28/07/14 | Posted in cheese & dairy by Bronwen Percival

Wooden boards are widely used for aging cheese in Europe and America. While their use has an established track record for utility and safety, it has also been the subject of debate. Food safety regulations on both sides of the Atlantic require food contact surfaces to be sanitizable, and wooden boards are rough and porous, and therefore difficult to disinfect. Public health authorities have expressed the reasonable concern that wooden boards harbor persistent microbial communities and may therefore act as a vector for contamination.

Despite this potential drawback, there is clear practical evidence that the use of wood for cheese ripening has significant benefits. It is inexpensive and lightweight compared to alternative surfaces such as plastic and stainless steel. It is also resilient; wooden boards may be serviceable for many years if properly maintained.

Most important is wood’s ability to regulate moisture at the cheese surface, which is imperative for proper rind development. Because of their porosity, wooden boards can both absorb and release humidity depending on the circumstances. In an environment that is too dry, they help to maintain humidity levels and prevent damage to the cheese from desiccation. Cheese maturation is a process that involves controlled loss of moisture, and wooden boards play an active role in this process, wicking excess humidity away from damp cheese surfaces early on and preventing moisture from being trapped between the cheese and its support. Such trapped moisture results in soft or wet patches on the rind and provides conditions that are ideal for the proliferation of spoilage microbes.

Rather than an affectation, wood is an essential tool for the proper maturation of natural-rind and many washed-rind cheeses.

Below is a summary of pertinent research papers on biofilms, with references, that help to outline the main principles at work within the system. The questions addressed include:

• What families of bacteria make up the biofilms on wooden boards?
• What effect do typical biofilms on wooden boards used for smear-ripened cheese have on the growth of Listeria monocytogenes?
• What effects do different cleaning techniques have on biofilms?
• Is it possible completely to disinfect boards that have become contaminated with resident populations of Listeria?
• What factors affect the capacity effectively to clean food-contact surfaces more widely regarded as sanitizable, such as stainless steel?

Click here for a summary of relevant journal articles and full references

Practical Implications

Persistent biofilms on wooden boards used for cheese ripening needn’t be regarded as a dangerous or negative phenomenon. Maintaining the boards correctly to promote a healthy population of the right organisms is extremely important; the aim–and challenge–is to maintain cleanliness without sterility. The Reblochon makers studied by Mariani et al. used a scrubbing machine and cold water to clean their boards. Images taken with a scanning electron microscope showed a clear reduction in the numbers of bacteria and fungi on the surface of the boards, and ‘most yeasts and bateria were no longer visitble by [scanning electron microscopy] on the surface.’ Nevertheless, cultural analysis showed that the cleaning and drying process only resulted in a decrease of 0.5 log₁₀CFU/cm² (Mariani et al. 2011).

The same team studied the microbial communities on 50 wooden shelves from eight different farm producers who used similar methods for ripening and board maintenance. They found that the biofilms present at the end of cheese ripening closely matched the populations of the cheese rinds, and while they found low levels of staphylococci and pseudomonads on some of the boards, they found nothing that would present a risk. They concluded from their study that the method used by the Reblochon producers, carefully followed and monitored, was highly effective at ensuring that the boards did not pose a risk (Mariani et al. 2007).

The inhibitory effect of biofilm microflora on Listeria growth was demonstrated repeatedly with boards from multiple facilities (Mariani et al. 2011). The sterilization of wooden boards between each use by definition eliminates this important extra source of protection against the proliferation of pathogens should contamination occur. Particularly striking was the fact that this effect was so widespread and general, rather than specific to a particular facility or individual strains of biofilm microbes.

The study done by Guillier et al. examined the competition from biofilm microflora (BM) taken from various wooden boards and two different strains of Listeria monocytogenes. They did not find that any molecules with an inhibitory effect on Listeria growth were secreted by the biofilm microflora, but they did find that the biofilm microflora grew more rapidly than Listeria under the same conditions, both separately and together. When Listeria and biofilm microflora were grown together, exponential growth was observed in both populations until the point when the biofilm microflora reached the stationary phase. At that point, growth of Listeria stopped too. When Listeria was inoculated into a community where the biofilm microflora had already reached a stationary phase (as would be the case on a clean wooden board), no exponential growth was observed, despite the fact that models would suggest that Listeria would grow under the conditions present (Guillier et al. 2008).

The authors suggest that this is an example of the ‘Jameson Effect’: a phenomenon based on non-specific competition for nutrients that can operate in environments where resident microbes have reached a stationary phase. The fact that the biofilm microflora grew more rapidly than the Listeria under the same conditions meant that they were effective at outcompeting it. The authors pointed out that under real-world conditions, contamination with Listeria is likely to take place at very low levels compared to the resident biofilm microflora on the cleaned boards. Therefore ‘the potential for growth of L monocytogenes on smear cheese wooden shelves is rather limited as long as the biofilm microflora is preserved’. This result is in accordance with the findings of Mariani et al. (2011) above. It could also help explain why the inhibition occurred consistently regardless of which cheese facility the boards came from. The authors also suggest that other undesirable microorganisms (pathogens or spoilage microbes) could be subject to inhibiton by the resident biofilm microflora by the same principle, making this a particularly significant finding.

However, if pathogenic bacteria do take up residence on wooden boards, standard cleaning methods such as scrubbing and the use of strong chemicals are unlikely to be sufficient completely to remove the contamination. In these cases, the boards must be cleaned as thoroughly as possible and then subjected to thorough disinfection using extensive heat treatment. Prolonged heat treatment (kilning or immersion in a boiler) is effective at sanitizing wooden boards contaminated with undesirable microorganisms including Listeria (Zangerl et al. 2010).

Biofilms also form on surfaces more widely regarded as sanitizable by the food industry, such as stainless steel, and make it extremely difficult to remove or destroy microbes that have adhered themselves to the surface (Frank et al. 1990). A study done on cleaning in food production factories found that their standard cleaning procedures (low pressure rinse, disinfection, rinse) only produced a 1.18 log reduction in viable counts on stainless steel surfaces on which biofilms had formed: not far behind the efficacy of cleaning the wooden boards with cold water scrubbing and no chemicals. Vigorous mechanical action (pressure washing or use of a mechanical floor scrubber) was shown to be essential in order to disrupt adhered colonies of cells sufficiently for sanitizers to perform effectively on stainless steel (Gibson et al. 1999). We should not assume that simply using stainless steel or plastic surfaces guarantees a properly ‘sanitized’ or microbiologically healthy working environment.

Conclusions

The majority of these studies have examined wooden boards used for maturing Reblochon (a soft, high-moisture, low-acid cheese with a mixed coryneform/Geotrichum rind that is washed at specific points during its maturation). These, and other washed rind cheeses, are regarded as the highest risk type of cheese to be matured on wood, because their rinds present an ideal physiochemical environment for the proliferation of pathogens like Listeria, and because their rinds are edible, increasing the chance that pathogens present would cause harm to the consumer. As shown above, these studies show that it is entirely possible to use wooden boards for these ‘high-risk’ cheeses safely and successfully.

Other types of cheese, including clothbound hard cheeses like Cheddar, plasticoted Goudas, or lower-moisture natural rinded hard cheeses, should present even less risk. Even if a cheese’s rind is considered inedible, presence of Listeria in a facility that handles food is unacceptable, and careful monitoring and control is just as important.

Regardless of whether stainless steel, plastic, or wood is used for aging cheese, proper monitoring is essential to ensure that inputs, equipment, and environment are free from contamination. A sterile aging environment is not achievable or necessary, and the evidence shows that surfaces without biofilm microflora are more susceptible to supporting pathogen growth. However, producers’ quality systems must demonstrate that risks are under control and that shelves are monitored at the appropriate frequency to verify this. In the unlikely event of contamination, thorough sanitization using prolonged heat treatment (or barring that, replacement of the boards) will ensure that contamination of future batches is prevented. Just as importantly, the initial source of contamination must be identified and eliminated so that it does not recur.

Further Research

The work by Mariani et al. and Guillier et al. on shelves used for soft smear cheeses provides a fascinating picture of the role of ‘positive pressure’ in promoting a healthy microbial environment that promotes cheese safety and resistance to pathogen growth. Further research on how these microbial communities form and persist, and how different board handling and washing practices affect them, will be of immense value in helping outline a set of best practice guidelines to support the maintenance of these healthy biofilms on wooden boards for all types of cheese maturation.

Another review, by Lortal et al. (2014) of the use of wooden tools in cheesemaking (including shelves and wooden vats) can be found here.

Article and photo by Bronwen Percival