The Evolution of Food Preservation and Packaging

Chapter 15

The Evolution of Food Preservation and Packaging

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Alvin F. Bopp* Department of Chemistry, Southern University at New Orleans, New Orleans, Louisiana 70122, United States *E-mails: [email protected]; [email protected].

With the introduction of agriculture, the storage of harvest surpluses became critically important for the viability of society. Storage techniques evolved to keep that surplus suitable for consumption or viable as seed for the next season’s crop. Naturally occurring decay mechanisms had to be identified and understood. With the development of cities and trade, a need also arose to accommodate those food items transported from rural to urban areas. In modern times, food security has also become a growing issue. Packaging has coevolved with societal expectations as a visible means of not only displaying and preserving food but also keeping it secure and free of contamination. This chapter provides a brief overview of decay processes in foods and how packaging has evolved to extend shelf life and nutritional value.

Background For most of our time as humans (Homo sapiens) we were hunter-gatherers who hunted game and collected plant foods by foraging rather than by growing or tending crops. Most hunter-gatherer societies are small nomadic or seminomadic groups living in temporary settlements typically constructed using impermanent building materials or natural shelters, where available. They stay in one place until available resources are depleted and then move on, generally following the migrating herds. Some hunter-gatherer cultures, however, such as the indigenous peoples of the Pacific Northwest coast, lived in uniquely rich environments that allowed them to build and occupy permanent settlements. The hunter-gatherer diet probably consisted of meat (including eggs and fish), fruits and nuts (requiring little or no processing), and some vegetables. They would have had access to few carbohydrates and no dairy past childhood. Also, food items would have been consumed in a relatively short period of time, thus limiting the need for food storage and preservation technologies. The transition from hunter-gatherer to farmer was gradual, and something akin to gardening was probably an intermediate step. But this would not have been gardening as we know it. Forest gardening is a food production system still in use in various parts of the world. Family groups identify © 2019 American Chemical Society

Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

and protect useful tree and vine species. Undesirable species are eliminated, and over time other useful species are selected and incorporated into the “garden.” The Natufian culture in the Middle East (12,500–9000 BCE) provides insight into the transition from nomadic hunter-gatherers to a semiagricultural society living in permanent settlements. Although primarily hunter-gatherers, Natufians lived in permanent settlements, and there is some evidence that they cultivated cereal crops and baked bread (1). Carl Sauer (1889–1975) postulated that hunter-gatherers would have known wild plants and how they grew, and would have incorporated plant cultivation with foraging as part of an overall food collection strategy (2). Eventually, the transition between simple foraging, where nomadic bands followed the plants and animals, to sedentary agricultural societies, where the people stayed in one place and grew crops, was complete. The foragers changed from collecting wild fruits, nuts, and cereals to cultivating them. The development of agriculture has been an evolutionary process and not a revolutionary one.

Domestication of Plants and Animals Domestication turns out to be more than simply taming a plant or animal. It involves its adaptation for human use, not only as a food source but also for work, medicine, fiber (for clothes or building materials), and even ornamental uses. Permanent settlements and agriculture started in the Levant (eastern Mediterranean and west of Mesopotamia—Syria, Lebanon, and Israel) around 10,000 years ago. Plants were domesticated before animals, and the first plants to be domesticated were probably wheat, barley, and lentils (3, 4). In the Far East and Southeast Asia, millet, a whole grain, was domesticated around 8000 BCE, and rice was being grown around 7000–8000 BCE (5). Potatoes have been archeologically dated in South America to 8000 BCE (6, 7). The partnership between dogs (Canis lupus familiaris) and humans is ancient and occurred much earlier than other animals, possibly as early as 14,000 BCE, and took place over a span of thousands of years (8, 9). That partnership likely originated from humans’ needs for help with hunting, an early alarm system, and a source of food in lean times. In return, dogs received companionship, protection, shelter, and a reliable food source. The domestication process was clearly demonstrated in the Soviet Farm-Fox Experiment. Starting in 1959, foxes were selectively bred based on a behavioral trait (friendliness); within 10 generations, the offspring developed dog-like characteristics, including a desire for human companionship and reproductive systems more typical of domestic animals. The experiment continues (10). Similarly nomadic peoples probably nurtured and fed wolves based on their hunting abilities and warning reliability; thus, they unconsciously selectively bred dogs. Shortly after domesticating plants and adopting a non-nomadic lifestyle, humans began domesticating animals. The exact transition from wild to domesticated may never be known, but, as with plants, active hunting led to animals being managed, then captured, tamed, and finally bred for food. Approximate dates are shown in Table 1 (11, 12). Table 1. Timeline for Animal Domestication Approximate Time Period (BCE)

Animal Sheep, pigs

9000

Goats, cattle

7000

Horses

3000

Poultry

2000

212 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

The use of animals for nonfood tasks (e.g., draft animals) followed. Oxen were first domesticated around 4000 BCE and the water buffalo somewhat later in Southeast Asia. Over the millennia, the yield and reliability of the harvest continued to advance due to improvements in crop species through breeding, the greater use of draft animals, and ultimately mechanization. However, the quantum leap came with the green revolution. The green revolution was a series of research-and-development and technology transfer initiatives starting in the late 1940s that focused on the introduction of high-yield crop varieties and the modernization of farm management technologies, such as irrigation and the widespread use of chemical fertilizers and pesticides (13, 14). The green revolution was championed by Norman Borlaug (1914–2009) (13), who was awarded the Nobel Peace Prize in 1970 for his contributions to world peace by increasing the food supply (15). The application of inorganic nitrogen-containing fertilizers (synthetic nitrogen), along with phosphate and pesticides, contributed greatly to increased crop yields. In addition to feeding a population, other benefits were also realized; for example, ranchers had access to less costly feed for their herds. Gains in yield continue as higher-yield varieties of staple grains and varieties that exhibit resistance to disease, drought, and pesticides continue to be introduced. Mechanization has also made farming more productive. Before 1800, five laborers could harvest 2 to 3 acres in a day. With the invention of the mechanical reaper by Cyrus McCormick (1809–1884) in 1831, productivity increased to 10 to 12 acres per day. By 1890, land productivity had increased more than 20-fold; it took only two men, aided by two horses, to process 20 acres of wheat in a day (16). Today a combine harvester averages 150 acres per day. The positive impacts of the green revolution have come with negative consequences. The green revolution was based on agricultural intensification, and as a result, there has been a loss of plant biodiversity in favor of varieties developed and selected for their high yields and agricultural value. The reliance on mechanization and a “business” model for agriculture has largely eliminated small farms and replaced them with agribusinesses. There has also been an overuse of pesticides and synthetic fertilizers, and water supplies for irrigation have been depleted. Thomas Malthus (1766–1834) observed that improvements in food production improved the nation’s standard of living but only temporarily, with those gains quickly being offset by population increases. Abundance fuels population growth at the expense of maintaining a higher standard of living. Populations grow and the lower economic classes suffer hardship, famine, and disease—the Malthusian trap (17). As such, population growth demands continued advances in food production and exacerbates the negative impacts that agriculture has on the environment. Modern agriculture is the result of a series of evolutionary changes and not a single revolutionary event. Hunter-gatherers no doubt knew seasonal animal migrations and plants, which served as food, as well as medicines and building materials. “Selective” gardening along with foraging became part of an overall food collection strategy. For true agriculture to take hold, humans had to change from a nomad’s lifestyle to a sedentary one (i.e., living in one place permanently). This transformation required access to a sufficient year-round food supply. Food Storage Further changes were in store. In a hunter-gatherer society, people went looking for food; in an agricultural society, the food came to the people, but at times it was determined by the specific crops and climate. Permanent settlements put an overwhelming stress on a local ecosystem such that hunting-gathering communities of any size cannot provide adequate sustenance for any prolonged


stretch of time. Crop surplus became necessary for survival, and with that surplus came a new issue: storing that surplus until it was needed, which involved understanding and controlling spoilage.

Spoilage Mechanisms Any change that makes food unfit for human consumption is termed “spoilage.” Such changes range from appearance and texture to biological contamination leading to illness. Food deterioration, and subsequent spoilage, is no accident. It is a naturally occurring process that begins when the crop is harvested or the animal slaughtered. When dealing with food quality, time is an enemy. Food produced during harvest time, which often lasts for only a few weeks, has to be preserved to enable its use for a much longer period of time, presumably until the next harvest. In addition, a portion must be saved as seed and kept viable until the next planting season. Meanwhile, its quality must be maintained while in storage. Harvested crops are typically dried and kept at a cool temperature to prevent spoilage or germination. Methods for food preservation, such as smoking, drying, and fermentation, were developed as were containers to store and secure food while also making it available when needed. Before developing any approach to maintain food quality and retard or even prevent its spoilage, pathways leading to food spoilage had to be identified and understood. The key to effectively storing food items is to understand the varied spoilage mechanisms: • • • • • • • •

Microorganisms; Endogenous enzymes; Air (oxidation); Ethylene; Light; Pests; Temperature and humidity; and Time.

Microorganisms Many types of microorganisms can cause food problems, and those microorganisms that cause foodborne illness are further labeled pathogens. Pathogens typically grow best at ambient temperatures (15–32 °C) but not well at refrigerator (1–4 °C) or freezer temperatures (–16 to –2 °C). However, some spoilage microorganisms, including certain bacteria, yeasts, and molds, can grow well at temperatures as low as 5 °C. When spoilage microorganisms are present and active, the food item will usually look, taste, or smell unappealing, although the item may still be edible. However, some pathogenic microorganisms grow in food without any noticeable change in odor, appearance, or taste. The microbial spoilage of some staple materials has been reviewed for minimally processed vegetables (18), meat (19), and fish (20). Endogenous Enzymes Endogenous enzymes are naturally present in food and commonly responsible for the ripening process in fruits and vegetables, generally evidenced by changes in the material’s texture, color, and flavor. The effects of endogenous enzymes can sometimes be controlled by denaturing these enzymes. Blanching is one such technique. Blanching is a precooking process that involves steaming or boiling fruits and vegetables for a short time. It denatures the enzymes that cause loss of texture, 214 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

color, and flavor; it also cleans off surface contaminants like dirt and unwelcome organisms, while helping retain the food’s nutritional value. One particularly important endogenous enzyme is polyphenol oxidase, an enzyme responsible for enzymatic browning in many fruits and vegetables. Enzymatic Browning Polyphenol oxidase (PPO) is an enzyme found in most fruits and vegetables. PPO catalyzes the hydroxylation of monophenols to orthodiphenols (o-catechols) and their subsequent oxidation to quinones. o-Quinones are naturally antiseptic and provide some protection for the fruit, but they can form melanin, a naturally occurring pigment, by reacting with proteins and amino acids (Figure 1). The enzyme (PPO) and substrates (phenols) are located in different “vessels” in an intact cell. When a fruit is damaged, for instance, by improper handling or cutting, the enzyme and substrates are brought together, oxidation occurs upon exposure to oxygen, and enzymatic browning results (21). PPO catalyzes reactions at pHs between 5.0 and 7.5, so coating the fruit with even a weak acid, such as ascorbic acid, lowers the pH and inhibits catalytic activity (22, 23).

Figure 1. Enzymatic browning pathway. Image courtesy of Mary Virginia Orna. PPO is important in the food industry. On the one hand, browning can make an item less visually attractive and impact its nutritional content, thus making it less marketable and causing economic loss. There are, however, applications in which the browning reaction, catalyzed by PPO, is desirable, such as with prunes, sultana grapes, black tea, and green coffee beans. Oxidation Though oxygen supports life, it can be a problem when it comes to food shelf life, quality, and, in some cases, safety. Elemental oxygen is a product of photosynthesis and is essential to respiration in plants and animals. However, in foods, its oxidizing activity can result in off-flavors, odors, and sometimes harmful compounds.


Oxidation can produce detrimental changes in flavor, color, and nutrient content as a result of reactions with any number of components in items of food. When fats oxidize, they become rancid and exhibit off-flavors and aromas from the products of these reactions. Chemically, C–C double bonds, found in many fatty acids, are particularly susceptible to oxygen attack. These reactions are enhanced by singlet oxygen produced from irradiation with ultraviolet or visible energies with plant components acting as photosensitizers (24). Hydroperoxides form and lead to the formation of aldehydes, ketones, alcohols, and other byproducts responsible for off-odors and flavors. In red meats, the iron in myoglobin (the oxygen-binding porphyrin complex found in muscle tissues responsible for oxygen storage) can be oxidized from the +2 state to +3 by oxygen resulting in a change in the color of the meat from red to brown (25). It should be noted that the brown color of meat is not, on its own, an indication of spoilage. Ethylene (C2H4) Ethylene is a phytohormone that regulates a plant’s growth and development as well as the speed at which these processes occur, much as hormones do in humans or animals (26). Ethylene is also given off during ripening. As seeds mature, they emit ethylene, signaling to the fruit that it is time to ripen to assist in the germination process and seedling development. Ethylene’s presence is a doubleedged sword that depends on the state of maturity or the nature of the fruit. It is used to accelerate fruit ripening but can lead to over-ripening. The effects of ethylene are particularly noticeable in fruits. Some fruits, such as apples and pears, emit a greater amount of ethylene gas rendering them more sensitive to ethylene presence than other fruits that produce very little ethylene gas, such as cherries or blueberries. Pests Just like humans, insects, rodents, birds, and other pests require food to survive. Some of what they eat is what we eat. In addition to this direct competition, they can also damage or otherwise contaminate food, making it unfit for human consumption and more vulnerable to further deterioration. Physical damage, such as bruises and cracks on raw produce, can leave areas where microorganisms may grow easily. Improperly packaged foods, dented cans, and broken packages provide places for air, light, microorganisms, or other pests to enter. Temperature, Time, and Humidity Like all chemical reactions, the rates of the biochemical and microbial processes, including those that cause food deterioration, increase at higher temperatures. Increased temperature reduces time to spoilage and, conversely, most microbial activity ceases at temperatures below about 5 °C. Time in storage can also be a critical element. At longer times, microorganism growth will be greater, and other chemical processes, such as oxidation and enzyme action, will have been able to proceed. Spoilage is not so much a chemical process but a race against time. This fact led to the adoption of the concept of a “shelf life.” The shelf life of a food product is the time it is not only safe to eat but also the time it has an acceptable appearance, texture, and taste. Moisture can be either beneficial or detrimental to a food commodity. On the one hand, water is key to maintaining the internal structure and integrity of various plant cell components, making the plant appear fresh. Even after harvesting, fruits and vegetables continue to respire and lose water and carbon dioxide primarily through their leaves, but they can no longer replace the water through a root 216 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

system. The vegetable or fruit shrinks and becomes limp and its skin wrinkles. It wilts. This process is particularly evident in leafy vegetables. Moisture loss also occurs in other foods like meat, fish, and cheese. On the other hand, humidity can be a problem when storing grains but with a different impact. One of the most critical physiological factors in grain storage is the temperature and moisture content in the storage environment. Warm temperatures and high humidity create conditions that promote fungal and insect growth, respiration, and seed germination. The moisture content in a growing crop is naturally high and only decreases as the crop reaches maturity. Seeds are typically dormant and germinate when re-wet by rain or as a result of their own residual water content. Grains continue to be biologically active and respire in storage. In addition to water, respiration also produces heat, so lowering the temperature of the grain in storage slows respiration and lengthens the shelf life. Insect and fungal activities are also affected by temperature. Lower temperatures reduce insect and fungal metabolism and consequently slow their activity. Temperature and humidity are typically controlled by blowing air through the bin where the grain is being stored. Uniform air flow is important because otherwise “hot spots” can form and provide conditions for regions of biological activity. The development of agriculture led to advances in storage technologies to preserve the food item from the time of harvest to consumption. More recently the urbanization of society with accompanying lifestyle changes forced another adaptation to food handling and storage: packaging.

Role of Packaging It became important to build on the knowledge and understanding of food spoilage to address the new requirement: food transportation and short-term storage by packaging. In marketing, packaging serves one or more of several purposes: • • • • •

It facilitates handling and shipping. It defines a purchase quantity. It showcases and conveys information about the product. It provides product security by being tamper-resistant or at least tamper-evident. It effectively eliminates any interaction between the product and the environment, thus protecting the product from contamination (and sometimes protecting the environment from the product).

Over the years, a variety of materials have served as packaging: natural materials, ceramics, paper and paper products, glass, metals, plastics, and polymer films. Packaging materials have evolved with a greater understanding of how food spoils, and user demands often drive advances in materials science. Packaging now plays a critical role in the preservation, transportation, display, and storage of foods (27, 28). Packaging Materials Natural Substances Until the 19th century, there was little sophistication in packaging materials, and packaging as we know it did not exist. Containers made from natural materials were utilized for transportation and storage. Grasses, reeds, and thin strips of wood were often woven into baskets for solid food transportation, and storage and earthenware pots were created to store and transport liquids. 217 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

Ceramics Pottery is probably one of our earliest materials industries. Once humans discovered that clay could be formed into objects by mixing it with water, shaping it, and then firing it, an industry was born. In an area now within the Czech Republic, animal and human figurines made from fired clay have been found and dated to as early as 24,000 BCE. The first use of functional pottery vessels for storing water or grain and cooking is thought to be around 7000 BCE in Mesopotamia. Glazes were introduced later in Egypt, between 8000 and 5000 BCE, both to serve as decoration and to seal the vessel (29). Paper The use of paper and paperboards for food packaging dates to the 17th century. Paper and paperboard are based on an interlaced network of cellulose fibers generally derived from wood pulp produced by the Kraft process (30). In this process, wood chips are treated with a mixture of sodium hydroxide/sodium sulfide that dissolves lignins. The resulting pulp is further treated with strengthening agents and slimicides to enhance the properties of the paper product. Slimicides are broad spectrum biocides designed to control biofilm-producing organisms. Biofilms can be formed by aerobic bacteria, algae, yeasts, and molds (31). Certain steps in the production process provide conditions favorable for biological activity, and slimicides are used to protect both the product and process machinery. The U.S. Food and Drug Administration (FDA) regulates the additives used in paper and paperboard food packaging under Title 21 Part 176 of the Code of Federal Regulations (21 CFR Part 176) – Indirect Food Additives, Paper and Paperboard Components. Kraft pulp can also be bleached to produce high-quality writing paper. Paper and paperboards are commonly formed as corrugated boxes for transport, cartons, and wrapping paper. Plain paper is not used to protect foods for any extended period of time because it is not heat-sealable and is too porous to provide barrier protection. When used as primary packaging (that is, in the layer in contact with the food item), paper is almost always treated or coated with materials such as waxes or lacquers to improve functional and protective properties. Aseptic Processing Aseptic processing (packaging) is a technique by which thermally sterilized liquids are packaged under sterile conditions into previously sterilized containers producing shelf-stable products. Aseptic processing involves three primary steps: thermal sterilization of the product, sterilization of the packaging material, and conservation of sterility during packaging (32). The food materials are exposed to high-temperature, short-duration heat treatment (flash heating) that kills bacteria while preserving the quality of the food. Liquids, such as juice-based drinks and milk, can be processed aseptically and stored without refrigeration. Glass Containers Although glass-making developed in antiquity from knowledge gained from making pottery glazes, glass was first manufactured on a large scale in Egypt around 1500 BCE. The basic materials forming glass have not changed since then. The ingredients are melted together, incorporating additives for workability and color, and formed while hot. Since that early discovery, the mixing process and the ingredients have changed very little, but the techniques for making bottles have progressed. Glass vessels were handmade until the late 1890s when Michael Owens (1859–1923) 218 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

invented a bottle-making machine. A modern bottle-making machine can produce up to 20,000 bottles per day (33, 34). The use of glass as a beverage container advanced with the development of the crown cork seal—the first bottle cap—in 1892. The seal kept the contents under pressure and made carbonated beverages accessible to the average home. Even though other packaging products, such as metals and plastics, gained popularity in the 1970s, packaging in glass tended to be reserved for high-value products. Its weight, fragility, and cost have reduced glass usage in favor of lighter and less costly metal and plastic containers. However, for products that have a high-quality image and the consumer’s desire for flavor or aroma protection, glass remains an effective packaging material. Metal Containers Iron cans with tin coating were known in Europe as early as the 14th century, but the technology never took hold because of a belief that metals were poisonous. It was not until the 19th century that Nicolas Appert (1749–1841) showed that food sterilized in sealed containers could be preserved for extended periods. Subsequent approaches had the food placed in tin-lined cans (35, 36). Over time, steel became the basic metal of choice. Interestingly, tin foils were made before aluminum foils, and to this day people refer to aluminum foil as tin foil and steel cans as tin cans. It was not until the 1950s that aluminum made its appearance in food containers. Aluminum’s great advantages are that it is more malleable and less dense than steel, offering a lighter container that can be stamped from a sheet and easily formed into the container walls; a steel can involves welds along two seams. Improvements in quality control allowed for the use of thinner gauge aluminum sheets, further cutting weight and cost. Today, virtually all canned beverages are sold in aluminum cans (37). Modern metal cans are still tin-plated, but they are also manufactured with a polymeric lining to isolate the metal from the contents—to protect the food from metal migration, which can lead to taste issues, and to provide corrosion resistance from acidic contents. Plastic coatings must adhere to the base metal and be elastic, nontoxic, and chemically inert (38). The first plastic liners were based on oleoresins, and during the 1940s the first synthetic resins were introduced. In the 1960s, epoxy resins based on 4,4′-isopropylidenediphenol, or bisphenol A (BPA), came into widespread use. (BPA is a chemical that is a building block of both epoxy resins and polycarbonate polymers.) However, BPA metabolites were detected in urine, breast milk, and subsequently in infant urine, and any migration of a chemical into food proved to be unacceptable to the public (39). While BPA is nontoxic at the exposure levels derived from food cans, it is a potential endocrine disruptor. Even though BPA is not regulated by the FDA or the European Food Safety Authority, public opinion has led to its replacement in food containers and polycarbonate bottles. Acrylics and non-BPA epoxy resins have been developed for food service use instead. Polymer-Based Packaging The era of polymer-based packaging was ushered in with cellophane. Cellophane is a thin film of regenerated cellulose that emerged from efforts during the late 19th century to produce artificial materials from cellulose found in wood, cotton, hemp, and other sources. In 1892, English chemists Charles F. Cross (1855–1935) and Edward J. Bevan (1856–1921) patented viscose, a solution of cellulose treated with caustic soda and carbon disulfide that is better known as the basis for rayon. In 1898, Charles H. Stearn was granted a British patent for producing films from viscose. Later, 219 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

in 1908, the Swiss chemist Jacques E. Brandenberger (1872–1954) designed a machine for the continuous production of a strong, transparent film (40). He also coined the term cellophane by combining “cellulose” with “diaphane,” the French word for “translucent.” In 1923, E.I. du Pont de Nemours & Company acquired the U.S. rights to manufacture cellophane. Cellulose film has been manufactured continuously since the mid-1930s and is still used today in a variety of packaging applications. Cellophane is derived from natural sources such as wood, while plastic wrap is a petrochemical product. Unlike plastic, cellophane cannot be recycled, but it is biodegradable, so it can be composted or sent to a landfill in the regular garbage. Since the 1960s, however, cellophane films have steadily given ground to films made from synthetic polymers. Many other polymeric materials have also seen applications in food packaging or food containers. These polymers are not “natural” but synthesized from petroleum and natural gas. Common polymers and recognizable applications are: • High-density polyethylene (HDPE) is chemically inert and is used in milk cartons. • Low-density polyethylene (LDPE) is less rigid than HDPE and finds applications in films and squeeze bottles. LDPE has a greater amount of branching compared to HDPE, and these side chains interfere with packing and intermolecular bonding. As a result, HDPE packs better than LDPE and is denser. • Polyethylene terephthalate is used in carbonated beverage (soft drink) bottles. • Polyvinyl chloride is more commonly associated with plastic pipe, but it finds some use in film wraps. It is resistant to attack by oils, so it also finds some use in cooking oil containers. • Ethylene vinyl acetate is an additive in plastic wraps. • Polypropylene finds use in single-serving tubs, such as yogurt containers and microwavable packaging. • Polycarbonate is a polymer synthesized by reacting BPA with phosgene. It finds wide use in rigid plastics, such as water bottles, baby bottles, and hard-plastic food storage containers, plus many nonfood applications. • Polystyrene is the continuous medium in Styrofoam and still finds widespread use in food containers. • Polyvinylidene chloride was the original polymer in Saran wrap, seeing use from 1953 until around 2004. It was replaced by LDPE because of environmental concerns. It produces HCl upon decomposition. Food-grade plastics must follow strict formulation standards. In the United States, the FDA is responsible for regulating food-grade plastics. First, they must meet certain purity standards. The material cannot contain monomers, dyes, other additives, or recycled plastic products deemed harmful to humans. Use of recycled plastics must be closely monitored to ensure that contamination is not carried over to the new container. Food-grade plastic can contain some levels of recycled materials as long as those materials meet the FDA guidelines for plastics. Additionally, because some foods, particularly acidic foods, can leach contaminants from their containers, these foods must go into more product-specific containers that are free of potential contamination. Leaching can also be affected by temperature, and for this reason, labels warning against microwaving or direct heating are often placed on the container. Tamper-Resistant and Tamper-Evident Packaging Product tampering involves the deliberate altering or adulteration of a product or its package. Tampering was not a concern until 1982 when the Chicago Tylenol murders occurred (41). A 220 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

number of poisoning deaths resulted from drug tampering when the acetaminophen in Tylenol capsules was replaced with potassium cyanide. A total of seven people died in a first group of poisonings, and subsequent copycat crimes resulted in several more deaths. The perpetrators were never found. Later tampering crimes followed. In 1989, baby food manufacturers were victimized by a tampering scare. Other major producers were extortion victims by persons claiming to have spiked jars of baby food with poisons and replaced them on supermarket shelves. These incidents led to reforms in the packaging of over-the-counter foods and drugs and to legislation making product tampering a federal crime. Tamper-resistant and tamper-evident packaging was developed to counter these threats. Plastic wraps, mechanical seals that join the cap to the container (such as the ones on 2 L soda bottles), and foil or paper inner seals all resulted from these threats to food security. The Tylenol incident also prompted the pharmaceutical industry to phase out capsules, which were easy to open, replace contents, and then reseal without obvious signs of tampering. The capsule was replaced with the solid “caplet,” a tablet made in the shape of a capsule for drug delivery. Active Packaging The purpose of food packaging is to preserve the quality and safety of the food contents from manufacture until the time it is used by the consumer. A new approach to preservation is active food packaging technology. Active packaging refers to packaging systems that function beyond simply being an inert material that provides passive containment of the product. Active packages have some ability to regulate the package’s internal environment or be formulated to provide enhanced protection against spoilage (42). Additives embedded into the packaging material range from antimicrobial particles to antioxidants. The next step in active packaging is intelligent packaging. These next-generation systems are engineered with the ability to monitor the conditions (that are important to quality) inside the package and give this information to the merchant and customer. Active packaging does not have to be complex. A 2016 patent from the People’s Republic of China discloses a three-layer antimicrobial food packaging paper (43). The packaging is a laminate consisting of a surface layer, an intermediate layer, and an inner layer. The surface and inner layers are manufactured from paperboard material. The intermediate layer is a multifunctional layer that confers both antimicrobial properties and bonds the surface and inner layers into a laminate. The package retards microbial invasion and aids in quality retention during transport and storage. Other approaches to active packaging are modified atmosphere packaging (MAP) and engineered materials. Manufacturers of some consumer products are using active packaging to convey warnings. Blue Lizard sunscreen has blended dyes into their plastic sunscreen lotion bottles that turn blue when exposed to UV-A and UV-B. If the bottle is blue, it is time to put on sunscreen. Modified Atmosphere Packaging (MAP) MAP is the practice of modifying the composition of the atmosphere in a package (commonly food or drugs) in order to retard spoilage and improve the shelf life (44, 45). The need for this technology for food arises from the short shelf life of meats or certain fruits and vegetables in the presence of oxygen or ethylene. Many meats, seafood, poultry, and baked goods are packaged under modified atmospheres. When exposed to air, oxygen is readily available for the oxidation of lipids and iron in meat myoglobin. Oxygen also helps maintain high respiration rates of fresh produce and promotes the growth of aerobic microorganisms. In a MAP application, the air in the head space of the package is removed; the package is then refilled with an inert gas. Most commonly, air is replaced with mixtures of nitrogen and carbon dioxide. Different gas mixtures are used with different products. The 221 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

removal of oxygen or ethylene and the replacement with selected gas mixtures can delay oxidation and microbiological spoilage. Oxygen scavengers may also be incorporated into the package to reduce enzymatic oxidation (browning). Vacuum packaging systems were introduced in the 1940s and are essentially the first MAP application. The food item was placed in a plastic sleeve, the air was pumped away from the food item, and the sleeve was sealed. Removing oxygen extends the product’s shelf life by delaying the growth of aerobic bacteria and retarding respiration. Since the package is sealed, water cannot escape. The shelf life is extended and freezer burn (a condition caused by dehydration and oxidation that affects the quality of a product) is eliminated. The selection of polymers for MAP materials is also important. When fruits and vegetables are packaged in MAP films, the gas permeability and water vapor transmission rates become as important as its mechanical properties, transparency, and sealing reliability. Gas retention and exchange are critical properties of these films. The basic physical chemistry is shown in Figure 2. Gas exchange is modeled using Henry’s Law to calculate the solubility of the gaseous species going into or out of the film, and Fick’s Law of Diffusion provides gas mobility through the film.

Figure 2. Physical model of a film barrier. Image courtesy of Mary Virginia Orna. The true utility of MAP is that the atmosphere inside the package can be tailored to meet the needs of the product. For example, once coffee beans are roasted, they emit carbon dioxide and are particularly susceptible to oxygen. Oxygen reacts with oils native to the bean, leading to flavor loss. To address this issue, certain coffee brands have adopted MAP. Figure 3 shows a one-way valve on a whole-bean package. After roasting, the bag is evacuated and refilled with nitrogen and sealed. As carbon dioxide is emitted, the pressure is relieved through the valve without reintroduction of air (and oxygen). Fruits and vegetables are respiring products, so there is a need to transmit oxygen through the film and away from the product but retain moisture in the package. Films are classified as either permeable, if they allow gas transfer, or barrier films, if they prevent the exchange of gases. Fresh fruits and vegetables are often packed in permeable packaging, and barrier films are mainly used with nonrespiring products like meat and fish. Traditional packaging films such as LDPE, polyvinyl


chloride, ethylene vinyl acetate, and polypropylene may not be sufficiently permeable for highly respiring products. Filling a MAP with product and modifying its atmosphere is typically carried out in a batch process. The product is introduced into a preformed wrap and placed in a chamber, which is then evacuated. The modified atmosphere is introduced and the package is heat-sealed in the chamber. This process is labor- and time-intensive; however, it is less expensive than automated packaging machines. An alternative method uses a snorkel to replace the atmosphere in a package before final sealing. The food product is placed in the packaging material and positioned into the packaging machine. A nozzle, or snorkel, is inserted into the package that vacuums out the air and then flushes the modified atmosphere into the package. The nozzle is removed and the package is heat-sealed. This method is suitable for bulk and large operations.

Figure 3. One-way valve of a whole-bean coffee package (left, outer; right, inner). Photographs courtesy of the author. Nanoparticles Nanotechnology has found many applications in science and technology, now including food packaging. Nanoparticles of molecules with antimicrobial properties have been used in food-contact materials to preserve food products for longer periods. Inorganic and metal nanoparticles serve as antimicrobial agents (in particular, Ag, Cu, or ZnO nanoparticles) or oxygen scavengers, such as Fe(II) oxides, have been incorporated into films to produce active food packaging materials that extend the shelf life (46). Packaging containing these nanoparticles offers numerous advantages, such as reduction of chemical preservatives and enhanced inhibition of microbial growth. Nevertheless, the safety issues stemming from particle migration from the film to the food and subsequent chronic exposure to nanoparticles remain (47). Radio Frequency Identification Tags Radio frequency identification tags are being incorporated into packages to create a “smart” packaging system. Basically, the technology enables the tracking of objects with attached tags, and because the communication occurs through radio waves, the tag does not have to be in the line of sight of the receiver, like a bar code reader. Many diverse applications use this technology, ranging from tracking livestock to automobiles on an assembly line, and, not surprisingly, numerous patents have been awarded. When coupled with an appropriate sensor, a radio frequency identification tag can transmit data on the atmosphere or conditions inside a vegetable package or a temperature–time profile as indicators of freshness. Together, the sensor and transmitter make up an intelligent package. 223 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

There are varied applications of sensor–transmitter combinations for product real-time testing ranging from food quality to the detection of urine in incontinence products. Nutritional Information Ever since packaging began, the package has been used to both identify and promote the product. At a minimum, information on the product and the quantity contained therein has been prominently displayed. Now, food packages present attractive product displays, showing images of the product, ingredient information, and a Nutrition Facts label. Many countries now require nutritional information on most forms of packaged food. In the United States, the Nutritional Facts label not only defines what constitutes a portion but also lists the Recommended Dietary Allowance and how much of these constituents a portion provides. In the United States, there is also a requirement for ingredients to be listed in order from highest to lowest quantity by weight. Casings and Other Edible Coatings Edible coatings are not a new concept but have been in use for centuries. In antiquity, wax films were placed on fruits to reduce water loss, but they also impeded the exchange of other gases and negatively impacted quality. Sausage stuffings have been placed inside edible casings for millennia. Originally animal intestines were used to make sausage casings. By the 20th century, collagen and cellulose were introduced, and today some plastics even find use in casings. Edible films can also be formed directly on the surfaces of the food and become part of the food item, remaining on the food through consumption. Edible coatings are sometimes used on breakfast cereals to improve taste and provide some resistance to liquid (milk) intrusion. Likewise, apples are not naturally shiny. Processors put a thin layer of shellac on the fruit to replace natural waxes lost in washing. The coating improves appearance and also prolongs shelf life. They also have the potential to carry and hold antioxidants, antimicrobials, and vitamins at the food surfaces, thus further improving nutritional value and product safety. Further, when edible films and coatings act as the primary layer, edible packaging can reduce the cost and complexity of the packaging process and minimize package waste (48). Unintended Consequences In modern society, food packaging has become a single-use item that controls contamination of the food item because it isolates the food item from the environment and is the only material that comes into contact with the item. This belief underlies the whole approach to packaging. But for all the benefits previously discussed, food packaging can be a source of chemical contaminants, and this issue is largely left out of any discourse on packaging. For example, printing inks have been shown to migrate through paperboard into dry foods. Monomers and any additives formulated into the packaging film (antimicrobials, antioxidants, plasticizers, etc.) can migrate into the food item. Chemicals that can act as endocrine disruptors or mutagens are of particular concern. Even at low concentrations, they can have effects that clinically present in a variety of ways. Engineered nanomaterials show some promise for packaging foods, but only limited studies on their possible toxicity and risk of exposure have been done so far. The overall concern does not seem great because the benefits of modern packaging are many and the quantities of the migrating chemicals are low. Another major concern with plastic packaging, in particular, is the “second life” these materials have after they have been discarded. In just over a century, plastics have become ubiquitous and have found use in almost every facet of modern life from packaging to building and construction materials. 224 Orna et al.; Chemistry’s Role in Food Production and Sustainability: Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 2019.

Many of these materials are designed to be used just once, do not burn readily, and are resistant to biodegradation. Uncontained, they break down into smaller pieces—some into the nanoparticle size range. It is estimated that over 160 million tons of plastics are used as packaging, much of it single-use, of which 9 million tons end up in the ocean every year (49). While obviously harmful to wildlife, its second life in the food chain, which in many cases ends with human consumption, is largely unaddressed. Recent attention to the overall problem is leading to action. The European Parliament and the State of California are moving to limit single-use plastics (50), and the Our Ocean Conference brought together governmental and corporate interests to pledge to recycle more and eliminate problematic packaging. Major corporations that use significant quantities of plastics, such as Coca-Cola and PepsiCo, are signatories (51). What Is the Future of Packaging? The future of food packaging will no doubt be interesting with the greater use of active packaging. Polymeric films formulated to ensure the desired atmosphere remains in contact with the product will be developed and manufactured as will biodegradable plastics. Embedded in these engineered films will be sensors that measure atmospheric composition and temperature–time profiles as an indication of product freshness. Some of these materials are already in use commercially. The need for such materials to be developed for the detection of microbiological activity will take on greater importance as the Earth’s temperature rises and with it the occurrence of increases in biological activity. It will be an interesting era.

Acknowledgments The author would like to acknowledge and thank Mary Virginia Orna for her invaluable assistance with graphics, comments, and suggestions, and encouragement and energy throughout this adventure.

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The Evolution of Food Preservation and Packaging

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