Trevor Suslow, of the Department of Vegetable Crops at UC, Davis, says practices in agricultural production for more wholesome foods have triggered tougher strains of microorganisms.
Even these, however, are prey for a fungus known as Muscodor albus that lives within plants. It destroys the tiny pathogens, which become resistant to both physical removal and disinfectants such as chlorine and ozone, lodged between cell layers of produce.
"We are coming to understand that different strains of pathogens have very different resistance to environmental stress. We believe we may be selecting for different strains that weren’t there before changes in agricultural production," he said.
Speaking at the 22nd Annual Agribusiness Management Conference in Fresno recently, Suslow said the new strains form a protective biofilm of dead cells on the surface of produce, allowing for explosive growth of a few remaining beneath.
Like methyl bromide
Muscodor penetrates that growth. Recovered from Central American rainforests and being developed under exclusive license by AgraQuest, Inc. of Davis, Calif., it produces a natural fumigant that acts like the chemical fumigant methyl bromide.
AgraQuest seeks to market Muscodor as a replacement for the once widely used chemical, which is being phased out of use. The company also envisions applications of Muscodor for control of mold and bacteria that cause "sick building syndrome."
The fungus, discovered during an expedition by Gary Strobel, a professor at Montana State University, produces volatiles that prevent growth of both plant and human pathogens on tomatoes, bell peppers, and cantaloupes.
Suslow said harmful, superficial fungi are important to food safety because they can breach natural barriers, such as the rind of melons, and move inside.
Turning to general comments on food safety, Suslow, who has probed risk assessment of pathogens such as Salmonella, E. coli, and Shigella, said the likelihood is very low that a person in the U.S. will become ill from a definitely identified contamination of fresh foods prior to preparation.
However, he added, "It is equally clear that disease outbreaks linked to fresh produce from various production areas have occurred and that those outbreaks have affected large numbers of individuals across many states and into Canada." At particular risk are the young, the very old and those having weak immune systems.
Global sourcing of foods on a year-round basis and discoveries that certain produce has cancer-fighting and other benefits have intensified the importation of foods to the U.S. Precise tracking of the origin of produce and centralized distribution help officials locate the source if contamination occurs.
But at the same time, Suslow said, with consolidation of produce distribution, disease outbreaks may be widespread and therefore require amplification of food safety standards.
In addition to disease outbreaks, other events have brought focus and concern for microbial food safety of produce. Those include positive detection of human pathogens on random samples of domestic and imported produce, research reports of difficulty in cleaning and disinfecting produce surfaces, and reports documenting the potential for pathogens to move inside produce during postharvest handling.
Focusing on prevention of contamination, the produce industry has adopted Good Agricultural Practices as guidelines and criteria for handling. These range from use of clean, disinfected water and ice in packing to pest control and sanitation for workers and equipment.
Nevertheless, Suslow added, "Implementation of sensible and practical GAP programs has been hampered by the pervasive misconception that most microbial risk reduction is just common sense hygiene."
The "common sense" approach may be adequate in many instances, but, he said, recent data from around the world reveal persistence of pathogenic microbes in places previously thought to be safe from them.
To expand the understanding of how pathogens move and how they persist, several scientific disciplines are being assembled to learn how the pathogens move in soil and in water.
Meanwhile, he added, crop management and post harvest technologies are being marshaled and evaluated for their effect on risk and risk reduction.
Packers, he said, "have to make sure that produce that goes into fresh-cut processing is not already contaminated before they get it, because there’s almost nothing that can be done to guarantee there are no pathogens on fresh produce that is not cooked."
Since there is no "one size to fit all" when it comes to microbial food safety, Suslow said efforts have to be at least regionalized and may have to be tailored to the ranch level. "You have to know how your operation influences food safety, all the way from seeding to the shelf and beyond," he said.
Use of portable field toilets in the field is an important sanitary practice, but so is proper disposal of wastewater from hand washing and even attention to footwear that can track contaminants into a field or processing plant.
"More and more, people in the fresh produce industry are realizing that rather than dealing in raw agricultural commodities, they are food handlers," he said.
While the movement toward improved measures for food safety advances, researchers have become aware of the need for "better and more discriminating tools for segregating, for identifying, and for rapidly typing" harmful microorganisms."
Suslow warned that there is no silver bullet to eliminate contamination once it is in produce. "The more we study it, the more we learn how difficult it is to remove microorganisms without cooking."