You might not see bugs and bacteria on fresh fruit and vegetables, but they’re there. Texas A&M food engineers are experimenting with new technologies to eliminate these threats to keep our produce safe and healthy.
Many Americans assume fruits and vegetables sold in supermarkets are safe, wholesome foods that are good for us and won’t make us sick. This can be a deadly assumption. Despite rigorous national standards, spinach infected with Escherichia coli and salmonella-tainted tomatoes have hit the market, in recent years, sickening more than 1,000 people in the United States.
But food engineering researchers at Texas A&M are perfecting several methods to ensure the safety of fresh produce: electron beam, or e-beam, irradiation, which kills disease-causing organisms that survive conventional decontamination methods, as well as several advanced packaging techniques.
“Irradiating produce reaches bacteria inside the vegetables, not only the organisms that are on the surface,” says food engineer Rosana Moreira, professor in the Department of Biological and Agricultural Engineering. “Irradiation kills bacteria without damaging produce or making the product unsafe to eat.”
The Centers for Disease Control says that food irradiation holds great potential for preventing many foodborne diseases in meat, poultry, fresh produce and other foods without harming the nutritional value of food or making it hazardous to human health.
Moreira, Elena Castell-Perez and Carmen Gomes are working to calculate the best methods of using electron beam irradiation to eliminate dangerous bacteria and maintain the nutritional content of fresh produce.
Electron beams are streams of high-energy electrons. The beams are not radioactive, and they can be turned on and off like your TV or a flashlight. Applying ionizing radiation to food was introduced more than 100 years ago. Food processors in 50 countries rely upon irradiation to make their food safer, but it’s fallen out of favor in the United States — believed largely the result of consumer fear and lack of understanding of “radiation” and its diverse applications.
“The idea of eating food that has been irradiated concerns some consumers,” Moreira says. “But irradiated food is completely safe, and in some ways may be better than food that has not been irradiated.”
Almost all fresh fruits and vegetables sold commercially in the U.S. are treated with chemicals before reaching grocery stores. Although beneficial for eliminating many contaminants, some of the chemicals used have been found to leave residues that can become harmful once in the consumer’s hands — for instance, when cooking fruits and vegetables at high temperatures, common in the home canning process. And chemical cleaning reaches only bacteria on the surface of the produce, and it may not even eliminate all of that.
But Gomes says food irradiation has several advantages over chemical decontamination methods.
“When we treat with chemicals, we just treat the surface of the produce,” Gomes says. “Irradiation penetrates the product and helps decontaminate it from harmful bacteria that may have found its way inside lettuce, for instance.”
Irradiation also eliminates problems associated with other food safety treatments such as nutrient degradation or changing the produce’s color, texture and flavor. The researchers are now working to determine precisely how much irradiation is enough.
“Quality is very important,” Castell-Perez says. “We want to maintain everything there — vitamins, color, shelf life — but get rid of things like Salmonella.”
Computed tomography, or CT, scans can help to map produce to calculate the dosimetry, the absorbed dose of ionizing radiation, for certain fruits and vegetables. Using computer simulation, they can cut a cantaloupe, for example, into thousands of layers to create a model that enables the researchers to calculate the smallest dose of radiation needed to reach every part of the product.
But because no fruit or vegetable is exactly alike, mapping out a standardized dose is a sizable challenge.
“First, as engineers, we need to understand how to make the energy distribution uniform,” Moreira says. “Once you understand the uniformity, which is a big issue, we need to know how much energy to put in the fruit or vegetable to make sure there is no degradation of quality.”
The researchers’ work has shown that irradiation can slow down ripening and spoilage to extend shelf life. Recent studies on mushrooms have shown that when mushrooms are impregnated with calcium lactate or ascorbic acid, the fungi’s shelf life increases. The team is also exploring how irradiation can actually increase nutritional value of fruits that are high in antioxidants, such as blueberries.
And yet another challenge is reducing pathogen contamination in raw nuts. The high oil content in nuts makes irradiation treatment challenging because the energy accelerates fat degradation, which affects the taste and quality of the nuts. Irradiation combined with advanced packaging methods reduces this radiation sensitivity to oxidation while assuring decontamination.
In fact, developing new and improved packaging for produce is another major area of focus for Castell-Perez, Moreira and Gomes.
The team is working to improve the sheet plastic that wraps much of the fresh produce found in supermarkets by adding a combination of techniques — radiation physics and biology, food science, packaging materials, and computer methods — to enable the plastic wrap to fight off unwanted germs.
Such a comprehensive approach to enhancing plastic packaging for food safety has not been done in quite this way before, where an emphasis on irradiation combined with several other technologies is producing a new generation of protective packaging.
“The idea to improve effectiveness of packaging came to us when we first irradiated a bag of spinach,” Castell-Perez says. “The applied dose was too much for a food sensitive to radiation and though the process eliminated the pathogens, it also destroyed the food. So we thought, can the package be ‘active’ and help maintain the quality as well as the safety of the spinach?”
To do this, researchers apply natural enzymes and natural extracts, such as cinnamon, garlic, clove, thyme and rosemary, into plastic films used for food packaging. These spices have shown to be powerful antimicrobial substances, Gomes says.
A major challenge is making sure the spice used for protection does just that, without leaving anything behind — like its flavor. While garlic may enhance the bacteria-fighting ability of the plastic packaging, garlic’s distinctive flavor would not be a good addition to berries, for instance.
These extracts are embedded into a basic FDA-approved plastic film coating and then the extracts are mixed with a natural polymer.
“We use microencapsulation,” Gomes says. “We coat our compound with another substance, making a capsule, and when it’s in contact with the food, the compound migrates to the food.”
This effect must be carefully designed so that the exact amount of the extract at the desired rate is released into the food.
In addition to experiments with natural extracts, the researchers are evaluating the feasibility of using the plastic film in combination with certain gases such as air, 100 percent oxygen, combinations of nitrogen and oxygen, and ozone.
When a bag of spinach is irradiated, the air inside the bag is also exposed to ionizing radiation. This creates active radicals, meaning they are ready to react with another compound, such as ozone, hydroxide ions and even carbon dioxide. Reactions between these compounds are harmless to the consumer, but they destroy unwanted bacteria.
Moreira and Castell-Perez are taking a new approach in the field of packaging. Combining the antimicrobial packaging with atmospheres, an application of modified atmosphere packaging, could increase the radiation sensitivity of the pathogen in question, thus requiring a smaller dose while ensuring wholesome, safe and long-lasting spinach.
“When you talk about using those gases in combination with irradiation, then there is a synergistic effect so that the irradiation converts that gas into some compounds that will be antimicrobial,” Moreira says.
The Texas A&M researchers are working toward a common goal: improving packaging safety. As they work through these challenges, they’re also building synergy that has fueled innovation for attacking the challenge from all directions. Among them: applications including extracts, different types of atmospheres, and irradiation treatments that are leading to new discoveries in the packaging world.
“Irradiation and better packaging do not excuse dirty or mishandled produce,” Castell-Perez says. “But these are preventive steps and we are collecting scientific data that proves this point.”