can change the crystallization kinetics, the crystalline morphology, the crystal forms, and the crystallite size. As a result, these particles may improve the mechanical and barrier properties of starch. Montmorillonite (MMT) is the most commonly used natural clay and has been successfully applied in numerous nanocomposite systems.[14] The popularity of MMT nanoclays in food contact applications derives from their low cost, effectiveness, and high stability.[15] The nanoclays have an average thickness of about 1 nm and average lateral dimensions ranging between a few tenths of a nanometer to several microns. The incorporation of MMT into starch matrices has been used to enhance mechanical and barrier properties.[16] In some studies, the safety of packaging materials made with nanoclays was investigated. The main risk of consumer exposure to nanoclay packaging is through migration of nanoparticles or other substances from packages into foods that are then ingested. However, migration studies are few in number. Avella et al.[17] showed that vegetables in contact with clay/starch nanocomposite films exhibited no changes in their iron or magnesium content. Their results demonstrated either no appreciable migration of the constituent elements of the clay nanoparticles into the food or migration within the limits set forth by the current food regulations.

Furthermore, nanocomposite films are excellent vehicles for incorporating a wide variety of additives such as antioxidants and antimicrobial agents. The effect of these additives may result in improvement of food quality and safety. The antibacterial activities of the films are determined in part by the release rate of antibacterial agents—too slow of a release and microbial growth is not inhibited sufficiently; too fast of a release and inhibition is not sustained.[18] Nanoclay can potentially be used to control the release rate of antimicrobial agents from film materials.

Currently, consumers are more conscious about the potential health risks associated with the consumption of synthetic additives in food products for preservation and other applications. Indeed, the shift toward the incorporation of antimicrobial agents into packaging rather than directly adding them to food products has been a significant focus of packaging research.[19] Several natural antimicrobials have been incorporated into food packaging such as plant essential oils, nisin, and chitosan (a natural biopolymer extracted from crustacean shells). However, nisin has a limited spectrum of activity, does not inhibit Gram-negative bacteria or fungi, and is effective only at low pH.[20] There is a concern with allergenicity of chitosan incorporated into packaging. Thyme essential oil (TEO) has been evaluated and effectively exhibited antimicrobial activity against pathogenic microorganisms.[9]

Scientists at North Carolina A&T State University created a biodegradable nanocomposite film for food packaging that incorporates TEO and biodegradable MMT nanofillers into sweet potato starch (SPS) to address issues of food safety, environmental impact, and agricultural sustainability. The physico-mechanical and barrier properties of the film were characterized. Then the antibacterial activity of the developed film was tested against two foodborne pathogens (Salmonella and E. coli) on inoculated baby spinach leaves during 8 days of refrigerated storage. To comprehend consumer preferences, a sensory test on uninoculated samples was conducted as well.

The results showed that incorporating MMT and TEO into SPS film greatly improved water solubility and water vapor permeability by more than 50 percent.21 Water vapor permeability is the time rate of water vapor transmission through a unit area of thickness induced by a unit vapor pressure difference between two surfaces. Water vapor permeability is one of the most important parameters for biodegradable films. Since the main function of food packaging is to avoid or minimize moisture transfer between the food and the surrounding atmosphere, water vapor permeability should be as low as possible to optimize the food package environment and potentially increase the shelf life of the product. Furthermore, the combined effect of MMT and TEO (up to 4%) improved tensile strength (which is defined as extending the film at a given rate upon breakage and registering the strength versus time or distance), elongation (which is defined as the percentage of elongation at the moment of rupture of the tested sample), and tristimulus color values. Also, our results showed that the incorporation of TEO into the film significantly reduced the population of E. coli and S. Typhimurium on fresh baby spinach leaves below detectable levels within 5 days, whereas the control samples without essential oil maintained approximately 4.5-log CFU/g.[22] Thymol and carvacrol are the most active components in TEO. It has been reported that TEO has inhibitory effects against microorganisms through the breakdown of the outer membrane of the microorganism and leads to an excessive leakage of essential elements, causing bacterial cell death. Also, the effects of TEO refer to its ability to increase the permeability of the cell’s outer membrane, leading to the release of lipopolysaccharides and increased adenosine triphosphate loss.[23] Furthermore, the ratios of the major and minor components describe the chemical composition of the essential oils. The baby spinach samples wrapped in SPS films without TEO showed a decrease in bacterial or yeast and mold populations. This might be due to superior oxygen barrier properties of starch-based films, which limit the growth of aerobic bacteria on the samples wrapped in the films.[24] Films containing 2 percent TEO showed lower numbers of microorganisms compared with the control samples after storage at 4 °C for 2 and 8 days. However, when the baby spinach samples were wrapped in films with 4 and 6 percent TEO, no bacterial or yeast and mold growth was detected over the storage time. This indicates that TEO incorporated into SPS/nanocomposite films could strongly inhibit the growth of microorganisms. Sensory scores of spinach samples wrapped in films containing TEO were higher than those of controls. Overall, this study suggests that SPS incorporated with TEO may provide a viable solution to the waste disposal of plastic packaging materials for foods and shows a strong potential for use as active packaging.   

Reza Tahergorabi, Ph.D., is a Research Assistant Professor in the Food and Nutritional Sciences Program at North Carolina A&T State University.

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