Cheeseboards on the chopping block : Survival on wood and plastic surfaces

A few days ago, the FDA began issuing citations to cheese makers in New York State for the use of wooden shelves and boards for aging cheese, a practice that is permissible by state law and has been used for hundreds of years. Not surprisingly, this made a lot of people pretty angry. A petition to reverse the policy has accumulated nearly 100,000 signatures in only three days.

Today, it seems that the FDA is backing down. Maybe. Kinda. It’s hard to tell.

The basis of the citations is that under the Food Safety Modernization Act, signed into law by President Obama in 2011, directs the FDA to endeavor to prevent, rather than only respond to food borne illness. According to the CDC, one in six Americans (48 million people) are infected by food borne pathogens every year, 128,000 of whom are hospitalized, and 3,000 ultimately die.* Getting ahead of these outbreaks and stopping them before they start seems like a very good thing.

The problem is that if we actually knew how to do this, we would probably already be doing it. The actual movement ecology of microorganisms is very poorly understood. This appears to be a regulatory mandate the agency does not have the science to implement. As a result, many of the preemptive actions taken by the FDA under these new rules have been controversial.

What do we know about wood surfaces as a vector for transmission of food borne pathogens? It turns out that the question is a bit complicated. If wood were not a relatively inhospitable material for bacteria, there wouldn’t be very many trees around. On the other hand, the things we build out of wood are not the same thing as trees. What starts as a tree is sawed into pieces, baked, compressed and saturated with a variety of other compounds, and then stored for a long time (sometime for centuries). The tree typically does not survive this process, and so its immune system isn’t alive to help kill bacteria.

The property of wood that the FDA seems to be concerned about is its porosity. Wood indeed is porous, and microbes do indeed get into all those little nooks and crannies, and under certain circumstances, are recovered large numbers using contact plates. However, it turns out that plastic and metal surfaces, especially ones that get scrubbed and cleaned often, also develop a rough and porous surface.

As anyone whose ever tried to build a cabinet knows, wood changes shape when it dries. A drawer that runs straight in dry weather will jam in humid weather. At the microscopic level, the fibers in the wood move dramatically. It seems that the configuration of cavities that might harbor microbes change as the wood fiber move, exposing them to the air. Tests done on dry wood surfaces seem to indicate an that it is an extremely hostile environment for bacteria, although how hostile depends on the species of tree, the humidity and the temperature.

A brief review of the literature on the topic seems to indicate that most of the researchers working on the hygienic properties of wood are in Europe, particularly Germany and France. As with work on the eradication of Salmonella in the poultry industry, there seems to be a disconnect between American regulators and the scientific community, particularly when the center of gravity of the relevant part of the scientific community is in Europe.

An excellent brief summary of the state of research on the hygiene of wood products in food handling can be found in the introduction of Survival of bacteria on wood and plastic particles: Dependence on wood species and environmental conditions, by Annett Milling, Rolf Kehr, Alfred Wulf and Kornelia Smalla.

Numerous scientific studies have evaluated the hygienic potential of wood compared to plastics and stainless steel and resulted in completely different observations. On the one hand, contamination experiments showed that plate counts from wood were greater than those from all the boards made of plastics or metal tested (Kelch and Palm 1958; Rodel et al. 1994). Furthermore, also after different cleaning procedures, high rates of bacteria were recovered from the wooden surfaces, indicating that those surfaces could not be decontaminated efficiently (Gilbert and Watson 1971; Kampelmacher et al. 1971; Borneff et al. 1988a,b; Abrishami et al. 1994; Rodel et al. 1994). On the other hand, the results published by Ak et al. (1994a,b) indicated that wood is safer in contact with foodstuffs than plastics. After contamination of cutting boards made of nine different hardwoods and plastics with several hygienically relevant gram-negative bacteria, significantly fewer viable bacteria were detected on the wooden boards than on the plastic boards, regardless of new or used status of the boards. In the frame of the Nordic Wood Project ‘‘Wood in Food’’, the hygienic properties of wood, plastic, and steel were compared (Koch et al. 2002). The survival of Bacillus subtilis and Pseudomonas fluorescens, found in the meat industry, was followed in two sets of experiments: (I) on boards made of beech, oak, and ash, representing commonly used materials for tabletops or (II) on spruce and pine, simulating the usage of pallets. A remarkably great difference in the survival of the bacteria on the surface of the samples was observed between wooden samples and plastic and steel. Oak showed the highest decrease rate in bacterial titre, followed by beech and ash. Bacteria survived longest on plastic followed by stainless steel. [emphasis added] In the experiments with pine and spruce, pine performed better than spruce both at low and at high moisture content. A recent study performed by our group (Schonwalder et al. 2000, 2002) showed that the survival of the test bacteria Escherichia coli pIE639 and Enterococcus faecium depended on different factors such as tree species, the initial inoculum size, and the characteristics of the inoculated bacterium. Pine-wood boards exhibited better hygienic performance than other wooden boards made of spruce, beech, and poplar or plastic boards. The study indicated an antibacterial effect of wood, especially pine, presumably caused by a combination of the hygroscopic properties of wood and wood extractives.

Nevertheless, it is difficult to compare the different studies because the very wide disparities between the experimental conditions such as type of wood, surface state of wood, orientation of wood fibres in cutting boards, humidity level, and fouling of wood prior to contamination. Also, the origin and type of micro-organisms used to contaminate surfaces, the method of surface contamination, and sampling methods have a strong impact on the obtained results (Carpentier 1997).

In this study, wooden sawdust of seven different European woods (pine, spruce, larch, beech, maple, poplar, and oak) and polyethylene chips was inoculated with Escherichia coli pIE639 and Enterococcus faecium, and survival was measured using cultivation dependent and molecular methods. They found significantly lower survival on wood sawdust, particularly pine and oak.

It’s a simple enough experiment to perform, and there seems to be great enthusiasm for it. A quick Google Scholar search for similar papers turns up more than a hundred hits, many of which are straightforward replications of the same experiment, arriving at the same findings. For example, there is “Hygienic aspects of wood and polyethylene cutting boards regarding food contaminations. A comparison,” by Gehrig, Schnell, Zürcher and Kucera. This study inoculated European maple (Acer pseudoplatanus), beech (Fagus sylvatica), oak (Quercus robur) and polyethylene with Escherichia coli, incubated under different temperature and humidity conditions, and then measured colony forming units. They found that under humid conditions, all surfaces remained heavily contaminated. Under dry condition, the contamination on wooden surfaces decreased dramatically. Examination with scanning electron microscopy revealed a possible mechanism; in normal use (particularly if washed frequently and vigorously), the normally smooth polyethylene surface developed what the authors describe as “a very rough and cavernous surface similar to wood (but with less profound porosity).” Wooden surfaces developed similar structures, but putative reservoirs for bacteria opened up during the drying process.

Of course, this is perhaps missing the central issue in this controversy. Laboratory experiments in which various surfaces are inoculated with various lab strains of various organisms and CFUs and OTUs tabulated are interesting and suggestive, but don’t go far enough to actually elucidate the role these surfaces play in microbial movement ecology — not to mention in public health. The FDA seems to be acting in the absence of any evidence that wooden surfaces in cheese making have, in fact, been vectors an actual outbreak.

There is a good reason the FDA, like the FAA, has traditionally responded to rather than preempted disasters. The regulator and the industry it regulates have finite resources, and those resources must be deployed against real risks. Unfortunately, it is often the case that the only way to know if a risk is real or not to find oneself on the wrong side of it. This is why the FAA thoroughly investigates every aviation accident, why it maintains detailed statistics on its findings, and why it grounds its regulations firmly in these statistics.

Unfortunately, the FDA does not, in fact, investigate every outbreak of food borne illness. In the outbreaks that are investigated, the FDA does not necessarily pinpoint the precise cause. There are simply too many outbreaks, too few investigators, and too few resources. It also doesn’t help that the FDA often doesn’t have jurisdiction to perform any sort of meaningful investigation. In most agricultural settings, they must give way to the USDA, and in many other situations they must give way to state and municipal public health authorities. They do not enjoy the FAA’s unchallenged jurisdiction over their area of responsibility, nor the FAA’s ability to marshal a vast amount of manpower, data and expertise in each of the relatively small number of accidents they investigate. It’s important not to hold the agency responsible for a problem it lacks the authority to solve.

However, this new preemptive approach is problematic. The retrospective approach to regulation taken by the FAA is largely responsible for transforming aviation from one of the most dangerous modes of transportation into the safest thing you can do with your time. So few people die in aviation incidents that it is difficult to compute a meaningful annual average. The retrospective approach works extremely well when the regulator has the authority to carry it out.

If the FDA is going make this preemptive approach work, it will have to get a lot more serious about basic research. Simply noting that wood is porous and might harbor microbes is not a sound basis for preemptive action.
Cheese Aging Cave

* Update : Joanna Reuter from Chert Hollow Farm points out that numbers I pulled from the CDC’s website are estimates of the number of cases of foodborne illness, not confirmed cases. Based on the most current data, the CDC’s estimates for the rates of gastroenteritis from foodborne pathogens are between 6.6 and 12.7 million for major pathogens and 19.8 61.3 million for unspecified agents. That is a confidence interval of about +/- 32% and 73%, respectively, at a 90% credible interval. So, these estimates bracket the rates of infection to within roughly an order of magnitude.

4 thoughts on “Cheeseboards on the chopping block : Survival on wood and plastic surfaces

      1. Hmm. This particular paper wasn’t my really point. My point was that there are papers out there that seem to have been ignored. Dozens, or perhaps hundreds of papers looking at different factors that are pertinent to the FDA’s concerns about how wood interacts with microbes and food. Moreover, the experiments are not difficult to set up (as Nick Loman evidently did something on this in high school). It’s just… weird that the FDA doesn’t seem to have actually run these experiments.

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Russell Neches

A microbiology graduate student at UC Davis, working with Jonathan Eisen @phylogenomics . Studies evolution & ecology. Advocate of Open Hardware & Open Access.