Pundit Special Science Report:
Part 2 — The Science Of Waterborne Bacteria
Jim Prevor’s Perishable Pundit, December 15, 2006
In addition to exploring the differences in environmental conditions and growing practices between Salinas and Yuma, we still wanted to know more about Yuma and particularly water issues there, so Mira Slott, Pundit Investigator and Special Projects Editor, spoke with Charles Sanchez of the Yuma Ag Extension Service:
Charles Sanchez, University of Arizona, Yuma Ag Extension Service, Director and Professor of Soil, Water and Environmental Sciences
Q: Wasn’t there an issue several years ago in Yuma regarding potentially dangerous perchlorate levels in water used for production of leafy greens?
A: There had been a lot of speculation about toxic levels of perchlorate in the leafy vegetables, but research has shown they are perfectly safe to eat. Essentially, perchlorate isn’t a viable food safety issue.
Q: Could you explain the science behind that?
A: There are trace levels of perchlorate in the Colorado River, but the levels in lettuce are a small fraction of the reference dose considered safe as recommended by the National Academy of Science. I’ve measured spinach as well. The levels are a little higher, but certainly less than 10 percent of the reference dose.
Q: How did perchlorate become an issue in the first place?
A: There had been plants at a site near Las Vegas in Henderson, where perchlorate had leaked out through Lake Mead to the Colorado River. The contamination could have been there a long time, but we only had the technology a few years ago to detect it at the parts per billion level. In fact, due to bio remediation near the site of the contamination, the levels of perchlorate in the river have dropped substantially over the last few years.
Q: Has the water used for production been chemically treated?
A: Growers are rigorously testing water for pathogens, but to my knowledge, water is not being chemically treated in Yuma. In any case, it would be challenging to do so based on the production system here. Growers do spray with hyperchorloide in the field, but that’s not related to chemically treating water.
Q: Why would it be difficult?
A: The water is diverted from the river to canals on to the fields. We do furrow irrigation here. Logistically, it would be very difficult to chemically treat the volumes of water we use. We’re talking about approximately 5 billion cubic meters of water in the Yuma and Imperial Valley area that are used. Growers will use 40 inches of water for a lettuce crop. The water is let out of canals on to the field by gravity. You can’t treat those volumes of water. Well, it would be extremely challenging. Obviously if there was a need, we’d figure out a solution, but beyond that there hasn’t been a need to chemically treat it.
Q: Can you elaborate on that?
A: The pathogen testing hasn’t shown a need to chemically treat. During the production times starting in November through late March, the counts and surface water are extremely low, and there’s no evidence of harmful pathogens in leafy greens. Jorge Fonseca can attest to that.
So perchlorate is an old issue we don’t have to worry about. But what should we worry about? We went back to Salinas and Pundit Investigator and Special Projects Editor Mira Slott talked to Trevor Suslow of UC Davis:
Trevor Suslow: Extension Research Specialist at University of California, Davis. Learn more here.
Q: What was the impetus behind your in-depth studies of the water used for vegetable production in the Salinas, California area?
A: The California Lettuce Research Board in Salinas commissioned us about 4½ years ago to study water used for production, particularly looking to assess the safety of the reservoirs and identification of risks, analyze what kind of testing is appropriate, what the results mean, and what needs to be done to improve food safety. We had started doing work funded by the Board before then, examining livestock, manure issues, and moving to other areas including irrigation concerns predominantly around Romaine.
Q: Why the emphasis on water?
A: Nothing is black and white. In essence, obviously the water and water quality has always been a primary concern and point of control as it relates to fruit and vegetable production, disease control and food safety. Our primary focus was E. coli. In other areas of study, our primary concern was salmonella.
Most of the water testing has been focused on the Salinas Valley and the Central Coast, but we’ve extended it to other areas for comparative reasons.
Q: Could you bullet point the key findings for us?
A: In the reservoir study, we did comprehensive testing of 19 different reservoirs and found the water to be extremely safe. The majority of water at the times we were taking samples throughout the seasons contained very, very low populations of non-pathogenic E. coli. The amounts tend to fluctuate from sample date to sample date, and there is some variability based on a number of issues such as location and time, but by sampling over a couple of years, we established a base line for comparison.
We extended our research to capture water coming out of sprinklers at the farm level when irrigating. Typically, we found the background non-pathogenic bacteria lower in that water than from grab samples at the same reservoir from the shore line area. We simulated the way a grower would be testing the water from the reservoir for more meaningful numbers.
The final step was to look at populations of the same non-pathogenic bacteria on the plants themselves, a series of snapshots from the irrigated field, and the majority of plants showed very low or non-detectable E. coli.
We never found E. coli 0157:H7 in the samples taken from an irrigation reservoir. We have been taking samples over the last three and a half years, still continuing to do studies, but more narrowly focused based on our findings.
Q: Are you conducting studies to help in discovering answers to the spinach/E. coli outbreak?
A: That’s a tough question. Obviously, we at UC Davis and within our lab are more than willing to provide diagnostic testing and analysis if asked by the industry, the public, or the government. At this point, I don’t believe anyone here has been involved in the trace-back investigation. But we have been involved in the follow-up understanding of what led to the outbreak. Obviously we are paying very much attention to what is going on and want to use our experience and expertise to provide options for better implementation of food safety.
Q: What are some of the areas that need more research?
A: There are similarities and differences in production regions. None of these environmental connections are as straight forward as one tends to think. We need to go into more depth to understand what leads to pathogens’ survival and growth.
Local climates and micro climates in the Salinas region could play an important role. Temperature is a cause of bacteria multiplying on lettuce. Temperature, moisture and the method of delivery of irrigation are important factors.
We have studied differences between furrow and overhead irrigation as it relates to the presence of bacteria on the leaf. Expect slightly higher populations with overhead irrigation. If contamination does occur, we need to understand the likely outcomes of survival and growth, and the potential of plant internalization. We need to do these studies to protect the public. What I don’t like is speculation or increased testing for the sake of testing that gives everyone a false sense of security.
Q: Has this outbreak created hyper sensitivity to the dangers of eating produce?
A: An outcome of this very tragic situation is that consumers who had a lack of awareness are looking at food safety in a way they haven’t done before. The general public is confronting the world around them with the realities of the microbial world, questioning what they’re exposed to. That’s what we’ve been working on for 11 years at UC Davis, food safety concerns and challenges. Unfortunately, there’s only so much people will do until a catastrophe.
Q: But hasn’t the produce industry put food safety as a top priority long before this outbreak?
A: The thing that needs to have a bright spot put on it is that people and sectors in the produce industry have been working hard to develop the highest food safety standards. But there are still too many non-compliances and the industry needs to ferret out some of those lapses. With minimal participation, everyone falls down together.
I worked within the fresh produce industry before I came back to the University. Even with the best intentions, mistakes happen. You can have the best plan, but if you don’t have 100 percent buy-in from the top down throughout the organization, you can have very serious consequences.
Q: Will this involve extensive testing from the field to the processing operation to the finished product before it leaves the plant?
A: Testing and monitoring has a role in the right place and with the right frequency. It is not enough to just extend a lot of resources. In fact excessive testing can give a false sense of safety. First, there are not enough resources to go around, even for all the research that’s needed. You have to pick priorities and target areas where the information will be effective. Testing has a role and can be a great guide. But the best use of testing is where control points are used with strategies to minimize change, using the dollars to monitor those areas and make necessary adjustments.
Q: But wouldn’t more testing throughout the supply chain reduce the chance of food outbreaks?
A: The problem with product testing is that you can never do enough sampling. It’s just like me saying I never found a pathogen in a reservoir. The testing is only as good as the number of samples at that time on that date. You can identify some changes in what’s normal, establish a two-year baseline on 19 reservoirs and know what is in and out of range.
So we need to guard against false security from product testing and we need 100% buy-in from top down as to the priority of food safety.
Additionally, we sent Pundit Investigator and Special Projects Editor, Mira Slott, to talk to Richard Smith of the Cooperative Extension in Monterey County, California:
Richard Smith, Vegetable Crop and Weed Science Advisor, University of California Coop Extension in Monterey County. Learn more here.
Q: What can Salinas County do at this point to alleviate concerns about spinach production?
A: We are left looking at a plethora of possibilities that exist for E. coli contamination from the field to the harvest operation to the processing and packing plants. At any of those places, there is the potential for something to happen.
The reality is that growers take extensive measures. There is example after example of quite extraordinary efforts in the Salinas Valley and in general with the vegetable industry to make food safe. Part of the problem is that everybody has a theory. We’ve dealt with so many calls. Without all the data, it’s anyone’s guess, and there may be factors not even being considered.
Q: There have been concerns that if water were to be contaminated, Salinas irrigation systems could exacerbate the problem. Could you comment on this?
A: My colleague Steve Koike has worked with Trevor Suslow [see above] at U.C. Davis conducting an extensive 2 1/2-year evaluation of the water in Salinas County, looking at the most likely source of contamination; the piece of the puzzle where water is pumped out of the ground and goes into reservoirs. That’s a pretty solid piece of research that I don’t think people are giving enough credence to. The reality is you can find generic E. coli, but not necessarily the strain that’s the problem.
Q: Could you discuss how irrigation systems work in Salinas?
A: We do use overhead irrigation sprinklers here, and that is something we have to deal with if water turns out to be the issue. If the grower was using contaminated water, overhead irrigation could create a greater possibility of it getting on the plant. There is potential for that to spread the problem. With furrow irrigation, the water is at the base of the plant, so you wouldn’t be splashing water on the foliage.
Q: Would it make sense for growers to change irrigation strategies?
A: It would be very huge to make a change in our irrigation methods. First of all, the way spinach is produced in Salinas, it grows on 80-inch wide beds, so they don’t lend themselves to furrow irrigation. Furrow is not an option for that wide of a bed, so the other option would be drip irrigation. That would be a dramatic and costly change, because currently I don’t think there is even one acre of that.
Growers use a lot of drip irrigation on 40 inch beds. But high density spinach plantings would be a radical modification of our agricultural practices. Not that we can’t or wouldn’t make the change. It could be done, but we’re at the very bottom of the learning curve on that.
Q: Another issue being examined is the proximity of vegetable fields to livestock and other wild animals.
A: Growers started last year putting up plastic fencing in spinach fields within close proximity of ranches and to keep out rodents, coyotes and other wildlife. If growers don’t take heightened precautions to minimize food safety risks, they’ll lose their market.
Q: When an outbreak occurs, everyone loses, even growers with the most stringent standards…
A: We all feel terrible about the outbreak and the very serious illnesses and deaths. The problem is that all through this thing, I’ve been hearing off the cuff conjecture and assumptions from so many people. They may mean well, but when we have something this serious with illness and death, and millions and millions of dollars lost, we need to be prudent in researching and evaluating theories before making determinations.
The problem is the world is not going to wait for the research to fully understand the problem. We have to act on the best available information, even while redoubling our efforts to learn more.