Properties of wood
This is an important part of FEFPEB’s activities. It inherits the strategy of Nordic Timber Council’s Wood-Food project and aims to continue promoting an international platform and network to promote the use and hygienic advantages of wood. It is an essential tool for communicating, collecting and spreading all available information.
Wood has traditionally been used for centuries in the preparation packaging, storage and transportation of food. However, the hygiene credentials of wood have been disputed based on the fact wood is an absorbent and porous material.
Results from R&D projects tell a different story, indicating that wood has excellent hygienic properties. Good manufacturing quality, good handling practice and proper sanitation treatments makes wood a highly suitable material for most applications in the food industries. The results necessitate a review of the existing guidelines and regulations for the use of wood in the food industry.
The network will be the wood sector’s forum for exchange of information and knowledge sharing, aiming a better acceptance of wood in the food industry.
Wood packaging is used to pack, transport, handle, preserve, present and give and added value to the following food products and sectors:
- Fruit and vegetable
- Fish and seafood
- Wines and spirits
- Cheese and dairy
- Meat and conditioned meat
- Bread and bakery
- Dried fruits
On this page we maintain a database of information about the hygiene properties of wood. Here, you can will find information about wood in the food industry as well as the latest news and the most recent research- and development results:
- Tradition vs. Prejudices
- Scientific support
- Species of wood
- Chemical safety
- Low risk of migration
- Migration studies
- Porosity: more pros than cons
- The biofilm on “impermeable” material
- Low biohazard
- Antibacterial effect
- Bactericidal effect
- Wooden vs. plastic crates
- Hygienic use and reuse of box-pallets and pallets
- Rules for handling and storage pallets
Wood is a complex natural material and therefore can interact with food, like any other material for packaging. However, history (and now modern research) shows us that this does not cause problems. Wood in contact with food is traditionally used not only in single-use packages or reusable packaging but also in cutting boards and countertops, utensils and kitchen utensils, kebab skewers, toothpicks, ice pops, wine barrels and more. If we use all these items without trouble, we can do it in packaging, too.
Wood can be perceived as a less suitable material for single-use containers compared with those made of smooth materials. However, comparative studies refute this fact. Wood has even considered more difficult to clean when used for refillable and reusable containers; but effective sanitation protocols have been developed and make it extremely viable.
There is no perfect material for every situation. It is important to know the intrinsic qualities of each material and its suitability for use given factors which include: the type of food, surface and contact time, and operating conditions such as temperature and humidity,.
There remains a significant lack of legislation and methods for characterization of food contact material compared with plastics. This is mainly due to the safety of wood and food security priorities of the European Union.
However, there are many studies on the hygienic properties of wood that confirm that wood is as good as other materials for use in the food industry, whether on pallets, packages or containers.
Various characterizations of wood cutting boards, worktops, pallets, etc. highlight its bactericidal effect by "physical inhibition”, (experts believe the methodologies should be harmonized).
The compilation of wood scientific studies conducted by Schönwälder (2002) and Milling (2005)find that the negative trend on the qualities of wood in contact with food is reversed during the 1990s, when more research finding in favour of wood came about as evaluation techniques developed and scientists began to get a better understanding of phenomena related to food contact materials.
There is a wide range of wood species and the most commonly used are from continental origin: poplar, pine, spruce, beech, ash, oak, etc.
French law has a regulation in 1945, updated in 1980, which includes a list of timber species which are suitable for food contact. Most is hardwood (no conifers), except pine, which was also included as it is used extensively without problems.
The following woods are accepted for contact with all food types:
- Douglas Fir
- Olive trees
- Maritime Pine
- Scots Pine
Solid foods are restricted to poplar, beech, walnut and elm.
There is lack of references to the authorisation of resinous and tropical woods for contact with food, or withstanding the well-established use of properly cured softwood without health problems. Fruit and vegetable pallet boxes or tables to cure cheese are good examples of this.
In the Nordic countries, pine and spruce are traditionally used forfish, meat and dairy products; while Denmark produces most of ice cream, lolly or other food sticks that we use.
In the United States, wooden and kitchen utensils are manufactured with coconut, cherry, mahogany, poplar, walnut, teak, maple, oak, mulberry, pear, elm, apple, yew and other woods. Meanwhile, cutting boards are produced using ash, balsa, basswood, beech, birch, walnut and maple.
Fir, willow, beech or birch with basswood and alder, in good condition, is considered adequate, even for fatty foods. With these woods, it is important to use dry material.
Wood in contact with food is regulated - so it should not be treated with chemicals for preservation or phytosanitary treatment. In some cases in wood packaging manufacture it undergoes a final drying process only (reduction of moisture content below 20%).
There is a clear difference between the level of content of certain natural compounds from that are found in the timber, and the level of those that may result from treatment or contamination.
Special coatings are rarely used in wooden packaging. This is more common in household and kitchen utensils of wood, where certain natural coatings (solvents, waxes and oils)are permitted to improve their properties
The transfer of constituents from materials in contact with food, into the food itself, is called migration. EU has only established migration limits for plastic materials (in 2009)
Although properties and exchange of wood components in the diet may even be desirable, as active material -for instance in food production such as wines, cheeses and so on -, to comply with current legislation wood must not transfer compounds in an amount that would alter the composition or the taste of food or that are likely to pose a risk to human health.
In all packaging materials, the substances with the potential for migration should be identified, and acceptable limits established, using a methodology to analyze the process scientifically. This is well covered in plastic, a synthetic material with increased risk, but very little for other materials.
In wood, the main substances involved in potential migration are natural volatile organic compounds (VOCs) and those that can be extracted by liquids. Wood, as a natural material, has been studied more for its useful extractive substances, rather than the identification of natural VOC potential to be transferred to what scientists call the headspace or gas phase.
Greenaway et al. studied the migration of VOCs from poplar containers to fruits, vegetables, poultry and cheese, identifying polyphenols fenilaldehydes and ketones.
Mousavi, meanwhile, developed a mathematical model to predict migration from spherical and cylindrical wooden forms through gas chromatography and mass spectroscopy, and with benzaldehyde as marker. He concluded that the model, in the absence of published scientific data, would be useful to legislating for wood in contact with food.
Mousavi concludes "After this study, we conclude that the above timber volatile substance canmigrate to food. However, its low concentration mass (between 0.5 and 17 mg / g in the poplar veneer) in packaged food, its form and its diffusion coefficient greatly reduce the risk of poisoning. Moreover, the drying of wood before use reduces further the concentration of pollutants.
Waymel also concluded that different pine species were suitable for food contact based on an assessment after a study ofthe migration of alpha-pinene, the principal monoterpene of wood, along with other compounds such as beta-pinene, 3 carene, camphene, myrcene and limonene.
The structure of wood is complex and mostly porous. Capillary properties and ability to retain moisture in the fibers (hygroscopic), far from being a problem, impart desirable characteristics. People are often unaware of their positive antibacterial effect.
Unfortunately, the exclusion of timber from parts of the food sector, and replacement with smooth materials, is not usually based on scientific evidence but simply on popular prejudice from decision makers that are unaware of wood’s excellent performance under sanitary conditions.
A smooth and impermeable material apparently is not the “panacea” of hygiene, especially in reusable containers. Some bacteria will adhere to hard surfaces, multiply and produce extracellular polymeric substances, called "biofilm"
Bacteria trapped in the "biofilm" can be well protected from active compounds used for cleaning, especially in the presence of fatty deposits. The pathogens are of particular concern, since the biofilm may facilitate their spread across surfaces and their transfer to the product packaging.
Several studies have shown that these cells appear to be more resistant to disinfectants (Schwach & Zottola, 1984, Frank and Koffi, 1990; Wirtanen and Mattila-Sandholm, 1992a, b).
In food there are several possible sources of contamination as well as vectors (contact, and diffusion). Bacteria and fungi are among the biological ones.
As with any other material, temperature and specific humidity conditions are important as they determine the viability of the medium on the proliferation of contaminants. This aspect is directly related to the absorption capacity of wood and the drying effect. The bacteria, according to Schönwälder et al in 2002, are more sensitive than fungi to this effect.
Despite a lack of consensus on the methodology, there are numerous studies on microbial spread, in terms of cross contamination, especially with cutting boards comparing different species of wood (ash, linden, beech, walnut, birch, cherry, balsa, maple oak and others) with polypropylene or polyethylene, stainless steel or ceramics.
Studies on bacteria and different wood surfaces behave identically when it comes to pollution, whatever the type of wood. Differentiation of wood versus other materials is based on its porous structure, and the physical inhibitory effect of capillarity and moisture.
The rapid disappearance of surface contamination in the timber poses a lesser risk of cross contamination to other non-porous surfaces such as plastics. The porosity of the wood is not a source of microcavities and roughness conducive to the growth of bacteria, as would occur with non-porous materials. In fact, some researchers (Gilbert & Watson, 1971, Ak et al. 1994b; Abrishami et al., 1994) have concluded that the shredded plastic surfaces are harder to clean.
The wood packaging for single use and dry (with a moisture content less than 20%) is not conducive to the propagation or survival of micro-organisms (Ak et al. 1994a, 1994b; Abrishami et al. 1994; Revol-Junelles et al. 2005). Moreover, according to Abrishami et al., 1994 wet wood surfaces have a behavior similar to plastic surfaces, despite the fact they are less absorbent than dry surfaces. Ak also concluded that plastic tables could not be considered more hygienic than wood.
Abrishami et al. (1994) showed that 88% of new cells introduced into dry wood were attached after 10 min. In fact, it was seen by scanning electron microscopy (SEM) that many bacteria are associated with the dry regions of the cytoplasm of structural and vegetative tissue of the xylem (woody part of the timber).
Gehrig, Schnell, Zürcher and Kucera in 2002 compared the hygienic aspects of wood and polyethylene cutting boards regarding food contaminations. The study compared the hygenic aspects of cutting boards made of wood (European maple, beech and oak) and polyethylene (PE) and the risk of food contamination through Escherichiacoli bacteria contamination. This showed that a humid environment affected bacteria colonies equally whereas a drier environment lead to less bacteria in porous wood surfaces. On all materials a significant decrease of bacteria count was achieved by manual washing with detergent and brush followed by rinsing under warm water. For wood, an even higher degree of disinfection could possibly be achieved with the microwave method suggested by Park and Cliver (1996).
Schönwälder et al in 2002, and Friedrich et al in 2007, concluded there is a possibility that bacteria are transferred into the timber by absorption with no evidence of subsequent release.
Revol-Juelles et al in 2005 compared the evolution of dry poplar wood crates and glass surfaces contaminated with Escherichia coli or Bacillus cereus spores by impedance microbiology. Physical and chemical properties of the wood greatly and the number of cells rapidly decreased after different contact times, whereas growth occurred in inert and nonporous materials. Wood exhibited growth-inhibiting properties and cells were no longer metabolically active, concluding that the potential for cross contamination of foods stored directly in contact with previously contaminated poplar wood crates is low under experimental conditions.
Moore et al in 2007, refers to a reduction of pollution or proliferation in porous media versus those smooth or nonporous material.
Despite the fact that timber has been questioned for the processing of meat, the meat industry defends the use of wood as a cutting surface and preparation of its products.
In fact, Cliver, based on their own investigations and epidemiological evidence, and other partners, concluded that wooden boards were better than plastic or glass. This was based on data from a case study on Salmonella, which revealed that those using wooden planks in the kitchen are less than half as likely to contract the disease than those who use plastic or glass surfaces.
Cliver concluded that the bactericidal effect of the wood could be a combination of grip and the drying effect on cells.
Chiu et al, Schönwälder et al, Moore et al, Gough et al, and Milling et al, also concluded that: wood surfaces lead to decreased proliferation of inoculated pollutants than other surfaces, and reduces the survival time of bacteria in wood. This is more evident the higher the amount of pollution and the duration of contact. Schönwälder et al in 2002 concluded that these properties are independent of the age of the wood.
The Danish Technological Institute (www.teknologisk.dk), leading a broader joint research with research institutes in the Nordic countries, Germany and Switzerland, inoculated bacteria similar to Salmonella Listeria.en Camphylobacter in different species of wood (oak, pine, Nordic fir , beech and ash) in cutting boards, pallets, containers of fish and food packaging, comparatively with plastic and steel, concluding that the first hada superior bactericidal effect.
This study also highlighted special health qualities of pine, not only for its porosity, but for its antibacterial extracts.
Further investigations proved the survival of selected bacteria (Bacillus subtilis and Pseudomonas fluorescens) which are commonly found in meat, in different conditions in the industry. They were tested on brushed beech, oak and ash, as well as untreated pine and fir wood packaging, with reference to plastic and stainless steel.
The study concluded that the wood reduces the bacterial concentration before plastic and steel, and that wood species behave differentially; oak has a better performance than beech or ash, and Scotch pine is better than fir.
See further information on the study “Part report No 10. Wood plastic and steel- a comparison of hygienic properties” Report_No_10
According to Schönwälder et al in 2002 and Milling et al in 2005, the structure of wood is not the only criterion of differentiation in their response to pollution, since the presence of certain molecules and the chemical composition, are even more than a factor in this differentiation.
For instance, Scots pine shows strong antibacterial properties (Välimaa et al in 2007), even higher than other woods and plastic. In spruce, beech and poplar similar properties are observed, and slightly above or equal to polyethylene. This makes it important to consider the combination of the undifferentiated structure of wood next to the antibacterial properties of each species.
A 2008 study at the Faculty of Science and Technology of New University of Lisbon by researchers Fernando Abrantes, Garcia, and Mendes, held at the food market in the Region of Lisbon (MARL) concluded that wood is as an hygienic packaging material as plastic, compared to what is popularly believed.
The research assessed the microbial contamination through parallel samples of bacterial flora on the surface of boxes of wood (pine) and boxes of plastic (HDPE and PP) containing the same horticultural produce.
They analyzed the most common micro-organisms in plant products that may contaminate the container, or those resulting from normal handling and storage, such as Enterococci, Escherichia coli, Clostridium perfringens, Pseudomonas and Bacillus cereus.
Having determined the bacterial concentration at different temperatures, it was concluded that no significant differences between the bacterial load of wood and plastic.
The study concluded that to prevent proliferation of microorganisms, the sanitation, cleaning and disinfection, as well as care in the storage conditions is important in reused packaging .
Lightwight wooded packaging is considered to be single-useg, although the hygienic reuse of it as transport container is still accepted in several European countries.
Re-use is more widespread in industrial packaging, in agricultural box pallets, for example, or in pallets where there is no direct contact with food.
In this sense, the Danish Technological Institute (www.teknologisk.dk), leading a joint research with Nordic, German and Swiss research institutes, studied the incidence of bacteria on pallets used in 14 food industries (salted fish, meat, dairy, vegetables and bread) on a sample of 15,000 wooden pallets and plastic (polyethylene and high density polyethylene).
The bacterial count on the pallets of different wood species showed to be on average 15% lower than on plastic pallets. The study concluded that wood is a hygienic material because it kills bacteria by offering poorer living conditions than plastic or steel. Similarly, the study concluded that the cleaning of the pallets with pressurised water also kills bacteria.
A Nordic study, financed by Nordic Wood, Nordic Industrial Fund and national funds focused on the comparison of the hygienic properties of six different types of wooden pallets (pine, spruce and beech) and two types of plastic pallets (PE and HD-PE), used for storing and transporting food products. The difference between wood and plastic pallets was negligible when determined by analysis of variance (p>0,05). The pallets were cleaned with high-pressure cold water and the cleaning efficiency was evaluated very good for all kind of pallets tested.
The German Institute for Food Technology tested the antibacterial effect of wood in use, carring out field tests in 14 companies - in the meat, dairy, vegetable and bakery sectors - with commercial wooden pallet, a specially treated hygienic wood pallet and plastic pallets. The pallets were used for six months at the premises as a transport device and periodically checked for their hygienic condition.
In this research project, the antibacterial effect of pine heartwood in a practical application was tested. The results demonstrated the hygienic suitability of wooden pallets in the food industry, showing have much more hygienic surfaces compared to plastic pallets. It also demonstrated the antibacterial effect of pine heartwood.
In order to keep good hygienic conditions there are some general rules for handling and storage of pallets
- Use clean, dry pallets for use with food products
- To avoid biological, physical and chemical contamination, wooden pallets should not be stored unprotected outdoors
- Keep pallets separated – special pallets for hygienic zones
- Use pallet inverters. To avoid contamination, a cheap and easy solution is to use wooden pallets with a slip-sheet on top
- When depalletising one pallet, the receiving pallet also has a slip-sheet on top. The pallets can be kept in separate zones and the slip-sheets can be made of different materials, expendable or reusable.
- One method to clean pallets is to use high pressure water sprinkling. The pallets can be pasteurised by using:
- Heat treatment by adding an additional drying cycle in a kiln
- High temperature treatment
- Microwave technology seems to be one very promising method.